Instructor s Manual Chapter 6

Transcription

1 Chapter 6 Table of Contents Chapter Outline Learning Outcomes Chapter Summary Lecture Suggestions Enrichment Topics Common Misconceptions Consider This Key Terms Internet Sites, Videos, Software, and Demonstration Aids Chapter Outline Introduction Sidebar: Erosion and Natural Resources LO1 How Are Earth Materials Altered? LO2 How Does Soil Form and Deteriorate? LO3 Weathering and Resources LO4 Sediment and Sedimentary Rocks LO5 Types of Sedimentary Rocks LO6 Sedimentary Facies LO7 Reading the Story Preserved in Sedimentary Rocks LO8 Important Resources in Sedimentary Rocks Learning Outcomes After reading this unit, the students should be able to do the following: LO1 Explain how Earth materials are altered LO2 Explain how soil forms and deteriorates LO3 Know how weathering and resources are related LO4 Identify sediment and sedimentary rocks LO5 Explain how sedimentary rocks are classified LO6 Understand sedimentary facies LO7 Read the story preserved in sedimentary rocks LO8 Recognize important resources in sedimentary rocks 44

2 Chapter Summary Mechanical and chemical weathering disintegrate and decompose parent material, so it is more nearly in equilibrium with new physical and chemical conditions. The products of weathering include solid particles and substances in solution. Mechanical weathering includes such processes as frost action, pressure release, salt crystal growth, thermal expansion and contraction, and the activities of organisms. Particles liberated by mechanical weathering retain the chemical composition of the parent material. The chemical weathering processes of solution, oxidation, and hydrolysis bring about chemical changes of the parent material. Clay minerals and substances in solution form during chemical weathering. Mechanical weathering aids chemical weathering by breaking parent material into smaller pieces, thereby exposing more surface area. 45

3 Mechanical and chemical weathering produce regolith, some of which is soil if it consists of solids, air, water, and humus and supports plant growth. Soils are characterized by horizons that are designated, in descending order, as O, A, B, and C. Soil horizons differ from one another in texture, structure, composition, and color. Soils called pedalfers develop in humid regions, whereas arid and semiarid region soils are pedocals. Laterite is a soil that results from intense chemical weathering in the tropics. Soil erosion, caused mostly by sheet and rill erosion, is a problem in some areas. Human practices such as construction, agriculture, and deforestation can accelerate losses of soil to erosion. Sedimentary particles are designated in order of decreasing size as gravel, sand, silt, and clay. Sedimentary particles are rounded and sorted during transport, although the degree of rounding and sorting depends on particle size, transport distance, and depositional process. Any area in which sediment is deposited is a depositional environment. Major depositional settings are continental, transitional, and marine, each of which includes several specific depositional environments. Lithification involves compaction and cementation, which convert sediment into sedimentary rock. Silica and calcium carbonate are the most common chemical cements, but iron oxide and iron hydroxide cements are important in some rocks. Detrital sedimentary rocks consist of solid particles derived from preexisting rocks. Chemical sedimentary rocks are derived from substances in solution by inorganic chemical processes or the biochemical activities of organisms. Geologists also recognize a subcategory called biochemical sedimentary rocks. Sedimentary facies are bodies of sediment or sedimentary rock that are recognizably different from adjacent sediments or rocks. Some sedimentary facies are geographically widespread because they were deposited during marine transgressions or marine regressions. Sedimentary structures such as bedding, cross-bedding, and ripple marks commonly form in sediments when, or shortly after, they are deposited. Geologists determine the depositional environments of ancient sedimentary rocks by studying sedimentary textures and structures, examining fossils, and making comparisons with present-day depositional processes. Intense chemical weathering is responsible for the origin of residual concentrations, many of which contain valuable minerals such as iron, lead, copper, and clay. 46

4 Many sediments and sedimentary rocks, including sand, gravel, evaporites, coal, and banded iron formations, are important resources. Most oil and natural gas are found in sedimentary rocks. Lecture Suggestions 1. Point out the importance of soils to everyday activities. For example, if a student has a hamburger for lunch, everything which he/she eats depends on soil: the lettuce, tomato, and wheat for the bun were grown in soils, and the beef cattle had to eat plants grown in soil. If the hamburger was wrapped in aluminum foil, the aluminum came from bauxite. 2. 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 rather a size category. 3. 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. 4. 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. 5. Stress the distinction between weathering and erosion. 6. 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 seafloor. 7. Be sure students understand how sedimentary facies reflect the different locations of a shoreline as sea level rises and falls. 8. 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, and clay. Partly fill the jar with water. Thoroughly shake the jar and then set it on the table, allowing the sediment to settle in a graded fashion. This shows how graded bedding can be used to locate the top direction, and tells something about the environment of deposition. 9. 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. 47

5 Enrichment Topics Topic 1. Soils and a Positive Feedback Mechanism for Global Warming. Soils are rich with carbon, which microbes eat. Warmer temperatures are causing microbes to decompose soil carbon faster and release it into the atmosphere. This increases atmospheric CO 2, which increases global temperatures and increases the rate at which microbes decompose soil carbon, etc. This is called a positive feedback mechanism for global warming. There are ways to increase the storage of carbon in the soil, but the effect would be small compared to the potential effect of the release of carbon from soil due to warmer temperatures. The only way to reduce the loss of soil carbon is to reduce greenhouse gas emissions into the atmosphere. Topic 2. Soil Erosion and Coral Reefs. Corals cannot exist in muddy water because sediment clogs their feeding apparatus and inhibits oxygen diffusion. Extensive deforestation in tropical areas is resulting in dramatic increases in the rates of soil erosion. This, in turn, is threatening the health and diversity of coral reefs, which exhibit incredible biodiversity and are nurseries for important commercial fish species. It is estimated that excess sediments due to deforestation have killed 75% of the coral reefs in the Philippines and in the Caribbean Sea off of Costa Rica. Australia s Great Barrier Reef receives five to ten times as much sediment as it did before the arrival of Europeans. Topic 3. Reducing Soil Erosion. Various methods of reducing soil erosion have been developed, but it is necessary to be sure that the solution does not do more damage than the problem and that the solution is effective. A new environmentally-friendly solution called WoodStraw covers the ground with strands of wood that reduced erosion by 98%. There are potential problems with this, such as where the wood will come from. Also, while this might work for construction sites, it will not help in agricultural areas. Topic 4. Philippine Logging Ban. The Philippine government has taken a drastic step to reducing soil erosion. In 2004, the Philippine government banned logging because soil erosion on deforested slopes brought about massive landslides that killed hundreds or thousands of people during storms. In February 2006, for example, a massive landslide killed more than 1,000 people and buried portions of the town of Guinsaugon. So many slopes have been deforested that it will be a long while before landslides are reduced in number and severity. Topic 5. Modern Agriculture and Soil Loss. Intensive farming greatly increases the erosion rate of farmland. Plowing loosens the earth, and monoculture farming (planting only one crop) leaves it exposed for part of the year. Without protection, the ground is vulnerable to rain, wind, and gravity. Loose soil exposed to the elements is easily and rapidly eroded. Usually, the soil that erodes is the nutrient-rich topsoil. Artificial fertilizers must be added to make up for losses of soil nutrients, or the land will not be as productive. About one-third of the world s farmland is currently eroding at 7 to 100 times the rate that soil is forming. China has the highest rate of soil loss, with a national annual average of about 18 tons per acre (40 metric tons per hectare). In some locations, soil is lost at 25 tons per acre (48 metric tons per hectare) per year. If every location lost soil at that rate, the entire world s topsoil would be 48

6 eroded in 150 years. Since the arrival of Europeans, the United States has lost about one-third of its topsoil. Topic 6. 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. There is a flip side to this seemingly good news. Methane is a potent greenhouse gas, about 23 times more effective at trapping heat than CO 2. 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 the 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: Common Misconceptions Misconception: Modern agricultural practices, including intensive farming, have been universally good by allowing many more people to live on the planet than would otherwise be possible. Fact: The Green Revolution has kept alive about 1 billion people who would not otherwise have been able to be fed. However, the costs in pollution and topsoil erosion, among many other things, will eventually need to be paid. Allowing 1 billion more people contributes to overpopulation, which causes many more problems. Misconception: Granite and marble are for eternity: What do you want on your tombstone? Fact: No rock will retain an inscription and last as a monument forever. All rocks exposed at the surface undergo weathering. The type and rate of weathering depend on several factors, with climate being most important. The acid content of the rainwater is also an important factor. Perusing headstones in old cemeteries is very informative along this line. Misconception: 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 that serve as traps. Misconception: Petroleum formed from the remains of dinosaurs. Fact: Petroleum and natural gas form from the remains of microscopic organisms that exist in the seas and in 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. 49

7 Consider This 1. The ecosystem encompasses the interactions among the lithosphere, hydrosphere, atmosphere, and biosphere. Soils are very different in different ecosystems. What role do they play in deserts, forests, tundra, and other ecosystems? 2. How did the rates of weathering and erosion change after the evolution of land plants? 3. Our attitudes toward the land and our place in nature largely determine how land is used. What are the three general attitudes toward land use commonly found within this and other societies? 4. What are some of the ways in which the rate of soil erosion can be reduced? 5. Should mineral resource-rich laterite soils in tropical regions be mined? Why or why not? 6. Is soil a renewable or nonrenewable resource? 7. What effect might increased soil erosion have on global warming? 8. 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? 9. Has the abundance of some sedimentary rock types changed over the duration of Earth s existence? 10. What types of sedimentary rocks are evidence of arid conditions and of tropical climates? 11. 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? 12. If fossils did not exist, would geologists have discovered that Earth has a history longer than recorded human history? 50

8 Key Terms bed frost action salt crystal growth biochemical sedimentary rock graded bedding sediment carbonate rock hydrolysis sedimentary facies cementation laterite sedimentary rock chemical sedimentary rock lithification sedimentary structure chemical weathering marine regression soil compaction marine transgression soil degradation cross-bedding mechanical weathering soil horizon depositional environment mud crack solution detrital sedimentary rock oxidation spheroidal weathering differential weathering parent material strata erosion pedalfer talus evaporite pedocal thermal expansion and exfoliation dome pressure release contraction fossil regolith weathering ripple mark Internet Sites, Videos, Software, and Demonstration Aids Internet Sites 1. Sedimentary Rocks: Picture Gallery of the Most Common Rock Types: Photos and articles covering all types of sedimentary rocks from geology.com. 2. Natural Resources Conservation Data: U.S. Department of Agriculture site to help people understand soils. 3. American Association of Petroleum Geologists: About careers in petroleum geology, with videos and other items for sale. 4. Grand Canyon Geology: The National Park Service has a Geology Training Manuel for people interested in the sedimentary rocks and the formation of the Grand Canyon. 5. Sediment Thickness of the World s Oceans: 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, DVDs (2008, 4 40 min.) Touring the national monuments of the Pacific Northwest in search of geologic features, such as fossil beds and lava flows. 51

9 2. Elements of Earth Science: Rocks, Minerals and Soils. Insight Media, DVD (2005, 30 min.) The rock cycle, the main types of rocks, and how fossil fuels are used. 3. Rock Cycle. Insight Media, DVD (2003, 30 min.) How minerals form rocks and how rocks alter from one type to another. 4. Sedimentary Environments. Insight Media, DVD (1995, 19 min.) Sedimentary rock formation, names, and classifications. 5. Sedimentary Rocks and Their Formation. Insight Media, DVD (2004, 18 min.) 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 min.) 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 min.) The chemical and ecological features of soil. 8. Earth Revealed. Annenberg Media: (1992, 30 min., free video): #8: Earth s Structures. Exploring rock layers, sedimentation, structures, and petroleum, using the Grand Canyon as a study side. #15: Weathering and Soils. A comparison of weathering in a city versus a remote desert. #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, Mac/Windows CD-ROM (2001) The main rock and mineral types and how they are identified. 2. Society for Sedimentary Geology Slide Sets: Rivers and Their Deposits Coastal Erosion 3. Educational Images, Ltd. Slide Sets: Fossils and Fossilization Sediments, Faults and Unconformities Geomorphology and Computer Programs Erosion, Slides and Surface Features Rocks and Topography 52

10 4. GeoPhoto Publishing. 35 mm transparencies or digital images: Weathering, Soils, and Erosion Sedimentary Rocks 5. Science Stuff: Sedimentary Rock Collection 53