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BRIEF PLATE TECTONICS
Plate Tectonics is a recent (1960s) theory explaining Earth's topography. Oceans, continents, volcanos, and earthquakes are explained by the movement (tectonics) of pieces (plates) of Earth's crust. See This Dynamic Earth: The Story of Plate Tectonics (from which the images shown on this page came with permission) or Volcano World's Introduction to Plate Tectonics for a thorough explanation of the theory of Plate Tectonics and its history. Below, are just the fundamental concepts of plate tectonics.
Convection Cells and Plates
Explosive and Blooping Volcanoes
Types of Plate Boundaries: the crust's plates can move 3 different ways with respect to each other, resulting in 5 different plate boundaries.
1) Divergent Plate Boundaries: plates move apart from each other, creating new ocean floor. This occurs where a convection cell comes up, breaks through the crust (creating a rift -- deep crack in crust), and deposits dark, heavy lava on the surface by "blooping" volcanoes (this becomes the ridge, and the new oceanic crust)
2) Ocean-Continent Convergent Plate Boundaries: one plate with oceanic crust at the front collides with another plate with continental crust at the front. The denser oceanic crust subducts beneath the lighter continental crust, creating a trench and dragging "scum" down that melts and explodes back up in explosive volcanoes.
3) Ocean-Ocean Convergent Plate Boundaries: one plate with oceanic crust at the front collides with another plate with oceanic crust at the front. The denser of the two crusts subducts beneath the other, creating a trench and dragging "scum" down that melts and explodes back up in explosive volcanoes.
4) Continent-Continent Convergent Plate Boundaries: one plate with continental crust at the front collides with another plate with continental crust at the front. Rather than subducting, both crusts buckle, folding mountains upward.
5) Transform Plate Boundaries: two plates slide past each other.
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