11_drift_paleomag_09_post - 11: Continental Drift,...

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Unformatted text preview: 11: Continental Drift, Paleomagnetism Why is this important? Continental drift is the forerunner to plate tectonics, which explains the first-order topography of Earth’s surface; the distribution of mountain belts, fossils, and climate-sensitive sedimentary rocks; how the arrangement of continents and oceans has changed through time; and the locations and ultimate cause of earthquakes and volcanoes. It also provides a framework for exploring for natural resources. Drift, Paleomag: 1 Drift, Paleomag: 2 Drift, Timeline Timeline 1858: Antonio Snider-Pellegrini proposed that all continents were previously assembled in one giant continent that subsequently broke apart due to Noah’s flood. 1620: Sir Francis Bacon--the similarity in the shape of the coastlines of South America and Africa is “no mere accidental occurrence.” Wikipedia Wikipedia Drift, Paleomag: 3 Drift, Drift, Paleomag: 4 Drift, Continental drift--evidence Timeline 1912 (lecture)-1915 (book: Die Entstehung der Kontinente und Ozeane): Alfred Wegener summarized previous work along with his own research to propose the theory of continental displacement (wrongly translated as continental drift) and the former existence of the supercontinent of Pangea • Jigsaw fit of continents; matching coastlines Wikipedia Fig. 3.2 Fig. 3.2 Drift, Paleomag: 5 Drift, Paleomag: 6 Drift, Continental drift--evidence • Continuity of similar rock units (now separated by oceans) when continents are restored Continental drift--evidence • Similarity of non-marine fossils now separated by oceans Fig. 3.5 Fig. 3.6 Drift, Paleomag: 7 Drift, Drift, Paleomag: 8 Drift, Continental drift--evidence Continental drift--evidence • Rocks now exposed near equator: evidence of past glaciations • Rocks now exposed near equator: evidence of past glaciations Fig. 3.3 Fig. 3.3 Drift, Paleomag: 9 Drift, Paleomag: 10 Drift, Continental drift--evidence • Past tropical conditions in now polar regions Continental drift--cause • Continents plowed through oceanic crust; caused by Earth’s rotation 1924: Harold Jeffreys determines that Wegener’s proposed driving mechanism of continental drift was too weak to move continents through strong oceanic crust. Thus, most geologists rejected Wegener’s theory www.york.ac.uk Fig. 3.4 Drift, Paleomag: 11 Drift, Drift, Paleomag: 12 Drift, Review Questions Review Questions 11-1. This scientist proposed the theory of continental drift: A. Charles Darwin B. James Hutton C. Charles Lyell D. Alfred Wegener 11-2. Wegener suggested all of the continents were once ____________. A. aligned north to south along the prime meridian during the late Cenozoic B. aligned east to west along the equator during the late Mesozoic through the Cenozoic C. combined to form a supercontinent (he termed Rodinia) in the Proterozoic D. combined to form a supercontinent (he termed Pangaea) in the late Paleozoic through the early Mesozoic 11-3. Late Paleozoic (Pangea-aged) glacial deposits are not found in which of the following places? A. India B. southern Africa C. North America D. South America 11-4. Abundant swamps led to the formation of coal during the Late Paleozoic in which of the following places? A. India B. southern Africa C. North America D. Antarctica 11-5. Distinctive rock sequences on South America terminate at the Atlantic Ocean but reappear on the continent of ____________. A. Africa B. Europe C. North America D. Australia Drift, Paleomag: 13 11-6. Late Paleozoic (Pangea-aged) glacial deposits ____________. A. are more difficult to explain than in the modern continental configuration B. are much more readily explained than in the modern continental configuration C. makes very little sense in either the Pangaea configuration or the modern configuration 11-7. Which of the following is evidence in support of continental drift? A. continuity of similar rock units on different continents now separated by an ocean B. similar non-marine fossils found on widely separated continents C. jigsaw fit of South America and Africa D. presence of glacial deposits in rocks currently located at the equator E. all of the above 11-8. A. True / B. False: According to Wegener, oceanic crust plowed through the continental crust. 11-9. Wegener’s idea of continental drift was rejected by American geologists because ____________. A. his English was too poor to be understood by them B. he could not conceive of a valid mechanism that would cause continents to shift positions C. he had very little evidence supporting the existence of a supercontinent Drift, Paleomag: 14 Drift, Importance of isostasy Isostasy: wood block analogy Those who opposed continental drift required submerged land bridges to explain why now-widelyseparated continents contain similar non-marine fossil species but different modern species. However, can continental crust sink into the mantle? • All blocks have the same density. Most mass • Tallest block floats highest. www.geo.arizona.edu Drift, Paleomag: 15 Drift, Drift, Paleomag: 16 Drift, Isostasy: wood block analogy • Principle of isostasy: at some depth within a fluid, the pressure due to weight of overlying material is constant • All blocks have the same mass. Highest ! Lowest ! Isostasy: wood block analogy Fig. 4.1 Highest ! • Least-dense block floats highest. Drift, Paleomag: 17 Isostasy • Principle of isostasy: at some depth within a fluid, the pressure due to weight of overlying material is constant Drift, Paleomag: 18 Drift, Isostasy & topography • Continental crust (higher elevation) -- lower !, thicker • Oceanic crust (lower elevation) -- higher !, thinner Fig. 4.1 Drift, Paleomag: 19 Drift, Drift, Paleomag: 20 Drift, Isostasy Importance of isostasy Submerged land bridges to explain the fossil record require continental crust to sink into the mantle… • Mountains: very thick, low-density crust …but isostasy indicates that this is physically impossible. Fig. 11.29 www.geo.arizona.edu Drift, Paleomag: 21 Drift, Paleomag: 22 Drift, Magnetic field Magnetic field: iClicker • Bar-magnet analogy • Field lines give orientation of magnetic field Q1. The magnetic field points ___ the center of the Earth at the North Pole and ___ the center of the Earth at the south pole. A. away, away B. away, toward C. toward, away D. toward, toward Fig. 2.2 Drift, Paleomag: 23 Drift, Fig. A.2 Fig. A.2 Drift, Paleomag: 24 Drift, Magnetic field: iClicker • Magnetic inclination (I): angle between magnetic field line and Earth’s surface Q2. The inclination of the magnetic field is ___ at the magnetic pole and ___ at the magnetic equator. A. 0°, 0° B. 0°, 90° C. 90°, 0° D. 90°, 90° Paleomagnetism Lava flows, upon cooling, attain a magnetic field that parallels the Earth’s magnetic field. Below critical temperature Fig. A.5 Fig. A.4 Drift, Paleomag: 25 Drift, Paleomag: 26 Drift, Paleomagnetism Paleomagnetism Above critical temperature Fig. A.6 Magnetic minerals in sediments align with magnetic field Fig. A.5 Drift, Paleomag: 27 Drift, Drift, Paleomag: 28 Drift, Paleomagnetism Paleomagnetism: iClicker Magnetic inclination gives paleolatitude and location of magnetic pole. If magnetic poles changed location through time, PWP’s should be same for all continents. They are not. Polar wander path: curve showing locations of magnetic poles for rocks of varying ages from same locality Fig. A.9 Drift, Paleomag: 29 Paleomagnetism Two possibilities: 1) Continent fixed, pole moves 2) pole fixed, continent moves Q3. Although the polarwander paths for North America, Europe, and Africa are different, the paths converge at the north pole. A. True B. False Fig. A.10 Drift, Paleomag: 30 Drift, Paleomagnetism Restoration of continents to Pangea geometry: PWP’s become more similar: proof of continental drift Fig. A.11 Fig. A.10 Drift, Paleomag: 31 Drift, Drift, Paleomag: 32 Drift, Paleomagnetism: iClicker Magnetic field • No bar magnet at core • Magnetic minerals lose magnetism at high temps. • Electromagnetism: “flowing” current in liquid outer core X Q4. Paleomagnetic studies on 225 million year old rocks in Nova Scotia, Canada, and Morocco indicate that the magnetic inclination, I, is 25° for both sets of rocks. Using the graph below, determine the magnetic latitude the rocks occupied when they were magnetized. A. +13° B. -13° C. +42° D. -42° Fig. A.2 Drift, Paleomag: 33 Paleomagnetism: iClicker Q5. If the current latitude of Nova Scotia is 45°, the North American continent has drifted north by ___. A. 13° B. 25° C. 32° D. 45° Q6. If the current latitude of Morocco is 35° the African continent has drifted north by ___. A. 13° B. 22° C. 25° D. 35° Drift, Paleomag: 34 Drift, Review Questions 11-10. A. True / B. False: If two blocks of wood have the same density but different heights, the taller block will float higher in water than the shorter block. 11-11. A. True / B. False: If two blocks of wood have the same mass but different densities, the higher-density block will float higher in water than the lowerdensity block. 11-12. A. True / B. False: Oceanic crust stands at a lower elevation than continental crust because oceanic crust is thicker and more dense. Q7. At 225 Ma, North America (Nova Scotia) and Africa (Morocco) occupied the same latitude and were part of the supercontinent of Pangea. Since then, the continents have separated and drifted to their current positions. A. True B. False Drift, Paleomag: 35 Drift, 11-13. A. True / B. False: The magnetic field is parallel to the Earth’s surface at the magnetic equator and perpendicular to the Earth’s surface at the magnetic poles. 11-14. A. True / B. False: Igneous rocks lose their paleomagnetism if they are cooled below a certain critical temperature. Drift, Paleomag: 36 Drift, Review Questions 11-15. The apparent tendency of a magnetic pole to vary in position over time is termed ____________. A. dipole B. magnetic declination C. magnetic inclination D. polar wander 11-16. A. True / B. False: Because polar wander paths are different for different continents but become at least partially more similar when continents are restored to former positions, it is more likely that the continents have moved and the magnetic pole has remained fixed. 11-17. The apparent polar-wander paths for continents that were not connected over some span of geologic history will likely ____________ concerning the positions of the ancient magnetic pole. A. agree B. disagree 11-18. A. True / B. False: The Earth's magnetic field is caused by magnetic iron minerals in the inner core. Drift, Paleomag: 37 ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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