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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.

Earth's Structure:

• Crust: the relatively thin, solid, outer layer of Earth.
  • Continental Crust: composed of relatively lightweight (and light in color) rocks. This is the "scum" of the Earth, the lightest weight stuff that has floated to the surface of the Earth and upon which we walk.
  • Oceanic Crust: composed of relatively heavy (and dark in color) rocks. This is the stuff forced to the surface by convection cells (see image). Because it is so dense and heavy, it is thinner and sinks deeper into the mantle, thus produces ocean basins.
• Mantle: the thick, semi-solid, middle layer of Earth that slowly flows circular convection cells (see image). The uppermost part of the mantle is solid and, together with the crust, forms what is called the lithosphere. The layer of the mantle below the lithosphere, that is malleable like butter, is called the aesthenosphere and this is the layer that the lithosphere depresses and moves across.
• Core: the core consists of two layers, an outer liquid layer and an inner solid layer.

Convection Cells and Plates

• Convection Cells are the slow moving currents of mantle material from deep in the mantle, up to the surface, then along the surface, then eventually sinking (subducting) back deep in the mantle. They are powered by Earth's interior heat and break the crust into pieces (called plates).
• Plates are pieces of the Earth's crust (see image on right), broken apart and moving by the action of convection cells. In the image showing convection cells above, the piece of lithosphere with the arrows pointing to the right is one plate, and the piece of lithosphere with the arrows pointing to the left are another plate and both are subducting beneath other plates.

Explosive and Blooping Volcanoes

• Explosive (Strato-) Volcano: Occurs when "scum" is dragged into the mantle by subduction, melts, and comes up explosively. For example, Mt. Saint Helens.
• Blooping (Shield) Volcano: occurs when heavy mantle material if forced to the surface. For example, Hawaii.

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)

• Result: Rifts and Ridges, new crust, and oceans
• Example: Mid-Atlantic Rift and Ridge, and the new crust under the Atlantic Ocean

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.

• Result: trench and volcanic mountain chains
• Example: Philippine Trench and Sierra Nevada Mountain Range

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.

• Result: trench and volcanic island arcs
• Example: Aleutian Trench and Aleutian Islands

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.

• Result: upfolded mountains
• Example: Himalyas and Appalacian Mts.

5) Transform Plate Boundaries: two plates slide past each other.

• Result: strike-slip faults
• Example: San Andreas Fault