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Transcript
10-1 Continental Drift 10-2 The Theory of Plate Tectonics 10-3 The Changing Continents In 1912, a German scientist named Alfred Wegener proposed a hypothesis called continental drift. He proposed that the continents once formed part of a single landmass called a supercontinent. According to Wegener, this supercontinent began breaking up into smaller continents during the Mesozoic Era (250 million years ago). It has taken millions of years for these continents to drift to their present locations. Some mountains may be the result of two plates colliding. Fossil evidence has been used to support this hypothesis. Fossils of the same plants and animals have been found in areas that could have been joined together at one time. Geologic evidence also supports Wegener’s hypothesis of continental drift. The ages and types of rocks in the coastal regions of widely separated areas, such as western Africa and eastern South America, match closely. If the continents are assembled into a model supercontinent, the mountains of similar age fit together in continuous chains. Climatic patterns suggest that the continents have not always been located where they are now. Geologists have discovered layers of debris from ancient glaciers in southern Africa and South America. Fossil plants that are tropical such as ferns have also been found in much colder climates like Antarctica. The biggest problem with this theory was that there was no known way for the continents to move. Mid-Ocean Ridges – the evidence that Wegener needed to make his theory of continental drift work was found on the bottom of the ocean. The undersea mountain ridges are mountains through which a steep, narrow valley runs down the middle. Though observations scientists realized that the ocean floor closer to the ridge was younger than the ocean floor farther from the ridge. They also discovered that seafloor rocks were younger than rocks on land. Seafloor spreading - Harry Hess proposed that the valley at the center of the ridge was a crack or rift in the Earth’s crust. At this rift, molten rock, or magma, from deep inside Earth rises to fill the crack. As the ocean floor moves away from the ridge, rising magma cools and solidifies to form new rock that replaces the ocean floor. Seafloor spreading is the mechanism for causing continents to move. The study of paleomagnetism showed the magnetic properties of the rocks. Plate tectonics is the theory that explains why and how continents move and is the study of the formation of features in Earth’s crust. The upper part of the mantle forming the lithosphere are broken up into several blocks, called tectonic plates. The asthenosphere is the layer just below the lithosphere that is “plastic” rock that flows very slowly under heat and pressure. The tectonic plates float on top of the asthenosphere like blocks of wood. The earth’s crust is classified into two types: Oceanic crust-is dense and made of rock rich in iron and magnesium. Continental crust-has a low density and is made of rock that is rich in silica. Tectonic plates can include continental crust, oceanic crust, or both. The continents are carried along on the moving tectonic plates. The Earth is broken into 15 major tectonic plates. Scientists identify the plate boundaries mostly by studying the data from earthquakes. Frequent earthquakes in an area are evidence that there are two or more plates that meet in the area. The locations of volcanoes also indicates a tectonic plate boundary. The Pacific Ring of Fire is a zone of active volcanoes that surround the Pacific Ocean. 3 Types of plate boundaries Divergent Boundaries-two plates that move away from each other Convergent Boundaries-where two plates collide Transform boundaries-two plates slide past each other Scientists do not fully understand the force that drives plate tectonics. They believe it may be due to convection and ridge push. Convection is the movement of substances due to differences in temperature. The hot material rises and as it cools, the cooler material sinks back down to the bottom of the layer. Ridge push is the movement of sinking lithosphere that pushes downward away from the mid-ocean ridge.