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8-1 The Scientific Method in Action: The Ideas of Alfred Wegener Plate Tectonics In the early 20th century Alfred Wegener took these earlier ideas and made a case that the continents indeed move. Early evidence for continental drift: He gathered evidence from a variety of areas: The idea that the continents move is not new. 1. He showed how the continents could be put together to form one massive continent: Pangaea. In 1620, shortly after explorers first created maps of the new world, Francis Bacon noticed how nicely South America and Africa fit together. In 1858 Antonio Snider showed how these and other continents fit together like a jigsaw puzzle. Perhaps the continents were once together and drifted apart? 2. Common Fossils He showed that several fossils from the paleozoic age found on separate continents are quite similar. e.g. South America, Africa, India, and Australia Indeed, in Wegener’s reconstruction, these continents fit close together. 3. Common Rock Formations If India hasn’t moved what would a glaciated India suggest about the Earth’s climate at this time? In addition to fossils, similar rock formations are found in India, Africa, South America, Australia, and Antarctica. Also North America and Europe. Again, these continents are close together in Wegener’s reconstruction. 4. Paleoclimates Glacial features appear on the southern continents: South America, Africa, India, and Australia. India?!? Yet no such evidence on the northern continents. Indeed, evidence suggests these continents were quite warm at this time: Coal is created in tropical swamps. Coal in Norway? Despite this evidence, many geologists remained unconvinced. Why? 8-2 Why did Geologists Resist Wegener’s Ideas? 2. Geography: Much of Wegener’s evidence came from the southern hemisphere. 1. No one could explain what forced the continents to move: Wegener’s original thought was that the continents somehow plow through the oceanic crust. Did not seem plausible given what was known about the strength of rocks. Most northern geologists had not seen this evidence firsthand. Not surprisingly, southern hemisphere geologists were more impressed with the idea of continental drift. Thus, the idea of continental drift (and plate tectonics) languished for half a century. The Revival of Continental Drift Evidence from Paleomagnetism During the 1940s and ’50s evidence continued to trickle in. The Earth generates a magnetic field. The poles of the magnetic field are near the geographic poles of rotation. In particular, evidence along two lines proved important in the revival of continental drift: 1. Investigations of the sea floor 2. Geophysical observations, particularly of rock magnetism (This is why compasses work—they point towards the magnetic pole which, for most purposes is close enough to the rotation poles). While the generation of the magnetic field is not fully understood, it is thought to be related to the Earth’s rotation. Thus, it is likely that the two have always been close. Interestingly, however, the magnetic poles appear to reverse periodically. 8-3 Certain minerals (e.g. magnetite) can act like little bar magnets. ==> Rocks containing these minerals are magnetic as well. These rocks contain records of the past magnetic field of the Earth. Thus, when a rock is formed from molten magma it will record the strength and direction of the Earth’s magnetic field at the time of its formation. Once solidified this magnetic signature is frozen into the rock. This signature can be used to determine where the magnetic pole was when the rock was formed. How? The magnetic crystals will point towards the pole. If one heats up a magnet to a certain temperature (the Curie temperature) it loses its magnetism. ==> direction of the pole. What about distance? If one then cools a magnet (or magnetic rock) below this temperature it will become magnetized along whatever magnetic field is present. The distance from the pole is determined by the dip of the crystals. How will they dip near the equator? Polar Wander Magnetic rocks within continents indicate that the pole has changed position in time. This suggests one of two possibilities: In the northern hemisphere? Either the pole has moved in time or, In the southern hemisphere? the continent has moved in time. However, rocks from different continents suggest different positions for the pole at the same time: 8-4 Study of the Sea Floor What about the problem of how continents move? Sea-floor spreading provided a plausible mechanism for the motion of the continents. Nice theory. Is there any evidence for this? Studies of the sea floor led to a modification of Wegener’s original hypothesis in this regard. The discovery of the mid-oceanic ridge led Harry Hess in 1962 to suggest that it was the sea-floor, not the continents, which moved. This proposal became known as sea-floor spreading because he proposed that the sea floor spread away from the mid-oceanic ridge. He proposed mantle convection as the driving force. Wegener’s continental drift and sea-floor spreading have been combined into the theory of Plate Tectonics. Plate Tectonics The basic idea behind the theory: Theory of plate tectonics has revolutionized geology. Prior to plate tectonics people tended to look for local explanations for geologic features. The theory states that rigid lithospheric plates move with respect to each other on the surface of the planet. How might plates move with respect to each other? Plate tectonics has provided a global, unifying, theme to geology. 8-5 Divergent Plate Boundaries Convergent Plate Boundaries Sea floor spreading centers. As we have seen, new lithosphere is continually being created at divergent plate boundaries. Would you expect volcanism at such boundaries? The Earth is not getting bigger—thus if surface area is being added at some places what must be occurring elsewhere? Tensional geologic features. (Normal faults). Lithosphere is consumed at subduction zones. Numerous earthquakes but typically not large and generally quite shallow. Oceanic crust is denser than continental crust, thus where oceanic crust meets continental crust the oceanic plate dives under the continent. Where two oceanic plates collide, one usually dives under the other. Associated with substantial volcanism and earthquakes. Continent-continent collision Transform Plate Boundaries Continental crust is low density. Regions where plates are sliding by each other. Thus it cannot be subducted very far. Examples? Thus when two continents collide neither plate dives into the mantle. Instead, the plates smash into each other greatly deforming and folding up the rocks near the plate boundary. This increases the thickness of the material leading to a new mountain range. Examples: Associated geologic activity? 8-6 Measuring Plate Motion The theory of plate tectonics gained acceptance in the 1960s even before we could actually measure the motion of the plates. However, since that time it has become possible to directly measure the actual plate motions. Hot Spots Hot spots appear to be regions where thermal plumes from deep within the Earth’s interior impinge on the base of a lithospheric plate. Sometimes these plumes can “burn through” the overriding plate creating volcanoes. With the use of GPS one can determine ones location on the surface of the planet to within feet. If they reach the surface they can become islands. With GPS the motion of the continents has been directly measured. The hawaiian islands are the classic example. Rather direct proof that plate motion does actually take place! If we look at the hawaiian islands we see that their ages increase to the northwest. Only the big island currently has active volcanism. Given this information, which direction is the Pacific plate currently moving? A new island is currently forming to the southeast of Hawaii. In fact, the hawaiian islands are but part of a larger chain of islands, seamounts and guyots running to the northwest and north to the Aleutian trench. Distance and age difference between the islands can be used to determine the speed of motion. For example: (Midway, the site of a decisive WWII sea battle is part of this chain). These have been dated and found to become older to the northwest. Islands created as the Pacific plate moved over the hot spot which periodically burned through the plate. If we can directly measure the motion of plates, why measure the speed from hot spots? 8-7 What Causes Plate Motions? The mechanism responsible for the motion of the lithospheric plates is currently an unresolved issue. 2. Magma pushing plates apart If plates are moved by magma forcing the plates apart what kind of stresses would the rocks in the plates be put under? Several mechanisms have been proposed: 1. Mantle Convection (Mantle Drag) —Hess’s original proposed mechanism for sea floor spreading. If this is the case what kind of features would we expect to see? 2. Magma intruding into and pushing plates apart (other books call this ridge push) 3. Plate Sliding (your book calls this ridge push) 4. Slab-Pull 3. Plate-Sliding 4. Slab-Pull Because it is warm and thus less dense the oceanic crust is raised at the midoceanic ridge. At subduction zones oceanic plate dives down into the mantle. Away from the ridge it cools off and sinks. Plate is cold relative to surrounding mantle --> more dense. Further, cooling away from the ridge results in a thickening of the lithospheric plate. Gravity thus pulls the descending slab downwards. Pulls the rest of the plate with it. For both these reasons the plate at the ridge is significantly higher than nearer the edge of the continents. Thought to be more powerful than plate-sliding. May explain why plate motions into subduction zones tends to be faster than at divergent boundaries. Plate can thus slide downhill much like a child sliding down a slide.