* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Exam Block #5
Survey
Document related concepts
Geochemistry wikipedia , lookup
Anoxic event wikipedia , lookup
Post-glacial rebound wikipedia , lookup
Physical oceanography wikipedia , lookup
Spherical Earth wikipedia , lookup
Magnetotellurics wikipedia , lookup
Schiehallion experiment wikipedia , lookup
History of geomagnetism wikipedia , lookup
Tectonic–climatic interaction wikipedia , lookup
Age of the Earth wikipedia , lookup
History of Earth wikipedia , lookup
Oceanic trench wikipedia , lookup
History of geology wikipedia , lookup
Abyssal plain wikipedia , lookup
Mantle plume wikipedia , lookup
Transcript
Exam Block #5 • Chapter 12 – Earth’s Interior • Chapter 13 – Divergent Boundaries • Chapter 14 – Convergent Boundaries How to study for this class: 1. Read the chapter and answer the Review Questions. As you read, follow along with the Chapter Outline (download from our class site below) and these PowerPoint Notes. 2. Review each chapter using the GEODe: Earth CD-ROM in your book. 3. Go online and take the Concept Quizzes and Chapter Test for each chapter at: www.prenhall.com/tarbuck 4. Check your grades online at: http://pages.sbcglobal.net/solanogeo/index.htm BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 1 of 20 Chapter 12 – Earth’s Interior 1 2 At their birth, planets form from an accumulation of nebular debris (Nebular Hypothesis) but quickly begin to form layers. The planet’s gravity causes the densest material (iron) to settle to the center and form the core. Less dense rock rises to form the mantle and even less dense rock forms the crust. The best way to learn about the Earth’s interior is to dill a hole, but the deepest well in only 12.3 km – not even through the crust! (Earth’s diameter is 6371 km). Most of our knowledge of Earth’s interior comes from the study of earthquake waves. Mantle convection. Gravity and Layered Planets Gravity and Layered Planets 3 4 Probing Earth’s Interior The seismic waves from large earthquakes (> 6 magnitude) are well-recorded by seismographs all around the globe and provide the means to “see” into our planet. Energy travels as wave fronts but often is shown as rays perpendicular to wave fronts. • Transition Zone: bottom of the asthenosphere (upper mantle). • D” (D double prime): bottom of the mesosphere (lower mantle). 5 Seismic Body Waves P waves – compressional, travel through solids and liquids. S waves – shear, cannot travel through liquids because they have no shear strength. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Nature of Seismic Waves 6 Travel times of P (compressional) and S (shear) waves through the Earth vary depending on the properties of the materials. Variations in the travel times correspond to changes in the materials encountered. Velocity depends on the density (g/cm3) and elasticity of the intervening material. Within a given layer the velocity generally increases with depth due to pressure forming a more compact elastic material. In all cases, P waves travel faster than S waves. When seismic waves encounter a sudden change in physical properties (discontinuity) they may refract (bend) or reflect (bounce) or diffract (go around). Page 2 of 20 7 Reflection vs. Refraction 8 Search For Oil and Gas Q: Which are reflected and which are refracted? Seismic waves can locate the location of oil and gas reserves within the Earth. 9 Refracting Ray Paths in the Earth Refracted seismic rays have a bent path, indicating that seismic velocity increases with depth. Earth’s Crust 11 Crust – the thinnest of all the Earth’s layers. Eggshell-thin layer compared to the Earth’s size. Oceanic crust - about 7 km thick, basaltic composition, average density of 3.0 g/cm3, and up to 160 million years old. Continental Crust: average thickness is 40 km, but more than 70 km thick in mountain belts, granitic composition (typical Sierra Nevada batholiths), average density of 2.7 g/cm3, and up to 4.0 billion years old. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Compositional/Mechanical Layers Early in Earth’s history, heavier elements sank and lighter ones floated upward, forming a layered Earth. 10 Pressure and temperature greatly affect the mechanical strength of materials. Earth’s Major Boundaries – Moho 12 Moho – boundary that separates the crust and mantle (Andrija Mohorovičič in 1909). Identified by a change in the velocity of P waves with distance from the source. P wave velocity 6 km/sec Crust P wave velocity 8 km/sec Mantle Page 3 of 20 The Mantle 13 Mantle – over 82% of Earth’s volume; it is solid because S waves travel through it. Kimberlite Pipes – are volcanic features associated with diamonds and have brought up direct samples of the upper mantle from 200 km in depth to the surface and they are peridotites. Mantle – divided into the: Upper Mantle (asthenosphere) – base of crust to 660 km in depth. Transition Zone at bottom of the upper mantle. Lower Mantle (mesosphere) – 660 km to top of core. D” at the bottom of the lower mantle. Mineral Physics – Diamond Anvil 15 Seismology alone cannot determine what the Earth is made of. Mineral samples are squeezed at great pressures by diamonds and heated by lasers to simulate conditions in the mantle. 14 Transition Zone At 410 km Depth – The mineral olivine (main component of peridotite) collapses into a more compact mineral β-spinel. Density increases and seismic velocities increase. At 660 km Depth – The mineral β-spinel collapses into a more compact mineral ringwoodite. Density increases and seismic velocities increase again. 16 ✓REVIEW QUESTIONS 1. What role does gravity play in the layering of planets? 2. What are two major reasons for the increase in density with depth within Earth’s mantle? 3. Why is seismology responsible for gasoline prices being so affordable? 4. List three differences between oceanic and continental crust. Lower Mantle 17 In the lower mantle, olivine and pyroxene take the form of the mineral perovskite. D” Layer – lowermost 200 km of mantle; partially molten layer; seismic velocities decrease; area may create mantle plumes that erupt as hotspots on the surface. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Core-Mantle Discovery 18 Richard Oldham discovered the P wave shadow zone in 1906. Beno Gutenberg calculated the size of the core in 1914. The dense core refracts rays and creates a zone where P waves are largely absent. Page 4 of 20 The Outer Core is Liquid 19 Since S waves cannot travel through liquids, the S wave shadow zone is much larger. A few weak S waves do diffract around the core into this region, but are largely absent. 20 The Inner Core is Solid Inge Lehmann discovered the inner core was a solid in 1936, by noting the refraction of P waves in this region. The inner core is rotating slightly faster than the rest of the Earth. ✓REVIEW QUESTIONS 21 6. How do S waves allow us to determine the mantle is solid? 8. What mineral phases occur at the top & bottom of the transition zone? 9. What layer of the Earth has the greatest volume? 11. True or False: No seismic waves arrive in the shadow zones? Explain. The Core 23 22 The Core The average density of the core is 11 g/cm3. Even under extreme pressure, silicate minerals would not compact enough to account for this density. Since the Earth accreted form meteorites, they provide important clues to the Earth’s internal composition. Metallic meteorites contain large amounts of iron compared to the composition of the Earth’s crust and mantle. Therefore, the core must be enriched in iron. Composition: Core is mostly iron, with 5% 10% nickel, and perhaps some sulfur and oxygen. Variations in P and S Wave Velocities 24 Origin – early in Earth’s history, the heavier iron sank to the core and the lighter materials floated up to form mantle and crust. In this early stage, the entire core was probably liquid. However, as Earth began to cool, iron in the core began to crystallize and the inner core began to form. The inner core continues to grow at the expense of the other core. The Earth’s magnetic field is produced in the liquid outer core by convection. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 5 of 20 Earth’s Temperature 25 Heat travels by 3 different mechanisms: How did the Earth get so hot? (1) by colliding particles during the formation of the Earth and collision of the Mars-sized object with the Earth that led to the formation of the moon. Convection – moving material in a fluid-like manner. This is the primary means by which heat is transferred within the Earth’s core and mantle. Conduction – slowly moving of heat through a material on an atomic scale; important in the lithosphere. Radiation – electromagnetic radiation of heat into space. (2) decay of short-lived radioactive isotopes (Al-26 & Ca-41). Why is the Earth still so hot? by decay of long-lived isotopes (uranium, thorium, and potassium) in the Earth’s crust & mantle. Mantle Convection 26 Heat Flow 27 28 Seismic Tomography 3-D images of the Earth can be generated from earthquake waves. Modeling Mantle Convection 29 Earth’s Temperature Profile * * * BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 30 Geothermal Gradient or Geotherm – change in temperature with depth. Remember: most of the Earth is solid due to pressure – except for a few locations (*). Page 6 of 20 Earth’s Magnetic Field 31 Dipole Field– the Earth’s magnetic field has a north and south magnetic pole (dipole), just like an electromagnet and a bar magnet. But remember the Curie Point (585° C): the Earth’s interior is much too hot to form a magnetic field from a solid bar magnet. Earth’s Dipole Magnetic Field 33 32 Earth’s Geodynamo Geodynamo – as the liquid iron in the Earth’s outer core convects, its path becomes twisted due to the Earth’s rotation. This produces a dipole magnetic field similar to an electromagnet. The dynamo is self-generating and self-reversing, due to changing patterns in the convection currents. 34 ✓REVIEW QUESTIONS 13. Why is Earth’s inner core growing in size? 15. What sources of heat caused Earth to get so hot? 16. What keeps the Earth so hot? 17. Explain the difference between conduction and convection. 19. Why is conduction more important than convection within Earth’s crust? 23. Why is the lithosphere stiffer than the asthenosphere? BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 7 of 20 Chapter 13 – Divergent Boundaries 1 2 Mapping the Seafloor If all water were drained from the oceans, the scenery would be nearly as varied as that on land: great linear mountains, volcanoes, deep canyons, and large plateaus. The first measurements of the depths of the oceans were made by the H.M.S. Challenger from 1872 to 1876 using a weighted line. Q: What is the old measurement of depth used at sea? Ocean floor: the last piece of the Plate Tectonic Puzzle. Mapping the Ocean Floor 3 Seismic Reflection Profile 4 Seafloor Topography from Satellites 6 A) In the 1920’s, the first echo sounders began to show topographic features of the ocean floor. B) Modern sonar can map narrow segments in much more detail using multiple sources and receivers. Seismic Reflection Profile 5 A satellite altimeter measures the variation in sea surface elevation, which mimics the shape of the seafloor, due to the gravitational attraction of the features on the ocean floor. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 8 of 20 7 Three Major Provinces of the Ocean (A) Continental Margins, (B) deep-ocean basins, and (C) mid-ocean ridges. ✓REVIEW QUESTIONS 8 1. Assuming that the average speed of sound waves in water is 1500 meters per second, determine the water depth if the signal sent out by an echo sounder requires 6 seconds to strike bottom and return to the recorder. 2. How can satellites orbiting Earth determine features on the seafloor? 3. What are the 3 major topographic provinces of the ocean floor? Q: Why does the water get deeper away from a ridge? 9 (A) Continental Margins Two main types of continental margins have been identified – passive and active. 1) Passive Margins – are found along most of the coastal areas that surround the Atlantic Ocean. They are NOT associated with plate boundaries; therefore, little volcanism and few earthquakes. Features of passive continental margins include: Continental Shelf – submerged part of the continent; however, it was above sea level during the last Ice Age when sea level dropped. Continental Slope – marks the boundary between continental crust and oceanic crust. (A) Continental Margins 11 2) Active Margins – occur where oceanic lithosphere is being subducted beneath the edge of a continent. The continental shelf is very narrow, if it exists at all. They ARE associated with plate boundaries and there is much volcanism and many earthquakes, like the Pacific Rim. Accretionary Wedge – Sediments from the ocean floor and pieces of oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the continent. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Passive Continental Margins 10 Found around the Atlantic Ocean. Active Continental Margins 12 Found around the Pacific Rim: Ring of Fire. Page 9 of 20 (2) Deep-Ocean Basins 13 Trenches – sites where oceanic lithosphere plunge back into the mantle. Deepest trench is 11,000 meters below sea level at the Mariana Trench. Location of volcanoes and earthquakes. Abyssal Plains – are the most level places on Earth in the deep-ocean basins. Flatness is due to a thick accumulation of sediment that have buried an otherwise rugged ocean floor. Seamounts – These volcanic peaks form near oceanic ridges and hot spots. As islands move away from ridges, they cool and sink and are cut by wave erosion, forming coral atolls and guyots. Formation of Coral Atolls & Guyots Trenches of the World’s Oceans 14 Be able to locate these on a map for the final. 15 Coral Atolls & Guyots 16 17 (3) Mid-Ocean Ridges 18 Coral reefs are constructed from the skeletal remains of corals and algae. They are mainly confined to warm, clear waters of the Pacific and Indian oceans. Darwin sailed the globe in the 1830’s and correctly proposed a hypothesis for the formation of coral atolls. As a volcanic island slowly cools and sinks, the coral reefs continue to grow upward, as long as the depth of less than 45 meters from the surface. If the island sinks more quickly, flat-topped guyots are formed. ✓REVIEW QUESTIONS 4. List three features that comprise a passive continental margin. What is the flooded extension of the continent? Which is the steepest? 5. Compare and contrast active and passive continental margins and give examples of each. 7. How does a guyot form? BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Mid-Ocean Ridges (MOR)– are characterized by an elevated position, extensive faulting, and numerous volcanic structures. The MOR system is the longest feature on Earth at 70,000 km (43,000 miles) in length. The ridges are offset by large transform faults. The central portion is a deep down-faulted structure called a rift valley. Tectonic plates move apart at ridges and upwelling magma generates new slivers of oceanic lithosphere (seafloor spreading). This process continues in a conveyorbelt fashion. Page 10 of 20 Mid-Ocean Ridges of the World 19 Be able to locate these on a map for the final. 20 Mid-Ocean Ridge Why does the lithosphere gradually become thicker away from an oceanic ridge? Seafloor Spreading 21 Age of Oceanic Crust 22 (3) Mid-Ocean Ridges 23 Structure of Oceanic Crust 24 Mid-Ocean Ridges (MOR)– are passive features. As plates are pulled apart, space is created, pressure is reduced, causing the upper mantle rocks to partially melt, producing new oceanic lithosphere. MOR are elevated because the newly created seafloor is hot, and occupies more volume, and therefore is less dense. As the ocean floor moves away from a ridge, it cools and contracts and becomes more dense. The oceanic lithosphere increases in thickness for about 80 million years and then remains relatively constant thickness until subducted. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 The ocean crust consists of four distinct layers, called an ophiolite complex. They are: (1) Sediments – settle on top of the seafloor. (2) Pillow lavas – ejection of magma directly into water creates these pillow-like features. (3) Sheeted dikes – vertical fractures that carried the magma to the surface. (4) Gabbro – is the coarse-grained equivalent of basalt and cooled at depth. This sequence was discovered by old pieces of oceanic crust thrust upon the land. Page 11 of 20 25 Ophiolite Sequence ✓REVIEW QUESTIONS 26 8. Describe the oceanic ridge system. 1 2 9. How are MOR mountains different than those found on continents? 3 4 10. What is the source of magma for seafloor spreading? 11. Why are MOR topographically elevated? Continental Rifting 27 New ocean basins begin with the formation of a continental rift – breakup of a landmass in four stages: 1. Thinning of lithosphere, normal faults, grabens: Basin and Range, Nevada. 2. Formation of narrow rift valley: East African Rift. 3. Formation of linear sea with basaltic oceanic crust: Red Sea. 4. Formation of mid-ocean ridge: form new ocean like the Atlantic Ocean. Mechanisms for Continental Rifting 28 Continental Rifting Basin & Range, Nevada East African Rift Valley Red Sea Atlantic Ocean East African Rift Valley & Red Sea 29 Supercontinents (like Pangaea) have existed sporadically during the geologic past. Two mechanisms have been proposed for rifting: 1. Mantle Plumes & Hotspots (this gets plates moving apart) – large amounts of heat and volcanism (flood basalts) may initiate rifting - as evidenced by the opening of the Atlantic Ocean – but forces are thought not to be great enough to disperse continental fragments. 2. Slab Pull & Slab Suction (this keeps plates moving apart) – a subducting slab creates a suction force that pulls the overriding plate toward the trench – this was also the case on the western edge of Pangaea as it broke apart. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Mechanisms for Continental Rifting 30 130 million years ago: the head of the Tristen Hotspot arrived and started the rifting of Southern Pangaea. Northern Pangaea had already started rifting apart from the Iceland Hotspot. Page 12 of 20 31 Mechanisms for Continental Rifting At a subduction zone, the slabsuction forces pull the continent westward toward the trench (trench retreat), causing tensional forces that contribute to the breakup of the continent on the eastern shore. Formation of San Andreas Fault Franciscan Formation 33 32 Trench retreat moved North America westward toward the spreading center (mid-ocean ridge). The spreading center subducted into the trench, both were mutually destroyed, and replaced by a newly generated San Andreas transform fault which continues to grow in length. Granite from the southern Sierras broke off from Southern California and has slid north along the coast to Pt. Reyes National Seashore. ✓REVIEW QUESTIONS 34 15. Describe the 4 layers of the ocean crust. The local Pt. Reyes Franciscan Formation is an accretionary wedge that contains a complex variety of sedimentary, metamorphic, and igneous rocks. ✓REVIEW QUESTIONS Granite 16. Describe how the sheeted dike complex forms and the layer below it. 17. Name a place to see a continental rift. Granite 18. Why role are mantle plumes thought to play in the rifting of a continent? 35 20. Explain why oceanic lithosphere subducts even though the oceanic crust is less dense than the underlying asthenosphere. 21. Why does oceanic lithosphere thicken away from a MOR? 22. What remains of the Farallon plate? BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 13 of 20 Chapter 14 – Convergent Boundaries 1 2 Mountain Building Orogenesis – mountain building processes. Mountain building has occurred during many different periods of geologic time: Recent Period (0-100 Million Years): American Cordillera, Andes Mts., Himalaya Mts., Japan, Philippines, and Sumatra. Older Than 100 Million Years: Appalachians, Caledonian Belt, Urals and Tasman Belt. Elk Mountains, Colorado Rockies Mountain Building 3 Major Tectonic Plates & Mountains 4 Know major mountain belts and ages for the final. Q: What do you notice about the location of the older mts. & plate boundaries? Convergence and Subducting Plates 5 Subducting zones are located along convergent boundaries and are sites of plate destruction – where oceanic lithosphere bends and plunges back into the mantle. Features include:(1) deep-ocean trench (a deep linear feature where the oceanic slab starts to descend into the mantle); (2) forearc region (nearest the trench, often form an accretionary wedge); (3) volcanic arc (volcanic activity on the overlying plate); and (4) backarc region (often dominated by tensional forces that thin and stretch the crust). BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Convergence and Subducting Plates 6 Water-rich fluids reduce the melting point of hot mantle rock and melting occurs – producing a basaltic magma. Page 14 of 20 7 Aleutian Islands – Volcanic Island Arc 8 Backarc Spreading Suction Force – think of being on a lifeboat as the Titanic sank – you would be pulled toward it! Active backarc basins: Mariana & Tonga Islands. Inactive backarc basins: South China Sea & Sea of Japan. ✓REVIEW QUESTIONS 9 1. Which type of plate boundary is most directly associated with mountain building? 2. List the four main structures of a subduction zone. 3. How does a backarc basin form? 4. Describe the process that generates most basaltic magma at subduction zones? Andean-Type Margins 11 As the oceanic lithosphere descends into the mantle, water is driven from the plate and partially melts the mantle rocks, which are made of periodotite. This produces a primary magma of basaltic composition. The newly formed magma is less dense and rises and ponds at the base of the crust. At this point, heavier iron-rich minerals settle out, leaving the remaining melt richer in silica and lighter; this is called magmatic differentiation. New low-density melts continue to rise with a composition of andesite (intermediate) and with assimilation form felsic (granitic) batholiths. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Subduction and Mountain Building 10 There are two types of subduction: 1. Oceanic-Oceanic: When two oceanic lithospheres converge, the older, colder slab subducts beneath the younger one, creating island arc volcanics. Examples include: Aleutian Islands, Japan, Philippines; Tonga, and Mariana. 2. Oceanic-Continental: Also known as “AndeanType Margin” – oceanic plate subducts beneath the continental plate; this builds a continental volcanic arc and well developed accretionary wedge. Active examples: Andes & Cascade Range; inactive example: Sierra Nevada & Coast Range (accretionary wedge). Andean-Type Mountain Building 12 A) Passive margin: East Coast of US. B) Active subduction: Andes. C) Inactive: Sierra Nevada Coast Range Great Valley Sierra Nevada Plutons (made of granite) are large magma bodies that collect in the mid-crust, cool, and then are exposed by erosion – like the granite in the Sierra Nevada. Many plutons form a batholith. Page 15 of 20 Sierra Nevada Mountains 13 Active Sierra Nevada Before the San Andreas Fault (SAF), there was active subduction across California that created the Sierra Nevada Mountains. This stopped about 30 million years ago when the SAF formed. The Franciscan Formation of the Coast Range is the accretionary wedge of material scraped off the subducting plate. Active Sierra Nevada Coast Ranges Inactive Sierra Nevada Continental Collisions ✓REVIEW QUESTIONS 14 5. How are magmas with intermediate to felsic composition thought the be produced from mantle-derived magmas at Andean-type plate margins? 6. What is a batholith? In what modern tectonic setting are batholiths being generated? 7. In what ways are the Sierra Nevada and the Andes similar? How are they different? 8. What is an accretionary wedge? How does it form? 15 Continental Collisions 16 Himalayas 18 When a continent collides with another continent, the subduction of the oceanic lithosphere stops and forms a suture zone – where the continents become welded together. Fold-and-thrust belts are tightly folded belts of sedimentary rock resulting in considerable crustal shortening and thickening. Himalayas – An active collision that began about 45 million years ago. The Indian block is much older and has remained intact, while deforming the younger continental block of southeast Asia. Himalayas 17 BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 16 of 20 Continental Collisions 19 Appalachian Mountains 20 Terranes and Mountain Building 22 Appalachians – These formed from the closing of the ancestral Atlantic Ocean 250-300 million years ago, when Africa collided with North America. The collision created the tightly folded Valley and Ridge Province. Shortly after the formation of the Appalachian Mountains, the newly formed supercontinent of Pangaea began to break apart. The new rifting occurred east of the old suture between Africa and North America, leaving a remnant of Africa welded to the east coast of North America. The Alps (active) and Urals (inactive) are also the result of continental collisions. Appalachian Mountains 21 Terranes are accreted crustal blocks along the continental margins by Andean-type subduction zones. These terranes have many sources, including: microcontinents (like Madagascar off Africa), island arcs (like Japan and the Philippines), and submerged crustal fragments (like submerged oceanic plateaus – created by massive outpourings of basaltic lavas associated with hot spot activity). All these units are either too thick or too light to subduct into the mantle at subduction zones. When they collide with the continental margin, they accrete and increase the size of the continent. Accretion of Foreign Terranes 23 Accretion of Foreign Terranes 24 Mountains may also occur when smaller continental fragments merge and accrete to the continental margins. Most of western North America has been added by this method. Paleomagnetic data indicate these terranes originated thousands of kilometers to the south of their present locations. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 17 of 20 Terrane Formation 25 ✓REVIEW QUESTIONS 26 9. Give examples of passive and active continental margins. 10. Explain how volcanic island arcs like Japan can be just one step in the development of a major mountain belt. 11. What tectonic structure is the Coast Ranges of California? 12. Suture zones are often described as being ‘welded together’ – explain why this may be misleading. ✓REVIEW QUESTIONS 27 13. Why was India not deformed as much as Asia in the continental collision? 14. Where might magma be generated in a newly formed continental collision? 28 18. Briefly explain the three types of compressional mountain building. 15. How could a sliver of oceanic crust be found in the interior of a continent? 16. How can the Appalachian Mountains be considered a collision-type range when the nearest continent is 3000 miles away? Fault Block Mountains ✓REVIEW QUESTIONS 17. How does the plate tectonics theory help explain the existence of fossil marine life in rocks atop compressional mountains? 19. Define the term terrane. 20. In addition to microcontinents – what other structures can terranes consist of? 29 Basin and Range Province 30 Fault Block Mountains – are created by not by compressional forces, but by tensional force, resulting in high-angle normal faults. When the crust is pulled apart, it thins and stretches and produces volcanism (decompressional melting). The high heat flow causes the crust to expand and rise and contributes to the extension in the region. Important examples today are the Sierra Nevada of California, the Grand Tetons of Wyoming, and the Basin and Range Province of Western USA. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 18 of 20 Grand Tetons, Wyoming 31 Vertical Movements of the Crust 32 Isostasy – concept of a floating crust in gravitational balance. Think of a floating wooden block or iceberg. Isostatic Adjustment 33 A) Young mountains with thick roots. B) Erosion lowers mountains and crust rises. C) Erosion and uplift continue until “normal” crustal thickness is reached. Postglacial Rebound – Hudson Bay 35 Q: What happens if you add a block on top? A: Postglacial Rebound – Isostatic Adjustment 34 After the continental glaciers melted, the crust was unloaded (like a ship) and the crust has been rebounding ever since (see Fig. 18.26). Vertical Movements of the Crust 36 Mantle Convection – the up-and-down convective flow in the mantle also affects the elevation of Earth’s major landforms. The buoyancy of hot rising material accounts for broad upwarping in the overlying lithosphere, while downward flow causes downwarping. Southern Africa is one region where large uplift has produce expansive plateaus of nearly 5000 feet in elevation – convective upflow – even though this region has not experienced a plate collision for nearly 400 million years. Large downwarped structures, like the Michigan Basin are thought to be linked to convective down flow. BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 19 of 20 Vertical Movements of the Crust 37 ✓REVIEW QUESTIONS 38 21. Describe the differences between the evolution of the Appalachians Mountains and the NA Cordillera. 22. Compare fault-block mountains to the other types. 23. Give evidence that supports crustal uplift. 24. Isostatic adjustment - see Slide #32. 25. What elevates portions of southern Africa? BLOCK EXAM #5 - CHAPTERS 12, 13 & 14 Page 20 of 20