Transcript Slide 1
VOCABULARY CH. 10 Continental Drift Mid-Ocean Ridge Sea Floor Spreading Paleomagnetism Plate Tectonics Divergent Boundary Convergent Boundary Transform Boundary Rifting Rift Terrane Pangaea Panthalassa Asthenosphere Lithosphere Subduction Zone Craton Shields Trench OBJECTIVES: Summarize Wegeners hypothesis of continental drift Describe the process of sea-floor spreading. Identify how paleomagnetism provides support for the idea of sea-floor spreading. Explain how sea-floor spreading provides a mechanism for continental drift. In 1912, a German meteorologist by the name of ALFRED WEGENER, proposed the idea of continenetal drift. Wegener hypothesized that the continents once formed part of a single landmass called a supercontinent. Wegener called this supercontinent Pangaea. Fossil evidence was found to help support Wegener’s theory. The plant glossopteris, has been found on the following continents. 1. 2. 3. 4. 5. 6. South America Africa Madagascar India Antarctica Australia Evidence from Rock Formations The rocks on the east coast of S. America and the west coast of Africa match in age and in the type of rocks layers Mountain chains that end on one continent continue on other continents across oceans. For example the Appalachian Mountains continue into Greenland and match mountains in Scotland and Europe in composition and structure. Climatic Evidence is also found. Glacial evidence is found in South America and Africa. Regions that today are far to warm for glaciers. Fossil evidence shows that tropical and subtropical swamps and forests existed in areas like Michigan that are too cold for such features today. Wegener theorized that if the continents were connected and in different positions, the climatic changes could be explained. Wegener believed that Pangaea began its breakup during the Mesozoic era. Wegener also proposed that the collision of continents and the crumbling of the crust was responsible for the building of mountain ranges. Despite the evidence that Wegener collected to support his theory, other scientist rejected his ideas because Wegener could not come up with a feasible means for how the continents moved. Wegener suggested that the continents plowed through the rocks on the oceans floor. This was easily disproved by geologic evidence. Wegener died in 1930 before he could discover the mechanism that drove the continents. MID OCEAN RIDGES The evidence Wegener needed was discovered almost 20 years after his death. Geologist found that the sediments nearest the ridge were thinner than those farther away. The rocks also got older as you went farther from the ridge. The ocean floor is also much younger than the continental rock. There are no rocks on the ocean floor older than 175 million years old, while rocks on land are as old as 3.8 billion In the late 1950’s a geologist named Henry Hess proposed a new hypothesis. He proposed that the valley at the center of the ridge was a crack, or rift, in the earth’s crust. Hess suggested that magma from deep inside the earth would rise through these cracks as the ocean floor moved away. The magma would cool and form new ocean floor. This process became known as seafloor spreading. Hess suggested that if the ocean floor is moving, the continents might be moving too. Hess thought this might be the mechanism that Wegener failed to find. When rock on either side of the ridge hardens, the magnetic orientation of the minerals becomes permanent. This residual magnetism is called paleomagnetism. This helped to support the idea that the ocean floor moved. The similar age of the rocks on each side of the rift gave support to Wegeners theory.