Chapter 14: Plate Tectonics
Continental Drift
- Continental Drift was proposed by Alfred Wegener in 1912.
- Wegener suggested that all of the continents were once one large "supercontinent" that he called Pangaea.
- It was never accepted because he couldn't explain how or why the continents moved.
- The observations he used were:
- Fossil Clues - Mesosaurus
- Plants - Glossopteris
- Climate - old glacial deposits are found in now tropical areas
- Rocks - the Appalachian Mountains in the Eastern United States are similar to mountains found in Greenland and Western Europe.
Seafloor Spreading
- Seafloor spreading was proposed by Harry Hess in the 1960's. This theory answered the how or why that Wegener could not.
- His idea came from the maps of the seafloor made in the 1940's and 1950's using sound waves. The mid-ocean ridges were discovered in the making of these maps.
- The evidence for seafloor spreading was magnetic anomalies. There were stripes that paralleled the mid-ocean ridges that recorded reversals in the Earth's magnetic north. This happened because the iron minerals in the magma would line up with the direction of the magnetic pole. There have been times in the past when your compass would not have pointed north, as we now know it to do, but south.
Theory of Plate Tectonics
- The theory of Plate Tectonics came about in the 1960's with the combination of Alfred Wegener's continental drift and Harry Hess' seafloor spreading.
- Earth Structure
- Lithosphere - composed of the crust and upper mantle. (Think of the shell of an egg.)
- Asthenosphere - plasticlike layer below the lithosphere. (Think of the white part of an egg.)
- Plate Boundaries:
- Divergent Boundaries - plates that are moving apart. They cause landforms such as rifts (ex. East Africa), mid-ocean ridges (ex. Mid-Atlantic Ridge), and ocean basins (ex. Atlantic Ocean).
- Convergent Boundaries - plates that are coming together. These cause several different landforms depending on the types of plates that are colliding.
- Ocean-Ocean Collision - trenches (ex. Marianas Trench)
- Continent-Ocean Collision - volcanoes (ex. Cascade Range of Western United States)
- Continent-Continent Collision - mountains (ex. Himalayas)
- Transform Boundary - plates that are sliding past each other. (ex. California - San Andreas)
- The movement in plate tectonics is due to convection in the mantle.
- To explain convection, think of a lava lamp. It is heated up by the light underneath and becomes less dense. This causes the "lava" to rise to the top of the lamp. Once at the top, the "lava" cools, becoming more dense. It then sinks back to the bottom of the lamp where the process starts over again. In short, convection is the heating, rising, cooling and sinking of a material.
- The next time you wait for your water to boil while you make macaroni and cheese, you can witness convection!!
(Source: Glencoe Science, Level Green, 2003.)
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