The Theory of Plate Tectonics

1. The Theory of Plate Tectonics By Jessica Johnston A6

2. Context • The Theory was developed in the 20th Century. • It is a scientific theory that describes the large scale movement of the Earth’s crust and outer mantle (Lithosphere). • The theory builds on the older concepts of continental drift.

3. Creator • The theory was developed by a man called Alfred Wegener. • He hypothesized that the continents were slowly drifting around the Earth but was unable to demonstrate a mechanism for continental drift. • His evidence was mostly circumstantial. • His hypothesis was not accepted until the 1950s, when several discoveries provided evidence of continental drift.

4. Evidence • The shapes of many continents look like they are separated pieces of a jig-saw puzzle. • Many fossils along the edges of continents look like they fit together which suggests that the two continents were joined at some point. • There is a lot of volcanic activity along the plate boundaries. • There are ridges, such as the Mid-Atlantic Ridge, where plates are separating • There are mountain ranges being formed where plates are pushing against each other (e.g., the Himalayas, which are still growing).

5. Boundaries • Divergent: where two tectonic plates are moving away from each other. They eventually form ocean basins. Divergent boundaries within continents initially produce rifts. Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill. • Convergent: where two (or more) move toward one another and collide. As a result earthquakes and volcanoes are common near convergent boundaries.

6. Volcanoes • Composite: Tall volcanoes that erupt with lots of compressed gas. The lava is viscous. They are formed by layers of rock and lava. • Shield: Low flat volcanoes, with sloping sidea, that ooze out thin lava.