CH-5-8 Running Water and Groundwater – Earth Science : Book Guide

This Document Contains Chapters 5 to 8 Chapter 5 Running Water and Groundwater Running Water and Groundwater opens with an examination of the hydrologic cycle and the exchange of water between the oceans, atmosphere, and land. The discussion of running water includes an investigation of drainage basins, river systems, and the factors that control streamflow. Erosion and transportation of sediment are presented, including erosional and depositional features of narrow and wide valleys. Running water concludes with a look at flooding and flood control measures. After an examination of the importance, occurrence, and movement of groundwater, springs, geysers, wells, and artesian wells are investigated. Following a review of some of the environmental problems associated with groundwater, the chapter ends with a look at the formation and features of caves and karst topography. FOCUS ON CONCPTS After reading, studying, and discussing the chapter, students should be able to: 5.1 List the hydrosphere’s major reservoirs and describe the different paths that water takes through the hydrologic cycle. 5.2 Describe the nature of drainage basins and river systems. Sketch four basic drainage patterns. 5.3 Discuss streamflow and the factors that cause it to change. 5.4 Outline the ways in which streams erode, transport, and deposit sediment. 5.5 Contrast bedrock and alluvial stream channels. Distinguish between two types of alluvial channels. 5.6 Contrast narrow V-shaped valleys, broad valleys with floodplains, and valleys that display incised meanders. 5.7 Discuss the formation of deltas, natural levees, and alluvial fans. 5.8 Discuss the causes of floods and some common flood control measures. 5.9 Discuss the importance of groundwater and describe its distribution and movement. 5.10 Compare and contrast springs, wells, and artesian systems. 5.11 List and discuss three important environmental problems associated with groundwater. 5.12 Explain the formation of caverns and the development of karst topography. TEACHING RUNNING WATER AND GROUNDWATER This chapter has fewer actual examples that you can bring to class as in previous chapters. It is not possible, for example, to bring a river or a water table to class. However, the text has many good illustrations of these examples. Utilize these figures to clearly explain all parts of them to your students; what may seem inherently obvious to you may be very confusing to those who have not studied it before. List the values for percent groundwater available for different uses listed under “The Importance of Groundwater.” Have students in groups or individually construct pie charts representing relative distributions or uses of the groundwater to help them see how it is distributed. Use virtual field trips (see Additional Resources) to show students caves and cave development, especially if you are unable to take a real field trip to a cave. For students who have difficulty remembering the difference between stalactites and stalagmites, tell them that stalactites hang “tight” to the ceiling. To help students understand the difference between porosity and permeability, consider doing an inclass demonstration. For porosity, bring one beaker filled with rough gravel and another filled with a finer-grained sediment, such as sand. If you pour water into each, students should be able to see the water filtering more quickly through the coarser sediment. To demonstrate permeability, use a coffee filter rubber banded over one beaker and a piece of wax paper secured over a second beaker. Carefully pour water on each, explaining that while both beakers have pieces of paper over them, one paper is permeable and the other is not; use this as an analogy for soil permeability. When discussing groundwater as a nonrenewable resource, consider completing National Geographic’s Water Footprint Calculator as a group (see Additional Resources) or assign it as homework prior to that class. Most people do not realize how much water they actually use. One analogy you might use for discussing land subsidence caused by groundwater removal is drinking a slushee or similar drink very quickly; if you quickly suck the liquid portion out with a straw, you are left with a lot of compacted ice crystals. Use animations or graphics to demonstrate changes in rivers over time. Many students have never heard of oxbow lakes or yazoo tributaries and will have difficulty envisioning them. CONCEPT CHECK ANSWERS Concept Check 5.1 Describe or sketch the movement of water through the hydrologic cycle. Once precipitation has fallen on land, what paths might it take? Answer: Precipitation may fall into the ocean or into smaller freshwater streams on land. It may fall on the land surface and become runoff or it may fall on the land and penetrate the ground as with infiltration. Water from oceans, streams, or land surfaces may be evaporated back into the atmosphere and plants may transpire water vapor into the atmosphere also. What is meant by the term evapotranspiration? Answer: When plants respire, they respire water in a process called transpiration. Evaporation is when water evaporates off the surface into the atmosphere. Evapotranspiration is the collective term for both of these events that return water to the atmosphere. Over the oceans, evaporation exceeds precipitation, yet sea level does not drop. Explain why. Answer: Precipitation exceeds evaporation over the land. Runoff from the land into the ocean keeps the system in relative stasis. Concept Check 5.2 List several factors that influence infiltration. Answer: Intensity and duration of rainfall, amount of water already in the soil, nature of the surface material, slope of the land, and nature and type of vegetation. Draw a simple sketch of a drainage basin and divide and label each. Answer: See Figure 5.3. A drainage basin, also known as a watershed, is an area of land where all surface water flows to a common outlet. It is divided into tributaries (smaller streams or rivers) that feed into a main river or body of water, typically illustrated as a network of branching lines converging toward a central point. What are the three main parts (zones) of a river system? Answer: Sediment production, sediment transport, and sediment deposition. Prepare a sketch of the four drainage patterns discussed in this section. Answer: See Figure 5.6. Drainage patterns include dendritic (tree-like), trellis (parallel main streams with shorter tributaries), radial (flowing outward from a central point), and rectangular (right-angled bends and tributaries). Concept Check 5.3 Contrast laminar flow and turbulent flow. Answer: Laminar flow is where water flows in more or less of a straight line. Laminar flow is more typical of slower moving streams and unobstructed stream channels. Turbulent flow is where water moves in a more erratic fashion and increased stream velocity generally increases turbulence, as well as obstructions in the path of the moving water. Summarize the factors that influence flow velocity. Answer: • Gradient – how steep or shallow the slope of the stream bed is. • Channel shape, size, and roughness – larger, smoother channels have more rapid flow. • Discharge – how much water is supplied from the drainage basin. What is a longitudinal profile? Answer: This is a cross-section view of a stream from its headwaters to its mouth. A longitudinal profile is a cross-sectional view or diagram that shows the elevation changes of a river or stream along its course from its source (headwaters) to its mouth (where it empties into another body of water or the ocean). It typically depicts how the gradient (slope) of the river changes, as well as the features such as rapids, waterfalls, and meanders along its path. What typically happens to channel width, channel depth, flow velocity, and discharge between the head and mouth of a stream? Briefly explain why these changes occur. Answer: Channel size and discharge generally increase between the head and the mouth. As the stream approaches the mouth, more tributaries feed into the main channel, increasing the discharge. To accommodate the increased water volume, the channel size also increases. Flow velocity increases downstream because of the increase in channel size and discharge, plus the fact that stream roughness decreases also. Concept Check 5.4 List two ways in which streams erode their channels. Answer: Strong water flow can dislodge unconsolidated particles in the channel, causing erosion. Hydraulic force of the streamflow can also cut into the bedrock, eroding the channel. In what three ways does a stream transport its load? Which part of the load moves slowest? Answer: Dissolved load, suspended load, and bed load. Bed load moves the slowest. What is the difference between capacity and competency? Answer: Capacity is the maximum load of solid particles the stream can transport within a certain time frame. Competence is a measure of a stream’s ability to transport particles based on size of the particles. What is settling velocity? What factors influence settling velocity? Answer: Settling velocity is how fast a particle falls through a still fluid. Larger, denser, rounder particles will fall more quickly than smaller, flatter, less dense particles. Concept Check 5.5 Are bedrock channels more likely to be found near the head or the mouth of a stream? Answer: The head. Bedrock channels are more likely to be found near the head of a stream, closer to its source. As a stream flows downstream towards its mouth, it often transitions from a bedrock channel to a wider, deeper channel with sediment (alluvium) accumulation, especially in lower gradient areas. Describe or sketch the development of a meander, including how an oxbow lake forms. Answer: Meanders develop as small meanders migrate onto the floodplain. Velocity and turbulence are greatest on the outside edge of this meander, causing erosion of the edges of the stream and an ultimate meandering path. An oxbow lake is a former meander in a meandering stream that has been cut off. Sometimes the original stream cuts a straight path through the narrow neck of the meander, and sediment gets deposited, isolating the oxbow lake. Describe a situation that might cause a stream to become braided. Answer: When a streamflow slows down, it cannot carry the sediment load it had been. This sediment gets deposited as bars in the stream, causing a braided channel. Concept Check 5.6 Define base level and distinguish between ultimate base level and temporary base level. Answer: Base level is the lowest extent to how deep a stream can erode. Ultimate base level is sea level. Temporary base levels are also called local base levels and include lakes, resistant layers of rock, and main streams that are base levels for their tributaries. Explain why V-shaped valleys often contain rapids and waterfalls. Answer: Rapids and waterfalls occur where there is a significant increase in the stream’s gradient, which is typical of a V-shaped valley where rocks have different erodabilities. Describe or sketch how an erosional floodplain develops. Answer: River meanders generate continuous side-to-side erosion. Eventually this produces a broad, flat valley floor that becomes covered with alluvium during floods. Relate the formation of incised meanders to changes in base level. Answer: As base level drops or the land containing the meanders uplifts, the meanders will start downcutting again, resulting in incised meanders. Concept Check 5.7 What feature may form where a stream enters the relatively still waters of a lake, an inland sea, or the ocean? Answer: A delta. Where a stream enters the relatively still waters of a lake, an inland sea, or the ocean, it may form a feature called a delta. Deltas are sedimentary landforms that form where rivers deposit their sediment load as they flow into standing water, typically creating fan-shaped or triangular landforms. What are distributaries, and why do they form? Answer: Distributaries are several small channels within a delta. They form as the main channel becomes sediment choked and the water finds other ways to make its path to the base level. Briefly describe the formation of a natural levee. How is this feature related to back swamps and yazoo tributaries? Answer: Natural levees are built over time as streams repeatedly flood their banks and leave raised deposits of sediments along the edge of the stream channel. When the area behind the levee floods it is often poorly drained and marshes called back swamps form. Sometimes tributary streams called yazoo tributaries form when they cannot enter a river because the natural levees block them from doing so. How does an alluvial fan differ from a delta? Answer: An alluvial fan occurs where a high gradient stream in highly mountainous areas with narrow valleys opens onto a broad, flat plain or valley floor. Sediment is dumped rapidly in a cone or fan shape. Deltas represent a stream’s more gradual descent to a broader body of water and are marked with distributaries. Concept Check 5.8 Contrast regional floods and flash floods. Which type of flood is more deadly? Answer: Regional floods occur when rivers overflow their banks because of weather such as rain or rapid snow melting. Flash floods happen much more quickly in a more limited area, often due to sudden, heavy rains in conjunction with mountainous areas with rapid runoff. Flash floods are more deadly. List and briefly describe three basic flood-control strategies. What are some drawbacks of each? Answer: • Artificial levees – mounds built on the banks of rivers to artificially increase the amount of water the river can contain. Many are unable to withstand periods of extreme flooding. Flood-control dams – store floodwater and let it out slowly; acts to lower flood crest. Reservoirs created by these dams may end up covering farmland, forests, historic sites, and scenic valleys. They also contribute to erosion downstream by trapping sediment. Channelization – altering a stream channel to speed water flow; this prevents the water from reaching flood height. However, this can cause accelerated erosion of the riverbanks, requiring further intervention. What is meant by a non-structural approach to flood control? Answer: Instead of building structures to control floods, this approach involves good floodplain management by recognizing appropriate flood zone land use. Concept Check 5.9 About what percentage of freshwater is groundwater? How does this change if glacial ice is excluded? Answer: About 30%; if you exclude glacial ice, almost 96%. What are two geologic roles for groundwater? Answer: Erosional agent and water storage/streamflow equalizer. When it rains, what factors influence the amount of water that soaks in? Answer: Steepness of the slope it falls on, nature of the surface material, intensity of the rainfall, and type and amount of vegetation. Define groundwater and relate it to the water table. Answer: Groundwater is fresh water, most of which is beneath the surface. The water table is a subsurface storage region of water buildup. Distinguish between porosity and permeability. Contrast aquifer and aquitard. Answer: Porosity is the percentage of the rock that contains pore spaces. Permeability is a measure of a surface’s ability to transmit a fluid. An impermeable layer that prevents water movement is an aquitard and permeable rocks or sediments that allow free movement of groundwater are aquifers. What factors cause water to follow the paths shown in Figure 5.29? Answer: As you move deeper into the zone of saturation, water pressure increases. Therefore, looping curves followed by water result from a combination of gravity and the tendency to flow towards lower pressure zones. Concept Check 5.10 Describe the circumstances that created the spring in Figure 5.27. Answer: An aquitard exists above the main water table and as water moves downward, some of it collects in the aquitard. A localized saturation zone results that we call a perched water table. What is the source of heat for most hot springs and geysers? Answer: Magma pools and hot igneous rocks. Describe what occurs to cause a geyser to erupt. Answer: Water in the bottom of a geyser is under a great deal of pressure, thus increasing its boiling point. As the water is heated, it expands, and some water is forced out at the surface. This reduces pressure within the chamber, lowers the boiling point, and the process begins again. Relate drawdown to cone of depression. Answer: Drawdown is when the water table around a well is lowered due to withdrawal of water from the well. The resulting depression within the water table is called a cone of depression. In Figure 5.27, two wells are at the same level. Why is one successful and the other not? Answer: The unsuccessful well does not penetrate the zone of saturation. One well may be successful and the other not due to variations in the aquifer's permeability and the specific location of water-bearing layers relative to the wells' positions, influencing the availability of groundwater at each location despite their similar depths. Sketch a simple cross section of an artesian system with a flowing artesian well. Label aquitards, the aquifer, and the pressure surface. Answer: See Figure 5.33. An artesian system cross-section shows an aquifer between two aquitards with a pressure surface above the aquifer and a flowing artesian well where water rises above the aquifer's top due to pressure. Concept Check 5.11 Describe the problem associated with pumping groundwater for irrigation in parts of the High Plains. Answer: The natural recharge of the aquifer is low and there are high irrigation demands for farming in the region; therefore the groundwater has become depleted. Explain why ground may subside after groundwater is pumped to the surface. Answer: As water is removed, water pressure drops, causing the burden of the weight of the land surface above to be put on the sediment. The increased sediment becomes more tightly compressed under the pressure, and the ground subsides. Which aquifer would be most effective in purifying polluted groundwater: coarse gravel, sand, or cavernous limestone? Answer: Sand. A coarse gravel aquifer would generally be most effective in purifying polluted groundwater due to its high permeability, which allows for rapid movement of water and better filtration of contaminants through physical and chemical processes. Concept Check 5.12 How does groundwater create caverns? Answer: Most caverns are made at or below the water table. Acidic groundwater finds lines of weakness in the rock, and slowly dissolves it along those joints. Over much time, enough rock is dissolved to create caverns. How do stalactites and stalagmites form? Answer: As water seeps through a cave’s top, calcite dissolved in the rock precipitates as the water it was dissolved in evaporates. Sometimes this water drips onto a cave’s floor and evaporates, generating stalagmites similarly, except growing upwards instead of downwards. Describe two ways in which sinkholes form. Answer: • Over time, limestone immediately below the soil is dissolved by rainwater that contains carbon dioxide. A roof of a cavern may suddenly collapse under its own weight. GIVE IT SOME THOUGHT ANSWERS A river system consists of three zones, based on the dominant process operating in each part of the river system. On the accompanying illustration, match each process with one of the three zones: a. Sediment production (erosion) Sediment deposition Sediment transport Answer: Zone # 1: a. sediment production; Zone # 2: c. sediment transportation; Zone # 3: b. sediment deposition What factors influence how much of this rain will soak into the ground compared to how much will run off? Answer: Slope of the land, amount and nature of the vegetation present, how saturated the soil already is, how heavy the rainfall is. If you collect a jar of water from a stream, what part of its load will settle to the bottom of the jar? What portion will remain in the water indefinitely? What part of the stream’s load would probably not be represented in your sample? Answer: The suspended load will settle. The dissolved load will remain indefinitely. The bed load would not have been picked up if you just took water from the top of a stream. This satellite image shows portions of the Ohio and Wabash Rivers in May 2011. What is the base level for the Wabash River? What is the base level for the Ohio River? (Hint: Referring to Figure 5.4 will help.) Are either of the base levels you just noted considered ultimate base level? Explain. Answer: Both of these rivers are part of the Mississippi River drainage basin. The base level for both of these rivers is ultimate base level because they both eventually flow into the Gulf of Mexico. The Middle Fork of the Salmon River flows for about 175 kilometers (110 miles) through a rugged wilderness area in central Idaho. Is the river flowing in an alluvial channel or a bedrock channel? Explain. What process is dominant here: valley deepening or valley widening? Is the area shown in this image more likely near the mouth or the head of the river? Answer: a It is a bedrock channel. Underlying strata are exposed and alluvial channels tend to have more sediment deposition features such as braiding. b Valley deepening. c Head of the river. Which one of the three basic rock types (igneous, sedimentary, or metamorphic) is most likely to be a good aquifer? Why? Answer: Best porosity and permeability would come from a sedimentary rock, where grains are not interlocking as in metamorphic or igneous. What is the likely difference between an intermittent stream (one that flows off and on) and a stream that flows all the time, even during extended dry periods? Answer: The main difference between an intermittent stream and one that flows all of the time, even during extended dry periods, is the level of the water table. The intermittent stream has a much lower water table that doesn’t always intersect the stream channel. The cemetery in this photo is located in New Orleans, Louisiana. As in other cemeteries in the area, all of the burial plots are above ground. Based on what you have learned in this chapter, suggest a reason for this rather unusual practice. Answer: Much of New Orleans is situated below sea level, or lower than base level. That would make the cemetery prone to saturation and flooding. During a trip to the grocery store, your friend wants to buy some bottled water. Some brands promote the fact that their product is artesian. Other brands boast that their water comes from a spring. Your friend asks, “Is artesian water or spring water necessarily better than water from other sources?” How would you answer? Answer: I would explain to my friend that neither one has “better” water—springs occur where the water table intersects Earth’s surface while artesian systems represent wells where the water rises above the level where it was first encountered. Also, I would explain to them that in some instances artesian springs occur as a combination of the two phenomena explained above. Imagine that you are an environmental scientist who has been hired to solve a groundwater contamination problem. Several homeowners have noticed that their well water has a funny smell and taste. Some think the contamination is coming from a landfill, but others think it might be a nearby cattle feedlot or chemical plant. Your first step is to gather data from wells in the area and prepare the map of the water table shown here. Based on your map, can any of the three potential sources of contamination be eliminated? If so, explain. What other steps would you take to determine the source of the contamination? Answer: a It cannot be the landfill since the landfill is situated downhill from the houses. b I would investigate if there were other locations at higher elevations from the houses that might be releasing contaminants into the ground. I would check the locality for sewage or highways that might contribute pollution. This black-and-white photo from the 1930s shows Franklin Roosevelt enjoying the hot springs at the presidential retreat at Warm Springs, Georgia. The temperature of these hot springs is always near 32°C (90°F). This area has no history of recent volcanic activity. What is the likely reason these springs are so warm? Answer: There can be a magma pool beneath the surface; there does not necessarily have to be volcanic activity. EXAMINING THE EARTH SYSTEM ANSWERS This photo shows the famous Thousand Springs along the Snake River in southern Idaho. The springs are natural outlets for groundwater. Is there water in this image that can’t be seen? If so, where might it be? Prepare a story that speculates on the answers to the following questions. Suggest several possibilities for both questions: What journeys did the water in this image take to get to this place? What paths might the water follow when it makes its way back to the ocean? Answer: There is water underground that cannot be seen. The water was evaporated from the ocean, condensed in clouds, and fell as rainfall. Some of the rainfall fell directly on the area and infiltrated the soil to the underground water table. Some rain fell elsewhere and via runoff made its way into the local soil. Mountainous regions may also contribute water via ice and snow melt. When the water makes its way back to the ocean, it may be evaporated off the land and rain into the oceans. It may be taken up by plant respiratory processes, transpired into the atmosphere, and precipitated back into the ocean or back onto the land where it runs off into the ocean. Building a dam is one method of regulating the flow of a river to control flooding. Dams and their reservoirs may also provide recreational opportunities and water for irrigation and hydroelectric power generation. This image, from near Page, Arizona, shows Glen Canyon Dam on the Colorado River upstream from the Grand Canyon and a portion of Lake Powell, the reservoir it created. How did the behavior of the stream likely change upstream from Lake Powell? How might the behavior of the Colorado River downstream from the dam have been affected? Given enough time, how might the reservoir change? Speculate on the possible environmental impacts of building a dam such as this one. Answer: The stream would have slowed and begun stream widening processes. Sediment would build up as its path is blocked. Downstream there may have been an increase in erosion, since the source of sediment input will have been blocked. The reservoir might be depleted over time in the event of drought or excessive use for local water needs. Recreational use might pollute the reservoir. It is in an arid area so evaporation could also deplete the reservoir. It can block fish and aquatic life migration patterns and it can trap excess sediments. It changes the nature of the water environment from free-flowing to more slack. Damming can change the habitat of the riparian species that rely on the banks of the river and it can alter the deltas and downstream environments that previously relied on river flow from upstream. Imagine a water molecule that is part of a groundwater system in an area of gently rolling hills in the eastern United States. Describe some possible paths the molecule might take through the hydrologic cycle if: It were pumped from the ground to irrigate a farm field. There was a long period of heavy rainfall. The water table in the vicinity of the molecule developed a steep cone of depression due to heavy pumping from a nearby well. Answer: evaporation and transpiration precipitation runoff and infiltration Combine your understanding of the hydrologic cycle with your imagination and include possible short-term and long-term destinations and information about how the molecule gets to these places via evaporation, transpiration, condensation, precipitation, infiltration, and runoff. Remember to consider possible interactions with streams, lakes, groundwater, the ocean, and the atmosphere. Answer: In the hydrologic cycle, a water molecule may begin its journey as precipitation falling onto land or directly into oceans. If it lands on land, it can infiltrate into the soil (infiltration), percolating downward to recharge groundwater aquifers. Some water may be taken up by plants and released through transpiration back into the atmosphere. Water in lakes and streams may evaporate directly into the atmosphere, while groundwater can slowly flow underground towards rivers or the ocean (groundwater discharge). Eventually, water in rivers and streams flows into larger bodies of water like lakes and oceans (runoff), where evaporation occurs again, completing the cycle. A glance at the map shows that the drainage basin of the Republican River occupies portions of Colorado, Nebraska, and Kansas. A significant part of the basin is considered semiarid. In 1943, the three states made a legal agreement regarding sharing the river’s water. In 1998, Kansas went to court to force farmers in southern Nebraska to substantially reduce the amount of groundwater they used for irrigation. Nebraska officials claimed that the farmers were not taking water from the Republican River and thus were not violating the 1943 agreement. The court ruled in favor of Kansas. Explain why the court ruled that groundwater in southern Nebraska should be considered part of the Republican River system. How might heavy irrigation in a drainage basin influence the flow of a river? Answer: The groundwater in Nebraska flows into the Republican River. Heavy irrigation can deplete the water table, inhibiting the flow of a river. Over the oceans, evaporation exceeds precipitation, yet sea level does not drop. Why? Answer: Sea level does not drop because water is also being added to the sea via runoff and infiltration. Over oceans, evaporation exceeds precipitation, but sea level does not drop significantly because the excess water vapor in the atmosphere eventually returns to the Earth's surface as precipitation. This precipitation can fall back into the oceans or onto land, maintaining a balance where the total amount of water entering the oceans (through precipitation) equals the amount leaving (through evaporation and other processes). Thus, sea level remains relatively stable over time despite variations in evaporation and precipitation rates. ADDITIONAL RESOURCES DVDs and Movies Earth Revealed, Episode 19: Running Water I: Rivers, Erosion and Deposition (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Earth Revealed, Episode 20: Running Water II: Landscape Evolution (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/ resources/series78.html Earth Revealed, Episode 21: Groundwater (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Freshwater: Returning the Colorado River to the Sea. National Geographic, 5 minutes. Available for free streaming from http://video.nationalgeographic.com/video/environment/freshwater/freshwater- colorado-delta/ The Weather (2003) BBC Video. Subheading “Wet” Section 8, “Water Power” shows the power of flash flooding. Websites Virtual Field Trip – Caves. Two Texas cave features are explored in a short, visual virtual field trip that loads easily and is readily understandable. http://www.esi.utexas.edu/outreach/caves/ virtualtours.php Virtual Field Trip – Geyser and groundwater. Although this field trip refers to earthquakes, the pictures show the mass-wasting effects and how this impacted a geyser. http://www.eeducation.psu.edu/geosc10/l2_p4.html Water Footprint Calculator. Figure out how much water you use. From National Geographic. http://environment.nationalgeographic.com/environment/freshwater/change-the-course/waterfootprint-calculator/ Formation of River Meanders and Oxbow Lakes. Animation from Cumbria and Lancashire Education Online, UK. http://www.cleo.net.uk/resources/index.php?ks=4&cur=10 Stream Processes and Floodplain Development. Animation from “Geology Guy” in California. http://geology-guy.com/teaching/iac/animations/stream_processes.htm Chapter 6 Glaciers, Deserts, and Wind Glaciers, Deserts, and Wind begins by examining the types, the movement, and the formation of glaciers. Also presented are the processes of glacial erosion and deposition, as well as the features associated with valley glaciers and ice sheets. The Pleistocene epoch and some indirect effects of Ice Age glaciers are also discussed. The section closes by examining some of the theories that attempt to explain the causes of glacial ages. The study of deserts begins with a review of the roles of weathering and water in arid climates. A discussion of the evolution of much of the Basin and Range region of the United States provides insight into the processes that shape desert landscapes. The chapter ends with an investigation of wind erosion and deposition, including the various types of sand dunes. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 6.1 Explain the role of glaciers in the hydrologic and rock cycles and describe the different types of glaciers, their characteristics, and their present-day distribution. 6.2 Describe how glaciers move, the rates at which they move, and the significance of the glacial budget. 6.3 Discuss the processes of glacial erosion and the major features created by these processes. 6.4 Distinguish between the two basic types of glacial deposits and briefly describe the features associated with each type. 6.5 Describe and explain several important effects of Ice Age glaciers other than erosional and depositional landforms. 6.6 Discuss the extent of glaciation and climate variability during the Quaternary Ice Age. 6.7 Summarize some of the current ideas about the causes of ice ages. 6.8 Describe the general distribution and extent of Earth’s dry lands and the role that water plays in modifying desert landscapes. 6.9 Discuss the stages of landscape evolution in the Basin and Range region of the western United States. 6.10 Describe the ways that wind transports sediment and the features created by wind erosion. 6.11 Explain how loess deposits differ from deposits of sand. Discuss the movement of dunes and distinguish among different dune types. TEACHING GLACIERS, DESERTS, AND WIND To many students, the inclusion of all of these topics within one chapter might not seem logical. However, you can use this opportunity to stress the interconnectedness of all parts of the Earth system by showing students how these topics are, in fact, related. All are geologic processes or features that are somehow related to the local climate. Before beginning the chapter, challenge students to determine the connection between what might seem like random topics that are put together in this chapter. Give them the opportunity to discover the connections themselves before exploring together how these topics are linked. If you live in a part of North America that was once covered by the Laurentide or Cordilleran Ice Sheets of the last Ice Age, ask students what features of the local landscape might have been caused by those glaciers. These may be features they have already observed or features they could potentially observe. If you know of any localities near you that have even large boulders, suggest that your students generate a story about how those features may have formed. If you live where there has not been glacial activity, ask students what glacial features are lacking in your area. For example, much of the southern U.S. lacks glacial erratics such as large boulders that were displaced during the last Ice Age. Some notable locations in the U.S., such as Long Island and Martha’s Vineyard, are terminal moraines from the Laurentide Ice Sheet. Show these to students on a map and ask them to describe what these areas might have looked like 10,000 years ago as the ice sheet was retreating. Students may have difficulty understanding the concept of glacial rebound. It seems unfathomable to them that the weight of ice can depress something as large and (to them) unmovable as Earth’s crust. You can demonstrate this with a small basin of water such as a plastic shoebox, a toy boat or floating container such as a plastic food storage container, and some weights or small toys (Legos work well). Float the empty boat or container in the water. Make a mark where the water touches the outside. Add the weights or toys, or have students do this, and then make a note of the new water line as the boat or container slowly subsides. Explain how Earth’s crust works in a similar, albeit more large scale, fashion. Many students harbor the misconception that Earth’s orbit is highly elliptical. When discussing the Milankovitch theories, it is important to emphasize that Earth actually has an orbit with low eccentricity. One way to help dispel this is to have students sketch in their notebooks their own idea of Earth’s orbital path. Then show them Earth’s actual orbit. Many students who thought the path was highly eccentric may be surprised but will remember this after having been proven wrong. The concept of plastic flow in something as solid as ice is foreign to some students. Silly Putty is a useful tool for teaching this. If you roll some Silly Putty into a ball at the beginning of class, show students the ball, and set it on the table, you can check the ball later in the class or towards the end of the period and show students how, without you touching it, the ball has deformed and started to flow outwards. You can use this as an illustration of plastic flow. New York City’s Central Park is well known, even to people who do not live near it or who are not from there. It has been featured in a number of popular movies including Elf and Enchanted. You might show short clips from these films where the boulders of the park are prominently featured in the scene and discuss how they arrived there. Supplement with pictures of the glacial striations on the boulders as evidence of ice activity (see Additional Resources). If you have a plastic container such as a plastic wash bin or underbed box, you can put a layer of sand in it. Wheeled, lidded underbed boxes are easiest to maneuver when they contain sand. You can demonstrate the formation of ephemeral streams by slightly tilting up one end of the sand-filled box and pouring a volume of water at the top to generate an ephemeral stream. If you decide to use a sand-filled box, you can also sculpt sand dunes if you have a source of wind such as a small fan and/or a hair dryer. It takes some preparation and practice ahead of time to be able to figure out how to best sculpt different dune types. Small fishtank plastic plants make effective “vegetation” for creating parabolic dunes. You can challenge students to try to create various types of dunes also if you have the space and the time. You can compare historical and current satellite imagery to show both glacier retreat and increasing desertification of some areas. As is the case with teaching much of geology, having good pictures and figures, such as those found in the text, is important for helping students virtually explore the world. Be sure that if you are showing pictures or figures, you explain all the features and anything geologically interesting about them. It is easy to assume that it is obvious what you are looking at, but many of these students are new to the topic and have never seen these things before. CONCEPT CHECK ANSWERS Concept Check 6.1 Where are glaciers found on Earth today, and what percentage of Earth’s land surface do they cover? Answer: Glaciers are largely found in polar regions and high mountains and they cover about 10% of Earth’s surface. Describe how glaciers fit into the hydrologic cycle. What role do they play in the rock cycle? Answer: When precipitation falls in cold regions such as those at the poles or at high altitudes, it may become part of a glacier. Glaciers are part of the rock cycle because they act as erosional agents that can accumulate, transport, and deposit sediment. List and briefly distinguish among four types of glaciers. Answer: • Valley (Alpine) glaciers – relatively small, exist in high mountainous areas and occupy valleys that once contained streams. Flow is downvalley. Ice sheets – flow out in all directions from one center. Generally are very large and found at polar latitudes. Ice shelves – exist along parts of the Antarctic coastline. Glacier ice flows into the ocean, creating ice shelves, which are fairly large and flat masses of floating ice. Others – ice caps are on some uplands but smaller than ice sheets, piedmont glaciers form where valley glaciers emerge from their valleys, outlet glaciers can become floating ice shelves and icebergs. What is the difference between an ice sheet, sea ice, and an ice shelf? Answer: Sea ice is frozen seawater and includes the ice that covers the Arctic Ocean. It floats on the ocean. An ice sheet exists over land. An ice shelf extends from the Antarctic continent and while it floats on the ocean, it remains attached to the land. Concept Check 6.2 Describe two components of glacial movement. Answer: Plastic flow involves ductile flow within the ice due to the pressure of the overlying ice mass. Another component is sliding, where the glacial mass slips along the ground. How rapidly does glacial ice move? Provide some examples. Answer: Some glacial ice moves extremely slowly while others move several meters per day. On the Antarctic ice sheet, parts of some outlet glaciers move more than 800 meters (2600 feet) per year but some ice in more interior regions moves less than 2 meters (6.5 feet) per year. What are crevasses, and where do they form? Answer: Crevasses form on the tops of glaciers. These are large gaping cracks that form because the brittle ice cracks as the glacier moves over uneven terrain. Relate glacial budget to two zones of a glacier. Answer: Glacial budget is the balance or lack thereof between the zone of accumulation at the upper end of the glacier and the zone of wastage at the bottom end. Under what circumstances will the front of a glacier advance? Retreat? Remain stationary? Answer: A glacier will advance if the zone of accumulation grows due to increased ice accumulation. A glacier will retreat if warming increases wastage or if snowfall decreases input to the zone of accumulation. When there is a balance between accumulation and wastage, a glacier will remain stationary. Concept Check 6.3 How do glaciers acquire their load of sediment? Answer: Glaciers are erosional agents and scour the land in front of them as they advance, picking up sediment. How does a glaciated mountain valley differ in appearance from a mountain valley that was not glaciated? Answer: A glaciated mountain valley is wide, straight, and U-shaped whereas a river-carved mountain valley is Vshaped. Describe the features created by glacial erosion that you might see in an area where valley glaciers recently existed. Answer: • Hanging valleys – where valleys of melted tributary glaciers are seen above the main melted glacier channel. Cirque – bowl shaped feature. Horn and arête – where two or more cirques carve out an area near the top of a mountain. Fiords – deep inlets from the sea typically found at high latitudes. Concept Check 6.4 What is the difference between till and stratified till? Answer: Till is glacial debris, such as rock, left behind as a glacier melts. Stratified till is when the debris is sorted by size and weight. Distinguish among terminal end moraine, recessional end moraine, and ground moraine. Relate these moraines to the budget of a glacier. Answer: A terminal end moraine is glacial till left behind from the furthest advance of a glacier. These occur when wastage exceeds nourishment. A recessional end moraine is an end moraine that forms if the glacier stabilizes during its retreat. In these cases, accumulation temporarily exceeds wastage, but then wastage again takes over. A ground moraine is a gently rolling layer of till that is deposited as a glacier retreats when wastage exceeds accumulation. Describe the formation of a medial moraine. Answer: When two valley glaciers merge into a single ice stream, the lateral moraines of each create a stripe of debris in the middle portion of the newly coalesced glacier. List four depositional features other than moraines. Answer: Outwash plains, kettles, drumlins, eskers, valley trains, kames. Concept Check 6.5 List four effects of Ice Age glaciers, aside from the formation of major erosional and depositional features. Answer: Sea level changes, adjustments in Earth’s crust, migration of plants and animals, extinction of some species. Compare the two parts of Figure 6.21 and identify three major changes to the flow of rivers in the central United States during the Ice Age. Answer: Great Lakes drainage basin did not exist, head of the Ohio River only reached Indiana, merging of the Upper Missouri River and Lower Missouri River. Examine Figure 6.23 and determine how much sea level has changed since the Last Glacial Maximum. Answer: Sea level was about 100 meters, or 330 feet, lower than today. Contrast proglacial lakes and pluvial lakes. Answer: Proglacial lakes are formed by trapped meltwater just beyond the outer limits of a glacier or ice sheet. Pluvial lakes are formed in arid or semiarid regions by rain during the cooler, wetter climate of the ice ages. Concept Check 6.6 About what percentage of Earth’s land surface was affected by glaciers during the Quaternary period? Answer: 30% Where were ice sheets more extensive during the Ice Age: the Northern Hemisphere or the Southern Hemisphere? Why? Answer: Northern Hemisphere due to the higher portion of landmasses. Concept Check 6.7 How does the theory of plate tectonics help us understand the causes of ice ages? Answer: Glacial features exist in parts of Earth that are now tropical or subtropical. In addition, similar glacial features exist on separate landmasses, indicating they were likely once joined. Does the theory of plate tectonics explain alternating glacial-interglacial climates during the Quaternary period? Why or why not? Answer: No. The rate of plate movement is very slow and any climate changes caused by moving plates would be on the order of millions of years, not within the span of the Quaternary. Concept Check 6.8 Define dry climate. How extensive are the desert and steppe regions of Earth? Answer: A dry climate is a region where any type of water deficiency exists. Deserts and steppes comprise about 30 percent of Earth’s surface. How does the rate of rock weathering in dry climates compare to the rate in humid regions? Answer: The rate of rock weathering is less in dry climates due to the lack of moisture and lack of plants that secrete organic acids. What is an ephemeral stream? Answer: These are temporary streams that only flow after specific rainfall episodes. When a permanent stream such as the Nile River crosses a desert, does discharge increase or decrease? How does this compare to a river in a humid area? Answer: It decreases. In contrast, rivers in humid areas experience a discharge increase. What is the most important agent of erosion in deserts? Answer: Water. Concept Check 6.9 What is meant by interior drainage? Answer: There is a discontinuous pattern of ephemeral streams that never flow out to the ocean. Describe the features and characteristics associated with each stage in the evolution of a mountainous desert. Answer: Early stage – uplift and steep mountains generate alluvial fans, cones of sediment dumped on flat areas after being transported through steep valleys by streams. Playa lakes may form in the center of relatively flat basin floors after heavy rainfall. Middle stage – when playa lakes evaporate, the sediment left behind is a playa. As alluvial fans enlarge and merge with other alluvial fans, a bajada is formed. Late stage – erosion has worn down much of the mountainous part of the desert. A few large knobs that are mountain remnants may remain; these are called inselbergs. Where in the United States can each stage of desert landscape evolution be observed? Answer: Basin and Range region has all 3. Early stages are found in southern Oregon and northern Nevada. Death Valley and southern Nevada are in the middle stage, and southern Arizona has a late stage. Concept Check 6.10 Why is wind erosion relatively more effective in arid regions than in humid areas? Answer: Lack of vegetation cover makes the soil easier to mobilize, as does the lack of soil binding by moisture. What are blowouts? What term describes the process that creates these features? Answer: These are shallow depressions that occur as wind lifts and removes loose material. The process that creates these is deflation. Briefly describe two hypotheses used to explain the formation of desert pavement. Answer: Deflation lowers the surface of the desert containing poorly sorted deposits. Over time the coarser particles concentrate at the surface. The surface initially is covered with cobbles and pebbles. As dust settles in the spaces between these small rocks, they gradually sift or percolate downward via gravity or infiltrating rainwater. Concept Check 6.11 Contrast loess and sand dunes in terms of composition and how they form. Answer: Loess is windblown silt. It forms extensive blankets. Sand dunes are windblown sand blown into mounds and ridges. Sand dunes accumulate where there is an obstruction blocking the path of the windblown sediments. How are some loess deposits related to glaciers? Answer: Loess is often sediment that was part of glacial deposits. As glaciers retreated, sediment locked in the ice flowed out with the meltwater and was deposited. Describe how sand dunes migrate. Answer: Wind blows sand into dune shapes that are asymmetrical. Sand accumulates on the leeward side of the dune, allowing the slope to steepen. Eventually sand slides under its own weight and the dune slowly migrates. What is cross bedding? Answer: Sand is deposited in regular patterns consistent with the direction the wind is blowing. If the wind changes direction, the pattern of sand deposition also changes, giving the cross section of the dune a crisscross appearance. List and briefly distinguish among basic dune types. Answer: Barchan – U or crescent shaped with tips pointing downwind. Medium sized, form with limited sand supply on flat surfaces. Transverse – long ridges that form where sand is abundant. Form at right angles to prevailing wind and common in arid areas. Barchanoid – scalloped rows of sand at right angles to prevailing wind. Longitudinal – long sand ridges that parallel wind direction. Sand supplies are generally moderate. Parabolic – form where there is some vegetation in the sand. U shaped similar to barchan dunes except tips point upwind. Star – isolated sand hills with complex forms. Wind directions are variable and form sharp-crested ridges of sand. GIVE IT SOME THOUGHT ANSWERS The accompanying diagram shows the results of a classic experiment used to determine how glacial ice moves in a mountain valley. The experiment was carried out over an 8-year span. Refer to this diagram and answer the following: What was the average yearly rate of ice advance in the center of the glacier? About how fast was the center of the glacier advancing per day? What was the average rate at which ice advanced along the sides of the glacier? Why was the rate at the center different than the rate along the sides? Answer: 920 meters / 8 years = 115 meters per year. 115 meters / 365 days = 0.32 meters or 32 centimeters per day. 320 meters / 8 years = 40 meters per year or 40 meters / 365 days = 11 centimeters per day. The center of the glacier moved faster due to the frictional drag along the sides of the valley. Studies have shown that during the Ice Age, the margins of some ice sheets advanced southward from the Hudson Bay region at rates ranging from about 50 to 320 meters per year. Determine the maximum amount of time required for an ice sheet to move from the southern end of Hudson Bay to the south shore of present-day Lake Erie, a distance of 1600 kilometers. Calculate the minimum number of years required for an ice sheet to move this distance. Answer: 1600 kilometers / 50 meters per year = 32,000 years. 1600 kilometers / 320 meters per year = 5000 years. The accompanying image shows the top of a valley glacier in which the ice is fractured. What term is applied to fractures such as these? In what vertical zone do these breaks occur? Do the fractures likely extend to the base of the glacier? Explain. Answer: Crevasses Zone of fracture No, because deeper into the glacier you would find plastic flow obstructing the formation of fractures. If the budget of a valley glacier were balanced for an extended span of time, what feature would you expect to find at the terminus of the glacier? Now assume that the glacier’s budget changes so that wastage exceeds accumulation. How would the terminus of the glacier change? Describe the deposit you would expect to form under these conditions. Answer: You would expect to find a moraine due to the melting of ice at the terminus of the glacier. If wastage begins to exceed accumulation, the glacier would begin to retreat. Ground moraines would form under the glacier as it retreats along with recessional moraines depending on the nature of the retreat. Also, various deposits of stratified drift would form in response to the melting ice. Assume that you and a nongeologist friend are visiting Alaska’s Hubbard Glacier, shown here. After studying the glacier for quite a long time, your friend asks, “Do these things really move?” How would you convince your companion that this glacier does indeed move, using evidence that is clearly visible in this image? Answer: Some evidence that the Hubbard Glacier is really moving include the fractured and broken surface of the ice due to brittle deformation caused by movement, the trail of sediment extending through the center of the photograph where the two glaciers come together, the sediment at the terminus of the glacier that has been deposited by the moving ice, and the dark sediment along the margins of the glacier picked up along the walls of the valley. If Earth were to experience another Ice Age, one hemisphere would have substantially more expansive ice sheets than the other. Would it be the Northern Hemisphere or the Southern Hemisphere? What is the reason for the large disparity? Answer: The Northern Hemisphere would have more expansive ice sheets because there is far more land mass in the Northern Hemisphere than in the Southern Hemisphere. Is either of the following statements true? Are they both true? Explain. Wind does its most effective erosional work in dry places. Wind is the most important agent of erosion in deserts. Answer: Statement “A” is true—wind is more effective in arid regions as compared to humid areas. The loose, dry particles in arid regions are more influenced by wind than are particles in humid places where the moisture serves to make the sediment more cohesive and also heavier. However, statement “B” is not true—water is actually the most important agent of erosion in arid regions. This is an aerial view of the Preston Mesa dunes in northern Arizona. Which one of the basic dune types is shown here? Sketch a simple profile (side view) of one of these dunes. Add an arrow to show the prevailing wind direction and label the dune’s slip face. These dunes gradually migrate across the surface. Describe this process. Answer: Parabolic dunes. The prevailing winds are blowing from the bottom of the photo towards the top. As the higher portion of the dune accumulates sand, eventually the buildup will cause the sand to collapse down the slip face due to gravity. Wind picks up the sand and moves it further along the surface. Alternately, if the vegetation becomes disturbed, sand that was stabilized by the vegetation will be migrated by the wind. Bryce Canyon National Park, shown in the accompanying photo, is in dry southern Utah. It is carved into the eastern edge of the Paunsaugunt Plateau. Erosion has sculpted the colorful limestone into bizarre shapes, including spires called “hoodoos.” As you and a companion (who has not studied geology) are viewing the scenery in Bryce Canyon, your friend says, “It’s amazing how wind has created this incredible scenery!” Now that you have studied arid landscapes, how would you respond to your companion’s statement? Answer: I would respond that wind is not the dominant erosional agent in arid regions and that water was more likely the agent responsible for the most prominent features of the landscape. Compare the sediment deposited by a stream, the wind, and a glacier. Which deposit should have the most uniform grain size? Which one would exhibit the poorest sorting? Explain your choices. Answer: Sediment deposited by streams is generally well-sorted as a function of increasing or decreasing stream velocity. Particles are rounded despite the variety of sizes transported by running water. Wind tends to transport only very fine sediment—silt or sand-sized at best and the particles are often angular and have a frosted appearance. Glacial deposits are the most poorly sorted due to the indiscriminate nature of ice as a transporting medium—boulders and mud may be moved together in the same part of a glacier. The particles show a large disparity of sizes and they are often polished and scratched by the ice. Wind deposits would have the most uniform grain sizes (due to the limited ability to move larger particles) while glacial deposits are the most poorly sorted. EXAMINING THE EARTH SYSTEM ANSWERS Assume that you are teaching an introductory Earth science class and that you have just assigned. Chapter 6 of this text. A student in the class asks why glaciers, deserts, and wind are treated in the same chapter. Formulate a response that connects these topics. Answer: Answers will vary, but these are all topics that are influenced by the prevailing climate. Glaciers, deserts, and wind are linked in Chapter 6 because they all play significant roles in shaping Earth's surface through erosion and deposition processes driven by water and wind dynamics. This image is a close-up of Surprise Glacier in Alaska’s Prince William Sound. Prepare two brief descriptions that relate to Surprise Glacier (and to all other glaciers as well). One should describe how Surprise Glacier fits into the hydrologic cycle, and the second description should explain how Surprise Glacier fits into the rock cycle. Is Surprise Glacier a part of the hydrosphere? Is it a part of the geosphere? Some scientists think that ice should be a separate sphere of the Earth system, called the cryosphere. Does such an idea have merit? Explain. Answer: Because glaciers are water, and they also erode, transport, and deposit sediment, and eventually melt, they can be considered part of the hydrosphere. However, they are also solid and behave as many rocks do and therefore may also be included in the solid Earth. However, since ice and glaciers have their own distinct behaviors and influences on Earth, the concept of a cryosphere is valid. These two images show an ephemeral stream in Niger, a country in North Africa’s Sahara Desert. What is the local term for an ephemeral stream in this part of the world? Prepare a brief story that would explain the contrast between the two images. Try to include all four of Earth’s spheres in your story. Answer: The term for an ephemeral stream there would be a wadi. (Answers will vary.) A rare heavy rainfall has occurred (atmosphere). The soil (geosphere) is unable to soak up all the excess rainfall and runoff flows unhindered and rapidly, generating a temporary flowing water channel (hydrosphere). The lack of vegetation (biosphere) also allows the water to flow rapidly with no obstructions. Wind erosion occurs at the interface of the atmosphere, geosphere, and biosphere and is influenced by the hydrosphere and human activity. With this in mind, describe how human activity contributed to the Dust Bowl, the period of intense wind erosion in the Great Plains in the 1930s. You might find it helpful to research Dust Bowl on the Internet using a search engine such as Google (www.google.com) or Yahoo (www.yahoo.com). You may also find the Wind Erosion Research Unit site to be informative: www.weru.ksu.edu. Answer: The expansion of mechanized agriculture along with prolonged drought left the unprotected farm fields vulnerable to severe wind erosion. The result was soil loss, crop failures, and economic hardship. ADDITIONAL RESOURCES DVDs and Movies Glacial Rock Rap. National Geographic, 2 minutes. Includes a slide show of modern-day remnants of glacial activity. Free streaming from http://video.nationalgeographic.com/video/the-magazine/themagazine-latest/ngm-pet-rock-rap/ Extreme Ice. Nova, PBS. 53 minutes. Explores Arctic glaciers. Available for free streaming from http://www.pbs.org/wgbh/nova/earth/extreme-ice.html. DVD also available for purchase. Fastest Glacier. Nova ScienceNOW, PBS, 7 minutes. Exploration of fast-melting Greenland glacier. Available for free streaming from http://www.pbs.org/wgbh/nova/earth/fastest-glacier.html Booming Sands. Nova ScienceNOW, PBS, 7 minutes. Exploration of sounds made by sand dunes around the world. Available for free streaming from http://www.pbs.org/wgbh/nova/earth/ booming-sands.html Peru with a View. National Geographic, 1.5 minutes. Sand dunes in Peru. Available for free streaming from http://video.nationalgeographic.com/video/places/digital-nomad/b2a_peruwithview/ Earth Revealed, Episode 22: Wind, Dust, and Desert (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Earth Revealed, Episode 23: Glaciers (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Websites Photo Gallery: Desert Landscapes. Various desert landscapes from National Geographic. http://environment.nationalgeographic.com/environment/photos/desert-landscapes/#/black-rockgeyser_87_600x450.jpg Photo Gallery: Antarctica Warming. Pictures of Antarctic glaciers from National Geographic. http://environment.nationalgeographic.com/environment/photos/antarctica-gallery/ Glacier Animation: Accumulation, Ablation, Glacial Landsape Features. Animations of how glaciers form, with a model so you can alter variables and change your own glacier. From University of Kentucky. http://ees.as.uky.edu/sites/default/files/elearning/module13swf.swf Life Cycle of a Glacier Animation. Available as interactive and non-interactive slide show format. From PBS. http://www.pbs.org/wgbh/nova/vinson/glacier.html Glacier Simulation Interactive. Adjust snowfall and temperature and measure glacier thickness, velocity, and budget. From University of Colorado. http://phet.colorado.edu/en/simulation/glaciers Sand Dunes. Includes an animation of how the Great Sand Dunes of the San Luis Valley, U.S.A. formed, plus information about and pictures of sand dunes. From the National Park Service. http://www.nps.gov/grsa/naturescience/sanddunes.htm Great Sand Dunes National Park, Colorado, U.S.A. Photographs from the National Park Service. http://www.nps.gov/media/photo/gallery.htm?id=FB0DAFDA-155D-451F-6723353CA677D3C2 Sand Dune Types. Includes animations showing how some types of dunes form. From the National Park Service. http://www.nps.gov/grsa/naturescience/dune-types.htm Glacial Erratic Photograph, Central Park, NYC. Photograph of a large boulder with glacial striations in New York. http://geology.about.com/od/glaciers_ice/ig/glacier-pictures/erratic.htm Chapter 7 Plate Tectonics: A Scientific Revolution Unfolds Plate Tectonics: A Scientific Revolution Unfolds opens with a discussion of historical views about the placement of continents and ocean basins. It continues by examining the lines of evidence that Alfred Wegener used in the early 1900s to support his original continental drift hypothesis. This evidence included the fit of the continents, fossils, rock types, structural similarities between continents, and paleoclimates. Also presented are the main objections to Wegener’s ideas, particularly the lack of a driving mechanism. Following a brief overview, the theory of plate tectonics is examined in detail. The distinction between Wegener’s idea of continental drift and the science of plate tectonics is addressed. The movement of lithospheric plates and different types of plate boundaries are examined extensively. This chapter looks at the features associated with different types of plate boundaries and how these features shape Earth’s surface. Ages and distribution of ocean-basin sediments, hot spots, and paleomagnetism are used to provide additional support for plate tectonics. The chapter closes with comments about the driving mechanism of plate tectonics. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 7.1 Discuss the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents. 7.2 List and explain the evidence Wegener presented to support his continental drift hypothesis. 7.3 Discuss the two main objections to the continental drift hypothesis. 7.4 List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory. 7.5 Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere. 7.6 Compare and contrast the three types of convergent plate boundaries and name a location where each type can be found. 7.7 Describe the relative motion along a transform plate boundary and locate several examples on a plate boundary map. 7.8 Explain why plates such as the African and Antarctic plates are getting larger, while the Pacific plate is getting smaller. 7.9 List and explain the evidence used to support the plate tectonics theory. 7.10 Describe two methods researchers use to measure relative plate motion. 7.11 Summarize what is meant by plate–mantle convection and explain two of the primary driving forces of plate motion. TEACHING PLATE TECTONICS Plate tectonics is an important topic in geology. It can be more challenging to teach some later book chapters, especially volcanoes, earthquakes, mountain building, and the ocean floor before having established a good foundation in plate tectonics. When you introduce plate tectonics, you may find that many or most students are at least somewhat familiar with the fact that the continents once fit together and have since moved. It is important to stress, however, that this idea has been widely accepted only recently. This is a good opportunity to remind students that while Earth is very old and people have made observations about the Earth for a long time, geology itself is a relatively young science. When discussing Wegener and his hypothesis along with the theory of plate tectonics, this is a good time to revisit or discuss the scientific method with students. You can discuss the difference between a hypothesis and a theory and how a hypothesis might become a theory. The evolution of the continental drift hypothesis to the plate tectonic theory is a nice illustration of the scientific method at work. Mantle upwelling is part of the mantle convection process. You can demonstrate convection with a hot light bulb and a piece of paper cut into a swirl shape. Explain that similar to in the mantle, the warmer substance moves upwards where the cooler substance subsides. Alternately, you can describe the example of a boiling pot of water to relate this topic to something with which students are already familiar or bring in a lava lamp to show this process. The use of animations that show historical continent locations and future projections of continent locations is helpful for visualizing how the surface of the Earth has changed over time due to plate tectonics. See Additional Resources. Animations of motion at the three different types of plate boundaries are helpful supplements to the illustrations in the text. They allow students to see how these plates move relative to each other. After introducing the three types of plate boundaries, have students draw them in their notebooks along with the relative motions of each without referencing anything you have discussed. It will help them reinforce these boundary types and how they differ. When you talk about subduction, it is a good time to revisit the topic of igneous rocks and remind students of the composition of basalt and of granite. Remind students that the denser basalts will be more likely to subduct, and you can relate this back to mineral content. It keeps students from forgetting earlier course concepts and makes the topic more cohesive by tying earlier units into newer ones. If you don’t mind food in your classroom, Oreos or similar sandwich style cookies can be used to show plate motion and plate boundary types. Break the top portion of the cookie in half. It will then slide about on the semi-solid filling layer. This is especially good for illustrating transform fault motion. Print off several copies of a world map with plate boundaries and plate names on it. If you can print this on a large piece of paper, it works a bit better. Cut apart the map at plate boundaries. Distribute one “plate” to each student and have them find other “plates” to create a complete map. This gives students an idea of how many plates exist and where they are found relative to each other. Although earthquakes and volcanoes are prominent plate boundary features, save in-depth discussions of these two topics for the chapters dedicated to them. It is important to mention them in the context of plate tectonics, but tell students you will discuss them in more detail later. CONCEPT CHECK ANSWERS Concept Check 7.1 Briefly describe the view held by most geologists regarding the ocean basins and continents prior to the 1960s. Answer: Ocean basins and continents had fixed geographic positions and were very old. What group of geologists were the least receptive to the continental drift hypothesis? Explain. Answer: North American geologists were the least receptive because most of the evidence was from Africa, South America, and Australia; these continents were unfamiliar to these geologists. Concept Check 7.2 What was the first line of evidence that led early investigators to suspect that the continents were once connected? Answer: Similarity of continental coastlines and jigsaw-like fit of the continents. Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa, but nowhere else, supports the continental drift hypothesis. Answer: Mesosaurus was a freshwater reptile and could not have migrated across the ocean. In addition, there is no evidence of any bridge or other connector between these two continents. Early in the twentieth century, what was the prevailing view of how land animals migrated across vast expanses of open ocean? Answer: Lower sea level and land bridges, rafting, and island stepping stones. How did Wegener account for the existence of glaciers in the southern landmasses at a time when areas in North America, Europe, and Asia supported lush tropical swamps? Answer: Pangaea; southern continents were one landmass situated about the South Pole. Concept Check 7.3 1. What two aspects of Wegener’s continental drift hypothesis were objectionable to most Earth scientists? Answer: Gravitational force of the Sun and Moon moved the continents the way they moved the tides and the idea that continents plowed through the oceanic crust. Concept Check 7.4 What major ocean floor feature did oceanographers discover after World War II? Answer: Mid ocean ridge. After World War II, oceanographers discovered the major ocean floor feature known as mid-ocean ridges. These are underwater mountain ranges where new oceanic crust is formed through volcanic activity. Compare and contrast the lithosphere and the asthenosphere. Answer: The lithosphere is the hard outer portion of the Earth’s crust. The asthenosphere lies beneath and is more ductile, allowing the lithospheric plates to “float” and be mobile. List the seven largest lithospheric plates. Answer: North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic. List the three types of plate boundaries and describe the relative motion at each of them. Answer: Divergent – plates move apart. Convergent – plates move towards each other. Transform – plates slide laterally past each other. Concept Check 7.5 Sketch or describe how two plates move in relation to each other along divergent plate boundaries. Answer: Plates move apart, with new rock being formed between them due to magmatic upwelling. What is the average rate of seafloor spreading in modern oceans? Answer: 5 cm (2 in) per year. List four facts that characterize the oceanic ridge system. Answer: Long, continuous underwater mountain chain, divergent plate boundary, marked by formation of new basaltic rock, rift valleys along the crest. Briefly describe the process of continental rifting. Where is it occurring today? Answer: Mantle plumes beneath continental crust generate magmatic upwelling and forces that pull the crust apart, generating a new rift valley. The rift valley eventually widens to create a new ocean basin. It is occurring today in East Africa. Concept Check 7.6 Explain why the rate of lithosphere production roughly balances with the rate of lithosphere destruction. Answer: Subduction at convergent boundaries, which destroys old lithosphere, occurs at the same rate as seafloor spreading and new rock generation at divergent plate boundaries. Compare a continental volcanic arc and a volcanic island arc. Answer: A continental volcanic arc occurs on the edge of a continental land mass near an ocean-continent convergent plate boundary. These volcanoes tend to produce more felsic to intermediate magma. A volcanic island arc occurs at ocean-ocean plate convergent plate boundaries and produces an island chain in the ocean; these volcanic islands are basaltic. Describe the process that leads to the formation of deep-ocean trenches. Answer: When two oceanic plates collide, one is subducted. As one plate descends beneath the other, a trench is formed; deeper trenches tend to form where older, denser basaltic ocean floor can descend very deeply beneath the other plate, often at very steep angles. Why does oceanic lithosphere subduct, while continental lithosphere does not? Answer: Oceanic lithosphere is denser than continental lithosphere. Continental lithosphere is too buoyant to allow for subduction. Briefly describe how mountain belts such as the Himalayas form. Answer: As two continental plates collide, the crust buckles, thickens vertically, and fractures. This leads to the uplift of major mountain ranges such as the Himalayas. Concept Check 7.7 Sketch or describe how two plates move in relation to each other along a transform plate boundary. Answer: The plates slide laterally past each other. See Figure 7.21 for a diagram. At a transform plate boundary, two plates move horizontally past each other in opposite directions. Imagine two adjacent segments of a mid-ocean ridge extending into the ocean. As these plates slide past one another, they create a transform fault. This fault typically runs perpendicular to the direction of plate movement, forming a boundary where earthquakes frequently occur due to the stress and friction between the plates. This type of movement prevents the creation or destruction of crust and primarily redistributes it horizontally along the Earth's surface. Differentiate between transform faults and the other types of plate boundaries. Answer: Transform faults do not generate new lithosphere or destroy old lithosphere. The movement involves two plates sliding past each other rather than moving apart or coming together. Concept Check 7.8 What two plates are growing in size? Which plate is shrinking? Answer: The African and Antarctic plates are growing and the Pacific plate is shrinking. What new ocean basin was created by the breakup of Pangaea? Answer: The Atlantic. Briefly describe some major changes to the globe when we extrapolate present-day plate movements 50 million years into the future. Answer: North America’s Baja Peninsula and parts of southern California will be closer to Alaska. Africa will collide with Eurasia, closing the Mediterranean Sea. Australia will migrate towards the equator, North and South America will separate, and the Atlantic Ocean basin will be larger. Concept Check 7.9 What is the age of the oldest sediments recovered using deep-ocean drilling? How do the ages of these sediments compare to the ages of the oldest continental rocks? Answer: The oldest ocean sediments are 180 million years old, while the oldest continental rocks are more than 4 billion years old. Assuming that hot spots remain fixed, in what direction was the Pacific plate moving while the Hawaiian islands were forming? When Suiko Seamount was forming? Answer: The Pacific plate was moving northwest when the Hawaiian islands were forming and moving north when the Suiko Seamount was forming. How do sediment cores from the ocean floor support the concept of seafloor spreading? Answer: Sediment cores can be radiometrically dated, and their distance from a spreading center recorded. Data have shown that older cores are further from the mid-ocean ridges while the youngest rocks and sediments are near the ridges. Describe how Fred Vine and D.H. Mathews related the seafloor-spreading hypothesis to magnetic reversals. Answer: They found that new rock made from cooling magma magnetizes itself in the current direction of Earth’s magnetic polarity. They found lateral symmetry in magnetic patterns on either side of mid-ocean ridges, showing that these separated stripes were formed at the same time at the mid-ocean ridges. Concept Check 7.10 What do transform faults that connect spreading centers indicate about plate motion? Answer: These transform faults are aligned parallel to the direction of spreading. Measuring their alignments carefully will reveal the direction of plate movement. Refer to Figure 7.35 and determine which three plates appear to exhibit the highest rates of motion. Answer: The Pacific plate, the Australian-Indian plate, and the Nazca plate. Concept Check 7.11 Describe slab pull and ridge push. Which of these forces appears to contribute more to plate motion? Answer: As old, dense lithosphere subducts, the slab is pulled deep into the mantle by gravity and destroyed. The elevation of mid-ocean ridges allows new lithosphere to slide down; this is ridge push. Slab pull seems to be the dominant force. What role are mantle plumes thought to play in the convective flow in the mantle? Answer: Mantle plumes are thought to originate deep within the mantle, close to the core. The heat from the bottom of these plumes rises through the mantle, generating convective flow. Briefly describe the two models proposed for mantle-plate convection. Answer: Whole-mantle convection – also called the plume model. Subducted lithosphere descends to the coremantle boundary and this is balanced by buoyantly rising mantle plumes that move hot material to the surface. Layer cake model – has two zones of convection; one is a thin, dynamic upper mantle layer and one is slower and thicker in the lower mantle. Slow convection in the lower level carries heat upward but the upper layer is what generates surface volcanism. GIVE IT SOME THOUGHT ANSWERS After referring to the section in the Introduction titled “The Nature of Scientific Inquiry,” answer the following: What observation led Alfred Wegener to develop his continental drift hypothesis? Why did the majority of the scientific community reject the continental drift hypothesis? Do you think Wegener followed the basic principles of scientific inquiry? Support your answer. Answer: The puzzle-like fit of the coastal outlines of South America and Africa. The continental drift hypothesis was rejected by the scientific community because Wegener lacked a credible mechanism to explain why the continents were moving or drifting. Also, he incorrectly proposed that the continents broke through the thinner oceanic crust although no evidence existed to support this idea. Yes, overall Wegener followed the basic principles of scientific inquiry. He proposed evidence to support his various explanations although not all of his evidence supported his conclusions. Referring to the accompanying diagrams that illustrate the three types of convergent plate boundaries, complete the following: Identify each type of convergent boundary. On what type of crust do volcanic island arcs develop? Why are volcanoes largely absent where two continental blocks collide? Describe two ways that oceanic–oceanic convergent boundaries are different from oceanic–continental boundaries. How are they similar? Answer: A = ocean to continent convergence, B = ocean to ocean convergence, C = continent to continent convergence Oceanic crust Volcanoes are absent at continent to continent convergent boundaries because there is no subduction and therefore no mechanism to produce partial melting of mantle rocks. Ocean to ocean convergent boundaries are different from ocean to continent convergent boundaries in that the resulting volcanism occurs on the ocean floor rather than on land and the composition of the volcanoes between the two would be somewhat different due to the interaction of continental rather than oceanic crust. They are similar in that both of them produce volcanism, they both involve subduction zones, and both are characterized by earthquakes. Some predict that California will sink into the ocean. Is this idea consistent with the theory of plate tectonics? Explain. Answer: California will not necessarily sink into the ocean, but the continued motion of the Pacific plate will separate a portion of California from the North American plate along the San Andreas Fault system. The broken segment will then move with the Pacific plate as a large island since it is composed of continental crust rather than oceanic. Refer to the accompanying hypothetical plate map to answer the following questions: How many portions of plates are shown? Are continents A, B, and C moving toward or away from each other? How did you determine your answer? Explain why active volcanoes are more likely to be found on continents A and B than on continent C. Provide at least one scenario in which volcanic activity might be triggered on continent C. Answer: five Continents A, B, and C are moving away from each other because of the divergent boundary that occurs between all three of them. Active volcanoes are found on both A and B because each of them has an oceanic plate that is subducting underneath the continental plate. Continent C does not have volcanoes because it does not have a plate boundary involving subduction or rifting. Hot spot activity or continental rift could perhaps produce volcanoes in the future. Volcanoes, such as the Hawaiian chain, that form over mantle plumes are some of the largest shield volcanoes on Earth. However, several shield volcanoes on Mars are gigantic compared to those on Earth. What does this difference tell us about the role of plate motion in shaping the Martian surface? Answer: The extremely large volcanoes on Mars suggests that either the tectonic plates were moving really slowly when the volcanism occurred or perhaps Mars was lacking tectonic plates and the large volcanoes resulted from hot, stationary plumes inside of Mars. Imagine that you are studying seafloor spreading along two different oceanic ridges. Using data from a magnetometer, you produced the two accompanying maps. From these maps, what can you determine about the relative rates of seafloor spreading along these two ridges? Explain. Answer: Along the first ridge, the plate movements were apparently steady and relatively fast (as evidenced by the narrow magnetic stripes). On the second ridge, the movements are relatively slower over the most recent geologic times, but the plates were apparently moving faster at some point further back in time. Australian marsupials (kangaroos, koala bears, etc.) have direct fossil links to marsupial opossums found in the Americas. Yet the modern marsupials in Australia are markedly different from their American relatives. How does the breakup of Pangaea help to explain these differences (see Figure 7.24)? Answer: The break-up of Pangaea some 200 million years ago allowed for the separation of once-joined landmasses and the isolation of common organisms to undergo a long period of evolutionary development. The resulting organisms today share a common ancestry because of their common origins on Pangaea, but the long period of time has allowed for distantly related groups to develop their own unique characteristics. Density is a key component in the behavior of Earth materials and is especially important in understanding key aspects of plate tectonics. Describe three different ways that density and/or density differences play a role in plate tectonics. Answer: Density or density differences in plate tectonics play a key role in 1) the subduction of more dense oceanic crust underneath continental crust; 2) the rising of hotter mantle material at divergent boundaries and the sinking of colder lithospheric plates at subduction zones, thus creating the “conveyor belt” model that somehow drives the plate tectonics engine; and 3) the collision and uplift of continental crust at continent to continent convergent boundaries due to lower densities of continental rocks. Refer to the accompanying map and the pairs of cities below to complete the following: (Boston, Denver), (London, Boston), (Honolulu, Beijing) List the pair of cities that is moving apart as a result of plate motion. List the pair of cities that is moving closer as a result of plate motion. List the pair of cities that is presently not moving relative to each other. Answer: London, Boston Honolulu, Beijing Boston, Denver EXAMINING THE EARTH SYSTEM ANSWERS The changing positions of the continents and the redistribution of land and water over Earth’s surface have had a significant impact on Earth’s atmosphere, hydrosphere, and biosphere through time. Atmospheric and oceanic circulations are interrelated systems driven by heat energy from the Sun. As continents moved about, the distribution of heat energy over Earth’s surface varied, which, in turn, caused changes in global wind patterns and ocean circulation. Different atmospheric and oceanic circulation patterns produced changes in temperature, precipitation, storm tracks, and global climates in general. Furthermore, when the continents were assembled into large landmasses, their size and location produced climates much different from today. Answer: Life on Earth was greatly affected by the distribution of landmasses and the resulting climates. At times landmasses may have been arid and may have promoted the evolution of certain adaptable species. Then, as a consequence of plate tectonics, the landmass may have split and/or changed global position and become tropical, forcing extinctions and/or adaptations. A good example of the impact of plate tectonics on life is found in the unique species that currently inhabit Australia. Assume that plate tectonics did not cause the breakup of the supercontinent Pangaea. Use the accompanying map of Pangaea to describe how the climate (atmosphere), vegetation and animal life (biosphere), and geologic features (geosphere) would be different from the conditions that exist today in the areas currently occupied by the cities of Miami, Florida; Chicago, Illinois; New York, New York; and your college campus location. Answer: Miami, FL: Miami would be situated on the equator, warmer, and perhaps a tropical rainforest. Without the breakup of Pangaea, Miami would be landlocked and continental rather than having beaches and a coast. Chicago, IL: Chicago would be far removed from any moisture sources (the Atlantic Ocean and Gulf of Mexico do not exist) and therefore rather arid. Its location in the subtropics would probably result in warm temperatures. New York, NY: New York would be part of present northwest Africa and, like Chicago, inland from any sources of moisture, dry, and subtropical. Pre-Pangaea continental collisions would have formed the Appalachian Mountains. Your college campus: (answers will vary) ADDITIONAL RESOURCES DVDs and Movies How the Earth Was Made (2008) Narrated by Alec Baldwin. History Channel, 1 hour 34 minutes. Inside Planet Earth (2008) Narrated by Patrick Stewart. Discovery Channel, 2 hours. Earth Revealed, Episode 5: The Birth of a Theory (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Earth Revealed, Episode 6: Plate Dynamics (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Planet Earth, Episode 1: The Living Machine (1986) WQED/Pittsburgh in association with the National Academy of Sciences, 1 hour. Discusses plate tectonics. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series49.html Plate Tectonics. Science Channel, 2 minutes. Dr. Neal Driscoll shows California coastline features to Bill Nye. Available for free streaming from http://www.sciencechannel.com/tv-shows/greatestdiscoveries/videos/100-greatest-discoveries-plate-tectonics.htm When Earth Erupts: Glaciers vs. Tectonics. Science Channel, 2 minutes. Exploration on New Zealand’s South Island. Available for free streaming from http://www.sciencechannel.com/tvshows/greatest-discoveries/videos/when-earth-erupts-glaciers-vs-tectonics-video.htm Peaceful Easy Obduction. Musical discussion of plate tectonic motion performed by Dr. Richard Alley of Penn State University. 5 minutes. http://www.youtube.com/watch?v=f_BWRlx6VpA Websites Plate Tectonics Animations. From the University of California, Berkeley. http://www.ucmp.berkeley.edu/geology/tectonics.html Mountain Maker, Earth Shaker. From PBS. Interactive plate tectonics activity that allows manipulation of tectonic plates. http://www.pbs.org/wgbh/aso/tryit/tectonics/# Thirteen different plate tectonics animations. From the U.S. Geological Survey and National Park Service. You can choose the one that best suits your needs. http://www2.nature.nps.gov/geology/ usgsnps/animate/pltecan.html Plate Tectonic Movement Visualizations. From Carleton College. A collection of various visualizations and descriptions of each. http://serc.carleton.edu/NAGTWorkshops/geophysics/ visualizations/PTMovements.html Chapter 8 Earthquakes and Earth’s Interior Earthquakes and Earth’s Interior begins with a brief description of the effects of the major earthquakes that have taken place in California within the last decade. Following an explanation of how earthquakes occur, the types of seismic waves, their propagation, and how they appear on a typical seismic trace are presented. This is followed by a discussion of earthquake epicenters—how they are located and their worldwide distribution. Earthquake intensity and magnitude are also explained. The destruction caused by seismic vibrations and their associated perils introduces a discussion of earthquake prediction. The chapter closes with an explanation of how earthquakes are used to discover Earth’s interior structure and a brief description of Earth’s interior composition. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 8.1 Sketch and describe the mechanism that generates most earthquakes. 8.2 Compare and contrast the types of seismic waves and describe the principle of the seismograph. 8.3 Distinguish between intensity scales and magnitude scales. 8.4 List and describe the major destructive forces that can be triggered by earthquake vibrations. 8.5 Locate Earth’s major earthquake belts on a world map and label the regions associated with the largest earthquakes. 8.6 Compare and contrast the goals of short-range earthquake predictions and long-range forecasts. 8.7 Explain how Earth acquired its layered structure and briefly describe how seismic waves are used to probe Earth’s interior. 8.8 List and describe each of Earth’s major layers. TEACHING EARTHQUAKES Most students are familiar with what an earthquake is. Use this as a starting point for opening discussion of earthquakes by building on any pre-existing knowledge students may have. Some students think the only place earthquakes occur is along transform faults. Have them revisit the concept of plate tectonics and think about what happens along any type of plate boundary and discuss why earthquakes can happen at any plate boundary. You can easily demonstrate the difference between P and S waves using a Slinky toy and a rope. Have a student or another person hold the other end of the slinky or rope to demonstrate the difference in motion between these two types of waves. Sometimes students do not understand how rock, which is brittle, can also deform. Silly Putty is useful for showing how the same substance can have both brittle and ductile properties. Leave a ball of Silly Putty in the front of the room as class is beginning. After 15 to 20 minutes, draw students’ attention to it, as it will have slowly flowed out of a ball shape. This illustrates how rocks can flow. Roll the Silly Putty back into a ball, place it on a hard surface, and hit it hard with a hammer. The Silly Putty will shatter, illustrating the brittle properties of rock. Often earthquakes are seen as rare catastrophic events. While this is true of many large earthquakes, many students do not realize that the Earth is continually shifting. Showing the map of current earthquake activity (see Additional Resources) provides students with real-time data of many small earthquakes happening around the world in real time. Showing a video or animation of tsunami propagation (see Additional Resources) can help students understand the magnitude of tsunami and how far and fast they are capable of travelling. Most students remember the recent Japan earthquake and subsequent tsunami; this is a good starting place for introducing the topic. In order for students to understand why a tsunami differs from a regular ocean wave, it is important to explain that regular ocean waves are generated by the wind and only affect the upper portion of the ocean. Show students how seafloor slips and displaces water on the ocean floor and how this subsequently sets the wave up to encompass the entire depth of the ocean, regardless of how deep. Ask if anyone has ever tried to lift a fishtank or other large vessel filled with water. Then stress that it is the entire mass and volume of the water column of the ocean involved in a tsunami, and this power is highly destructive when it reaches shoreline. If students have a basic understanding of oceanography or ocean waves, you can tell students that tsunami act as shallow water waves even in the deep ocean. Have students figure out the relative proportions of Earth’s different layers by composition and by structure. Provide them with thicknesses of each layer and have them determine and construct a scale model. They can do this in groups or individually in their notebooks or, if you have the space and the inclination, you can have them construct a larger model outside using sidewalk chalk. You can illustrate the concepts of reflection and refraction in the classroom. Putting a straw or a ruler into a clear-sided container of water demonstrates refraction. You can reflect a light beam off a mirror. After demonstrating this, show students the illustration of earthquake wave propagation through Earth’s interior and explain how these physical properties are similar. CONCEPT CHECK ANSWERS Concept Check 8.1 What is an earthquake? Under what circumstances do most large earthquakes occur? Answer: An earthquake is ground shaking caused by sudden and rapid movement along Earth fractures called faults. Most large earthquakes occur when there are huge amounts of stored energy in the rocks that suddenly is released. How are faults, hypocenters, and epicenters related? Answer: Slippage of rocks that generate earthquakes occur along faults. The hypocenter is the place slippage occurs between the rocks and the epicenter is the location on Earth’s surface directly above the hypocenter. Who was the first person to explain the mechanism by which most earthquakes are generated? Answer: H.F. Reid. The first person to explain the mechanism by which most earthquakes are generated was Harry Fielding Reid, an American geophysicist. He proposed the elastic rebound theory in 1906, which explains earthquakes as the sudden release of stored elastic strain energy in the Earth's crust due to the movement along faults. Explain what is meant by elastic rebound. Answer: Rocks can deform when stressed as with build up to an earthquake; when rocks snap back to their original shape after slippage, this is termed elastic rebound. What is the approximate duration of an earthquake that occurs along a 300-kilometer-long fault? Answer: 1.5 minutes. Defend or rebut this statement: Faults that do not experience fault creep may be considered safe. Answer: This is not true; different locations on the same fault may behave differently. Part of any fault may be building up strain and slip, generating an earthquake. What type of faults tend to produce the most destructive earthquakes? Answer: Megathrust faults associated with convergent plate boundaries. Concept Check 8.2 Describe the principle of the seismograph. Answer: The inertia of the suspended weight keeps it motionless while the recording drum is anchored to bedrock. The recording drum vibrates proportionally to the strength of seismic waves. List the major differences between P, S, and surface waves. Answer: P waves are body waves that travel through materials compressionally, like a spring. S waves are shaking waves that oscillate at right angles to the wave motion. P waves travel about 1.7 times faster than S waves. Surface waves are the slowest wave type and can cause the surface of the Earth to shake laterally or vertically. Which type of seismic waves tend to cause the greatest destruction to buildings? Answer: Surface waves tend to cause the greatest damage. Concept Check 8.3 What does the Modified Mercalli Intensity scale tell us about an earthquake? Answer: Intensity of the ground shaking. What information is used to establish the lower numbers on the Mercalli scale? Answer: Seismic records. How much more energy does a magnitude 7.0 earthquake release than a 6.0 earthquake? Answer: 32 times more energy. Why is the moment magnitude scale favored over the Richter scale? Answer: Because the moment magnitude scale estimates total energy released by the quake. Concept Check 8.4 List four factors that affect the amount of destruction that seismic vibrations cause to human-made structures. Answer: Intensity of shaking, duration of the vibrations, nature of the material on which the structures are built, and the nature of the building materials and construction practices of the region. In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes. Answer: Landslides, fire, tsunami. What is a tsunami? How are tsunami generated? Answer: A tsunami is a large ocean wave that spans the depth of the water column. Tsunami are generated when a large slab of seafloor is displaced along a fault, displacing the water above it. Tsunami can be very destructive if they make landfall. List at least three reasons an earthquake with a magnitude of 7.0 might result in more death and destruction than a quake with a magnitude of 8.0. Answer: If a region is on soft sediment, it may amplify the vibrations. Liquefaction may occur and generate destruction. Buildings might not be anchored as firmly or built to withstand earthquake vibrations. Concept Check 8.5 Where does the greatest amount of seismic activity occur? Answer: Along plate boundaries. What type of plate boundary is associated with Earth’s largest earthquakes? Answer: Convergent. Name another major concentration of strong earthquake activity. Answer: Alpine-Himalayan belt. Concept Check 8.6 Are accurate, short-range earthquake predictions currently possible using modern seismic instruments? Explain. Answer: No. Some earthquakes have detectable foreshocks and others do not. Sometimes slippage occurs with no warning that can be predicted. What is the value of long-range earthquake forecasts? Answer: These strategies use evidence that major fault zones slip in a cyclical pattern, producing earthquakes at similar intervals. They are important for establishing building codes for an area. Concept Check 8.7 How did Earth acquire its layered structure? Answer: Differential sorting during early formation of the solar system. Lighter materials became the outermost portion while heavier, denser elements sank into the core. Briefly describe how seismic waves are used to probe Earth’s interior. Answer: Waves change velocities depending upon the material they encounter. For example, waves travel more slowly through hotter material. Velocity also increases with depth and through stiffer rock. Seismic waves are reflected, refracted, and diffracted as they pass through Earth and encounter different layers or boundaries between layers. Concept Check 8.8 How do continental crust and oceanic crust differ? Answer: Continental crust is a less dense, granitic type of rock while oceanic crust is a denser, darker, basaltic rock. Contrast the physical makeup of the asthenosphere and the lithosphere. Answer: The lithosphere is the entire crust and uppermost mantle; it is the stiff and relatively cool outer shell of Earth. The asthenosphere is a weaker sphere that is hotter and more ductile. A small amount of melting make a semi-liquid layer at the top of the asthenosphere. How are Earth’s inner and outer cores different? How are they similar? Answer: They are both rich in iron and nickel. However, the inner core is a solid sphere while the outer core is liquid and generates Earth’s magnetic field. The outer core is slightly cooler than the inner core. GIVE IT SOME THOUGHT ANSWERS Draw a sketch that illustrates the concept of elastic rebound. Develop an analogy other than a rubber band to illustrate this concept. Answer: The concept of elastic rebound explains that earthquakes are caused by excess elastic strain energy being suddenly (catastrophically) released as the highly overstrained rocks snapped back (rebounded) to a state of much lower strain. Cool lithospheric rocks have elastic limits large enough to support earthquakecausing elastic strains. Hence, most earthquakes originate in the lithosphere. Because they are much warmer, asthenospheric rocks begin deforming by flowage (plastic deformation) at much lower stress magnitudes. Therefore, any stored elastic strain energies in the asthenosphere are too small in magnitude to produce a strong earthquake. Other than the idea of a rubber band, a plastic or wooden ruler would illustrate the same concept by bending it and allowing it to snap back or “rebound” to its original shape. The accompanying map shows the locations of many of the largest earthquakes in the world since 1900. Refer to the map of Earth’s plate boundaries in Figure 7.12 (page 218) and determine which type of plate boundary is most often associated with these destructive events. Answer: Most earthquakes occur at convergent plate boundaries. Use the seismogram located in the upper right column to answer the following questions: Which of the three types of seismic waves reached the seismograph first? What is the time interval between the arrival of the first P wave and the arrival of the first S wave? Use your answer from Question b and the travel-time graph on page 255 to determine the distance from the seismic station to the earthquake. Which of the three types of seismic waves had the highest amplitude when they reached the seismic station? Answer: P waves 6 minutes about 3000 miles or 4800 kilometers surface waves You go for a jog on a beach and choose to run near the water, where the sand is well packed and solid under your feet. With each step, you notice that your footprint quickly fills with water but not water coming in from the ocean. What is this water’s source? For what earthquake-related hazard is this phenomenon a good analogy? Answer: The water that fills your footprints is located between the grains of sands or in the pore spaces. This is a good analogy to liquefaction that occurs in water-saturated soils during an earthquake. Make a sketch that illustrates why a tsunami often causes a rapid withdrawal of water from beaches before the first surge. Answer: As the tsunami waves begin to “feel bottom” and the water begins to pile up, the result of the building surge or wave is a rapid withdrawal of water from the beaches prior to the wave coming ashore. This “piling up” effect of the waves pulls the water away from the beach, thus signaling the approach of the main surge. Why is it possible to issue a tsunami warning but not a warning for an impending earthquake? Describe a scenario in which a tsunami warning would be of little value. Answer: It is possible to issue a tsunami warning because they are possible after an earthquake has occurred. In other words, they result from the earthquake event. However, we cannot issue earthquake warnings because there are no precursors or signs that signal an impending quake. A tsunami warning would be of little value if a tsunami-generating earthquake happens right along a coastline; there would be not enough time to react. Using the accompanying map of the San Andreas Fault, answer the following questions: Which of the four segments (1–4) of the San Andreas Fault do you think is experiencing fault creep? Paleoseismology studies have found that the section of the San Andreas Fault that failed during the Fort Tejon quake (segment 3) produces a major earthquake every 135 years, on average. Based on this information, how would you rate the chances of a major earthquake occurring along this section in the next 30 years? Explain. Do you think San Francisco or Los Angeles has the greater risk of experiencing a major earthquake in the near future? Defend your selection. Answer: Segment 2 Chances are good that a major earthquake will occur. Based on long-term earthquake predictions, this segment is statistically likely to experience another earthquake of similar magnitude to the 1857 quake. Los Angeles, because it is located close to both the segment that produced the 1857 quake and the southernmost segment, which is way overdue according to the idea that major quakes occur approximately every 200 years. The accompanying image shows a doubledecked section of Interstate 880 (the Nimitz Freeway) that collapsed during the 1989 Loma Prieta earthquake and caused 42 deaths. About 1.4 kilometers (0.9 mile) of this freeway section, commonly called the Cypress Viaduct, collapsed, while a similar section survived the vibration. Both sections were subsequently demolished and rebuilt as a singlelevel structure, at a cost of $1.2 billion. Examine the map and seismograms from an aftershock that shows the intensity of shaking observed at three nearby locations to answer the following questions: a. What type of ground material experienced the least amount of shaking during the aftershock? What type of ground materials experienced the greatest amount of ground shaking during the same event? Which of the two sections of the Cypress Viaduct shown on the map do you think collapsed? Explain. Answer: Bedrock. Soft mud. Section #1 is on the soft mud. It would have experienced greater shaking and therefore was more likely to collapse. Strike-slip faults, like the San Andreas Fault, are not perfectly straight but bend gradually back and forth. In some locations, the bends are oriented such that blocks on opposite sides of the fault pull away from each other, as shown in the accompanying sketch. As a result, the ground between the bends sags, forming a depression or basin. These depressions often fill with water. a. What name is given to the depression in the accompanying photo? b. Describe what would happen if these two blocks began moving in opposite directions. Answer: Sag basin. The basin would enlarge. Using the Internet, compare and contrast the 2010 Haiti earthquake with the 2011 Japan earthquake. Include magnitude, type of plate boundary, and extent of destruction. Explain why the Japan earthquake produced a tsunami, while the Haiti quake did not. Answer: Results will vary with websites students use, but the Haiti plate boundary is transform while the Japan boundary is convergent. Tsunami require vertical slippage of Earth’s crust, which does not happen at a transform boundary. Here's a comparison of the 2010 Haiti earthquake and the 2011 Japan earthquake based on the specified criteria: 1. Magnitude: • Haiti (2010): Magnitude 7.0. • Japan (2011): Magnitude 9.0. 2. Type of Plate Boundary: • Haiti (2010): The earthquake occurred along a strike-slip transform boundary between the Caribbean Plate and the North American Plate. • Japan (2011): The earthquake occurred along a subduction zone boundary where the Pacific Plate subducts beneath the North American Plate. 3. Extent of Destruction: • Haiti (2010): The earthquake caused widespread devastation in the capital city of Port-au-Prince and surrounding areas, resulting in approximately 230,000 deaths and significant damage to infrastructure. • Japan (2011): The earthquake and subsequent tsunami devastated northeastern Japan, causing around 15,900 deaths, displacement of thousands, and substantial damage to infrastructure, including the Fukushima Daiichi nuclear power plant. 4. Tsunami Generation: • Japan (2011): The earthquake in Japan occurred along a subduction zone where the Pacific Plate was thrusting beneath the North American Plate. This abrupt movement displaced a large volume of water vertically, causing a tsunami. The tsunami waves traveled across the Pacific Ocean and affected many coastal areas. • Haiti (2010): The earthquake in Haiti occurred along a strike-slip transform boundary, where horizontal movement between plates does not typically displace a large volume of water vertically. Therefore, the Haiti earthquake did not generate a significant tsunami. Explanation: The primary reason why the Japan earthquake produced a tsunami while the Haiti earthquake did not lies in the nature of their respective plate boundaries. Subduction zone earthquakes, like the one in Japan, involve the vertical displacement of the seafloor due to the convergence of plates. This sudden movement vertically displaces water above the seafloor, generating tsunami waves that propagate outward. In contrast, transform boundary earthquakes, such as the one in Haiti, primarily involve horizontal movement along faults. This horizontal movement does not displace water vertically to the extent required to generate a tsunami of significant amplitude. In summary, while both earthquakes were devastating in their own right, the differing plate boundary types (subduction zone vs. transform boundary) played a crucial role in determining whether a tsunami was generated and the extent of destruction caused by these natural disasters. Describe the two different ways that Earth’s layers are defined. Answer: Earth’s internal layers are defined based on their composition and their physical properties or behavior (mechanical). Based on the properties of Earth’s layers and the mode of travel of body waves, predict the location in Earth’s interior where waves should (a) travel fastest and (b) travel slowest. Is there an exception for these generalities? Explain your answers. Answer: The outer core of the Earth does not transmit S waves and the reason for this behavior is thought to be because this region is composed of a dense liquid, probably liquid iron. In general, interior or body waves (mainly P waves) would travel fastest in the inner core because the highest densities are thought to occur there. The slowest interior waves would occur in the outer core or perhaps in the asthenosphere where a limited amount of partial melting is thought to occur. EXAMINING THE EARTH SYSTEM ANSWER 1. What potentially disastrous phenomenon often occurs when the energy of an earthquake is transferred from the solid earth to the hydrosphere (ocean) at their interface on the floor of the ocean? When the energy from this event is expended along a coast, how might coastal lands and the biosphere be altered? Answer: Tsunami, or seismic sea waves, are generated when the energy of an earthquake is transferred from the solid Earth to the hydrosphere at their interface on the floor of the ocean. Upon entering shallower coastal water, these destructive waves are slowed, and the water begins to pile up to heights that occasionally exceed 30 meters. The surge of water is capable of extending hundreds of meters inland. Severe wave erosion alters the landforms, and the rapid oceanward retreat of the water moves sediment along the coast. In addition to destroying coastal structures and causing human injuries and casualties, tsunami can severely alter the biosphere by uprooting trees and damaging coastal ecosystems. ADDITIONAL RESOURCES DVDs and Movies Earth Revealed, Episode 9: Earthquakes (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Earthquakes 101. National Geographic, 2 minutes, 38 seconds. Brief tutorial on earthquakes and their relationship to plate tectonics. Available at http://video.nationalgeographic.com/video/environment/ environment-natural-disasters/earthquakes/earthquake-101/ Inside Earthquakes. National Geographic, 2 minutes, 29 seconds. Includes home and security video footage of earthquakes and explanations from a seismologist. http://video.nationalgeographic.com/ video/environment/environment-natural-disasters/earthquakes/inside-earthquake/ Detecting Earthquakes (2010) NOVA Science NOW, PBS, 11 minutes. Geologists look at earthquake detection after the 2010 Haiti earthquake. http://www.pbs.org/wgbh/nova/earth/earthquakedetection.html Japan’s Killer Quake (2011) NOVA, PBS, 53 minutes. An account and investigation of the epic earthquake, tsunami, and nuclear crisis. http://video.pbs.org/video/1863101157/ Rare Video: Japan Tsunami (2011) National Geographic, 3.5 minutes. http://video.nationalgeographic.com/video/news/environment-news/japan-tsunami-2011-vin/ Watch the Line. Earthquake lyrics to a Johnny Cash song, performed by Dr. Richard Alley of Penn State University. http://www.youtube.com/watch?v=Ls2De3yF4Ps Tsunami Propagation. From NOAA. Shows propagation of a tsunami from a 2010 earthquake near Chile. Contains links to other tsunami animations. http://nctr.pmel.noaa.gov/animate.html Websites Earthquake photo gallery. Pictures of destruction from various global locations. From National Geographic. http://environment.nationalgeographic.com/environment/photos/earthquake-general/ Earthquake simulator. Build a building and see if it withstands an earthquake. From The Learning Channel. http://www.tlc.com/tv-shows/other-shows/games-and-more/earthquake-simulator.htm How Shifting Plates Caused the Earthquake and Tsunami in Japan. Animation from the New York Times. http://www.nytimes.com/interactive/2011/03/11/world/asia/maps-of-earthquake-and-tsunamidamage-in-japan.html?_r=0 National Earthquake Information Center. Includes link to current earthquakes map, recent large earthquakes, and today in earthquake history, as well as historic earthquake information. http://earthquake.usgs.gov/regional/neic/ Seismic Monitor. Monitor worldwide earthquakes in real time. http://www.iris.edu/seismon/ Virtual Earthquake. Geology Labs Online. Interactive where you can select the location of your earthquake and then read the seismogram and determine the epicenter. http://www.sciencecourseware.org/ virtualearthquake/vquakeexecute.html Pacific Tsunami Museum Online. Photos, Hilo Bay webcam, student information. http://www.tsunami.org/ Tsunami Visualizations. Assorted short film clips and animations showing tsunami and wave propagation from historical tsunami. http://serc.carleton.edu/NAGTWorkshops/hazards/visualizations/ tsunami.html Elastic rebound animation. From the University of California Santa Barbara. http://projects.eri.ucsb.edu/understanding/elastic/rebound.html Solution Manual for Earth Science Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa 9780321928092, 9780321934437