Elemental Geosystems, 5th Edition - Chapter 8

PART THREE: Earth's Changing Landscape Systems Overview Earth is a dynamic planet whose surface is actively shaped by physical agents of change. Part Three is organized around two broad systems of these agents—the internal (endogenic) and external (exogenic). The endogenic system (Chapters 8 and 9) encompasses internal processes that produce flows of heat and material from deep below the crust, powered by radioactive decay. This is the solid realm of Earth. “The Ocean Floor” chapter-opening illustration that begins Chapter 9 is used as a bridge between these two endogenic chapters. The exogenic system (Chapters 10–14) includes external processes that set water, air, waves, and ice into motion, powered by solar energy. This is the fluid realm of Earth's environment. These media are sculpting agents that carve, shape, and reduce the landscape. The content is organized along the flow of energy and material or in a manner consistent with the flow of events. To assist in preparing lecture materials for Part 3, I recommend that you obtain a copy of Geomorphology from Space—A Global Overview of Regional Landforms, edited by Nicholas M. Short and Robert W. Blair, Jr. Washington, D.C.: Scientific and Technical Information Branch, National Aeronautics and Space Administration, 1986. This is available from the Superintendent of Documents, U.S. GPO. The 700-page volume has chapters on regional landform analysis associated with tectonic, volcanic, fluvial, deltaic, coastal, karst, lacustrine, eolian, glacial, and planetary landform processes. Also included are sections on geomorphology mapping and a future look at global geomorphology. This book contains thousands of images and photographs, many in full color. Each chapter is accompanied by an informative text, references, and detailed source information for the images used. It is a great resource for teaching, lecture preparation, and classroom media development. Also there are two other basic reference works—Bates, Robert L., and Julia A. Jackson eds., Glossary of Geology, 3rd ed., Alexandria, VA: American Geological Institute (AGI), 1987; and a topical source book: Smith, David G. ed., The Cambridge Encyclopedia of Earth Sciences, New York (London): Cambridge University Press, 1981. 8 The Dynamic Planet The 20th century was a time of great discovery about Earth's internal structure and dynamic crust, yet much remains undiscovered. It was a time of revolution in our understanding of how the present arrangement of continents and oceans evolved. One task of physical geography is to explain the spatial implications of all this new knowledge and its effect on Earth's surface and society. The fact that the continents are actively adrift is still astounding and perhaps unknown to many. Physical geographers have a responsibility to turn people on to this information about our dynamic planet. Outline Headings and Key Terms The first-, second-, and third-order headings that divide Chapter 8 serve as an outline for your notes and studies. The key terms and concepts that appear boldface in the text are listed here under their appropriate heading in bold italics. All these highlighted terms appear in the text glossary. Note the check-off box () so you can mark your progress as you master each concept. Your students have this same outline in their Student Study Guide. The icon indicates that there is an accompanying animation or other resource on the CD. The outline headings for Chapter 8: Earth’s Varied Landscapes: Aerial Photo gallery endogenic system exogenic system The Pace of Change Applying Relative Dating Principles geologic time scale uniformitarianism Earth’s Structure and Internal Energy Earth in Cross-Section seismic waves Earth’s Core core Earth’s Magnetism geomagnetic reversal Earth’s Mantle mantle asthenosphere Lithosphere and Crust crust Mohorovicic discontinuity (Moho) granite basalt isostasy The Geologic Cycle The Rock Cycle Notebook Igneous Rocks Table Formation of intrusive igneous features Foliation (metamorphic rock) geologic cycle Rock Cycle mineral rock Igneous Processes igneous rock magma lava pluton batholith Sedimentary Processes stratigraphy sedimentary rocks lithification limestone evaporites Metamorphic Processes metamorphic rock Plate Tectonics Seafloor Spreading, Subduction Pangaea Breakup, Plate movements Plate Boundaries Notebook India Collision with Asia Transform Faults, Plate Margins A Brief History Pangaea Plate tectonics Seafloor Spreading and Production of New Crust Convection in a Lava Lamp sea-floor spreading mid-ocean ridges Subduction of Crust subduction zone oceanic trenches The Formation and Breakup of Pangaea Plate Motions Through Time Plate Boundaries Motion at Plate Boundaries Correlating Processes and Plate Boundaries Forming a Divergent Boundary Transform faults, plate margins Earthquakes and Volcanoes Hot Spots Hot Spot Volcano Tracks hot spots geothermal energy Summary and Review News Reports High Latitude Connection 8.1: Isostatic Rebound in Alaska News Report 8.1: Radioactivity: Earth’s Time Clock News Report 8.2: Drilling the Crust to Record Depths Focus Study 8.1: Heat from Earth—Geothermal Energy and Power _____________________________________________ The URLs related to this chapter of Elemental Geosystems can be found at http://www.prenticehall.com/christopherson Key Learning Concepts After reading the chapter and using the Student Study Guide, the student should be able to: • Distinguish between the endogenic and exogenic systems, determine the driving force for each, and explain the pace at which these systems operate. • Diagram Earth’s interior in cross section and describe each distinct layer. • Illustrate the geologic cycle and relate the rock cycle and rock types to endogenic and exogenic processes. • Describe Pangaea and its breakup and relate several physical proofs that crustal drifting is continuing today. • Portray the pattern of Earth’s major plates and relate this pattern to the occurrence of earthquakes, volcanic activity, and hot spots. Annotated Chapter Review Questions • Distinguish between the endogenic and exogenic systems, determine the driving force for each, and explain the pace at which these systems operate. 1. To what extent is Earth's crust actively building at this time in its history? The U.S. Geological Survey reports that, in an average year, continental margins and seafloors expand by 1.9 km3 (0.46 mi3). But, at the same time, 1.1 km3 (0.26 mi3) are consumed, resulting in a net addition of 0.8 km3 (0.2 mi3) to Earth's crust. The results are irregular patterns of surface fractures, the occurrence of earthquakes and volcanic activity, and the formation of mountain ranges. 2. Define the endogenic and the exogenic systems. Describe the driving forces that energize these systems. The endogenic system (Chapters 8 and 9) encompasses internal processes that produce flows of heat and material from deep below the crust, powered by radioactive decay. This is the solid realm of Earth. The exogenic system (Chapters 10–14) includes external processes that set air, water, and ice into motion, powered by solar energy. This is the fluid realm of Earth's environment. These media are sculpting agents that carve, shape, and reduce the landscape—all under the pervasive influence of gravity. 3. How is the geologic time scale organized? What is the basis for the time scale in relative and absolute terms? What era, period, and epoch are we living in today? The geologic time scale (Figure 8.1) reflects currently accepted names and the relative and absolute time intervals that encompass Earth's history (eons, eras, periods, and epochs). The sequence in this scale is based upon the relative positions of rock strata above or below one another. An important general principle is that of superposition, which states that rock and sediment always are arranged with the youngest beds “superposed” near the top of a rock formation and the oldest at the base—if they have not been disturbed. The absolute ages on the scale, determined by scientific methods such as dating by radioactive isotopes, are also used to refine the time-scale sequence. The figure presents important events in Earth's life history along with the geologic time scale. For interesting historical background see Lawrence Badash's “The Age-of-the-Earth Debate,” Scientific American (August 1989): pp. 90–96. Also see a good succinct review in John Thackray's The Age of the Earth, London: Her Majesty's Stationery Office for the Institute of Geological Sciences, 1980 (ISBN 0-11-884077-0), available in the U.S. from Cambridge University Press, New York. Although it is filled with European terminology, a good treatment of chronostratic and chronometric time scales and related dating methods is in Harland, W. Brian, Armstrong, Richard L., et al., A Geologic Time Scale, New York (London): Cambridge University Press (1990). 4. Describe uniformitarianism in Earth’s development. How can this flow of events and time be interrupted? The guiding principle of Earth science is uniformitarianism, first proposed by James Hutton in his Theory of the Earth (1795) and later amplified by Charles Lyell in his Principles of Geology (1830). Uniformitarianism assumes that the same physical processes active in the environment today have been operating throughout geologic time. “The present is the key to the past” describes this principle. Evidence unfolding from modern scientific exploration and from the landscape record of volcanic eruptions, earthquakes, and Earth’s processes support uniformitarianism. However, geologic time is punctuated by dramatic events, such as massive landslides, earthquakes, volcanic episodes, and extraterrestrial asteroid impacts. Within the principle of uniformi-tarianism, these localized catastrophic events occur as small interruptions in the generally uniform processes that shape the slowly evolving landscape. Here, the punctuated equilibrium (interruptions in the flow of events; jumps to new system operation levels) concept studied in the life sciences and paleontology might apply to aspects of Earth’s long developmental history. • Diagram Earth’s interior in cross section and describe each distinct layer. 5. Make a simple sketch of Earth's interior, label each layer, and list the physical characteristics, temperature, composition, and range of size of each on your drawing. See details in Figures 8.2, 8.3, and 8.4 as a basis for this sketch. For an up-to-date survey of the science of Earth’s structure, see “Research: From the Core to the Crust,” in Geotimes, July 1999 issue (Vol. 44, No. 7). 6. What is the present thinking on how Earth generates its magnetic field? Is this field constant, or does it change? Explain the implications of your answer. The fluid outer core generates at least 90% of Earth's magnetic field and the magnetosphere that surrounds and protects Earth from the solar wind. A present hypothesis by scientists from Cambridge University details spiraling circulation patterns in the outer core region that are influenced by Earth's rotation; this circulation generates electric currents, which in turn induce the magnetic field. An intriguing feature of Earth's magnetic field is that it sometimes fades to zero and then returns to full strength with north and south magnetic poles reversed! In the process, the field does not blink on and off but instead oscillates slowly to nothing and then slowly regains its strength. (New evidence suggests the field fades slowly to zero, then when it returns it tends to do so abruptly.) This magnetic reversal has taken place nine times during the past 4 million years and hundreds of times over Earth's history. The average period of a magnetic reversal is 500,000 years, with occurrences as short as several thousand years possible. Earth's magnetic field presently is losing strength at the rate of approximately 7% per 100 years. The field was about 40% stronger 2000 years ago according to the latest published research. 7. Describe the asthenosphere. Why is it also known as the plastic layer? What are the consequences of its convection currents? The extreme upper mantle, just below the crust, is known as the asthenosphere, or plastic layer. It contains pockets of increased heat from radioactive decay and is susceptible to convective currents in these hotter (and therefore less dense) materials. The depths affected by these convection currents are the subject of much scientific speculation. Because of this dynamic condition, the asthenosphere is the least rigid region of the mantle, with densities averaging 3.3 g/cm3. This section of the mantle is known as the plastic layer due to its dynamic activity. About 10% of the astheno-sphere is molten in asymmetrical patterns and hot spots. Think of Earth's outer crust (densities of 2.7 g/cm3 for continental crust and 3.0 g/cm3 for oceanic crust) as floating on the denser layers beneath, much as a boat floats on water. With a greater load (e.g., ice, sediment, mountains), the crust tends to ride lower in the asthenosphere. Convection currents in the astheno-sphere disturb the overlying crust and create tectonic activity. In return, the movement of the crust-collision, divergence, may influence currents in the mantle. 8. What is a discontinuity? Describe the principal discontinuities within Earth. A discontinuity is a place where a change in physical properties occurs between two regions deep in Earth's interior. A transition zone of several hundred kilometers marks the top of the outer core and the beginning of the mantle. Scientists at the California Institute of Technology analyzed the behavior of more than 25,000 earthquakes and determined that this transition area is bumpy and uneven, with ragged peak-and-valley-like formations. Some of the motions in the mantle may be created by this rough texture at what is called the Gutenberg discontinuity. The boundary between the crust and the rest of the lithospheric upper mantle is another discontinuity called the Mohorovicic discontinuity, or Moho for short, named for the Yugoslavian seismologist who determined that seismic waves change at this depth, owing to sharp contrasts of materials and densities. 9. Define isostasy and isostatic rebound, and explain the crustal equilibrium (balance between buoyancy and gravity) concept. The principle of buoyancy (that something less dense, like wood, floats in denser things like water) and the principle of balance were further developed in the 1800s into the important principle of isostasy to explain certain movements of Earth's crust. The entire crust is in a constant state of compensating adjustment, or isostasy, slowly rising and sinking in response to its own weight, and pushed and dragged about by currents in the asthenosphere (Figure 8.4). 10. Diagram the uppermost mantle and crust. Label the density of the layers in gm/cm3. What two types of crust were described in the text in terms of rock composition. See Figures 8.2c and 8.4 as the basis for this diagram. The two types of crust discussed in the text were oceanic crust, composed of basalt, a rock high in silica and magnesium (earning its name as simatic crust), and continental crust, composed mostly of granite, a rock high in silica and aluminum (earning its name as sialtic crust). • Illustrate the geologic cycle and relate the rock cycle and rock types to endogenic and exogenic processes. 11. Illustrate the geologic cycle and define each component: rock cycle, tectonic cycle, and hydrologic cycle. See Figure 8.5 as the basis of this illustration. 12. What is a mineral? A mineral family? Name the most common minerals on Earth. What is a rock? A mineral is an element or combination of elements that forms an inorganic natural compound. A mineral can be described with a specific symbol or formula and possesses specific qualities. Silicon (Si) readily combines with other elements to produce the silicate mineral family, which includes quartz, feldspar, amphibole, and clay minerals, among others. Another important mineral family is the carbonate group, which features carbon in combination with oxygen and other elements such as calcium, magnesium, and potassium. Of the nearly 3000 minerals, only 20 are common, with just 10 of those making up 90% of the minerals in the crust. A rock is an assemblage of minerals bound together (such as granite, containing silica, aluminum, potassium, calcium, and sodium) or sometimes a mass of a single mineral, such as rock salt. Some background notes on economic aspects of rocks and minerals. Many different types of rock are useful and of economic importance to society. Clays of certain grades are used for pipe-making and pottery, some very fine clays are used to coat paper, such as the paper on which the textbook is printed. Pure sands, high in quartz content, are processed in glass making. Sands and gravels are an important aggregate in cement and for building construction. Lime derived from limestone is used in the making of cement and in agriculture. Phosphates from marine shales and limestone are important in the making of fertilizer. Gypsum, an evaporite derived from deposits related to sea water, is used in plaster. Marble, granite, and limestone are used as construction material, with pure white marble preferred for sculpture and decorative building facades. And, of course, salt (NaCl) is of importance to civilization throughout history. The rock cycle is intricately woven into society in many ways. An ore is a body of rock which contains minerals sought by society. If the concentration of the desired mineral is high enough, mining becomes economically feasible. A major concentration process in nature for iron, lead, zinc, mercury, copper, and other minerals involves hydrothermal solutions. Sometimes associated with an intrusive igneous pluton, and sometimes with contact metamorphism, high temperature moisture solutions dissolve minerals and carry them through cracks, joints, and fractures. These valuable elements precipitate out in vein formations. In many places, individual veins of ore are traceable back to the parent igneous pluton. Sometimes these ore deposits occur in association with other vein-filling minerals such as quartz and calcite. In other areas, hydrothermal solutions disseminate the mineral precipitate throughout an ore body, producing a low concentration of the desired mineral in a large mass of ore. Large-scale mining methods are used to extract such disseminated minerals. The Bingham Open Pit Copper Mine west of Salt Lake City, Utah is a prime example. The copper ore was so low-grade that economics dictated location of the concentrator, smelter, and refinery near the mine, thus reducing transportation costs. To get 6.4 kg of copper, almost 900 kg of ore had to be processed, which required the removal of 2.04 metric tons of overburden (14 lbs. of copper, 2000 lbs. of ore, 4500 lbs of overburden). Low copper prices worldwide kept the operation, along with other western copper mines, closed during much of the 1980s. Relative to the formation of mineral deposits on the ocean floor, current scientific thinking points directly to the actions of plate tectonics. For the first time, scientists aboard deep-sea submersibles saw mineral deposits actually forming on the ocean floor. Hot solutions of minerals spew from ocean floor vents that are associated with the mid-ocean ridge system. Minerals precipitate out when the hot solution comes into contact with the near-freezing temperatures of the deep ocean. Plate tectonics carries these accumulated deposits toward eventual collision and subduction beneath the continents. In areas where some of the “cargo” on the plate is not subducted but is instead raised and pasted onto the continental mass, the mineral-rich content is readily visible. On the island of Cypress in the Mediterranean Sea, deposits of copper were mined for the past 4000 years by various civilizations. Cypress is composed of old seafloor pressed upward by the African-Eurasian collision. The deposits that do subduct melt with the diving plate and work their way toward the surface, cooling and dissolving according to the specific nature of each element or mineral involved. Hot water trapped in the rock does the rest, dispersing the minerals in veins or disseminating the mineral ore bodies as we discussed above. Other formation processes were at work in Sudbury, Ontario, Canada. Nickel, iron, and copper came up with a mafic (high in magnesium and iron), intrusive body of magma that cooled and began crystallization. These elements, along with other minerals, settled out in the magma chamber in specific layers, forming a very rich resource body and the basis for an active mining district. 13. Describe igneous process. What is the difference between intrusive and extrusive types of igneous rocks? Rocks that solidify and crystallize from a molten state are called igneous rocks. Most rocks in the crust are igneous. They form from magma, which is molten rock beneath the surface (hence the name igneous, which means fire-formed in Latin). Magma is fluid, highly gaseous, and under tremendous pressure. It is either intruded into preexisting crustal rocks, known as country rock, or extruded onto the surface as lava. The cooling history of the rock—how fast it cooled, and how steadily the temperature dropped—determines its texture and degree of crystallization. These range from coarse-grained (slower cooling, with more time for larger crystals to form) to fine-grained or glassy (faster cooling). Notes on Igneous Rocks. The categorization of igneous rocks is usually done by mineral composition and texture. The two broad categories are: 1. Felsic igneous rocks—derived both in composition and name from feldspar and silica (SiO2). Felsic minerals are generally high in silica, aluminum, potassium, and sodium, with low melting points. Rocks formed from felsic minerals generally are lighter in color and density than mafic mineral rocks. 2. Mafic igneous rocks—derived both in composition and name from magnesium and ferric (Latin for iron). Mafic minerals are low in silica and high in magnesium and iron, with high melting points. Rocks formed from mafic minerals are darker in color and of greater density than felsic mineral rocks. The same magma which produces coarse-grained granite when slowly cooled beneath the surface also forms a fine-grained rhyolite as its rapidly cooled volcanic counterpart. If it cools rapidly, magma having a silica content comparable to granite and rhyolite may form the dark, smoky, glassy-textured rock called obsidian or volcanic glass. Another glassy rock called pumice forms when escaping gases bubble a frothy texture into the lava. Pumice is full of small openings, is light in weight, and low enough in density to float in water. On the mafic side, basalt is the most common fine-grained extrusive igneous rock. It comprises the bulk of the ocean floor and appears in lava flows such as those on the Galápagos Islands. Its intrusive counterpart, formed by slow cooling of the parent magma, is gabbro. 14. Briefly explain how the rock formation in Figure 8.9a demonstrates through its layers a record of past climates. This sedimentary sandstone, with siltstone below, demonstrates different iron content through its coloration change, therefore different parent materials. The weaker siltstones below are weathering faster, leaving the mass above developing into a balanced rock form. Drier times in the past were marked by sand dunes. These dune forms are lithified into rock near the upper third of the rock, layered at an angle. Wetter times, when more flowing water was present, are marked by horizontal beds, laid down as sediment and lithified into stone. The discontinuity between the horizontal and angular layers must have been a time of change as parts of the record were eliminated through weathering and erosional processes. 15. Briefly describe sedimentary processes and lithification. Describe the sources and particle sizes of sedimentary rocks. Most sedimentary rocks are derived from preexisting rocks, or from organic materials such as bone and shell that form limestone, mud that becomes compacted into shale, and ancient plant remains that become compacted into coal. The exogenic processes of weathering and erosion generate the material sediments needed to form these rocks. Bits and pieces of former rocks—principally quartz, feldspar, and clay minerals—are eroded and then mechanically transported (by water, ice, wind, and gravity) to other sites where they are deposited. In addition, some minerals are dissolved into solution and form sedimentary deposits by precipitating from those solutions; this is an important process in the oceanic environment. The cementation, compaction, and hardening of sediments into sedimentary rocks are called lithification. Various cements fuse rock particles together; lime (CaCO3, or calcium carbonate) is the most common, followed by iron oxides (Fe2O3) and silica (SiO2). Particles also can unite by drying (dehydration), heating, or chemical reactions. The two primary sources of sedimentary rocks—the mechanically transported bits and pieces of former rock and the dissolved minerals in solution—are known as clastic sediments and chemical sediments, respectively. The example of evaporites forming in Death Valley, shown in the documentary pair of photos in Figure 8.10, was a dramatic experience. Another pair of photos taken over these same two days appears in Chapter 13. During the 1982–83 El Niño event the southwest and California experienced record precipitation. On a day in 1983 Death Valley received 2.57 cm, or about 55% of the normal expected amount for an entire year. I waited overnight at a roadblock because the road into the valley was closed with playa flooding over the pavement. My wait was rewarded with incredible scenes of fluvial action in the desert. The lake you see in Figure 8.10a is approximately 3 km wide and 8 km long and only several centimeters deep. I walked out about a kilometer. The natural sorting process that began at the top of the alluvial fans had left large rocks and grains behind, leaving only a very fine clay—as fine as face powder—to squish between my toes. The scene was strange because the blocked roads had let few into the valley, making the aloneness, quiet, and stillness almost overwhelming. I returned one month later and matched the pictures using a slide viewer in one hand and a camera in the other to produce the picture in (b). A bed of borated-salt precipitate approximately 2 cm thick replaced the vast reflective water surface. The salt surface cracked underfoot exposing those very fine clays. As a result of the two trips I have matching pairs of about 50 scenes throughout Death Valley demonstrating that water is the major erosional force in the desert, however infrequently it occurs! Mono Lake Briefing. Another type of hydro-thermal activity produces evaporites and sedimentary formations near the Nevada-California border at Mono Lake, an alkaline lake high in carbonates and sulfates that presently has an overall salinity of 95‰ (the ocean average is 35‰, or 35 parts per thousand). Carbonates in Mono Lake interact with calcium-rich hot springs to produce a rock called tufa. The tufa deposits grew tall in Mono Lake as long as the springs were beneath the surface of the lake. However, since the 1940s, tributary streams have been diverted out of the region, lowering the lake's surface more than 20 m (65 ft), reducing its surface area by more than half, exposing these chemical sedimentary deposits and alkaline shorelines, and damaging the lake's natural ecology and habitats for wildlife and birds, particularly birds that perished in large numbers in the early 1980s. Some tufa towers now rise over 10 m (30 ft) above the shoreline, as shown in the foreground of Figure 5.18h. Strong winds in the area blow across the newly formed alkali flats of evaporites, producing days of severe air pollution. After much litigation and a case that went to the California Supreme Court, the state’s water law was significantly changed. Century-old appropriative water rights (first one at water’s edge withdraws whatever they want) were overturned in favor of placing this body of water in the public trust—benefits to the many overshadowed rights of a few. A master plan was completed and agreed to by all parties and the future looks bright for this water-troubled region. Tributary flows were restored by court order in April 1991, and the lake level is rising. Additional court and state agency rulings in 1994 assured victory for the lake. By May 2, 2006, the lake level stood at 1945.4 m (6383.2 ft), an increase of more than 2.5 m since 1994. The tributaries are restoring habitats, bird populations are increasing in numbers, and lake ecology improving—a real success story! (Mono Lake Committee office, 760-647-6595, Lee Vining, CA, http:/www.monolake.org). 16. What is metamorphism and how are metamorphic rocks produced? Name some original parent rocks and metamorphic equivalents. Any rock, either igneous or sedimentary, may be transformed into a metamorphic rock by going through profound physical and/or chemical changes under increased pressure and temperature. (The name metamorphic comes from the Greek, meaning to change form.) Metamorphic rocks generally are more compact than the original rock and therefore are harder and more resistant to weathering and erosion. See Table 8.1, p. 268. • Describe Pangaea and its breakup and relate several physical proofs that crustal drifting is continuing today. 17. Briefly review the history of continental drift, sea-floor spreading, and the all-inclusive plate tectonics theory. What was Alfred Wegener's role? In 1912, German geophysicist and meteorologist Alfred Wegener publicly presented in a lecture his idea that Earth's landmasses migrate. His book, Origin of the Continents and Oceans, appeared in 1915. Wegener today is regarded as the father of the concept called continental drift. Wegener postulated that all landmasses were united in one supercontinent approximately 225 million years ago, during the Triassic period, Figure 8.15b. The fact that spreading ridges and subduction zones are areas of earthquake and volcanic activity provides further evidence for plate tectonics, which by 1968 had become the all-encompassing term for these crustal processes. 18. Define upwelling and describe related features on the ocean floor. Define subduction and explain the process. The worldwide submarine mountain ranges, called the mid-ocean ridges, were the direct result of upwelling flows of magma from hot areas in the upper mantle and asthenosphere. When mantle convection brings magma up to the crust, the crust is fractured and new seafloor is formed, building the ridges and spreading laterally. When continental crust and oceanic crust collide, the heavier ocean floor will dive beneath the lighter continent, thus forming a descending subduction zone (Figure 8.13). The world's oceanic trenches coincide with these subduction zones and are the deepest features on Earth's surface. Notes on Earth’s interior. The depth of convection currents in the mantle is still being investigated. Indirect evidence suggests that there are influences as deep as the Gutenberg discontinuity at the outer core-mantle boundary. Some of the undulating peaks and valleys at that transitional boundary are perhaps triggering some mantle motion. Principal regions of heating and diapir (upward moving hot plumes) formation are still probably within 300 km (185 mi) of the surface. See: Jason Phipps Morgan and Peter M. Shearer, “Seismic constraints on mantle flow and topography of the 660-km discontinuity: evidence for whole-mantle convection,” Nature, 365, October 1993: 506–11, among many published studies in Nature and Science. The lower mantle is thought to be almost uniform in composition and distinct from the upper mantle, containing high-density minerals of iron, magnesium, and silicates, with some calcium and aluminum—known as silicate perovskite. However, much scientific speculation surrounds the composition of the lower mantle. Some scientists think the upper and lower mantle remain separate and distinct. Others think that the 670 km transition zone presents no barrier to mixing and that convection occurs throughout the mantle. Various metaphors can be used here: the continental plates are barges or rafts floating on the denser asthenosphere. I will sometimes ask if any students have soft-cover textbooks that I can use for a demonstration. Then, standing next to a world physiographic map or “The Ocean Floor” mural map, I use one book as the Nazca plate and the other as the South American plate. As the plates migrate and collide (book spine to spine), I allow the sea-floor book to subduct beneath the continental book. The uplifted edge of the one book becomes the Andes crest. The original Moho project, designed to drill through oceanic crust, was abandoned in the 1960s. The Russians have been working on the deepest penetration of the crust since 1970—the Kola well under the control of the Russian Interdepartmental Council for the Study of Earth's Interior and Superdeep Drilling. The Russians have started several deep wells. Depths are now approaching 12,000 m (39,400 ft, 7.5 mi). The deepest well in the United States is over 9000 m (29,529 ft). See: Kozlovsky, Ye. A., “The World's Deepest Well,” Scientific American (December 1984): pp. 98–104. 19. What was Pangaea? What happened to it during the past 225 million years? See the sequence of illustrations in Figure 8.15, a through e. The supercontinent of Pangaea and its subsequent breakup into today's continents represents only the last 225 million years of Earth's 4.6 billion years, or only the most recent 1/23 of Earth's existence. 20. Characterize the three types of plate boundaries and the actions associated with each type. The boundaries where plates meet are clearly dynamic places. Divergent boundaries are charac-teristic of sea-floor spreading centers, where upwelling material from the mantle forms new seafloor, and crustal plates are spread apart. Convergent boundaries are characteristic of collision zones, where areas of continental and/or oceanic crust collide. These are zones of compression. Transform boundaries occur where plates slide laterally past one another at right angles to a sea-floor spreading center, neither diverging nor converging, and usually with no volcanic eruptions. • Portray the pattern of Earth's major plates and relate this pattern to the occurrence of earthquakes, volcanic activity, and hot spots. 21. What is the relation between plate boundaries and volcanic and earthquake activity? Plate boundaries are the primary location of Earth's earthquake and volcanic activity, and the correlation of these phenomena is an important aspect of plate tectonics because they are produced by plate/asthenosphere interactions at these boundaries. Earthquakes and volcanic activity are discussed in more detail in the next chapter, but their general relationship to the tectonic plates is important to point out here. Earthquake zones and volcanic sites are identified on the world plate map in Figure 8.18. 22. What is the nature of motion along a transform fault? Name a famous example of such a fault. Transform boundaries occur where plates slide laterally past one another at right angles to a sea-floor spreading center (Figure 8.15e). These plates are not diverging or converging, and there is usually no volcanic activity associated with transform boundaries. A famous example of a transform fault is the San Andreas fault; see Figures 9.11c, 9.12, and 9.13. 23. How is the Hawaiian–Emperor chain of islands and seamounts an example of plate motion and hot spot activity? Correlate your answer with the ages of the Hawaiian Islands. See Figure 8.19. Note the active island-forming eruptions are over the hot spot in the southeastern portion of the big island of Hawai‘i. The newest island named Lö‘ihi is still rising from beneath the ocean and will take another 10,000 years or more to be hit with tropical sunlight. The Hawaiian Island chain gets progressively older as you move away from the hot spot—as the Pacific Ocean plate moves northwestward. Note that Midway, produced by eruptions from this same hot spot, is 27.7 million years old and that the ages get progressively older as you move to the northwest. This is evidence of a stationary hot spot injecting materials into an overlying plate in motion. Overhead Transparencies 166. 8.1 Geologic time scale; pie chart of relative time spans 167. 8.2 a, b, c Earth in cross-section 168. 8.3 Distance from Earth’s center to crust compared to distances across North America 169. 8.4 a, b, c Isostatic adjustment of the crust 170. 8.5 a, b The geologic cycle—rock cycle, tectonic cycle, hydrologic cycle 171. 8.6 + 4 photos Rock-cycle schematic 172. 8.7 Igneous rock types 173. 8.10a, b Death Valley: wet and dry 174. 8.13 a, b, c Crustal movements, placed below blow-out image and map 175. 8.14 Magnetic reversals recorded 176. 8.15 a, b, c Breakup of Pangaea 177. 8.15 c, d Breakup of Pangaea 178. 8.16 and 8.18 Earth’s 14 lithospheric plates and their motions (top); Earthquake and volcanic activity locations (bottom) 179. 8.17 The ocean floor revealed—global gravity anomaly image 180. 8.19 Hot spot tracks across the Pacific; Emperor seamounts inset of ocean floor 181. F.S. 8.1.3 Geothermal potential map The Dynamic Planet • The Dynamic Planet • 102 • The Dynamic Planet The Dynamic Planet • 102 • The Dynamic Planet The Dynamic Planet • 102 • The Dynamic Planet The Dynamic Planet •