Chapter 31: From Continental Drift to Plate Tectonics: The Evidence

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(Note: This reading quiz contains some material from lecture that is not found in the textbook.) In this and the next chapter, we look first at the evidence that the continents move and the seafloors spread on the surface of the earth. Then we look at the model that geologists developed to explain how the earth works and some of the things it explains. Continental Drift: Alfred Wegener started it all in 1912 when he began to collect and publish evidence that the continents move. It opened a decades-long controversy and intellectual battle. The basic paradox that plagued the idea of continental drift was that there was no conceivable mechanism consistent with the laws of physics capable of moving continents through the solid rock of the . For every piece of evidence that the continents were moving, someone could think of an alternative explanation. Some of the early evidence for movement: existence of fold mountain belts; shape of the continents, particularly and South America; matching of physical features of continents, i.e., mountain chains and rock formations, across oceans; of the same creatures (plant or animal) on separated continents; glaciation patterns; Later evidence after 1950 included, magnetism patterns in igneous rocks on different continents. (Magnetism patterns in (Cenezoic, Paleozoic, Precambrian?) rocks in North America and in Europe look like the magnetic pole of the earth was in two different places at the same time long ago, but could be explained if the continents had moved.) Seafloor Spreading: Evidence began to accumulate that the seafloor in some places was spreading and getting larger. The basic paradox that plagued this idea was that it is impossible for the seafloors to get larger without something else getting smaller on an earth of unchanging size. Initially, little or no connection was made between continental drift and seafloor spreading. Evidence: heat seems to rise from the earth's center and spread sideways at the (midoceanic rift, abyssal plains, continental slope?) ; rocks nearest the rift are (younger, older?) than rocks farther away in a pattern that is parallel to the rift; there are magnetization patterns in the rock that are (parallel, perpendicular?) to the rift and correlate with the reversals of the earth's magnetic field, as if the seafloor were a giant tape recorder; sediments on the seafloor get (shallower, deeper?) as one moves farther from the rift in a pattern that suggests that the oceanic crust at the rift is (older, younger?) than the oceanic crust farther away. In 1967 the paradoxes that plagued continental drift and seafloor spreading were resolved and the two ideas were combined into one unified theory, the Theory of . The deciding evidence was the pattern of earthquakes on or near the earth's surface. These were seen not to be randomly distributed but to occur along the (abyssal plains, continental slope, trenches?) on the ocean floor. Moreover, the depth at which the earthquakes occurred below the surface (increases, decreases?) in a systematic pattern as one moves perpendicularly farther from the trench. Almost at once, people recognized that continents were not plowing through the solid rock of the crust, but were sitting on plates (pieces) of the lithosphere that were sliding on the mushy (lithosphere, asthenosphere, core?) . The seafloor could spread and become larger at the midoceanic (rift, abyssal plains, continental slope?) because crust was being consumed elsewhere on the earth's suface where one plate was sliding underneath another. Such places where crust is consumed are the (rift, trenches, abyssal plains?) on the ocean floor.

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