This Document Contains Chapters 7 to 10 Chapter 7 Sediment and Sedimentary Rocks Chapter Outline 7.1 Introduction 7.2 Sediment Sources, Transport, and Deposition 7.3 How Does Sediment Become Sedimentary Rock? 7.4 Types of Sedimentary Rocks 7.5 Sedimentary Facies 7.6 Sedimentary Rocks—The Archives of Earth History GEO-INSIGHT 7.1: Fossilization GEO-FOCUS 7.1: State Fossils 7.7 Sedimentary Rocks and Economic Geology Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • Sediments are deposited as aggregates of loose solids that may become sedimentary rocks if they are compacted and/or cemented. • Geologists use texture and composition to classify sedimentary rocks. • A variety of features preserved in sedimentary rocks are good indicators of how the original sediment was deposited. • Sedimentary facies can be traced across geographic areas to help geologists determine the geologic history of a region. • Most evidence of prehistoric life in the form of fossils is found in sedimentary rocks. • Sedimentary rocks host important resources like petroleum and natural gas, uranium, and iron. Chapter Summary • Detrital sediment consists of weathered solid particles, whereas chemical sediment consists of minerals extracted from solution by inorganic chemical processes and the activities of organisms. • Sedimentary particles are designated in order of decreasing size as gravel, sand, silt, and clay. • During transport, sedimentary particles are rounded and sorted. • Major depositional settings are continental, transitional, and marine, each of which includes several specific depositional environments. • Lithification takes place when sediments are compacted and cemented and converted into sedimentary rock. Silica and calcium carbonate are the most common chemical cements. • Sedimentary rocks are classified as detrital or chemical: a) Detrital sedimentary rocks consist of particles (gravel, sand, silt, and clay) derived from preexisting rocks. b) Chemical sedimentary rocks are derived from substances in solution by inorganic chemical processes or the activities of organisms. A subcategory called biochemical sedimentary rocks is recognized. • Carbonate rocks contain minerals with the carbonate radical (CO3)-2 as in limestone and dolostone. • Evaporites include rock salt and rock gypsum, both of which form by inorganic precipitation of minerals from evaporating water. • Coal is a type of biochemical sedimentary rock composed of the altered remains of land plants. • Sedimentary facies are bodies of sediment or sedimentary rock that are recognizably different from adjacent sediments or rocks. • Vertical sequences of rocks with offshore facies overlying nearshore facies form when sea level rises with respect to the land, causing a marine transgression. A rise in the land relative to sea level causes a marine regression, which results in nearshore facies overlying offshore facies. • Sedimentary facies are bodies of sediment or sedimentary rock that are recognizably different from adjacent sediments or rocks. • Vertical sequences of rocks with offshore facies overlying nearshore facies form when sea level rises with respect to the land, causing a marine transgression. A rise in the land relative to sea level causes a marine regression, which results in nearshore facies overlying offshore facies. • Sedimentary structure, such as bedding, cross-bedding, and ripple marks help geologists determine ancient current directions and depositional environments. • Fossils provide the only record of prehistoric life and are useful for correlation and environmental interpretations. • Depositional environments of ancient sedimentary rocks are determined by studying all aspects of the rocks and making comparisons with present-day sediments deposited by known processes. • Many sediments and sedimentary rocks, including sand, gravel, evaporates, coal, and banded iron formations, are important natural resources. Most oil and natural gas are found in sedimentary rocks. Enrichment Topics Topic 1. I Shall Return. In this hilarious article, the author describes how difficult it is to become a fossil, by trying to arrange to become one himself. Originally printed in Earth, April 1998. http://www.ocean.odu.edu/~spars001/common/articles/i_shall_return.pdf Topic 2. The Grand Canyon. Of course there’s no better place to learn about sedimentary rocks, facies and ancient environments than at the Grand Canyon. Learning about each rock layer, the metamorphism of the basement, the structures that break up the layering, can all help students to understand how sedimentary geology is a window into earth history. http://www.nps.gov/grca/naturescience/geologicformations.htm Topic 3. The Good and Bad of Methane Hydrates. Ice-like solids composed of gas molecules—largely methane—that have become trapped in the crystal lattice of water are known as gas hydrates. Gas hydrates that are abundant in the pore spaces of deep sea sediments and arctic permafrost could provide vast amounts of natural gas that could become important natural resources. Thousands of gigatons of methane are located in the oceans, equal to the world’s total amount of coal. The U.S. and other countries have a rich supply just offshore. The downside is that methane is a potent greenhouse gas, about 23 times more effective at trapping heat than CO2. Releasing methane into the atmosphere would exacerbate global warming. Perhaps more frightening is that methane hydrates need the correct temperature to keep them stable. If water temperatures rise beyond a certain threshold, the icy methane hydrates melt and methane gas is released. This is another positive feedback mechanism for global warming. This mechanism may be responsible for rapid increases in global temperature that occurred in the past. Oceanus, Fall-Winter, 2004. http://www.whoi.edu/oceanus/viewArticle.do?id=2441 Common Misconceptions Misconception 1. Oil and gas deposits accumulate in large underground caverns or pools. Fact: Oil and natural gas exist in the pores of sediments and sedimentary rocks. Large reserves of fossil fuels accumulate by upward migration of these materials, because of their lower specific gravity, to geologic structures which serve as traps. Misconception2. Petroleum is formed from the remains of dinosaurs. Fact: Petroleum and natural gas form from the remains of microscopic organisms that exist in the seas and some large lakes. When these organisms die, their remains settle to the sea or lake floor where little oxygen is available to decompose them. They are then buried by sediment, heated by depth of burial, and transformed into petroleum and natural gas. Lecture Suggestions 1. Stress that detrital sedimentary rocks are classified primarily according to particle size, not composition. Note that sand is not a compositional term indicating quartz particles, but a size category. 2. Conglomerate may be effectively compared to a natural concrete. Perhaps the latter could be used in a demonstration of the larger particle size, and the finer grained, poorly sorted matrix. 3. Note that clay may be used as a size or compositional term depending on the context in which it is discussed, and that mud may be used as a size term when clay is meant to refer to clay minerals. 4. Point out that, unlike detrital sedimentary rocks, the shell and skeletal particles that occur in biochemical sedimentary rocks are not transported by streams to depositional sites in marine environments, but that these form from the organisms that lived on, in, and above the sea floor. 5. Be sure students understand how sedimentary facies reflect the different locations of a shoreline as sea level rises and falls. 6. Perform a simple demonstration of how graded beds form using a sturdy glass or clear plastic jar. Into the jar, place various sizes of sediment—some small gravel, coarse sand, fine sand, clay. Then, partially fill the jar with water. Thoroughly shake the jar then set it on the table, allowing the sediment to settle in a graded fashion. This shows how graded bedding can be used to determine the top direction, and tells something about the environment of deposition. 7. To illustrate how certain depositional environments are recognized by geologists, offer a few sets of sedimentary rock types, textures, structures, and types of fossils (e.g., mud cracks, raindrop prints, and fossil plants or vertebrates) and have the students determine which depositional environments are most likely represented by the collective evidence. Consider This 1. If a sedimentary facies is deposited during a transgression or regression, is that facies of the same age everywhere? If so, why? If not, how might age equivalence within the facies be demonstrated? Answer: No, a sedimentary facies deposited during a transgression or regression is not necessarily of the same age everywhere. Facies can vary spatially due to changes in depositional environments and local conditions. Age equivalence within the facies can be demonstrated by correlating fossils, radiometric dating, or marker horizons across different locations. 2. Has the abundance of some sedimentary rock types changed over the duration of Earth’s existence? Answer: Yes, the abundance of sedimentary rock types has changed over Earth's history due to variations in climate, tectonics, and sea levels. For example, limestone was more prevalent during certain periods, while clastic sediments dominated others. 3. What types of sedimentary rocks are evidence of arid conditions and of tropical climates? Answer: Evaporites, such as salt and gypsum, indicate arid conditions, while coal and some limestones are evidence of tropical climates. 4. If soil is the terrestrial material that combines some components of the hydrosphere, atmosphere, biosphere, and lithosphere—and thus, is an embodiment of the ecosphere—what is the aquatic equivalent that combines all components of the marine ecosphere? Answer: The aquatic equivalent of soil in the marine ecosphere is marine sediment, which integrates components of the hydrosphere, atmosphere, biosphere, and lithosphere. 5. If fossils did not exist, would geologists have discovered that Earth has a history longer than recorded human history? Answer: Yes, geologists would have discovered Earth's long history through radiometric dating, rock strata, and geological processes, even without fossils. Internet Sites, Videos, Software, and Demonstration Aids Internet Sites 4. Sedimentary Rocks: Picture Gallery of the Most Common Rock Types http://geology.com/rocks/sedimentary-rocks.shtml Photos and articles covering all types of sedimentary rocks from geology.com. 5. American Association of Petroleum Geologists http://www.aapg.org/ About careers in petroleum geology, with videos and other items for sale. 6. Grand Canyon Geology http://www.nps.gov/grca/naturescience/geologicformations.htm The National Park Service has a “Geology Training Manuel” for people interested in the sedimentary rocks and the formation of the Grand Canyon. 7. Sediment Thickness of the World’s Oceans http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.html The thickness of sediments in the oceans yields very interesting information on the age of the ocean crust and on seafloor spreading. This site includes information and maps. Videos 1. America’s National Monuments: The Geologic West. Insight Media DVD (2008, 4- 40 min. DVDs) Touring the national monuments of the Pacific Northwest in search of geologic features, such as fossil beds and lava flows. 2. Elements of Earth Science: Rocks, Minerals and Soils. Insight Media DVD (2005, 30 mins.) The rock cycle, the main types of rocks and how fossil fuels are used. 3. Rock Cycle. Insight Media DVD (2003, 30 mins.) How minerals form rocks and how rocks alter from one type to another. 4. Sedimentary Environments. Insight Media DVD (1995, 19 mins.) Sedimentary rock formation, names and classifications. 5. Sedimentary Rocks and Their Formation. Insight Media DVD (2004, 18 mins.) Weathering, erosion, deposition and the formation of clastic and non-clastic sedimentary rocks. 6. From Rock to Sand to Muck: All the Dirt on Soils. Insight Media DVD (1996, 63 mins.) The breakdown of rock into sediments and their decomposition into soils; soil types. 7. The Once Good Earth: Understanding Soil. Insight Media DVD (2005, 46 mins.) The chemical and ecological features of soil. 8. Earth Revealed. Annenberg Media http://www.learner.org/resources/series78.html (1992, 30 mins., free video): • #8: Earth’s Structures. Exploring rock layers, sedimentation, structures and petroleum using the Grand Canyon as a study side. • #17: Sedimentary Rocks. Using the rocks of the Grand Canyon to understand the geologic past. Processes of sedimentation and sedimentary rock formation are discussed. • #19: Running Water I. Rivers, Erosion and Deposition. Landscapes formed by rivers, parts of a river and other information about streams. Slides and Demonstration Aids 1. Rocks, Minerals and Resources. Insight Media., (2001, Mac/Windows CD-ROM) The main rock and mineral types and how they are identified. 2. Society for Sedimentary Geology slide sets, www.sepm.org 1. Rivers and their Deposits 2. Coastal Erosion 3. Educational Images, Ltd. slide sets, http://www.educationalimages.com/cg120001.htm 1. Fossils and Fossilization 2. Sediments, Faults and Unconformities 3. Geomorphology and Computer Programs 4. Erosion, Slides and Surface Features 5. Rocks and Topography 4. National Geographic – images of erosion and weathering http://science.nationalgeographic.com/science/photos/weathering-erosion-gallery/ 5. Science Stuff, http://www.sciencestuff.com/ 1. Sedimentary Rock Collection Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 7.7 Coquina is made up of fragmented seashells, so would you expect it to accumulate on a Florida beach, on the deep seafloor, in a lake, or on a river’s floodplain? Answer: Coquina is a Clastic Sedimentary Rock, made up of pieces or clasts. The St. Augustine, Florida, deposit is a high-energy, littoral deposit sourced by wave action in the fore shore. The Anastasia Formation coquina originated from beach action. ❯❯ Critical Thinking Question Figure 7.8 Do you know of any commercial uses of rock salt and rock gypsum? Answer: Both “rocks” (one or more minerals = rock) are mined in Michigan. Halite is mined both physically underground (Detroit Salt Company) in one mine while other developers in the state use solution mining, bringing brine up in deep wells. Detroit’s product is rock salt dedicated to road use in the winter to melt snow and ice. The solution miners provide the table salt we eat. Gypsum, still mined by surface methods in Michigan, is used for Plaster of Paris, wallboard, and is added to cement. There are other uses such as in the chemical industry. ❯❯ Critical Thinking Question Figure 7.15 How can you determine whether the flow that formed the ripples shown in part (b) was from right to left or from left to right? Answer: In the photo (b), it is hard to tell which side has the steep face of these asymmetrical ripple marks. I think I can see the steep face on the left side of each ripple mark, therefore: the current would be from right to left. ❯❯ Critical Thinking Question Figure 7.16 What inferences can you make about the environment in which the mud in the image below was deposited. Answer: Mud was deposited in Montana in a wet, lucustrine environment; the lake dried up and in the desiccation of the sediment, the cracks formed, only to be preserved by a fine grained overlying deposit. Suggested Answer to Selected Short Answer Question (Answers to question 7 and question 9 provided in the appendix to the text) 15. Describe how deposits of mud and sand are lithified. Suggested Answer: Lithification is the transformation of loose sediment into solid rock through compaction and cementation. The detrital deposits of mud and sand are lithified through the same general process. In each case, the sediment consists of solid particles and pore spaces. These deposits are compacted by their own weight and the weight of any additional deposits. This compaction reduces the size of the pores. For mud, compaction alone is sufficient for lithification, but for sand, the added processes of cementation occurs from minerals precipitated into the pore space, acting as a cementing agent. Deposits of mud and sand are lithified through compaction and cementation. As sediments accumulate, the weight of overlying material compresses the lower layers, reducing pore space. Minerals precipitate from groundwater, filling the remaining pores and binding the particles together to form solid rock, such as shale from mud and sandstone from sand. Chapter 8 Metamorphism and Metamorphic Rocks Chapter Outline, 8.1 Introduction GEO-FOCUS 8.1: Asbestos: Good or Bad? 8.2 The Agents of Metamorphism 8.3 The Three Types of Metamorphism 8.4 How Are Metamorphic Rocks Classified? 8.5 Metamorphic Zones and Facies 8.6 Plate Tectonics and Metamorphism 8.7 Metamorphism and Natural Resources Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • Metamorphic rocks result from the transformation of other rocks by various processes occurring beneath Earth’s surface. • Heat, pressure, and fluid activity are the three agents of metamorphism. • Contact, dynamic, and regional metamorphism are the three types of metamorphism. • Metamorphic rocks are typically divided into two groups, foliated and nonfoliated, primarily on the basis of texture. • Metamorphic rocks with a foliated texture include slate, phyllite, schist, gneiss, amphibolite, and migmatite. • Metamorphic rocks with a nonfoliated texture include marble, quartzite, greenstone, hornfels, and anthracite. • Metamorphic rocks can be grouped into metamorphic zones based on the presence of index materials that form under specific temperature and pressure conditions. • The successive appearance of particular metamorphic minerals indicates increasing or decreasing metamorphic intensity. Chapter Summary • Metamorphic rocks result from the transformation of other rocks, usually beneath Earth's surface, as a consequence of one, or a combination, of three agents: heat, pressure, and fluid activity. • Heat for metamorphism comes from intrusive magmas, extrusive lava flows, or deep burial. Pressure is either lithostatic (uniformly applied stress) or differential (stress unequally applied from different directions). Fluids trapped in sedimentary rocks or emanating from intruding magmas can enhance chemical changes and the formation of new minerals. • The three major types of metamorphism are contact, dynamic, and regional. • Contact metamorphism results when a magma or lava alters the surrounding country rock. • Dynamic metamorphism is associated with fault zones where rocks are subjected to high differential pressure. • Most metamorphic rocks result from regional metamorphism, which occurs over a large area and is usually caused by tremendous temperatures, pressures, and deformation within the deeper portions of the crust. • Metamorphic grade generally characterizes the degree to which a rock has undergone metamorphic change. • Index minerals—minerals that form only within specific temperature and pressure ranges—allow geologists to recognize low-, intermediate-, and high-grade metamorphism. • Metamorphic rocks are primarily classified according to their texture. In a foliated texture, platy and elongate minerals have a preferred orientation. A nonfoliated texture does not exhibit any discernable preferred orientation of the mineral grains. • Foliated metamorphic rocks can be arranged in order of increasing grain size, perfection of their foliation, or both. Slate is fine grained, followed by (in increasingly larger grain size) phyllite and schist; gneiss displays segregated bands of minerals. Amphibolite is another fairly common foliated metamorphic rock. Migmatities have both igneous and high-grade metamorphic characteristics. • Marble, quartzite, greenstone, hornfels, and anthracite are common nonfoliated metamorphic rocks. • Metamorphic zones are based on index minerals and are areas of rock that all have similar grades of metamorphism; that is, they have all experienced the same intensity of metamorphism. • A metamorphic facies is a group of metamorphic rocks whose minerals all formed under a particular range of temperatures and pressures. Each facies is named after its most characteristic rock or mineral. • Metamorphism occurs along all three types of plate boundaries but is most common at convergent plate margins. • Many metamorphic rocks and minerals, such as marble, slate, graphite, talc, and asbestos, are valuable natural resources. In addition, many ore deposits are the result of metamorphism and include copper, tin, tungsten, lead, iron, and zinc. Enrichment Topics Topic 1. The Oldest Rocks. Considering the amount of time that has past since Earth first formed rocks it’s no surprise that the oldest rocks are metamorphic. It’s probably also not surprising that these rocks would be metamorphosed volcanic rocks since the early Earth was very hot and volcanically active. The rocks are an ancient greenstone belt. What is surprising is how old these rocks are. The Nuvvagittuq Belt on the coast of Hudson Bay in Northern Quebec is 4.28 billion years old. Since Earth formed 4.6 billion years ago and was very, very hot, that seems pretty old. http://www.livescience.com/2896-oldest-rocks-earth.html Topic 2. Shock Metamorphism. One of the great mass extinctions came at the end of the last Ice Age when a sudden cold snap reversed the warming trend that had been taking place. The cold lasted for 1,300 years and coincided with the extinction of large animals such as mammoths, mastodons, saber-toothed cats and others. The possibility that a comet airburst or meteorite impact caused the cold snap was explored by scientists who looked for nanodiamonds to indicate that an impact had taken place. Nanodiamonds are the result of shock metamorphism that occurs from an impact; they found none. Other lines of evidence for an impact have also been rejected and so other causes of the cold snap must be found. http://www.sciencedaily.com/releases/ 2010/08/100830152530.htm Topic 3. Useful Garnets. Garnets are useful as gemstones and for industrial purposes, but they are also useful for geologists. Garnets are important minerals for geothermobarometric, the measure of the previous temperature and pressure history of a metamorphic rock, since temperature-time histories are preserved in the compositional zonations that are typical of their growth pattern. A garnet with no zonation was likely homogenized by diffusion, which tells geologists something of the time-temperature history of the host rock. Garnets are also useful for determining the metamorphic facies of a rock. Rocks that had basalt composition often contain eclogite garnet. Pyrope is formed at high pressures, which means that a pyrope-containing peridotite formed at great depth, possibly as deep as 100 km. Silica-rich garnets are formed at even greater depths; if they are found as inclusions within diamonds they likely originated at 300 to 400 km depth in the crust. Common Misconceptions Misconception 1. Marble is forever. Marble is a good stone to use for permanent monuments, tombstones, sculpture, and other art objects because it will last for such a long time. Fact: Marble is made of the mineral calcite, CaCo3, and is not all that stable. Calcite has a low hardness (#3 on the hardness scale), and so can be scratched or abraded by anything harder. This, of course, is why marble has long been a favorite with sculptors, who can use tools made of harder stones or metals. In addition, calcite is easily cleavable, and it reacts to acid. This last property means that it is susceptible to solution by even mildly acid solutions, such as acid rain. Even normal rainwater, which is slightly acidic, will weather marble over time. Misconception 2. A preexisting rock can melt all the way and still become a metamorphic rock. Once a rock is completely molten, it is a magma. When it cools it becomes an igneous rock. Migmatite is the rock at that borderline place where some has melted all the way but some has not so it is still considered a metamorphic rock. Lecture Suggestions 1. When describing metamorphic rocks, it is important to stress that: a) foliation results from the preferred orientation of minerals; b) schist appears to be layered because its principle minerals (biotite and muscovite) are oriented with their flat cleavage surfaces parallel to one another; and c) the banded foliation of gneiss can be distinguished from other foliation types and from stratification by its coarse crystals and the differences in mineral composition from one layer to another. 2. Point out that foliation in a metamorphic rock does not have any necessary relationship to any preexisting layering, such as stratification, which may have been present in the parent rock. You could use a thick stack of cards (perhaps 5”x7” size) on the sides of which you rule a series of parallel lines to represent stratification. Be sure the “stratification” is at a distinct angle to the layered edges of the cards, which will represent “foliation.” 3. Note that metamorphic rocks can form from any parent rock type, even another metamorphic rock. Make frequent reference to the parent rock type(s) from which a given metamorphic rock formed. Make clear that although parent rock type is easily identified in some metamorphic rocks such as quartzite and marble, the parent rock types of others often cannot be identified without a whole-rock chemical analysis of the rock’s composition (e.g., gneiss can form from impure sandstones or from granitic igneous rocks). 4. Review the rock cycle, focusing on metamorphism and metamorphic rocks, and emphasize the link of metamorphism to plate tectonics. 5. Note that nonfoliated rock types also exhibit recrystallization, which usually results in increased crystal size, but often without changes in mineral or chemical composition. 6. Stress that although heat is involved in the metamorphism of rocks, the heat never, by definition, is great enough to melt all or even a significant portion of the parent rock. 7. Note that new minerals can form in metamorphic rocks without introduction of new or additional elements, because the rock is transformed as a new equilibrium adjustment to increased pressure and temperature is reached. When hot fluids are involved, however, these may introduce new or additional elements and remove others. Thus, in some metamorphic rocks, new minerals are formed by additions and losses of various elements. 8. When discussing the causes and tectonic contexts of metamorphism, stress that low-temperature, high-pressure metamorphism is usually associated with disruptions of the crust such as folds and faults which result from compressional forces. Consider This 1. Why can a glacier be considered a metamorphic rock? Answer: A glacier can be considered a metamorphic rock because, under pressure and over time, the ice undergoes recrystallization and deformation, transforming from snow into glacial ice, which has properties similar to metamorphic rocks. 2. How reasonable is it to assume that any metamorphic rock must be older than the igneous or sedimentary rock found in the same region? Answer: It is not always reasonable to assume that metamorphic rock must be older than igneous or sedimentary rocks in the same region, as metamorphism can occur at any stage of rock formation and may affect rocks of various ages. 3. Are diamonds the product of the metamorphism of anthracite coal? Answer: Diamonds are not the product of the metamorphism of anthracite coal; they form from carbon under extremely high pressure and temperature conditions, typically in the Earth's mantle, rather than from coal metamorphism. 4. What type(s) of metamorphism are likely to occur along continental-continental convergent plate boundaries? Answer: At continental-continental convergent plate boundaries, regional metamorphism and dynamic metamorphism are likely to occur due to high pressure and temperature conditions from collisional forces and tectonic activity. Important Terms aureole contact (thermal) metamorphism differential pressure dynamic metamorphism fluid activity foliated texture heat index mineral lithostatic pressure metamorphic facies metamorphic grade metamorphic rock metamorphic zone metamorphism nonfoliated texture regional metamorphism Internet Sites, Videos, and Demonstration Aids Internet Sites 1. Volcano World: Metamorphic Rocks http://volcano.oregonstate.edu/vwdocs/vwlessons/lessons/Metrocks/Metrocks2.html Basics of the rock cycle and metamorphic rocks and how they form. 2. Metamorphic Rocks http://csmres.jmu.edu/geollab/Fichter/MetaRx/ More basics of metamorphic rocks and how they form. Other rock types also. From James Madison University. Videos 1. Earth Revealed. Annenberg Media http://www.learner.org/resources/series78.html (1992, 30 mins., free video): • #18: Metamorphic Rocks. How metamorphic rocks form and the relationship of these rocks to plate tectonics processes. 2. Metamorphic Rocks. Insight Media, DVD (1999, 15 mins.) The causes of metamorphism and the types of rocks produced by metamorphic processes. 3. Rock Cycle. Insight Media, DVD (2003, 30 mins.) How minerals form rocks and how rocks alter from one type to another. Slides and Demonstration Aids 1. Educational Images, Ltd. slide sets, http://www.educationalimages.com/cg120001.htm a. Rock Specimens and Crystals. Educational Images, Ltd. 2. Science Stuff, http://www.sciencestuff.com/, • Metamorphic Rock Collection • Ores of Common Metals Collection Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 8.2 If the Acasta Gneiss (a metamorphic rock) is one of the oldest known rocks on Earth, why does Earth have to be older than 4 billion years? Answer: Age dating of the Acasta Gneiss dates back to the development of the highest grade of Metamorphism. This rock reached that grade ~4 Ga. The process of Progressive Metamorphism (Regional) takes a considerable period of time – often 100’s of millions of years. The process must also consider the “parent” rocks. The “parent” process most likely started with granite in the earliest of the Greenstone belts (Hadean?) with a weathering process to form some of the earliest sediments in fore arch basins, preparing to be crushed up against the oldest of North America’s cratons. Wow! Older than ~4 Ga? You bet! Earth must be older than 4 billion years because the Acasta Gneiss, while being one of the oldest known metamorphic rocks, formed from older protoliths that predate its current state. Therefore, the formation of the Acasta Gneiss implies that Earth itself must have existed long before the rock’s metamorphosis, pushing Earth's age beyond 4 billion years. ❯❯ Critical Thinking Question Figure 8.7 What criteria would you use to determine that what you see is indeed spheroidal weathering and not weathered pillow lavas? Answer: Joint patterns and crystal textures will be different. Joints should not be present between pillows (welded together during initial cooling). Pillows also show a crystal size zoning: very fine grain exterior and coarse interiors. To differentiate spheroidal weathering from weathered pillow lavas, examine the rock's texture and structure. Spheroidal weathering typically results in concentric, rounded layers within a rock mass, often seen in granitic or sandstone formations. In contrast, weathered pillow lavas exhibit characteristic pillow-shaped structures formed underwater, with distinct, often smooth, rounded features. Observing these features in context helps in accurate identification. ❯❯ Critical Thinking Question Figure 8.9 Why aren’t quartz or calcite index minerals that can be used to determine the metamorphic grade of a rock? Answer: Quartz may be found in metamorphic rocks, but is not necessarily the result of the process of metamorphism. Quartz formed during the crystallization of igneous rocks. It is stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. The calcite found in limestone has no value as an index mineral because it never changes. Quartz and calcite are not ideal index minerals for determining metamorphic grade because they are stable over a wide range of metamorphic conditions and do not provide specific temperature or pressure constraints. Index minerals like garnet or kyanite are more useful as they form or stabilize within narrower temperature and pressure ranges, offering better indicators of metamorphic grade. ❯❯ Critical Thinking Question Figure 8.12 How do you distinguish between bedding and cleavage in a metamorphic rock? Answer: This can be particularly interesting with the compressed, folded metasediments in the UP of Michigan like the Siamo Slate. You can see the uniform layers, in some places even vertical, cris-crossed in some places by slaty cleavage oriented east-west relative to north-south compression. As metamorphic grade increases the vestiges of sedimentary layering become obscure. To distinguish between bedding and cleavage in a metamorphic rock, observe their orientation and formation. Bedding refers to parallel layers of sedimentary rock, often visible in the original sedimentary structures. Cleavage, however, is a planar feature in metamorphic rocks that results from foliation or the alignment of minerals under pressure, typically cutting across the original bedding structures. Cleavage surfaces are usually more uniform and aligned compared to the irregular, sometimes uneven layers of bedding. ❯❯ Critical Thinking Question Figure 8.19 Go to a point that is represented by 200°C and 2 kbar of pressure. What metamorphic facies is represented by those conditions? If the pressure is raised to 12 kbar, what facies is represented by the new conditions? What change in depth of burial is required to effect the pressure change from 2 to 12 kbar? Answer: I would turn this chart bottom to top to suggest pressure increase with depth. At 200º C and 2 kilobars, your sample is in approximately the Zeolite facies. Pressure increase to 12 kilobars puts the sample in the Blue Schist facies; equal to an increase in burial depth of about 30 km; conditions common at subduction zones. At 200°C and 2 kbar, the metamorphic facies represented is typically the greenschist facies. When the pressure is increased to 12 kbar at the same temperature, the facies changes to the amphibolite facies. To achieve this pressure change from 2 to 12 kbar, an approximate increase in burial depth of about 30 km is required, given that pressure increases roughly by 1 kbar per 3 km of depth in the Earth's crust. Suggested Answer to Selected Short Answer Question (Answers to question 8 and question 9 provided in the appendix to the text) 7. Why is it important for people to know something about metamorphism, metamorphic rocks, and how they form? Suggested Answer: Not only do metamorphic rocks provide a tectonic history of a landscape, they also provide humans with valuable resources: • Marble, used for statuary and ornamental building stone • Slate, used for roofing, flooring, billiard/pool tables, and blackboards • Graphite, used in pencils and lubricants • Garnet and Corundum, used as gemstones and abrasives • Asbestos, formerly used as a heat insulator • Kyanite, Andalusite, and Sillimanite (aluminum silicates), used as raw materials in the ceramics industry • Iron and tin oxides deposits (hematite, magnetite, and cassiterite) • Precious metal deposits (gold) Understanding metamorphism and metamorphic rocks is crucial for several reasons: it aids in interpreting geological history, locating mineral resources, and assessing ground stability for construction projects. Knowledge of these processes also helps in understanding Earth's dynamic processes and the formation of valuable materials used in various industries. Chapter 9 Earthquakes and Earth's Interior Chapter Outline 9.1 Introduction 9.2 Elastic Rebound Theory 9.3 Seismology 9.4 Where Do Earthquakes Occur, and How Often? 9.5 Seismic Waves 9.6 Locating an Earthquake 9.7 Measuring the Strength of an Earthquake 9.8 What are the Destructive Effects of Earthquakes? GEO-INSIGHT 9.1: Designing and Building Earthquake-Resistant Structures 9.9 Earthquake Prediction GEO-FOCUS 9.1: Paleoseismology 9.10 Earthquake Control 9.11 What Is Earth’s Interior Like? 9.12 The Core 9.13 Earth’s Mantle 9.14 Seismic Tomography 9.15 Earth’s Internal Heat 9.16 Earth’s Crust Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • Energy is stored in rocks and is released when they fracture, thus producing various types of waves that travel outward in all directions from their source. • Most earthquakes take place in well-defined zones at transform, divergent, and convergent plate boundaries. • An earthquake’s epicenter is found by analyzing earthquake waves at no fewer than three seismic stations. • Intensity is a qualitative assessment of the damage done by an earthquake. • The Richter Magnitude Scale and Moment Magnitude Scale are used to express the amount of energy released during an earthquake. • Great hazards are associated with earthquakes, such as ground-shaking, fire, tsunami, and ground failure. • Efforts by scientists to make accurate, short-term earthquake predictions have thus far met with only limited success. • Geologists use seismic waves to determine Earth’s internal structure. • Earth has a central core overlain by a thick mantle and a thin outer layer of crust. • Earth possesses considerable internal heat that continuously escapes at the surface. Chapter Summary • Earthquakes are vibrations caused by the sudden release of energy, usually along a fault. • The elastic rebound theory is an explanation for how energy is released during earthquakes. As rocks on opposite sides of a fault are subjected to force, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape. • Seismology is the study of earthquakes. Earthquakes are recorded on seismographs, and the record of an earthquake is a seismogram. • An earthquake's focus is the location where rupture within Earth's lithosphere occurs and energy is released. The epicenter is the point on Earth's surface directly above the focus. • Approximately 80 percent of all earthquakes occur in the circum-Pacific belt, 15% within the Mediterranean-Asiatic belt, and the remaining 5% mostly in the interior of the plates and along oceanic spreading ridges. • The two types of body waves are P-waves (primary waves), which are compressional (expanding and compressing) and the fastest seismic waves, traveling through all material, and S-waves (secondary waves) , which are shear (moving material perpendicular to the direction of travel) and slower than P-waves and can travel only through solids. • Rayleigh (R-waves) and Love waves (L-waves) move along or just below Earth's surface. • An earthquake's epicenter is determined using a time-distance graph of the P- and S-waves to calculate how far away a seismic station is from an earthquake. The greater the difference in arrival times between the two waves, the farther away the seismic station is from the earthquake. Three seismographs are needed to locate the epicenter. • Intensity is a subjective or qualitative measure of the kind of damage done by an earthquake. It is expressed in values from I to XII in the Modified Mercalli Intensity Scale. • The Richter Magnitude Scale measures an earthquake's magnitude, which is the total amount of energy released by an earthquake at its source. It is an open-ended scale with values beginning at 1. Each increase in magnitude number represents about a 30-fold increase in energy released. • The seismic-moment magnitude scale more accurately measures the total energy released by very large earthquakes. • The destructive effects of earthquakes include ground shaking, fire, tsunami, landslides, and disruption of vital services. • Seismic risk maps help geologists in determining the likelihood and potential severity of future earthquakes based on the intensity of past earthquakes. • Earthquake precursors are changes preceding an earthquake and include seismic gaps, changes in surface elevations, and fluctuations of water levels in wells. • A variety of earthquake research programs are underway in various countries. Studies indicate that most people would probably not heed a short-term earthquake warning. • Although it is unlikely that earthquakes can ever be prevented, it might be possible to release small amounts of the energy stored in rocks and thus avoid a large, devastating earthquake. • Various studies indicate that Earth has an outer layer of oceanic and continental crust below which lies a rocky mantle and an iron-rich core with a solid inner part and a liquid outer part. • Density and elasticity of Earth’s materials determine the velocity of seismic waves. Seismic waves are refracted when their directions of travel change. Wave reflection occurs at boundaries across which the properties of rocks change. • Geologists use the behavior of P- and S-waves and the presence of the P- and S-wave shadow zones to estimate the density and composition of Earth's interior, as well as to estimate the size and depth of the core and mantle. • Earth's inner core is probably made up of iron and nickel, whereas the outer core is mostly iron with 10%- other substances. • Peridotite, an igneous rock composed mostly of ferromagnesiansilicates, is the most likely rock making up Earth's mantle. • Oceanic crust is composed of basalt and gabbro, whereas continental crust has an overall composition similar to granite. The Moho is the boundary between the crust and the mantle. • The geothermal gradient of 25°C/km cannot continue to great depths; within the mantle and core, it is probably about l°C/km. The temperature at Earth's center is estimated to be 6,500°C. Enrichment Topics Topic 1. A Rise in Earthquakes? Some people think that there are more earthquakes happening now than ever before. Certainly, we hear more about destructive earthquakes and tsunamis than ever. But are there more? What might cause a rise and fall in the number or magnitude of earthquakes? The truth is, there are not more earthquakes nor has any cycling ever been discovered. The USGS website can answer many questions about earthquakes: http://earthquake.usgs.gov/learn/ Topic 2. Intraplate Earthquakes. The great magnitude New Madrid earthquakes, 7.5 to 8, of 1811-12 occurred in the middle of the North American Plate far from any plate boundaries. The faulting in this region appears to be the result of compressional forces that occur as the plate absorbs strain from westward motion and convergence with the Pacific Plate. However, this fault zone is developed within and made possible by tectonic events which occurred during the rifting of Pangaea, and quite possibly the events which attended the breakup of a supercontinent some 600 million years ago. Rifting of the earlier supercontinent produced a zone of down-faulted crust which underlies a structure known as the Illinois basin and which remained active for some 300 million years. A second rifting event—that of Pangaea—produced a zone of down-faulted crust known as the Reelfoot, Delaware, and Oklahoma Rifts to the immediate south and west—in the vicinity of New Madrid. It is this rift zone which became the axis of the Mississippi River drainage system. Scientists now estimate that there is a 7 to 10 percent chance of a repeat quake and a 25 to 40 percent chance of a magnitude 6 quake in the next 50 years. Topic 3. Imaging Earth’s Interior with Tomography. There are many beautiful images available on the internet that show the results of seismic tomography studies. To find some, Google seismic tomography and choose images. Here’s one in which scientists have drawn in features of a subduction zone: http://www.whoi.edu/cms/images/lstokey/2005/1/v42n2-detrick2en_5301.jpg And here’s another showing blobs in the mantle: http://www.geo.cornell.edu/geology/classes/Geo101/graphics/s12fsl.jpg Common Misconceptions Misconception 1: When “the big one” hits, Southern California will break away from the rest of the continent and fall into the ocean. Fact: Although the state of California straddles the boundary between two plates of Earth’s crust, and movement along this boundary (the infamous San Andreas Fault zone) is responsible for many large earthquakes, nonetheless, this boundary is a transform fault. Thus the plates on either side of the transform fault are sliding past and tectonic forces are really fighting against separation of the plates. Los Angeles will someday be a suburb of California though. Misconception 2: Tidal waves are enormous ocean waves caused by earthquakes. Fact: A tidal wave is a small ocean wave that has nothing to do with earthquakes. The enormous waves caused by earthquakes or other shocks to the ocean are called tsunami. Since they are usually caused by earthquakes, they are sometimes called seismic sea waves as well. Lecture Suggestions 1. The students can choose a recent earthquake and discuss with the class the geological reason for that quake. Was the amount of destruction great or small? If great, where and why? If the magnitude of the quake was large but the destruction was small, why? 2. The difference between P- and S-wave motions can be effectively illustrated using a Slinky. 3. Check out the earthquake database of the National Earthquake Information Center, maintained by the U.S. Geological Survey in Golden, Colorado (see information below under Internet Sites, Videos, Software, and Demonstration Aids). Collect data on numbers and magnitudes of earthquakes that have occurred in the past week or two. The class will be interested to see how many quakes occur every day. 4. Demonstrate density differences among iron (meteorite, if available), peridotite, basalt, and granite to suggest how concentric layers in Earth’s interior may have developed. 5. Bring to class samples of the interior of Earth: granite and basalt for the two types of crust, peridotite for the mantle, and if possible, an iron-nickel meteorite for the core. Point out that these are not entirely accurate representation. Consider This 1. Does the observation, that lithospheric plates drag the underlying mantle along as they move, support either a slab-pull or ridge-push mechanism of plate movement? Answer: The observation that lithospheric plates drag the underlying mantle as they move supports the slab-pull mechanism of plate movement. This occurs because the sinking of a cold, dense plate into the mantle pulls the rest of the plate along with it. Ridge-push, on the other hand, involves the force generated by rising mantle material at mid-ocean ridges pushing plates apart, rather than dragging. 2. What tests could you design to determine whether mantle convection takes place only in the upper portion or if it involves the entire mantle? Answer: To determine whether mantle convection occurs only in the upper portion or involves the entire mantle, you could design tests such as: 1. Seismic Tomography: Analyze seismic waves' speed and behavior to map temperature and convection patterns throughout the mantle. 2. Geochemical Analysis: Study the composition of volcanic eruptions and mantle xenoliths to infer mantle mixing and convection depths. 3. Modeling: Use computational models to simulate convection dynamics and compare results with observed geological and geophysical data. 3. Suggest a reason why, as seismic tomography has indicated, there are large pockets of hot material beneath the interiors of continents rather than along the spreading ridges, and why there are zones of cold rock that encircle and extend inward beneath the continents on their Pacific margins. Answer: Large pockets of hot material beneath continents likely indicate plumes of mantle material rising from deep within the mantle, creating hotspots away from spreading ridges. Conversely, cold rock zones around Pacific margins represent subducting plates, where oceanic lithosphere sinks and cools as it descends into the mantle at convergent boundaries. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. National Earthquake Information Center, http://earthquake.usgs.gov/regional/neic/ To determine the size and location of all destructive earthquakes worldwide and to disseminate the information to interested parties. 2. USGS Earthquake Hazards Program http://earthquake.usgs.gov/ The latest earthquakes are reported here. 3. The Pacific Northwest Seismic Network: Earthquake Prediction http://www.pnsn.org/earthquakes/recentAll about earthquakes of the Pacific Northwest. 4. Northern California Earthquake Data Center (NCEDC) http://quake.geo.berkeley.edu/. An archive and distribution center for earthquake data for Northern and Central California. 5. Understanding Seismic Tomography http://www.see.leeds.ac.uk/structure/dynamicearth/flash_gallery/layered_earth/seismic_tomography.html A simple description of seismic tomography and its importance. Videos 1. Japan’s Killer Quake: NOVA presents a 53-minute episode, available online, which explores the enormous earthquake, tsunami and nuclear crisis that occurred in March 2011. http://video.pbs.org/video/1863101157/ 2. Tsunami: The Wave that Shook the World. NOVA, PBS DVD (2005, 60 mins.) The causes and consequences of the Indian Ocean tsunami that struck in 2004. 3. Earthquake Tsunami: Wave of Destruction. Insight Media DVD (2007, 30 mins.) The scientific causes of earthquakes and tsunamis with details about the 2004 Boxing Day tsunami in the Indian Ocean. 4. Earthquakes in the Midwest: NOVA scienceNOW DVD (2009) Earthquakes that strike away from plate boundaries as evidenced by those in the American Midwest. 5. Earthquake – The Science Behind the Shake. NOVA, PBS DVD Lives and dollars can be saved if earthquakes can be predicted. 6. Inside Planet Earth, Discovery Channel (2009, 84 mins.) An unusual and unique voyage into Earth’s interior 7. When the Earth Moves. Insight Media (DVD, 27 mins.) Earth movement from earthquakes, mass wasting, floods, glaciers and volcanoes and how to mitigate the problems caused by these movements. 8. Earthquakes. Insight Media (DVD, 13 mins.) The nature and consequences of earthquakes. 9. Earthquakes: Seismic Sleuths Insight Media (2001, 51 mins.) Scientists who attempt to predict earthquakes by learning all they can about why and where they occur. 10. Earth Revealed. Annenberg Media http://www.learner.org/resources/series78.html (1992, 30 mins., free video): • #3: Earth’s Interior. Using oil wells to understand what lies beneath the Earth’s surface. Geophysical methods, such as seismic wave studies, are described. • #9: Earthquakes. Footage of earthquakes and their aftermath including how earthquakes are studied. • #25: Living With Earth, Destructive Natural Phenomena. Annenberg/CPB Collection. How people cope with living in an earthquake zone, especially those who live along the San Andreas Fault. 11. The Great San Francisco Earthquake: American Experience Series, PBS DVD (1987, 56 mins.) The 1906 earthquake on the San Andreas Fault near San Francisco and its impact then and now. 12. Killer Quake. Nova. PBS DVD (1994) The 1994 Northridge earthquake in Los Angeles is studied. 13. Hidden Fury: The Danger Posed by the New Madrid Earthquake Zone. New Madrid Films (1993, 27 mins.) The 1811 New Madrid quake and the likelihood of another quake on that fault zone. 14. Aftershocks of the Loma Prieta Earthquake—Computer Animations. U.S. Geological Survey. The Open Video Project. http://www.open-video.org/details.php?videoid=770 Slides 1. National Geophysical Data Center (NGDC) Natural Hazard Slide Sets, including http://www.ngdc.noaa.gov/nndc/struts/results?eq_0=5&t=101634&s=0&d=1 • Earthquake Damage - General • Earthquake Damage in San Francisco, April 18, 1906 • Earthquake Damage in Mexico City, September 19, 1985 • Faults • Loma Prieta Earthquake, October 18, 1989 • Many others 2. National Information Service for Earthquake Engineering (NISEE) Slide Sets, including http://nisee.berkeley.edu/bertero/html/slides.html • Surface Faulting • Ground Shaking • Many others Answers to Figure-Related Critical Thinking Questions ❯ ❯ Critical Thinking Question Figure 9.3 Why isn’t the epicenter located where the fault emerges at Earth’s surface? Answer: The epicenter is located in the closest proximity at the Earth’s surface to the earthquake focus, where (all geology being the same) the surface wave damage should be the greatest. The epicenter is not located where the fault emerges at Earth's surface because it represents the point directly above the earthquake's focus (or hypocenter) within the Earth. The fault may extend to the surface at a different location, and the epicenter is specifically the point on the surface directly above the earthquake's origin, not necessarily where the fault reaches the surface. ❯❯ Critical Thinking Question Figure 9.4 Why are nearly all intermediate- and deep-focus earthquakes associated with convergent plate boundaries? Answer: It is only at the subduction zones of convergent plate boundaries that plate material is pushed past other material, in this case Mantle. At the other two boundaries, no plate material extends to the depth of intermediate- and deep-focus earthquakes. Intermediate- and deep-focus earthquakes are primarily associated with convergent plate boundaries because these areas involve one plate subducting beneath another. As the subducting plate sinks into the mantle, it causes intense stress and deformation at various depths, leading to earthquakes at intermediate to deep levels within the subduction zone. ❯❯ Critical Thinking Question Figure 9.11 Why are structures built on bedrock usually not as severely damaged during an earthquake as those sited on unconsolidated material? Answer: During an earthquake, unconsolidated material shifts with surface waves more violently than bedrock. Structures on bedrock are usually less severely damaged during an earthquake because bedrock is more rigid and transmits seismic waves efficiently, causing less amplification and shaking. In contrast, unconsolidated material, like loose soil, amplifies seismic waves and can lead to greater shaking and instability, causing more damage to structures. ❯❯ Critical Thinking Question Figure 9.24 If Earth were a homogeneous body, how would seismic waves behave as they moved through Earth? Answer: A homogeneous Earth would create a series of radiating body waves with no shadow zones as we see with today’s P and S Waves. If Earth were a homogeneous body, seismic waves would travel in straight lines at uniform speeds without bending or reflecting. The lack of variation in material properties would mean that waves would not experience refraction, diffraction, or significant changes in velocity, resulting in predictable and continuous wave paths. Suggested Answer to Selected Short Answer Question (Answers to question 7 and question 8 provided in the appendix to the text) 10. Creative Thinking Visual Question: At 3:02 a.m., on August 17, 1999, violent shaking from a magnitude-7.4 earthquake awakened millions of people in Turkey. Unfortunately for many, their houses or apartment buildings collapsed, causing an estimated 17,000 deaths, at least 50,000 injuries, and leaving more than 150,000 buildings moderately to heavily damaged. Like California, Turkey is situated in an earthquake-prone area. However, there are typically many more deaths and much greater destruction from earthquakes in Turkey than from earthquakes of similar size along the San Andreas Fault. Why is this so? What could some of the factors be that lead to so much more damage in Turkey than in California, even when the earthquakes are the same magnitudes? To answer this question, look at Figures 1a and b and think about how the types of construction, population density, building codes, the type of fault movement, and other factors generally lead to a greater number of deaths and more destruction in Turkey than in California. Suggested Answer: The tectonic setting in Turkey is more complex than that in California. The San Andreas fault marks the transform boundary between the North American and the Pacific plates, two of the dozen major mobile plates making up the mosaic of Earth's crust. The North Anatolian fault is the northern boundary of the small Turkish microplate, which is wedged between the Eurasian plate to the north and the Arabian plate to the south. The Turkish microplate is being squeezed westward as the Arabian and Eurasian plate converge. A marked difference in level of destruction and loss of lives, however, arises from the difference in policy and planning between the two locations. A major difference is that the earthquake in Turkey occurred in a densely populated region. Moreover, most of the buildings in Turkey were not built with earthquake-resistant design and good construction practices. The greater damage and higher death toll in Turkey compared to California, despite similar magnitudes, can be attributed to factors such as less stringent building codes, older or poorly constructed buildings, and higher population density in vulnerable areas. Additionally, Turkey experiences different fault movement and tectonic settings, which can affect shaking intensity and damage. In contrast, California's strict building regulations and modern construction practices reduce earthquake damage and fatalities. Chapter 10 Deformation, Mountain Building, and Earth’s Crust Chapter Outline 10.1 Introduction 10.2 Rock Deformation—How Does It Occur? 10.3 Strike and Dip—The Orientation of Deformed Rock Layers 10.4 Deformation and Geologic Structures 10.5 Deformation and the Origin of Mountains GEO-FOCUS: Engineering and Geology 10.6 Earth's Crust Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • Rock deformation involves changes in the shape or volume or both of rocks in response to applied forces. • Geologists use several criteria to differentiate among geologic structures such as folds, joints, and faults. • Correctly interpreting geologic structures is important in human endeavors such as constructing highways and dams, choosing sites for power plants, and finding and extracting some resources. • Deformation and the origin of geologic structures are important in the origin and evolution of mountains. • Most of Earth's large mountain systems formed, and in some cases, continue to form, at or near the three types of convergent plate boundaries. • Terranes have special significance in mountain building. • Earth's continental crust, and especially mountains, stands higher than adjacent crust because of its composition and thickness. Chapter Summary • Folded and fractured rocks have been deformed or strained by applied stresses. • Stress is compression, tension, or shear. Elastic strain is not permanent, but plastic strain and fracture are, meaning that rocks do not return to their original shape or volume when the deforming forces are removed. • Strike and dip are used to define the orientation of deformed rock layers. This same concept applies to other planar features such as fault planes. • Anticlines and synclines are up- and down-arched folds, respectively. They are identified by strike and dip of the folded rocks and the relative ages of rocks in these folds. • Domes and basins are the circular to oval equivalents of anticlines and synclines, but they are commonly much larger structures. • The two structures that result from fracture are joints and faults. Joints show no movement parallel with the fracture surface, whereas faults do. • On dip-slip faults, all movement is up or down the dip of the fault. If the hanging wall moves relatively down, it is a normal fault’ but if the hanging wall moves relatively upward, it is a reverse fault. Normal faults result from tension; reverse faults result from compression. • In strike-slip faults, all movement is along the strike of the fault. These faults are either right-lateral or left-lateral, depending on the apparent direction of offset of one block relative to the other. • Oblique-slip faults show components of both dip-slip and strike-slip movement. • A variety of processes account for the origin of mountains. Some involve little or no deformation, but the large mountain systems on the continents resulted from deformation at convergent plate boundaries. • Subduction of an oceanic plate beneath another oceanic plate or beneath a continental plate causes an orogeny. At an oceanic-oceanic boundary, a volcanic island arc intruded by plutons forms, whereas at an oceanic-continental boundary, a volcanic arc forms on the continental plate. In both cases deformation and metamorphism occur. • Some mountain systems are within continents far from a present-day plate boundary. These mountains formed when two continental plates collided and became sutured. • Geologists now realize that orogenies also involve collisions of terranes with continents. • Continental crust is characterized as granitic, and it is much thicker and less dense than oceanic crust that is composed of basalt and gabbro. • According to the principle of isostasy, Earth's crust floats in equilibrium in the denser mantle below. Continental crust stands higher than oceanic crust because it is thicker and less dense. Enrichment Topics Topic 1. Devastating Indian Earthquakes. Plate tectonics is responsible for earthquakes in India in which tens of thousands of people die. In January 2001, more than 10,000 people were killed in an intraplate earthquake on the Indian subcontinent. The Indian subcontinent is driving ever northward into Asia, so even where the plates don’t meet, weak spots are prone to earthquakes. India has suffered five such earthquakes since 1965; the January 2001 quake was on an ancient rift that originated when India separated from Gondwana 150 million years ago. At an even greater risk is the northeastern edge of the plate where it is colliding with Asia, and the strain is building quickly. ScienceNOW, January 29, 2001. Topic 2. Birth of a Mountain Range, But When? There are two camps in the debate on when the Sierra Nevada Mountains rose—one says that it was between 40 and 60 million years ago when a subducted ocean plate slid beneath the continent and raised it up; the other thinks that it occurred between 3 and 5 million years ago when a large chunk of crust broke off from beneath the continent, melted, became buoyant, and caused the range to rise. One way of trying to determine when the range rose is to look at hydrogen isotopes of ancient raindrops to distinguish the height of the cloud from which the drop fell. This technique favors the first explanation, that the Sierra Nevada rose about 50 million years ago, but the results are still controversial. ScienceNOW, July 6, 2006. Topic 3. Exotic Terrane. Remnants of Gondwanaland have been found in the Gulf Coast of the United States. Recognized by their fossils from proto-Africa, these sedimentary rocks were important to the development of plate tectonic theory. Just one of a set of terranes that were deformed and metamorphosed ancient volcanic arcs, the terrane helps geologists model ancient plate tectonic movements. Science News, Feb. 4, 2008. http://www.sciencedaily.com/releases/2008/02/080204212810.htm Common Misconceptions Misconception 1: Rocks are solid, permanent, unmovable, and undeformable, as shown in such expressions as “solid/hard as a rock,” “like a rock,” “I am a rock,” etc. Fact: Rocks respond to stresses just like other solid objects, and given sufficient stress conditions, which may include a long enough interval of time, rocks will deform like a plastic, taking on new orientations, shapes and dimensions. Misconception 2: Earth has always looked the same and always will look the same as it does today. Fact: Earth is a really dynamic planet. Plate tectonic processes cause mountains to be created and erosional processes cause mountains to be destroyed. Mountains are rising even now and are eroding at the same time. Lecture Suggestions 1. A large sample of “silly putty” can be used to illustrate how a given material can respond differently to different stresses or stresses applied at different rates. If a ball of the material is dropped from a short distance onto a table top, it will bounce; and although a small amount of the stress, resulting from the impact of the ball with the surface, is accommodated in a plastic manner (leaving a flat spot on the ball), most of the stress has been accommodated in an elastic fashion. The material can then be deformed plastically by squeezing it (compression) or stretching it out (tension), or simply by letting gravity pull on the ball as it sits on the table. Fracture can result, of course, if the material is pulled (by tension or shearing) too fast. Point out that even though “silly putty” is not a rock, rocks can respond in similar fashion under the right circumstances. 2. Strike and dip are difficult to visualize and need to be demonstrated in class. To illustrate strike, as well as apparent and true dip, stack some books and prop them at an angle. Let the binding’s trace be the strike and the cover be the bed’s dip surface. Take a pencil and orient it on the dip surface (cover) so that it is parallel to strike (the binding). Now slowly rotate the pencil so its eraser end remains fixed on the dip surface and its point moves away from the dip surface in a horizontal plane, until it lies perpendicular to the strike (binding edge). Notice that the distance between the pencil point and the dip surface (book cover) increases from zero (when the pencil is parallel to strike) to a maximum value, when it is perpendicular to the strike. The angle between the eraser and the pencil’s projection (or its shadow from an overhead light) on the cover will increase from zero to some maximum value. These are dip or inclination angles, and the maximum value is that of the true dip. 3. Students are likely to remember hanging wall and foot wall if they are told that the hanging wall is characterized as that wall from which a geologist could only hang (as opposed to walk), and the footwall as that wall which would be beneath a geologist’s feet. 4. Once they can recognize the hanging wall and footwall, it may help the students remember the types of dip-slip faults from the acronym FUN. This stands for “Footwall (moved) Up = Normal” fault. Obviously, the other type is HUR (“Hanging wall Up = Reverse”). 5. Ask students to compare and contrast the types of geologic features, structures, and activity that occur on continent-continent, continent-oceanic, and oceanic-oceanic convergent plate boundaries. Also compare and contrast the geologic features, structures, and activity along divergent, convergent, and transform plate boundaries. 6. The terms and structures of anticlines and synclines can be more readily understood if it is noted that “cline” means slope and “anti” means opposite or away from, while “syn” means together or toward. 7. A three-layered peanut butter and jelly sandwich, cut so as to illustrate a fault, can be used to demonstrate the types of forces and resulting structures which form along convergent, divergent, and transform boundaries. Consider This 1. What will happen to the western coast of California in 30 or 40 million years as the result of movements along the San Andreas Fault Zone? Answer: In 30 to 40 million years, the western coast of California will likely shift northward along the San Andreas Fault Zone, causing significant changes in its position relative to the rest of the North American continent. 2. Locate the shield areas of each of the continents. Answer: The shield areas on each continent are: • North America: Canadian Shield • South America: Brazilian Shield • Africa: African Shield • Asia: Siberian Shield • Australia: Australian Shield 3. If orogenesis is typically associated with the active margins of continents, and the Rocky Mountains are relatively young and not the product of suturing of two continents, how can the mid-continent location of the Rocky Mountains be explained by plate tectonic theory? Answer: The mid-continent location of the Rocky Mountains can be explained by intraplate tectonics where ancient collisions and reactivation of older faults caused uplift and orogenesis far from active plate boundaries. 4. What type of forces and geologic structures dominate the northern Rocky Mountains? Which dominate the middle Rocky Mountains? Answer: The northern Rocky Mountains are dominated by compressional forces and associated structures like thrust faults and folds. In contrast, the middle Rocky Mountains are influenced more by extensional forces and normal faults, resulting in features like basins and ranges. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. Himalayas: Where the Earth Meets Sky, library.thinkquest.org/10131/ The formation of the Himalaya Mountains Videos 1. Natural Landscapes of North America. Insight Media (1999, 21 mins.) The eight major geologic regions of North America with descriptions of the natural forces that created and continue to shape them. 2. Mountain Building and Continents. Insight Media (1999, 18 mins.) The evolution of the continents and major mountain belts. 3. Mountains and Mountain-Building Processes. Insight Media (2000, 23 mins.) Mountain-building processes in several mountain ranges around the world depicted by graphics and live-action footage. 4. Earth Revealed. Annenberg Media http://www.learner.org/resources/series78.html (1992, 30 mins., free video): • #7: Mountain Building. Animations reveal how mountains are built and how they erode with emphasis on plate tectonics, the rock cycle and other processes. 5. Before the Mountains. AAPG Bookstore DVD (1987, 29 mins.) The sedimentary rocks that came before the Rocky Mountains. 6. Birth of the Rockies. AAPG Bookstore DVD (1987, 28 mins.) The thrust sheets that form the Rockies. Slides 1. Educational Images slide sets. http://www.educationalimages.com/cg120001.htm • Sediments, Faults, Unconformities Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 10.5 Suppose it is 1 km from the right side of the diagram to the point where the uppermost sandstone layer plunges beneath the surface. Can you predict the depth at which this sandstone would be encountered if you drilled a hole at the right side of the diagram? Explain. Answer: Yes; trigonometry could be used to find the answer. If the uppermost sandstone layer plunges beneath the surface at a rate shown in the diagram, and it is 1 km from the right side to where it plunges, the sandstone would be encountered at a depth of 1 km below the surface where you drill, assuming a consistent plunge angle and no additional geological complications. ❯❯Critical Thinking Question Figure 10.9 Show on this illustration the direction of the forces that caused deformation. Answer: Compressional forces produce a series of anticlines and synclines. The forces would come from the right and the left. To show the direction of the forces causing deformation on the illustration, draw arrows indicating compressional forces converging toward each other for folds and thrust faults, or arrows diverging away from each other for extensional forces associated with normal faults. ❯❯ Critical Thinking Question Figure 10.14 If the arrows were not shown in Figure 10.14c, could you figure out the relative movement on this fault? Answer: With the fault breccia interrupting the connection between both sides, it might be very difficult to show the fault motion: but once you analyze the “drag” on opposite sides, it becomes more obvious Yes, you could figure out the relative movement on the fault by analyzing the displacement of rock layers. If the layers on one side of the fault are offset relative to those on the other side, you can determine the fault type and relative movement based on the direction and pattern of this displacement. Suggested Answer to Selected Short Answer Question (Answers to question 6 and question 7 provided in the appendix to the text) 9. Discuss how time, rock type, temperature, and pressure influence rock deformation. Suggested Answer: When rocks are under stresses greater than their own strength, they begin to deform, usually by folding, flowing, or fracturing. The change in shape or volume of a body of rock as a result of stress is called strain. At depths, where temperatures and confining pressures are high, rocks show ductile behavior, that is, they can change shape. The mineral composition and texture of a rock also greatly affect how it will deform. Rocks like granite and basalt that are composed of minerals with strong internal molecular bonds usually fail by brittle fracture. Sedimentary rocks that are weakly cemented or metamorphic rocks that contain zones of weakness—such as foliation—are more likely to deform by ductile flow. Rocks that are weak and most likely behave in a ductile manner when under force include rock salt, gypsum, and shale. Limestone, schist, and marble are of intermediate strength and may also behave in a ductile manner. In nature small stresses applied over long time spans play an important role in the deformation of rock. Forces that are unable to deform rock when first applied may cause rock to flow if the force is maintained over a long period of time. Time influences rock deformation by allowing more significant changes under stress, with rocks deforming more over long periods. Rock type affects deformation as different minerals respond differently to stress; some are more ductile or brittle. Temperature increases ductility, making rocks more pliable at higher temperatures, while pressure also increases ductility and alters deformation mechanisms. Solution Manual for The Changing Earth: Exploring Geology and Evolution James S. Monroe, Reed Wicander 9781285733418
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