Chapter 3 - Chapter 3 -First, melted asthenosphere flows...

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Chapter 3 -First, melted asthenosphere flows upward as magma and cools to form new ocean floor. Second, the new lithosphere slowly moves laterally away from the zones of oceanic crust formation on top of the underlying asthenosphere (seafloor spreading). Third, when the leading edge of a moving slab of oceanic lithosphere collides with another slab, the older, colder, denser slab turns downward and is pulled by gravity back into the asthenosphere (subduction). Last, the slab pulled into the mantle is reabsorbed. The time needed for this cycles is long, usually in excess of 250 million years. -Tectonic cycle comes from the Greek word “tekton” which means “to build.” -Francis Bacon first noticed how South America and Africa may have once fit together. -Alfred Wegener used similar rocks and fossils as proof of the continental drift idea. He was the most outspoken. -When lava cools below the Curie point and are magnetized, the minerals line up and point toward the pole. -Going out from the midocean volcanic ridge, there are layers of magnetized rock which is symmetrical on both sides. -Other evidence of plate tectonics: earthquake epicenters outline the plates, the process of hotspots moving (older volcanoes to younger volcanoes as the hot spot moves), oceanic mountains (areas of spreading where magma rises up to create volcanoes), oceanic trenches (the tops of subducting plates turning downward to reenter the mantle), systematic increases in seafloor depth (the progressive deepening of the seafloor with increasing age also testifies to the existence of seafloor spreading), the fit of the continents. -When oceanic lithosphere collides, the older and colder plate goes below the younger and warmer plate. -Hot spots send up hot, buoyant rock that form shield volcanoes on the seafloor. -Tension earthquakes are relatively small and don’t pose a huge threat.
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Chapter 3 - Chapter 3 -First, melted asthenosphere flows...

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