CH-9-12 Volcanoes and Other Igneous Activity – Earth Science : Book Guide

This Document Contains Chapters 9 to 12 Chapter 9 Volcanoes and Other Igneous Activity Volcanoes and Other Igneous Activity begins with a description of the catastrophic 1980 eruption of Mount St. Helens. A discussion of volcanism and the factors that determine the nature of volcanic eruptions (magma composition, temperature, and amount of dissolved gases) is followed by an examination of the materials that can be extruded during an eruption. The types of volcanic cones, their origins, shapes, and compositions, as well as the nature of volcanic landforms, are also presented. An examination of intrusive igneous activity includes the classification and description of the major intrusive igneous bodies—dikes, sills, laccoliths, and batholiths. The chapter closes with a discussion of the origin and distribution of magma and the relationship between plate tectonics and igneous activity. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 9.1 Compare and contrast the 1980 eruption of Mount St. Helens with the eruption of Kilauea, which began in 1983 and continues today. 9.2 Explain why some volcanic eruptions are explosive and others are quiescent. 9.3 List and describe the three categories of materials extruded during volcanic eruptions. 9.4 Label a diagram that illustrates the basic features of a typical volcanic cone. 9.5 Summarize the characteristics of shield volcanoes and provide one example. 9.6 Describe the formation, size, and composition of cinder cones. 9.7 Explain the formation, distribution, and characteristics of composite volcanoes. 9.8 Discuss the major geologic hazards associated with volcanoes. 9.9 List and describe volcanic landforms other than volcanic cones. 9.10 Compare and contrast these intrusive igneous structures: dikes, sills, batholiths, stocks, and laccoliths. 9.11 Summarize the major processes that generate magma from solid rock. 9.12 Relate the distribution of volcanic activity to plate tectonics. TEACHING VOLCANOES AND OTHER IGNEOUS ACTIVITY Students tend to be naturally drawn to large, spectacular geologic events, and volcanoes are no exception. You can use this leverage to draw students into the topic. Students can relate to viscosity in their everyday world. Consider bringing a jar of honey or other thick liquid and comparing it to water. Have students observe which one is thicker and make their own conclusions about viscosity. You can then relate this to magma viscosities. If you have the space and the means, you can pour a high-viscosity liquid and a low-viscosity liquid into separate pans. Have students observe which flows further the fastest. You can relate these properties to the shapes of shield and composite. Students also understand concentrated gases in carbonated beverages. You can open a new can or bottle of a carbonated beverage in class and ask what the hissing/fizzing noise is. You can relate gases under pressure in a container to gases under pressure in a volcanic setting. This unit is a good place to connect what has been learned about igneous rocks to actual settings in which they form. Using maps, you can show students where different types of igneous rocks are found. For example, you can show that basalts are shown at mid-ocean ridges, granites are in continental interiors, and intermediate rocks tend to form in Pacific Rim volcanoes, where oceanic crust meets continental crust. You can stress that increased silica content in the magma increases the viscosity. It may be helpful to show videos of basaltic lava flow eruptions, such as those in Hawaii (see Additional Resources), to demonstrate the lack of viscosity in low-silica magmas. You might consider showing all or part of the film “Dante’s Peak.” Although the movie itself was fictional, the plot centers on the eruption of a composite cone volcano and contains quite a bit of volcano, magma, and other volcanic-related footage. You can guide students to determine which of the movie scenes were realistic and which illustrate places where the directors exercised artistic license. See Additional Resources for a USGS web page that separates fact from fiction for this movie in particular. Ask students if they know any places they have seen volcanic rocks in commercial use. Often scoria is sold for gas grills as “lava rocks” and pumice is sold as a beauty aid for exfoliating skin. As with many topics in geology, figures and animations are helpful for students who are not familiar with the details of the topic. This is especially relevant for intrusive igneous structures such as batholiths, with which students may be less familiar. When discussing the locations of different types of volcanoes, particularly shield volcanoes and composite cones, it is useful to continuously show maps so that students are reminded of the types of magmas and rocks associated with each volcano type and location. CONCEPT CHECK ANSWERS Concept Check 9.1 1. Briefly compare the May 18, 1980, eruption of Mount St. Helens to a typical eruption of Hawaii’s Kilauea volcano. Answer: The eruption of Mount St. Helens was sudden and violent, generating a blast, ash, and projectiles. Kilauea generates relatively quiet gentle outpourings of more fluid lavas. Concept Check 9.2 Define viscosity and list three factors that influence the viscosity of magma. Answer: Viscosity is a fluid’s resistance to flow. Three factors that affect magma viscosity are temperature, composition, and presence of dissolved gases. Explain how the viscosity of magma influences the explosiveness of a volcano. Answer: More viscous magmas tend to generate more explosive eruptions. List these three magmas in order from most silica rich to least silica rich, based on their compositions: mafic (basaltic), felsic (rhyolitic), intermediate (andesitic). Answer: Felsic, intermediate, mafic. The eruption of what type of magma may produce an eruption column? Answer: Felsic. Why is a volcano that is fed by highly viscous magma likely to be a greater threat to life and property than a volcano supplied with very fluid magma? Answer: Volcanoes with viscous magma tend to produce violent eruptions that include clouds of hot gas and ash. The blast from the eruption can also cause collapse of part of the surrounding area. Concept Check 9.3 Describe pahoehoe and aa lava flows. Answer: Pahoehoe lava flows smoothly and sometimes generates smooth surfaces that look ropy or braided. Aa flows are rough with jagged blocks and spines of volcanic ejecta. How do lava tubes form? Answer: Lava tubes can form in pahoehoe flows when the inner part of the flow remains at a high temperature, allowing lava to flow through, while the outer portion cools and hardens. List the main gases released during a volcanic eruption. What role do gases play in eruptions? Answer: Water vapor, carbon dioxide, nitrogen, and sulfur dioxide. These gases escape as the confining pressure over a magma is released, releasing the gases into the atmosphere. How do volcanic bombs differ from blocks of pyroclastic debris? Answer: Volcanic bombs are emitted as molten rock and cool after being ejected. Pyroclastic debris blocks are ejected as solid materials. What is scoria? How does scoria differ from pumice? Answer: Scoria is a basaltic pyroclastic rock that is dark and riddled with vesicles from magma gas. It is darker and denser than pumice and does not float as pumice might. Concept Check 9.4 How is a crater different from a caldera? Answer: A crater is a small depression at the top of a volcano. A caldera is a crater that has a diameter greater than 1 kilometer (0.6 miles). Distinguish among conduit, vent, and crater. Answer: A conduit is the tube through which magma flows from the magma chamber to the vent. The vent is where magma is emitted from the volcano and a crater is a bowl-like depression at the top of the volcano. What is a parasitic cone, and where does it form? Answer: Parasitic cones form on the flanks of volcanoes and tend to form in mature volcanoes where magma erupts from fissures along the flank or base of the volcano. What is emitted from a fumarole? Answer: Gases. Concept Check 9.5 Describe the composition and viscosity of the lava associated with shield volcanoes. Answer: The lava will be basaltic and have low viscosity. Are pyroclastic materials a significant component of shield volcanoes? Answer: No. Where do most shield volcanoes form – on the ocean floor or on the continents? Answer: Ocean floor. Relate lava tubes to the extent of lava flows associated with shield volcanoes. Answer: Since shield volcanoes are not very steep, their lava flows fast and far from the vent by way of lava tubes. Therefore shield volcanoes tend to have lava tubes associated with them. Where are the best-known shield volcanoes in the United States? Name some examples in other parts of the world. Answer: Hawaii. Canary Islands, Galapagos, and Easter Island. Concept Check 9.6 Describe the composition of cinder cones. Answer: Basaltic; generally loose scoria. How do the size and steepness of slopes of a cinder cone compare with those of a shield volcano? Answer: Cinder cones are much smaller and steeper than shield volcanoes. Over what time span does a typical cinder cone form? Answer: Less than one year for most, less than a month for half. Concept Check 9.7 What zone on Earth has the greatest concentration of composite volcanoes? Answer: Ring of Fire that circles the Pacific Ocean rim. Describe the materials that compose composite volcanoes. Answer: Andesitic magma flows alternating with layers of ash and pyroclastic material. How does the composition and viscosity of lava flows differ between composite volcanoes and shield volcanoes? Answer: Composite volcanoes tend to have intermediate to felsic lava that is very viscous whereas shield volcanoes have basaltic lava that is low in viscosity. Concept Check 9.8 Describe pyroclastic flows and explain why they are capable of traveling great distances. Answer: These are hot gases mixed with ash and large fragments of lava. They are fast moving flows that are driven by gravity down the side of a volcano with little friction between the solid portions and the ground because of the high gas content. What is a lahar? Answer: These are volcanic mud flows that form when volcanic debris becomes saturated with water and flows quickly down the volcano. List at least three volcanic hazards other than pyroclastic flows and lahars. Answer: Volcano-triggered tsunami, volcanic ash stalling aircraft engines, dangerous gases breathed by humans and other animals. Concept Check 9.9 Describe the formation of Crater Lake. Compare it to the calderas found on shield volcanoes such as Kilauea. Answer: Crater Lake formed when a composite cone volcano erupted and the force of the explosion collapsed the crater into a caldera. Rainfall filled the depression, generating a lake. Calderas on shield volcanoes tend to form more gradually due to magma loss from a shallow magma chamber. Pyroclastic flows are associated with what volcanic structure that is not a cinder cone? Answer: Calderas. How do the eruptions that created the Columbia Plateau differ from eruptions that create large composite volcanoes? Answer: The Columbia Plateau was generated from fissure eruptions, or extrusion of basaltic magma from fissures in Earth’s crust. These magmas spread far and wide. Large composite volcanoes are made from andesitic magmas and are generally the result of much more violent eruptive events. Contrast the composition of a typical lava dome and a typical fissure eruption. Answer: A lava dome will tend to be felsic to intermediate while a fissure eruption will emit basaltic lava. What type of volcanic structure is Shiprock, in New Mexico, and how did it form? Answer: Shiprock is a volcanic neck. It consists of igneous rock that crystallized in the vent of a volcano. The less resistant rock surrounding this rock eroded away, leaving the volcanic neck standing high on the landscape. Concept Check 9.10 What is meant by the term country rock? Answer: Country rock is the existing rock that is invaded by magma. Describe dike and sill, using the appropriate terms from the following list: massive, discordant, tabular, concordant. Answer: Both dikes and sills are tabular. They are produced when magma intrudes upon rock with fractures or weak zones. Dikes are discordant, or not aligned with the existing layers where sills are concordant, or aligned with the existing layers. Compare and contrast batholiths, stocks, and laccoliths in terms of size and shape. Answer: Batholiths are the largest and can be as much as 100 km wide. They tend to be wide and bulbous. Stocks are smaller plutonic structures similar in shape to batholiths. Laccoliths are the smallest of the three and tend to be mushroom shaped. Concept Check 9.11 What is the geothermal gradient? Describe how the geothermal gradient compares with the melting temperatures of the mantle rock peridotite at various depths. Answer: The geothermal gradient is the increase in temperature with depth into the upper crust. The temperature at which peridotite melts is higher than the geothermal gradient at every depth. However, the difference between these two is greatest at the surface, they approach similar temperatures at 150 km depth, and then the temperatures become more different again. Describe the process of decompression melting. Answer: As hot mantle rock rises closer to Earth’s surface, it enters a zone where the pressure is reduced. The drop in confining pressure allows the rock to melt without an external heat source because its melting temperature is now changed under different pressure conditions. What role do water and other volatiles play in the formation of magma? Answer: Water and other volatiles lower the melting temperature of magmas. Therefore, rocks will melt more readily with less pressure. Concept Check 9.12 Are volcanoes in the Ring of Fire generally described as quiescent or explosive? Name an example that supports your answer. Answer: These volcanoes are generally explosive. Volcanoes such as Mt. St. Helens are in this ring. How is magma generated along convergent plate boundaries? Answer: As old oceanic crust is subducted, it descends into the lower crust and mantle. This process destroys the old crust, which melts and generates magma beneath the surface. Volcanism at divergent plate boundaries is most often associated with which rock type? What causes rocks to melt in these settings? Answer: Basalt. As the rock rises to the surface, the decrease in confining pressure generates decompression melting. What is the source of magma for most intraplate volcanism? Answer: Mantle plumes, or hot spots. At which of the three types of plate boundaries is the greatest quantity of magma generated? Answer: Divergent. GIVE IT SOME THOUGHT ANSWERS Match each of these volcanic regions with one of the three zones of volcanism (convergent plate boundaries, divergent plate boundaries, or intraplate volcanism): a. Crater Lake Hawaii’s Kilauea Mount St. Helens East African Rift Yellowstone Vesuvius Deccan Plateau Mount Etna Answer: convergent plate boundary intraplate volcanism convergent plate boundary divergent plate boundary intraplate volcanism convergent plate boundary intraplate volcanism convergent plate boundary Examine the accompanying photo and complete the following: What type of volcano is this? What features helped you make a decision? What is the eruptive style of such volcanoes? Describe the likely composition and viscosity of the magma. Which one of the three zones of volcanism is the likely setting for this volcano? Name a city that is vulnerable to the effects of a volcano of this type. Answer: It is a composite or stratovolcano. The shape is the best indication as to the type of volcano although the size is also consistent with a composite volcano. This type of volcano is characterized by explosive eruptions due to the highly viscous magmas/lavas associated with them. Typically they are rhyolitic or andesitic in composition. Composite volcanoes are found mainly at convergent plate boundaries. Seattle, Washington; Mexico City, Mexico; Tokyo, Japan; and Naples, Italy are all cities that could be impacted by future eruptions of composite volcanoes. Divergent boundaries, such as the Mid-Atlantic Ridge, are characterized by outpourings of basaltic lava. Answer the following questions about divergent boundaries and their associated lavas: a. What is the source of these lavas? What causes the source rocks to melt? Describe a divergent boundary that would be associated with lava other than basalt. Why did you choose it, and what type of lava would you expect to erupt there? Answer: Basaltic lavas at divergent boundaries are generated by partial melting in the upper mantle. Melting occurs due to decompression melting as the tectonic plates pull apart. A divergent boundary, such as the East African Rift, could have lavas other than basalt associated with it. The melting here could produce a composition of andesite or perhaps rhyolite because of the partial melting of continental crust. Explain why volcanic activity occurs in places other than plate boundaries. Answer: Volcanism occurs at areas other than plate boundaries (known as intraplate volcanism) due to a rising mass of hotter than normal mantle material that ascends towards the surface. The result is a localized region of volcanism called a hot spot. For each of the accompanying four sketches, identify the geologic setting (zone of volcanism). Which of these settings will most likely generate explosive eruptions? Which will produce outpouring of fluid basaltic lavas? Answer: convergent plate boundary with a continental volcanic arc divergent plate boundary involving oceanic plates divergent plate boundary in a continental plate (continental rifting) intraplate volcanism in an oceanic plate The convergent plate boundary will produce the most explosive volcanism while the divergent plate boundary and intraplate volcanism are both characterized by outpourings of fluid basaltic lava. The following image shows the Buddhist monastery Taung Kalat, located in central Myanmar (Burma). The monastery sits high on a sheersided rock made mainly of magmas that solidified in the conduit of an ancient volcano. The volcano has since been worn away. Based on this information, what volcanic structure do you think is shown in the photo? Would this volcanic structure most likely have been associated with a composite volcano or a cinder cone? Explain how you arrived at your answer. Answer: Vent Composite volcano. A cinder cone is generally easily eroded unconsolidated material and typically does not have magma erupt from the top. A composite volcano would have had a magma chamber, a conduit and a vent in which magma could have cooled into igneous rock, generating this structure. Imagine that you are a geologist charged with the task of choosing three sites where state-of-the-art volcano monitoring systems will be deployed. The sites can be anywhere in the world, but the budget and number of experts you can employ to oversee the operations are limited. What criteria would you use to select these sites? List some potential choices and your reasons for considering them. Answer: Student answers will vary. The primary criteria could be the selection of actively explosive volcanoes that are located near densely populated regions or major cities. Other criteria might include choosing volcanoes that are exhibiting signs of erupting in the near future and volcanoes that have documented histories of major eruptions. Some potential sites to consider based on the above criteria include major cities such as Tokyo, Seattle, or Mexico City, and other areas of active volcanism such as Mt. Etna in Italy or densely populated areas in the Philippines and Indonesia. Criteria for selecting volcano monitoring sites would include active volcanic activity, proximity to populated areas, and historical eruption frequency. Potential sites: Mount Vesuvius (Italy) due to high-risk population, Kilauea (Hawaii) for ongoing activity, and Mount Fuji (Japan) for historical significance and proximity to densely populated areas The accompanying image shows an igneous feature (dark color) located in southeastern Utah that was intruded into horizontal sedimentary strata. What name is given to this intrusive igneous feature? The light bands above and below the dark igneous body are metamorphic rock. Identify this type of metamorphism and briefly explain how it alters rock. (Hint: Refer to Chapter 2, if necessary.) Answer: Sill Contact metamorphism. It alters the rock thermally by contact with a hot magma source. Explain why an eruption of Mount Rainier would be considerably more destructive than the similar eruption of Mount St. Helens that occurred in 1980. Answer: A major eruption of Mt. Rainier, similar to the one that occurred at Mt. St. Helens, would be considerably more destructive because of the close proximity to the densely populated region in and around Seattle, Washington. Much of the human development near Seattle is located on recent volcanic deposits from earlier explosive eruptions. Any future, major eruptions of Mt. Rainier would certainly impact those areas. Possible indications that magma is moving through the crust beneath a volcano include changes in the pattern of volcanic earthquakes, surface inflation of the volcano, changes in the amount and/or composition of gases released from the volcano, and an increase in ground temperatures due to the emplacement of new magma. During a field trip with your geology class, you visit an exposure of rock layers similar to the one sketched on the following page. A fellow student suggests that the layer of basalt is a sill. You disagree. Why do you think the other student is incorrect? What is a more likely explanation for the basalt layer? Answer: The presence of vesicles near the top of the basalt unit suggests that it was extruded onto the surface as a lava flow. Afterwards, the area was submerged and the shale unit was deposited. Inspection of the contact between the shale and basalt units for no evidence of contact metamorphism could further strengthen the argument that the unit is a lava flow. Each of the following descriptions indicates how an intrusive feature appears when exposed at Earth’s surface by erosion. Name the feature. A dome-shaped mountainous structure flanked by upturned layers of sedimentary rocks A vertical wall-like feature a few meters wide and hundreds of meters long A huge expanse of granitic rock forming a mountainous terrain tens of kilometers wide A relatively thin layer of basalt sandwiched between layers of sedimentary rocks exposed on the side of a canyon Answer: laccolith dike batholith sill Mount Whitney, the highest summit (4421 meters [14,500 feet]) in the contiguous United States, is located in the Sierra Nevada batholith. Based on its location, is Mount Whitney likely composed of granitic, andesitic, or basaltic rock? Answer: Granitic. Mount Whitney, located in the Sierra Nevada batholith, is likely composed of granitic rock due to the predominance of granitic intrusions in the Sierra Nevada range, which is characterized by large masses of granite formed from magma intrusions during geological processes. EXAMINING THE EARTH SYSTEM ANSWERS Speculate about some of the possible consequences that a great and prolonged increase in explosive volcanic activity might have on each of Earth’s four spheres. Answer: A great and prolonged increase in volcanic activity will add substantial amounts of volcanic dust to the atmosphere, block sunlight, cause a global lowering of temperatures, and alter the general pattern of atmospheric and oceanic circulation. The reduction in both sunlight and temperature will have a substantial impact on the biosphere, perhaps resulting in mass extinctions. Lowering temperatures will result in increased cloud cover and precipitation. Consequently, erosion will be more pronounced, and additional sediment and water will be added to the oceans. The accompanying image is of Mount Unzen, Japan, a composite volcano that produced several fiery pyroclastic flows between 1900 and 1995. Despite the potential for devastating destruction, humans continue to live on or near active volcanoes such as this. Based on this image, suggest a reason the villagers continue to inhabit this area. Identify the path of the most recent destructive pyroclastic flows and any protective measure that may have been employed to contain them. Answer: Volcanic regions often offer rich, fertile, and productive soils along with majestic scenery. Families may also have ties to a local area and are reluctant to move. Proximity to a water conduit offers opportunities for travel and trade as well as recreation. It appears that a dike has been constructed to contain and channel destructive pyroclastic flows. ADDITIONAL RESOURCES DVDs and Movies Earth Revealed, Episode 13: Volcanism (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 14: Intrusive Igneous Rocks (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/ series78.html Dante’s Peak (1997) Universal Pictures, 108 minutes. Fiction but may be useful for instruction (See Teaching Volcanism). Available on DVD. Volcanoes 101. National Geographic, 3 minutes. Both creators and destroyers, volcanoes prove that beneath its calm surface, Earth remains a living planet. http://video.nationalgeographic.com/video/ environment/environment-natural-disasters/volcanoes/volcanoes-101/ Volcanic Lifestyle. National Geographic, 3 minutes. Life at the base of an active volcano. http://video.nationalgeographic.com/video/environment/environment-natural-disasters/volcanoes/ volcanic-lifestyle/ Ring of Fire. Geology version of a Johnny Cash song, performed by geologist Dr. Richard Alley. 3 minutes. http://www.youtube.com/watch?v=fq22bVmxfuk The Biggest Eruption in the Whole U.S.A. Musical rendition of the eruption of Mount St. Helens in 1980, by Dr. Richard Alley. 5 minutes. http://www.youtube.com/watch?v=qozdaqQMILM Websites Volcano World. Information on current and recent eruptions, plus general volcano information. http://volcano.oregonstate.edu U.S. Volcanoes and Current Activity Alerts. From the U.S. Geological Survey. http://volcanoes.usgs.gov Hawai’i Volcanoes National Park Eruption Updates. From the National Park Service. Current information on the active volcano. Includes links to the NPS-maintained park at the volcano. http://www.nps.gov/havo/planyourvisit/lava2.htm Yellowstone Volcano Observatory. From the U.S. Geological Survey. http://volcanoes.usgs.gov/ observatories/yvo/index.html Hot Spots. Pictures of various hot spot environments. From National Geographic. http://education.nationalgeographic.com/education/encyclopedia/hot-spot/?ar_a=1 Movies: Fact or Fiction? Addresses the plausibility of some of the scenes in the fictional volcano movie “Dante’s Peak.” http://volcanoes.usgs.gov/about/faq/faqmovie.php Chapter 10 Crustal Deformation and Mountain Building Crustal Deformation and Mountain Building begins with a brief examination of the processes of crustal deformation, including the factors that affect rock deformation. The various types of folds (anticlines, synclines, domes, and basins) and faults (both dip-slip and strike-slip) are investigated. Mountain building at convergent boundaries is discussed in detail, including subduction, continental collisions, and accretion of terranes. The chapter concludes with a discussion of fault block mountains and the concept of isostasy. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 10.1 Describe the three types of differential stress. Differentiate stress from strain. Compare and contrast brittle and ductile deformation. 10.2 List and describe five types of folds. 10.3 Sketch and briefly describe the relative motion of rock bodies located on opposite sides of normal, reverse, and thrust faults as well as both types of strike-slip faults. 10.4 Locate and name Earth’s major mountain belts on a world map. 10.5 Sketch a cross section of an Andean-type mountain belt and describe how its major features are generated. 10.6 Summarize the stages in the development of an Alpine-type mountain belt such as the Appalachians. 10.7 Explain the principle of isostasy and how it contributes to the elevated topography of young mountain belts like the Himalayas. TEACHING CRUSTAL DEFORMATION AND MOUNTAIN BUILDING This chapter builds heavily on a good foundation in plate tectonics. The concepts of the different types of differential stress and how they relate to large scale rock deformation should be well understood by students before delving into the topics in this chapter. Three-dimensional models of geologic structures are useful for showing students these features. For example, by using a movable 3-D dip-slip fault model, students can see not only the action of the fault itself but also how the underlying strata move relative to each other. For basins and domes, these models can be helpful for showing students where the older and younger rocks lie relative to each other, which many students find counterintuitive when learning about these structures. When using three-dimensional models, be sure to show the equivalent two-dimensional structure illustration. Some students have difficulty making the connection between these unfamiliar structures illustrated in 2-D and mentally rotating and realigning them. After studying plate tectonics, some students acquire the misconception that only continents move. Stressing the nature of active and passive continental margins can help dispel this myth where necessary, especially if you emphasize that along passive continental margins, the ocean basin is moving along with the continents. Sometimes students think that the ratio of continental landmasses to oceanic basins has remained constant throughout time. When discussing the topic of orogeny, stressing the idea that ocean fossils have been found even atop Mount Everest is a good way to illustrate to students that these ratios can change as the surface of the Earth changes. Isostasy is an easy topic to illustrate in the classroom once students have a basic understanding of how Earth’s lithosphere and asthenosphere interact. A small basin of water, a toy boat or other floating object, and some blocks or weights are all that is needed. Mark the floatation line on the empty boat in the water. Add blocks to it and mark the new water level. Explain that Earth’s crust responds similarly and emphasize how much water, frozen into thick glaciers, weighs as it exerts downward pressure on the crust. Give students an opportunity, either individually or in small groups, to make the connections between different types of stress and some of the different structures that you have discussed from this chapter. If the students need to think about why some structures come about, they are more likely to retain the information and make connections about how these structures formed. Sometimes students are confused by right-lateral and left-lateral faults because they are not sure from which side they should be looking at the fault line. It is easy for you to have a picture that you can invert and show them that it does not matter which side you are on; e.g. a right-lateral fault will have the opposing block moved rightward regardless of from which side you approach it. Because a discussion of orogeny so often incorporates vocabulary including how mountains are uplifted, some students may not fully understand that mountain building and many other Earth crust motions are due primarily to lateral motions. Vertical motion does exist, but it is not the driving factor behind mountain building. Students may acquire the misconception that rock deformation only occurs right at plate boundaries. It is important to stress that deformation occurs in wide zones around plate boundaries and that mountain uplift, where it occurs, happens near the boundary but not directly upon it. CONCEPT CHECK ANSWERS Concept Check 10.1 What is rock deformation? How might a rock body change during deformation? Answer: Rock deformation is a change in the shape or position of a rock in response to differential stress. A rock might develop folds, faults, or joints. List the three types of differential stress and briefly describe the changes they impart to rock bodies. Answer: Compressional – vertically thickens and horizontally shortens the rock. Tensional – pulls apart or elongates rock bodies. Shear – slides and tears the rock in different directions. What type of plate boundary is most commonly associated with compressional stress? Answer: Convergent. Compressional stress is most commonly associated with convergent plate boundaries, where two tectonic plates move towards each other. This movement often results in the collision and subsequent compression of crustal material, leading to the formation of mountain ranges and seismic activity. How is strain different from stress? Answer: Stress is the force that acts to deform rock bodies and strain is the resulting change in shape of the rock body. Describe elastic deformation. Answer: When stress is gradually applied to the rock, the rock is deformed by being stretched but not broken. If stress is removed, the rock returns to its original shape. How is brittle deformation different from ductile deformation? Answer: Brittle deformation is exhibited when rocks break apart into smaller pieces after their elastic limit is reached. Ductile deformation occurs when the rock or object changes shape but does not break or fracture. List and describe the four factors that affect rock strength. Answer: Temperature – higher temperatures make rocks softer and more malleable; therefore they are more prone to ductile deformation. Confining pressure – compresses the rock, making it stronger and harder to break; thus rocks tend to bend rather than fracture. Rock type – mineral composition of the rock. Rocks with weaker mineral composition tend to deform in a ductile manner where stronger mineralogical composition rocks tend to fracture. Time – gradual stress over time tends to produce rocks that show ductile behavior and bending or flowing. Sudden stress may produce fracture. Concept Check 10.2 Distinguish between anticlines and synclines, between domes and basins, and between anticlines and domes. Answer: An anticline is upfolded sedimentary layers, resulting in a ridge-like structure. A syncline is downfolded, or a trough. A dome occurs when upwarping in the basement rock produces a fairly circular uplifted structure of sedimentary layers. A basin results from downwarping, resulting in a bowl shape of sedimentary strata on the landscape. An anticline tends to be an elongated ridge where a dome is more circular. Draw a cross-sectional view of a symmetrical anticline. Include a line to represent the axial plane and label both limbs. Answer: See Figure 10.7. A symmetrical anticline in cross-section would show a U-shaped fold with two limbs that are mirror images of each other. The axial plane would run vertically through the center of the fold, dividing it into two limbs. The Black Hills of South Dakota are a good example of what type of geologic structure? Answer: A dome. The Black Hills of South Dakota are a good example of an uplifted dome structure. This geologic feature consists of a circular or elliptical area where the rock layers have been pushed upward and arched, often exposing older rocks in the center surrounded by younger rocks in concentric layers. Where are the youngest rocks in an eroded basin found: near the center or near the flanks? Answer: Near the center. The youngest rocks in an eroded basin are typically found near the center of the basin where deposition occurred most recently before erosion began to shape the basin's features. Describe the formation of a monocline. Answer: A monocline forms when ancient faults in basement rocks reactivate after sedimentary strata have been deposited over them. As the basement rock is displaced upward, the sedimentary strata fold and drape over the fault. Concept Check 10.3 Contrast the movements that occur along normal and reverse faults. What type of stress is indicated by each fault? Answer: A normal fault is when the hanging block moves down relative to the footwall, and a reverse fault is when the hanging block moves up relative to the footwall. Normal faults are associated with tensional stress and reverse faults are associated with compressional stress. What type of faults are associated with fault-block mountains? Answer: Normal faults. How are reverse faults different from thrust faults? In what way are they similar? Answer: Reverse and thrust faults both have the hanging wall block moving up relative to the footwall but differ in that thrust faults have dips less than 45 degrees where reverse faults do not. Describe the relative movement along a strike-slip fault. Answer: The displacement is horizontal and parallel to the direction of the fault. How are joints different from faults? Answer: Joints are fractures along which no displacement has occurred. Concept Check 10.4 Define orogenesis. Answer: Orogenesis is another term for mountain building. In the plate tectonics model, which type of plate boundary is most directly associated with Earth’s major mountain belts? Answer: Convergent. Earth's major mountain belts are most directly associated with convergent plate boundaries, where tectonic plates collide and crustal material is thrust upwards, leading to the formation of mountain ranges through compression and uplift. Concept Check 10.5 The formation of mountainous topography at a volcanic island arc is considered just one phase in the development of a major mountain belt. Explain. Answer: Some volcanic arcs are carried by subducting plates to the margin of continental blocks, where they become involved in large-scale mountain-building episodes. Describe and give an example of a passive continental margin. Answer: A passive continental margin is the edge of a continent that is not a plate boundary. One example is the East Coast of the United States. In what ways are the Sierra Nevada and the Andes similar? Answer: They are both generated along an Andean-type subduction zone. What is an accretionary wedge? Briefly describe its formation. Answer: When volcanic arcs are created, sediments on the subducted plate may be scraped off and plastered against the edge of the overriding plate. The resulting wedge-shaped sediment is called an accretionary wedge. What is a batholith? In what modern tectonic setting are batholiths being generated? Answer: A batholith is a massive igneous pluton that forms when magma hardens beneath Earth’s crust. The Sierra Nevada of California is one modern tectonic setting where batholiths are being generated. How are magmas that exhibit an intermediate-to-felsic composition thought to be generated from mantle-derived basaltic magmas? Answer: As basaltic magmas ascend through the crust, they undergo magmatic differentiation. Heavier ferromagnesian minerals settle out while lighter silica-rich minerals rise to the surface as intermediate or felsic. Concept Check 10.6 Differentiate between terrane and terrain. Answer: A terrane is an accretion of small crustal fragments that have been carried to the continental margin. Terrain describes the shape of surface topography. During the formation of the Himalayas, the continental crust of Asia was deformed more than India proper. Why was this the case? Answer: Much of India is a very old, strong Precambrian continental shield whereas the crust of Asia was much younger and had less strength to resist the collision. Where might magma be generated in a newly formed collisional mountain belt? Answer: In the deepest and most deformed part of the new mountain belt. How does the plate tectonics theory help explain the existence of fossil marine life in rocks atop compressional mountains? Answer: Sediments that were once below sea level were uplifted by orogenic events so that marine fossils now rest on mountain tops. Concept Check 10.7 Define isostasy. Answer: This is when Earth’s solid crust “floats” on the more ductile mantle. Give one example of evidence that supports the concept of crustal uplift. Answer: After the last ice age, Earth’s crust rebounded upward after being weighed down by the heavy ice. What happens to a floating object when weight is added? Subtracted? Answer: A floating object will float lower if weight is added, and will float relatively higher when weight is subtracted. Briefly describe how the principle of isostatic adjustment applies to changes in the elevations of mountains. Answer: As continental crust on mountains is eroded, the mountain isostatically adjusts by rising higher. Explain the process whereby mountainous regions experience gravitational collapse. Answer: If compressional forces no longer exist, mountains will gradually collapse under their own weight. GIVE IT SOME THOUGHT ANSWERS Is granite or mica schist more likely to fold or flow rather than fracture when subjected to differential stress? Explain. Answer: Mica schist is much more likely to flow or fold when subjected to differential stress because the foliation in metamorphic rocks imparts an internal weakness that is susceptible to ductile deformation. Refer to the accompanying diagrams to answer the following: What type of dip-slip fault is shown in Diagram 1? Were the dominant forces during faulting tensional, compressional, or shear? What type of dip-slip fault is shown in Diagram 2? Were the dominant forces during faulting tensional, compressional, or shear? Match the correct pair of arrows in Diagram 3 to the faults in Diagrams 1 and 2. Answer: Diagram 1 is a reverse fault and it is caused by compressional stress. Diagram 2 is a normal fault and it is caused by tensional stress. Diagram 1 = “a” and Diagram 2 = “b”. Refer to the accompanying photo to answer the following: The white line shows the approximate location of a fault that displaced these furrows created by a plow. What type of fault caused the offset shown? Is this a right-lateral or left-lateral fault? Explain. Answer: Strike-slip fault. It is a left-lateral fault because the crustal block on the opposite side of the fault has moved to the left. With which of the three types of plate boundaries does normal faulting predominate? Thrust faulting? Strike-slip faulting? Answer: Normal faulting predominates with divergent plate boundaries. Thrust faulting is predominant with convergent boundaries and strike-slip faulting would be most predominant with transform boundaries. Write a brief statement describing each of the accompanying photos, using terms from the following list: strike-slip, dip-slip, normal, reverse, right-lateral, left-lateral. Answer: This is a photo of a dip-slip fault. You can tell it is a normal fault because the hanging wall has moved down relative to the footwall; you can see this by looking at the matching strata on either side of the fault. This is a strike-slip fault. It is right lateral because the block opposite the fault line has moved to the right. The accompanying photo, taken near the bottom of the Grand Canyon, shows a quartz vein that has been deformed. What type of deformation is exhibited—ductile or brittle? Did this deformation most likely occur near Earth’s surface or at great depth? Answer: Ductile. At great depth. Refer to the accompanying photo to answer the following: Name the type of fold shown. Would you describe this fold as symmetrical or asymmetrical? What name is given to the part of the fold labeled A? Is point B located along the fold line, crest line, or axial plane of this particular fold? Answer: Anticline Symmetrical Syncline Fold line Suppose that a sliver of oceanic crust were discovered in the interior of a continent. Would this refute the theory of plate tectonics? Explain. Answer: Finding ocean crust in the interior of a continent would not refute the theory of plate tectonics. Ocean floor may be uplifted in mountain orogenies, causing sediments and rocks that were once part of the ocean to become part of the continents. Refer to the accompanying map, which shows the location of the Galapagos Rise and the Rio Grande Rise to answer the following questions: Compare the continental margin of the west coast of South America with the continental margin along the east coast. Based on your answer to the question above, is the Galapagos Rise or the Rio Grande Rise more likely to end up accreted to a continent? Explain your choice. In the distant future, how might a geologist determine that this accreted landmass is distinct from the continental crust to which it accreted? Answer: The west coast is an active continental margin where the east coast is a passive continental margin. The Galapagos Rise is more likely to be accreted because it is attached to the oceanic plate that is subducting underneath the South American plate. In the future, it could probably be recognized as an accreted terrane because the composition of the rocks (mainly basalt and other mafic rocks) is significantly different than the rocks that comprise the continental crust. The Ural Mountains exhibit a north–south orientation through Eurasia. How does the theory of plate tectonics explain the existence of this mountain belt in the interior of an expansive landmass? Answer: The Ural Mountains, a north–south range in west-central Russia, mark the closure site of an ancient marine basin that once existed between the European and Siberian parts of the Eurasian plate. As the two continents converged and joined, the sediments in the former marine basin were lithified, crumpled, and uplifted into a mountain range. Hence, they are now located in the interior of a massive landmass. Briefly describe the major differences between the evolution of the Appalachian Mountains and the North American Cordillera. Answer: Although the Appalachians and Urals share a similar origin (collision of continental plates), they are located in different parts of their respective continents. In North America, when Pangaea begin to break apart, it split just to the east of the modern Appalachians so that they are now located near the margin of the continent. However, when they formed, the Appalachians would have been located more in the center of the massive landmass of Pangaea. Which of the accompanying sketches best illustrates an Andean-type orogeny, a Cordilleran-type orogeny, and an Alpine-type orogeny? Answer: A is Alpine, B is Cordilleran, and C is Andean. Without specific sketches to reference, generally: 1. Andean-type orogeny would typically show a mountain range with a parallel volcanic arc along the coast of a continent, indicating a continental-continental convergent boundary. 2. Cordilleran-type orogeny would likely depict a mountain range along a coastline with a volcanic arc and possibly a deep ocean trench nearby, characteristic of a continental-oceanic convergent boundary. 3. Alpine-type orogeny would typically illustrate a high, rugged mountain range in the interior of a continent, resulting from the collision of two continental plates. These types of orogenies differ based on the tectonic settings and processes involved in their formation, leading to distinct geological features and mountain ranges. EXAMINING THE EARTH SYSTEM ANSWERS A good example of the interaction among Earth’s spheres is the influence of mountains on climate. Examine the accompanying temperature graph for the cities of Seattle and Spokane, Washington. Notice on the inset map that mountains (the Cascades) separate these two cities. The prevailing wind direction in the region is from west to east. (a) Contrast the summer and winter temperatures that occur at each city. Why are they different? (Hint: Check out the sections “Land and Water” and “Geographic Position” in Chapter 16.) The annual rainfall at Spokane (16.6 inches) is less than half that for Seattle (37.1 inches). Can you explain why? Answer: Seattle, located on the western, or windward, side of the Cascades receives more than twice as much annual precipitation as Spokane, about 360 kilometers to the east. Compared with Spokane, Seattle has warmer winters and cooler summers and a smaller annual range of temperature. The mountains act as a barrier to the flow of moist air from the Pacific. As the air crosses the mountains, it loses much of its moisture and is relatively dry when it reaches Spokane. Furthermore, the moderation of Seattle’s temperatures by the Pacific Ocean accounts for the less extreme conditions and smaller annual range. The greater continental influence and higher elevation of Spokane also affect its temperatures. The Cascades have had a profound effect on the amount and type of plant and animal life (biosphere) that inhabit the region around Spokane and Seattle. Provide several specific examples to support this statement. Answer: As a consequence of geographic position, Seattle, with its moderate temperature and greater annual precipitation, has lush forest areas of large trees, primarily of the needle-leaf type. In the drier highland area of Spokane, vegetation is less dense, primarily grasslands, and more suited to the semiarid conditions. ADDITIONAL RESOURCES DVDs and Movies Earth Revealed, Episode 7: Mountain Building (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 8: Earth’s Structures (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Aerials of the Alps and When Continents Collide. From BBC. Available at http://www.bbc.co.uk/ science/earth/surface_and_interior/mountain_formation Websites Fault Motion. Animations of different types of dip-slip faults. From IRIS. http://www.iris.edu/ gifs/animations/faults.htm Normal Fault. From MIT. http://video.mit.edu/watch/normal-fault-8182/ Animated graphics of faults, collision zones, and subduction. From Savage Earth, PBS. http://www.pbs.org/wnet/savageearth/animations/ Virtual Field Trip to the Virgin Anticline in Southwestern Utah. From the University of Georgia. Photographs and descriptions. http://www.gly.uga.edu/railsback/VFT/VFTVirginAnticline.html Virtual Field Trip to the Newfoundland Folds in New Jersey. From Rutgers University. http://www.rci.rutgers.edu/~schlisch/structureslides/VFT.html Animation showing terrane accretion. http://www.classzone.com/books/earth_science/terc/content/ visualizations/es0808/es0808page01.cfm?chapter_no=visualization Chapter 11 Geologic Time Geologic Time opens with a brief history of geology that spans the period from James Ussher (mid-1600s) to James Hutton (late 1700s) and Sir Charles Lyell (mid-1800s). The chapter continues with a discussion of the fundamental principles of relative dating, including the law of superposition, principle of original horizontality, principle of cross-cutting relationships, and the uses of inclusions and unconformities. How rock units in different localities can be correlated is also investigated. The types of fossils and their significance to understanding geologic time precede a discussion of the conditions favoring preservation. Also examined is the use of fossils in correlating and dating rock units. Following an explanation of radioactivity, the fundamentals and importance of radiometric dating are presented. The chapter concludes with an examination of the geologic time scale. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 11.1 Explain the principle of uniformitarianism and discuss how it differs from catastrophism. 11.2 Distinguish between numerical dates and relative dates and apply relative dating principles to determine a time sequence of geologic events. 11.3 Define fossil and discuss the conditions that favor the preservation of organisms as fossils. List and describe various fossil types. 11.4 Explain how rocks of similar age that are in different places can be matched up. 11.5 Discuss three types of radioactive decay and explain how radioactive isotopes are used to determine numerical dates. 11.6 Distinguish among the four basic time units that make up the geologic time scale and explain why the time scale is considered to be a dynamic tool. 11.7 Explain how reliable numerical dates are determined for layers of sedimentary rock. TEACHING GEOLOGIC TIME As you begin this unit, you may find some students are familiar with the general idea of geologic time but most do not fully comprehend the magnitude of it, nor do they realize how much of Earth’s history preceded humans. This misconception should be addressed at the beginning so that students can better understand the rest of the geologic time scale in context. The application of the concept of half-life and radiometric dating can be initially confusing to some students. Activities such as the M&M candy decay exercise can make this abstract concept more concrete for many and help them grasp how radioactive decay works. Many students come with the misconception that all radioactivity is bad. It is important to provide examples of radioactivity in everyday life that is not harmful; e.g. all things emit radiation, even them. The geologic time scale provides a framework for geologists to understand the relative order of events in Earth history, and radiometric dating can help attach numerical ages to this timeline. However, it is important to stress the fact that not all periods in geologic history are equally represented globally. You can tie this in with events such as plate tectonics, weathering, and other Earth processes that destroy rocks and sediments or prevent their deposition. Students often have difficulty with Steno’s principles because they are not sure how to order geologic events relative to each other. If you have them determine the sequence of events using worksheets with strata and various faults and unconformities on them, this can be useful for helping to sort out issues they have with understanding this topic. If students are unsure as to whether a dike predates the layers in which it is found, remind them that magma cannot invade rock layers that do not yet exist. The same is true for faults. All the rock layers cut by the fault must predate the fault, as a fault cannot cut through layers that do not exist. Figure 11.13 can help them start to grasp these concepts but they need to work through some blank geologic history diagrams similar to those before they will understand how they work. Steno’s principles involve some element of logic. Encourage students to understand them from a logical point of view rather than trying to memorize the details of them. Sometimes students think that everything becomes a fossil at one point or another. It is important to stress the conditions that favor fossilization and explain that the fossil record is not a detailed snapshot of all prehistoric life. In order to help students grasp the magnitude of the geologic time scale, it can be useful to have them create their own scale model of the time scale. Give them Figure 11.24 and have them create their own scale. They can use sidewalk chalk outside if you have the space and the permission of campus authorities to draw on the walkways. Alternatively you can use toilet paper or adding machine tape as a medium for marking beginnings and ends of geologic time periods (see Additional Resources). Students are often surprised at how little space is required for the Cenozoic and may have difficulty fitting all the time divisions on their scale model if they make their model too small. Encourage them to think big at the beginning. If you have local unconformities, a field trip to them might be useful for putting this topic in context. If not, you can show students the virtual field trip to a major unconformity in Colorado (see Additional Resources). If you have a natural history or science museum locally, this might also be a good way to show students different types of fossils. You can also consider bringing in fossil samples if you have them to show students the different types of fossils and connect this to how they form. CONCEPT CHECK ANSWERS Concept Check 11.1 Contrast catastrophism and uniformitarianism. Answer: Catastrophism is the view that Earth has been shaped largely by sudden, drastic events. Uniformitarianism embraces the idea that Earth has been shaped gradually by processes that have been in action for a long time. How did each philosophy view the age of the Earth? Answer: The philosophy of catastrophism viewed the Earth as being about 5000 or so years old, while uniformitarianism recognizes the Earth as being much older. Concept Check 11.2 Distinguish between numerical dates and relative dates. Answer: Numerical dates give us known ages of rocks and rock layers. Relative dates establish, based on the order of rock layers, which rocks are older or younger than others. Sketch and label four simple diagrams that illustrate each of the following: superposition, original horizontality, cross-cutting relationships, and inclusions. Answer: See Figures 11.2, 11.3, 11.4, and 11.7. 1. Superposition: Shows layers of rock stacked one above the other, with the oldest layer at the bottom and the youngest at the top. 2. Original Horizontality: Illustrates sedimentary layers deposited horizontally due to gravity and sedimentation processes. 3. Cross-Cutting Relationships: Depicts a geological feature (e.g., a fault or igneous intrusion) cutting across existing rock layers, indicating that the feature is younger than the layers it cuts through. 4. Inclusions: Shows one rock unit (inclusion) contained within another rock unit, indicating that the inclusion is older than the rock unit that contains it. These sketches illustrate fundamental principles used in relative dating and geological analysis to interpret the sequence of events and the relative ages of geological features. What is the significance of an unconformity? Answer: These breaks in the rock record represent times of significant geologic events. They can help us identify intervals of time not represented in the rock record. Distinguish among angular unconformity, disconformity, and nonconformity. Answer: An angular unconformity is an area in which rocks have been tilted and uplifted, then had horizontal deposition begin anew atop these tilted layers. A disconformity is a region where erosion rather than deposition occurred, as between two sedimentary layers widely separated in age. A nonconformity is a boundary between igneous or metamorphic rocks and sedimentary strata. Concept Check 11.3 Describe several ways that an animal or a plant can be preserved as a fossil. Answer: If a vertebrate animal dies and is buried quickly, its bones may be preserved as fossils. Sometimes animals are buried in sediment and dissolved by underground water, leaving behind a fossil mold. Plants are sometimes fossilized by way of carbonization when sediments and pressure squeeze out liquid and gaseous components of the plant and leave a black carbon residue behind. Some small organisms may get trapped in tree sap that turns into amber. List three examples of trace fossils. Answer: Burrows, tracks, coprolites, gastroliths. What conditions favor the preservation of an organism as a fossil? Answer: Rapid burial and possession of hard parts. Concept Check 11.4 What is the goal of correlation? Answer: Correlation can help geologists identify and match widely separated strata. State the principle of fossil succession in your own words. Answer: Older fossils will be deposited in rock strata that are in lower layers than younger fossils. Contrast index fossil and fossil assemblage. Answer: An index fossil is a fossil that is geographically widespread but has a narrow range of time during which it can be found. A fossil assemblage is a group of fossils typically found together than can help identify the layers in which they are found. In addition to being important time indicators, how else are fossils useful to geologists? Answer: Fossils are important environmental indicators. If you know the preferred habitat and environmental conditions of the fossilized organism, you can draw conclusions about what the area was like at the time that organism lived. Concept Check 11.5 List three types of radioactive decay. For each type, describe how the atomic number and the atomic mass change. Answer: Alpha emission – atomic number is decreased by 2 and mass is decreased by 4. Beta emission – mass does not change and atomic number increases by 1. Electron capture – mass does not change and atomic number decreases by 1. Sketch a simple diagram that explains the idea of half-life. Answer: See Figure 11.21. A simple diagram illustrating half-life would show a starting quantity of radioactive material decreasing by half over a specific period, demonstrating exponential decay. Why is radiometric dating a reliable method for determining numerical dates? Answer: The rates of decay for many isotopes have been precisely measured and do not vary under normal Earth conditions. For what time span does radiocarbon dating apply? Answer: Up to 70,000 years ago. Concept Check 11.6 What are the four basic units that make up the geologic time scale? List the specific ones that apply to the present day. Answer: Eons, eras, periods, and epochs. Phanerozoic Eon, Cenozoic Era, Quaternary Period, Holocene Epoch. Why is -zoic part of so many names on the geologic time scale? Answer: Time periods are classified according to what species lived; the suffix -zoic applies to animal life. What term applies to all geologic time prior to the Phanerozoic Eon? Why is this span not divided into as many smaller units as the Phanerozoic Eon? Answer: Precambrian. We do not have as complete a geologic record from this time and it is difficult to divide this span into smaller increments without more detailed data. To what does Hadean apply? Is it an “official” part of the geologic time scale? Answer: It refers to the earliest portion of Earth’s history. It is an informal term that refers to the “hellish” conditions that existed before the oldest known rocks formed. The term "Hadean" refers to the earliest eon in Earth's history, dating from the formation of the Earth about 4.6 billion years ago to approximately 4 billion years ago. It is an official part of the geologic time scale, recognized for the study of Earth's primordial processes before recognizable rocks and fossils appeared. Concept Check 11.7 Briefly explain why it is often difficult to assign a reliable numerical date to a sample of sedimentary rock. Answer: Sedimentary rocks are generally conglomerations of many different types of rocks that were all formed at different times, eroded, and became lithified into the sedimentary rock. How might a numerical date for a layer of sedimentary rock be determined? Answer: If the sedimentary layer is bracketed by dateable materials, such as igneous intrusions or layers of igneous rocks. GIVE IT SOME THOUGHT ANSWERS The accompanying image shows the metamorphic rock gneiss, a basaltic dike, and a fault. Place these three features in their proper sequence (which came first, second, and third) and explain your logic. Answer: Gneiss, dike, fault. Gneiss is metamorphic and must have formed before the dike, otherwise the igneous dike would have been metamorphosed also. The fault has split and separated the dike, therefore the dike must have formed before the fault occurred. A mass of granite is in contact with a layer of sandstone. Using a principle described in this chapter explain how you might determine whether the sandstone was deposited on top of the granite or whether the magma that formed the granite was intruded after the sandstone was deposited. Answer: A depositional contact or unconformity would be proven if detrital rock and mineral grains from the granite were found in the sandstone. Also the granite just below the contact might show reddish discoloration or other evidences of having been weathered before the sandstone was deposited. Bedding in the sandstone will be parallel or nearly parallel to the contact; there will be no evidence for contact metamorphism in the sandstone; and the sandstone will not be cut by the granitic dikes. If the contact is intrusive, the sandstone may be cut by granitic dikes and may show contact metamorphism. Rock and mineral grains in the sandstone will not show any direct correlation to the granite, and bedding in the sandstone will probably not be parallel to the contact. This scenic image is from Monument Valley in the northeastern corner of Arizona. The bedrock in this region consists of layers of sedimentary rocks. Although the prominent rock exposures (“monuments”) in this photo are widely separated, we can infer that they represent a once continuous layer. Discuss the principle that allows us to make this inference. Answer: The principle of lateral continuity says that sedimentary strata originate as continuous layers of horizontal strata that extend in all directions until they grade into a different type of sediment or phase out. The accompanying photo shows two layers of sedimentary rock. The lower layer is shale from the late Mesozoic era. Note the old river channel that was carved into the shale after it was deposited. Above is a younger layer of boulder-rich breccia. Are these layers conformable? Explain why or why not. What term from relative dating applies to the line separating the two layers? Answer: No, these layers are not conformable. In a former streambed, the fine-grained sediments that would eventually lithify into shale would have graded into coarser sediments before becoming something with such large pieces, as with breccia. Also in a streambed you would expect to find more rounded cobbles as they were weathered and eroded by the stream action; breccias have angular particles in them. The line separating these layers is a disconformity. If a radioactive isotope of thorium (atomic number 90, mass number 232) emits 6 alpha particles and 4 beta particles during the course of radioactive decay, what are the atomic number and mass number of the stable daughter product? Answer: Each time beta decay occurs, the atomic number rises by one and does not affect the mass number. Each alpha decay decreases the atomic number by 2 and the mass number by 4. Thus, for 6 alpha decays and 4 betas, the atomic number of the daughter would be (90 – (6 × 2) + 4) = 82, which is the atomic number of lead. The mass number of the daughter would be (232 – (6 × 4)) = 208. The stable daughter is lead-208. A hypothetical radioactive isotope has a half-life of 10,000 years. If the ratio of radioactive parent to stable daughter product is 1:3, how old is the rock that contains the radioactive material? Answer: A ratio of 1:1 would be produced in 10,000 years (one half-life). After two half-lives, 25 percent of the original parent would be left and 75 percent of the daughter would have formed. The ratio (25:75) is 1:3, so the sample is 20,000 years old (2 half-lives × 10,000 years in one half-life = 20,000 years). Solve the problems below that relate to the magnitude of Earth history. To make calculations easier, round Earth’s age to 5 billion years. What percentage of geologic time is represented by recorded history? (Assume 5000 years for the length of recorded history.) Humanlike ancestors (hominids) have been around for roughly 5 million years. What percentage of geologic time is represented by these ancestors? The first abundant fossil evidence does not appear until the beginning of the Cambrian period, about 540 million years ago. What percentage of geologic time is represented by abundant fossil evidence? Answer: The percentage is 5 × 103 yrs divided by 5 × 109 yrs × 100% which equals 1 × 10-4% or 0.0001%. The percentage is 5 × 106 yrs divided by 5 × 109 yrs × 100% which equals 1 × 10-1% or 0.1%. The percentage is 6 × 108 yrs divided by 5 × 109 yrs × 100% = 1.2 × 10% or 12%. These polished stones are called gastroliths. Explain how such objects can be considered fossils. What category of fossil are they? Name another example of a fossil in this category. Answer: These objects are considered fossils because they were once contained within a living organism. They are considered trace fossils. Animal tracks are also trace fossils. A portion of a popular college text in historical geology includes 10 chapters (281 pages) in a unit titled “The Story of Earth.” Two chapters (49 pages) are devoted to Precambrian time. By contrast, the last two chapters (67 pages) focus on the most recent 23 million years, with 25 of those pages devoted to the Holocene Epoch, which began 10,000 years ago. Compare the percentage of pages devoted to the Precambrian to the actual percentage of geologic time that this span represents. How does the number of pages about the Holocene compare to its actual percentage of geologic time? Suggest some reasons why the text seems to have such an unequal treatment of Earth history. Answer: (49/281) × 100% = 17.4% of the book is devoted to the Precambrian while 4,058,000,000 years constitute the Precambrian, which is (4.058/4.6) × 100% = 88.2% of time. (25/281) × 100%= 9% of the book is devoted to the Holocene whereas the last 10,000 years constitute the Holocene, which is (0.01/4600) × 100% = 0.00022% of time. This unequal treatment is due to the fact that we know much more about the Holocene, which is geologically recent, than we do about the Precambrian, which is geologically ancient. EXAMINING THE EARTH SYSTEM ANSWERS The accompanying photo shows a large petrified log in Arizona’s Petrified Forest National Park. Describe the transition of this tree from being part of the biosphere to being a component of the geosphere. How might the hydrosphere and/or atmosphere have played a role in the transition? Answer: The once-living tree was petrified (“turned into stone”) as the small internal cavities of the original wood were filled with precipitated mineral matter deposited by groundwater moving through the pores. The tree was originally a part of the biosphere and then it was buried by volcanic ash from magma that came from within the solid Earth. The atmosphere was not only the medium that carried the ash but also provided the precipitation that removed minerals from the ash once it settled to Earth. As groundwater, a part of the hydrosphere, moved through the cavities of the wood, the dissolved minerals were deposited in voids within the wood and the tree was petrified. Scotland’s Siccar Point shown in this photo was originally studied by James Hutton in the late 1700s. Describe in a general way what occurred to produce this feature. Suggest ways in which all four spheres of the Earth system could have been involved in producing Siccar Point. The Earth system is powered by energy from two sources. How are both sources represented in the Siccar Point unconformity? Answer: To produce the unconformity, sediment was originally deposited horizontally, tilted by crustal disturbance, uplifted, and eroded by wave action or running water. The area was then submerged, and new horizontal sediment was deposited on top of the inclined layers. All the spheres of the Earth system could have been involved. For example, the solid Earth provided the forces to tilt the sedimentary layers, the atmosphere supplied the water, the hydrosphere was associated with deposition of the sediment and subsequent wave erosion, and the biosphere could have supplied some of the sediment in the form of seashells. Earth’s internal heat source provided the energy to deform the sedimentary layers during mountain building. The Sun, the external heat source, furnished the energy for the ocean’s waves, tides, and currents that are responsible for the movement, deposition, and erosion of the sediments. This scene in Montana’s Glacier National Park shows layers of Precambrian sedimentary rocks. The darker layer contained within the sedimentary layers is igneous. The narrow, light-colored areas adjacent to the igneous rock were created when molten material that formed the igneous rock baked the adjacent rock. Is the igneous layer more likely a lava flow that was laid down at the surface prior to the deposition of the layers above it or a sill that was intruded after all the sedimentary layers were deposited? Explain. Is it likely that the igneous layer will exhibit a vesicular texture? Explain. To which group (igneous, sedimentary, or metamorphic) does the light colored rock belong? Relate your explanation to the rock cycle. Answer: The igneous layer is a sill because the adjacent layers on either side of it show evidence of having been in contact with magma. The igneous layer intruded upon the sedimentary layers and cooled underground. Vesicular textures are associated with volcanic activity and this is not a volcanic layer, it is intrusive igneous rock. The light colored rock is metamorphic. The sedimentary rock in contact with the intruding molten magma metamorphosed into metamorphic rock. ADDITIONAL RESOURCES DVDs and Movies Earth Revealed, Episode 10: Geologic Time (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html • How the Earth Was Made (2008) Narrated by Alec Baldwin. History Channel, 1 hour, 34 minutes. Prehistoric Earth: A Natural History (2008) From BBC, 563 minutes. This contains the subprograms Before the Dinosaurs: Walking with Monsters; Walking with Dinosaurs; Allosaurus; Walking with Prehistoric Beasts; and Walking with Cavemen. Available on DVD. Each segment represents a different portion of geologic time. Grand Canyon: The Great Unconformity. National Geographic, 2 minutes, 22 seconds. Available for free streaming from http://channel.nationalgeographic.com/channel/videos/the-great-unconformity/ Websites Toilet Paper Geologic Time Scale – Directions for creating a visual time scale analogy for a class demonstration. http://serc.carleton.edu/quantskills/activities/TPGeoTime.html Calculator Tape Time Scale – Similar to the toilet paper time scale except students do the calculations and create the time scale themselves. http://serc.carleton.edu/quantskills/activities/calculatortape.html • Geologic Time Scale. Clickable portions of the time scale link to photographs and more detailed information. From the University of California, Berkeley. http://www.ucmp.berkeley.edu/help/ timeform.php Animation showing formation of an unconformity. http://www.classzone.com/books/earth_science/ terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization Animation showing how fossils form. http://www.classzone.com/books/earth_science/terc/content/ visualizations/es2901/es2901page01.cfm?chapter_no=visualization Virtual Museum of Fossils. From Valdosta State University. http://fossils.valdosta.edu M & M Candy Radioactive Decay Activity. From Carleton College. http://serc.carleton.edu/ NAGTWorkshops/time/activities/60525.html Virtual Field Trip to a Major Unconformity. From the University of Georgia. http://www.gly.uga.edu/ railsback/VFT/VFTManitou.html Chapter 12 Earth’s Evolution Through Geologic Time Earth’s Evolution through Geologic Time opens with a presentation of the origin of Earth, including the physical and chemical differentiation during the early evolution of Earth. Also examined are the formation of Earth’s primitive atmosphere and how it changed over time. The geologic history of the Precambrian is presented along with a brief discussion of crustal evolution and the assembling of continents. The supercontinent cycle is presented in some detail and the chapter concludes with an examination of the significant geologic and biologic events that occurred during the Paleozoic, Mesozoic, and Cenozoic Eras. FOCUS ON CONCEPTS 12.1 List the principal characteristics that make Earth unique among the planets. 12.2 Outline the major stages in the evolution of Earth, from the Big Bang to the formation of our planet’s layered internal structure. 12.3 Describe how Earth’s atmosphere and oceans have formed and evolved through time. 12.4 Explain the formation of continental crust, how continental crust becomes assembled into continents, and the role that the supercontinent cycle has played in this process. 12.5 List and discuss the major geologic events in the Paleozoic, Mesozoic, and Cenozoic Eras. 12.6 Describe some of the hypotheses on the origin of life and the characteristics of early prokaryotes, eukaryotes, and multicelled organisms. 12.7 Discuss the major developments in the history of life during the Paleozoic Era. 12.8 Discuss the major developments in the history of life during the Mesozoic Era. 12.9 Discuss the major developments in the history of life during the Cenozoic Era. TEACHING EARTH’S EVOLUTION THROUGH GEOLOGIC TIME You may find that many students already have a natural interest in dinosaurs. Dinosaurs are popular in the culture and so almost everyone has been exposed to them in some form. You can use this as a reference point for discussing not only dinosaur fossils but other fossils as well. This topic lends itself well to videos and movies. The vastness of evolution through geologic time can be hard for many to imagine. Several of the movies listed in Additional Resources are excellent illustrations of the processes involved in the evolution of Earth and its species through time. Students often confuse the Big Bang theory with the nebular hypothesis. It is a good idea to stress that these are not the same event and that one formed the universe and the other formed the solar system. Having students think about why the four inner planets, or terrestrial planets, are rocky while the outer planets are gas giants can help them formulate coherent ideas as to why some solar system materials ended up where they did. Also stressing the size of these two groups of planets can help students understand that Earth’s gravitational attraction was not enough to retain the gases that make the gas giants huge. This is another good chapter to relate back to earlier concepts. You can help students to realize that the evolution of life on Earth is very closely tied to the geological processes that have occurred, such as plate tectonics and evolution of continental crust. Many students harbor the misconception that life has evolved randomly and that individual organisms evolved in response to environmental stressors. Use the major mass extinction events to discuss how organisms that already happened to have adaptations to the new environment that came upon them somewhat suddenly were those organisms that survived to reproduce. Individuals cannot themselves evolve but species can. Some students think that Pangaea was Earth’s “original” continent. Stress how Earth has been ever changing and dynamic and Pangaea is one example of a supercontinent that has existed. Encourage students to do the UC, Berkeley interactive websites (see Additional Resources) on their own, or, if your classroom is so equipped, on in-class computers. Have them generate one or two questions that were not answered by those activities and address them in class. This chapter deals with theories, such as the Big Bang theory, and hypotheses, such as those regarding the origins of life. You can use these real world examples to revisit the scientific method. Discuss why some of the topics in this chapter are theories and why some are hypotheses. Have students elaborate on the differences to fully comprehend what scientists know about the evolution of life on Earth. If you live near a science museum or a natural history museum, consider a field trip if at all possible to look at the fossils the museums have. You can write a set of guided questions to accompany this field trip to lead students to a deeper understanding of how fossils are formed, or fossil succession in the geologic record. You can use this opportunity to stress that one of the reasons fossils are found mainly in museums is because they tend to be rare and hard to find. It is a common misconception that fossils are easy to come by. CONCEPT CHECK ANSWERS Concept Check 12.1 In what way is Earth unique among the planets of our solar system? Answer: It is the only planet with life. It is the only planet with the right combination of liquid water, unhostile temperatures, correct size, and correct atmospheric conditions to support life. Explain why Earth is just the right size. Answer: If Earth were larger, its increased gravity would have retained a more hostile atmosphere of gases such as ammonia, methane, hydrogen, and helium. Why is Earth’s molten, metallic core important to humans living today? Answer: The molten core creates Earth’s magnetic field, which shields life forms from lethal cosmic rays. Why is Earth’s location in the solar system ideal for the development of higher life forms? Answer: If Earth were closer to the Sun it would be too hot to sustain life, whereas if it were further from the Sun it would be too cold. Its proximity to this modest-sized star with its consistent radiant energy has allowed the evolution of higher life forms. Concept Check 12.2 What two elements made up most of the very early universe? Answer: Hydrogen and helium. What is the name of the cataclysmic event in which an exploding star produces all the elements heavier than iron? Answer: Supernova. The cataclysmic event you're referring to is a "supernova nucleosynthesis," where the explosion of a star synthesizes elements heavier than iron through rapid neutron capture processes. Briefly describe the formation of the planets from the solar nebula. Answer: After the supernova explosion, there was a cloud of gas and dust. The nebular cloud begins to contract and become a flattened, rotating cloud. Over time, planetesimals form by way of accretion and these planetesimals eventually become planets. Describe the conditions on Earth during the Hadean. Answer: Earth was hot and molten. Conditions were hot enough to melt iron and nickel, which eventually settled into Earth’s core. Concept Check 12.3 What is meant by outgassing, and what modern phenomenon serves that role today? Answer: Outgassing is the release of gases from the Earth and was responsible for creation of the early atmosphere. Volcanoes serve that role today. Identify the most abundant gases that were added to Earth’s early atmosphere through the process of outgassing. Answer: Methane, ammonia, carbon dioxide, and water vapor. Why is the evolution of a type of bacteria that used photosynthesis to produce food important to most modern organisms? Answer: These bacteria respired oxygen into the atmosphere. Why was rainwater highly acidic early in Earth’s history? Answer: There were large quantities of sulfur dioxide in the atmosphere from volcanoes that dissolved into the rainwater, making it acidic. How does the ocean remove carbon dioxide from the atmosphere? What role do tiny marine organisms, such as foraminifers, play? Answer: Carbon dioxide is readily soluble in seawater so oceans can dissolve atmospheric carbon dioxide into them. Foraminifers extract calcium carbonate, a compound generated by the ocean’s uptake of carbon dioxide, and make their shells out of it. Concept Check 12.4 Briefly explain how low-density continental crust was produced from Earth’s rocky mantle. Answer: Partial melting of the material in Earth’s rocky mantle generates low-density silica rich rocks that are more buoyant than the mafic magma from which they originated. Denser mantle rocks are left behind. Describe how cratons came into being. Answer: Larger continental masses collided, accreted, and formed large regions of continental crust. How can the movement of continents trigger climate change? Answer: More greenhouse gases are released when the continents are more active, causing the climate to warm. What is the supercontinent cycle? What supercontinent preceded Pangaea? Answer: The supercontinent cycle is the concept that one supercontinent is dispersed by way of rifting and remains many separate components for a long time before reassembling into another supercontinent. Rodinia is the pre-Pangaea supercontinent. Explain how seafloor-spreading rates are related to sea-level changes. Answer: When seafloor spreading happens quickly, there is an abundance of warm oceanic crust, which is less dense than cooler crust. The warm crust occupies more ocean basin than cold crust causing sea level to rise during times of rapid seafloor spreading. Concept Check 12.5 During which period of geologic history did the supercontinent Pangaea come into existence? Answer: The Paleozoic Era. Where is most Cretaceous age coal found today in the United States? Answer: In the western regions such as Montana. During which period of geologic history did Pangaea begin to break apart? The Mesozoic Era. Describe the climate during the early Jurassic Period. Answer: The early Jurassic was dry, desert-like, and warm. Compare and contrast eastern and western North America during the Cenozoic Era. Answer: Both eastern and western North America were mostly above sea level during the Cenozoic. In addition, we have more information about this time period for both locations than we do for earlier time periods. Eastern North America was a stable continental margin that underwent extensive marine sedimentation during the Cenozoic. Isostatic adjustment of the eroded Appalachians also contributed erosional sediment to the eastern portion of the continent. Western North America, in contrast, was the site of an active plate boundary, and therefore experienced episodes related to this. The Laramide Orogeny finished building the Rocky Mountains during this time and other portions of the western continent were commonly experiencing volcanism and earthquakes. Concept Check 12.6 What group of organic compounds is essential for the formation of DNA and RNA and therefore necessary for life as we know it? Answer: Amino acids. The group of organic compounds essential for the formation of DNA and RNA, and thus necessary for life, are nucleic acids. These molecules carry genetic information and enable cellular functions crucial for life processes. Why do some researchers think that a type of asteroid, called carbonaceous chrondrites, played an important role in the development of life on Earth? Answer: These asteroids contain amino acid-like compounds. Some researchers believe that carbonaceous chondrites may have delivered organic molecules, including amino acids and other precursors of life, to Earth during its early history. These asteroids contain complex organic compounds and water, which are essential for the formation of life as we know it. What are stromatolites? What group of organisms is thought to have produced them? Answer: Stromatolites are mushroom-shaped algal mats. Cyanobacteria are thought to have produced them. Compare prokaryotes with eukaryotes. Within which of these two groups do all multicelled organisms belong? Answer: Prokaryotes are single-celled bacteria without a nucleus. Eukaryotes are multicellular organisms with distinct nuclei in their cells. Concept Check 12.7 What is the Cambrian explosion? Answer: This is the advent of many biodiverse multicellular life forms evolving and populating the planet. What animal group was dominant in Cambrian seas? Answer: Trilobites. What did plants have to overcome in order to move onto land? Answer: Obtaining water and staying upright. What group of animals is thought to have left the ocean to become the first amphibians? Answer: Lobe-finned fishes. Why are amphibians not considered “true” land animals? Answer: Part of their life cycle is dependent upon water habitats. What major development allowed reptiles to move inland? Answer: A shelled egg with amniotic fluid. Concept Check 12.8 What group of plants became the dominant trees during the Mesozoic Era? Name a modern descendant of this group. Answer: Gymnosperms. Modern descendants include pines, firs, and junipers. What group of reptiles led to the evolution of modern birds? Answer: Pterosaurs. What was the dominant reptile group on land during the Mesozoic? Answer: Dinosaurs. Name two reptiles that returned to life in the sea. Answer: Plesiosaurs and ichthyosaurs. Concept Check 12.9 What animal group became the dominant land animals of the Cenozoic Era? Answer: Mammals. Explain how the demise of the large Mesozoic reptiles impacted the development of mammals. Answer: Extinction of these large reptiles left many ecological niches open and available. Small mammals that survived the Cretaceous extinction event were able to flourish by filling these niches. Mammals also were warm-blooded and better able to adapt to a wider range of environments. Where has most of the evidence for the early evolution of our ancestors been discovered? Answer: East Africa. What two characteristics best separate humans from other mammals? Answer: Upright posture and bipedalism. Describe one hypothesis that explains the extinction of large mammals in the late Pleistocene. Answer: Early humans may have hunted to the brink of extinction species that were already a bit stressed by climate change. GIVE IT SOME THOUGHT ANSWERS Refer to the geologic time scale in Figure 12.3. The Precambrian accounts for nearly 90 percent of geologic time. Why do you think it has fewer divisions than the rest of the time scale? Answer: The Precambrian has fewer divisions than the rest of the geologic time scale (even though it contains about 90% of geologic time) because it is lacking in fossils and the rocks are highly deformed. These two factors make it much more difficult to interpret the details of the Precambrian. Also, rocks of this age are metamorphosed, extensively eroded, and obscured by younger overlying strata. Referring to Figure 12.4, write a brief summary of the events that led to the formation of Earth. Answer: Big Bang occurred, b) Milky Way galaxy formed, c) contraction of solar nebula and accretion of planetesimals to form Earth and other planets, d) bombardment and radioactive decay produce an ocean of magma on Earth, e) differentiation produces Earth’s layered structure, f) asteroid impact produces Earth’s Moon, g) outgassing produces Earth’s oceans and atmosphere. Describe two ways in which the sudden appearance of oxygen in the atmosphere about 2.5 billion years ago influenced the development of modern life forms. Answer: The appearance of a significant amount of oxygen in the atmosphere is related to life because a) it directly impacted the development of life on Earth as oxygen-dependent organisms began to thrive as levels increased; and b) increased oxygen levels led to the development of ozone in our atmosphere. Ozone serves as a filter or blocking agent for ultraviolet radiation and the appearance of ozone allowed for the evolution of organisms that were previously unable to survive in Earth’s atmosphere. The accompanying photograph shows layered iron-rich rocks called banded iron formations. What does the existence of these 2.5-billion-year-old rocks tell us about the evolution of Earth’s atmosphere? Answer: The existence of iron-rich rocks during the Precambrian indicates that Earth’s atmosphere contained much less oxygen than it does today. Such iron-rich rocks do not occur in rocks younger than the Precambrian due to the presence of oxygen. Five mass extinctions, in which 50 percent or more of Earth’s marine species became extinct, are documented in the fossil record. Use the accompanying graph, which depicts the time and extent of each mass extinction, to answer the following: Which of the five mass extinctions was the most extreme? Identify this extinction by name and when it occurred. What group of animals was most affected by the extinction referred to in Question a? When did the most recent mass extinction occur? During the most recent mass extinction, what prominent animal group was eliminated? What animal group experienced a major period of diversification following the most recent mass extinction? Answer: The Permian extinction 251 million years ago. Marine animals. 65.5 million years ago. The dinosaurs. Mammals. Currently, oceans cover about 71 percent of Earth’s surface. This percentage was much higher early in Earth history. Explain. Answer: The percentage of water covering Earth’s surface was much higher early in the history of our planet because it took several hundred million years for continental crust to form and become more abundant. Contrast the eastern and western margins of North America during the Cenozoic Era in terms of their relationships to plate boundaries. Answer: During the Cenozoic, the eastern continental margin of North America was tectonically stable and the site of abundant marine sedimentation. The western margin, on the other hand, was the leading edge of the North American plate. As a result, plate interactions during the Cenozoic gave rise to many events of mountain building, volcanism, and earthquakes in the West. Suggest at least one reason plants moved onto land before large animals. Answer: Although plants had to evolve a mechanism for absorbing water as they moved onto the land, they probably did so before animals because they were not limited by lower oxygen levels and they did not need ozone to protect them from the sun. Some scientists have proposed that the environments around black smokers may be similar to the extreme conditions that existed early in Earth history. Therefore, these scientists look to the unusual life that exists around black smokers for clues about how earliest life may have survived. Compare and contrast the environment of a black smoker to the environment on Earth approximately 3 to 4 billion years ago. Do you think there are parallels between the two? If so, do you think black smokers are good examples of the environment that earliest life may have experienced? Explain. Answer: While black smokers or deep-sea hydrothermal vents could have provided the organic material necessary for life to begin on Earth, saying that these vents are similar to the environments that existed on Earth 3 to 4 billion years ago is perhaps a stretch. There may be some chemical similarities between the two environments, but the heated waters of the geothermal vents, chemical differences in modern versus ancient oceans, and other differences between the ocean floor and Earth’s ancient surface suggest that modern hydrothermal vents on the seafloor are not good analogs for Earth’s early conditions. It should be pointed out that arguments have been made for and against the black smokers providing clues about Earth’s earliest life forms. About 250 million years ago, plate movement assembled all the previously separated landmasses together to form the supercontinent Pangaea. The formation of Pangaea resulted in deeper ocean basins, which caused a drop in sea level and caused shallow coastal areas to dry up. Thus, in addition to rearranging the geography of our planet, continental drift had a major impact on life on Earth. Use the accompanying diagram to answer the following: Which of the following types of habitats would likely diminish in size during the formation of a supercontinent: deep-ocean habitats, wetlands, shallow marine environments, or terrestrial (land) habitats? Explain. During the breakup of a supercontinent, what would happen to sea level? Would it remain the same, rise, or fall? Explain how and why the development of an extensive oceanic ridge system that forms during the breakup of a supercontinent affects sea level. Answer: Wetlands and shallow marine habitats would likely diminish. Instead of the presence of many coastlines and their associated wetlands and shallow marine environments, there would be one large coastline. The perimeter of one large supercontinent would be less than the many perimeters of several smaller landmasses. Sea level will rise. As the oceanic ridge system develops, a great deal of young, hot rock is formed. This young hot rock is less dense than older rock and will require more space in the ocean basin. As the water is displaced by these rocks, sea level rises. Suggest a geologic reason why the rift valley system of East Africa is so rich in human ancestor fossils. Answer: This region has conditions that favor fossilization, including a sedimentary basin and lack of destructive tectonic activity. In addition, this is the area where humans are thought to have originated and the population density of early humans here would have been greater than elsewhere. EXAMINING THE EARTH SYSTEM ANSWERS The Earth system has been responsible for both the conditions that favored the evolution of life on this planet and for the mass extinctions that have occurred throughout geologic time. Describe the role of the biosphere, hydrosphere, and solid Earth in forming the current level of atmospheric oxygen. How did Earth’s outer-space environment interact with the atmosphere and biosphere to contribute to the great mass extinction that marked the end of the dinosaurs? Answer: Earth’s original atmosphere was made up of the gases water vapor, carbon dioxide, nitrogen, and several trace gases that were released by outgassing from molten rock from the interior (solid Earth). Eventually, carbon dioxide became mixed in the primitive oceans (hydrosphere) as they formed on the cooling surface. Plants (biosphere), through the process of photosynthesis, began releasing oxygen. Once the available iron was oxidized, substantial quantities of free oxygen accumulated in the atmosphere. One hypothesis for the extinction of the dinosaurs is that about 65 million years ago a large meteorite collided with Earth. The huge quantities of dust blasted into the atmosphere by the impact reduced the amount of sunlight reaching Earth’s surface. Reduced sunlight and cooler temperatures caused delicate food chains to collapse, eventually triggering the mass extinction that marked the end of the dinosaurs. Most of the vast North American coal resources located from Pennsylvania to Illinois began forming during the Pennsylvanian and Mississippian periods of Earth history. (This time period is also referred to as the Carboniferous Period.) Using Figure 12.27, a restoration of a Pennsylvanian period coal swamp, describe the climatic and biological conditions associated with this unique environment. Next, examine the accompanying diagram that illustrates the geographic position of North America during the period of coal formation. Where, relative to the equator, was North America located during the time of coal formation? Why is it unlikely that a similar coal-forming environment will repeat itself in North America in the near future? (You may find it helpful to visit the University of California Time Machine Exhibit at www.ucmp.berkeley.edu/carboniferous/ carboniferous.html ) Answer: Carboniferous coal swamps were characterized by lush tropical-like vegetation such as large trees with scales, seed ferns, and scouring rushes, as well as large insects. During this time, North America’s equatorial location played a key role in establishing the warm, tropical conditions that aided in the growth and development of the lush vegetation that thrived in this unique environment, which extended through the central part of the continent. Owing to the present-day location and movement of North America, it is unlikely that the tropical/subtropical coal-forming environment will repeat itself in the near future. ADDITIONAL RESOURCES DVDs and Movies Earth Revealed, Episode 11: Evolution Through Time (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/ series78.html How the Earth Was Made (2008) Narrated by Alec Baldwin. History Channel, 1 hour, 34 minutes. Prehistoric Earth: A Natural History (2008) From BBC, 563 minutes. This contains the subprograms Before the Dinosaurs: Walking with Monsters; Walking with Dinosaurs; Allosaurus; Walking with Prehistoric Beasts; and Walking with Cavemen. Available on DVD. Each segment represents a different portion of geologic time. Dinosaurs: How Bones are Buried. Discovery Channel, 44 seconds. Available for free streaming from http://www.discovery.com/video-topics/other/dinosaur-videos/how-bones-are-buried.htm Playlist: Dinosaur Videos. Discovery Channel. List of videos ranging in length from about 1 to about 5 minutes, dealing mostly with dinosaurs but also with the process of fossilization and fossils in general. Websites Time Machine Exhibit. From the University of California, Berkeley. www.ucmp.berkeley.edu/ carboniferous/carboniferous.html Nebular Hypothesis Animation. http://home.snc.edu/takamasatakahashi/login/AstroSp10/Animations/ Active_Figures/nebular/ Virtual Dinosaur Dig. From the Smithsonian National Museum of Natural History Department of Paleobiology. http://paleobiology.si.edu/dinosaurs/interactives/dig/dinodig.html Life Has a History. Interactive website from the University of California, Berkeley. http://www.ucmp.berkeley.edu/education/explorations/tours/intro/Intro5to12/tour1nav.php Getting Into the Fossil Record. Interactive website demonstrating how organisms become fossils. From the University of California, Berkeley. http://www.ucmp.berkeley.edu/education/explorations/ tours/fossil/9to12/intro.html Famous Flora and Fauna. Interactive online map of North America. From the Paleontology Portal. http://www.paleoportal.org/index.php?globalnav=flora_fauna&sectionnav=map Solution Manual for Earth Science Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa 9780321928092, 9780321934437