Plates are composed either of a fragment of ocean basin, or (more commonly) a fragment of ocean basin with an attached continent. Plates are combinations of two units, continents and ocean basins. Observe that 7 different plates are labeled in the cross section. A look at the plate boundaries in the drawing may make plate boundaries appear confusing, but in fact there are only 3 kinds of plate boundaries. Everything between plate boundaries are part of some plate. Simplistically, the earth consists of plates, and plate boundaries, those zones where the plates contact and interact. It is this turning over of the convection cells the drives the plate movements. The cooled plastic rock then turns sideways and moves parallel to the earth's surface before descending back into the earth at subduction zones to become reheated. When the convection cells reach the base of the lithosphere they release heat to the surface at the divergent plate boundary in the form of volcanos to escape to space. The convection cells bring heat from the earth's interior out to the surface, but slowly. Within the asthenosphere are convection cells, slowly turning over hot, plastic rock. Everything below is asthenosphere, the hot, plastic interior of the earth with convection cells bringing heat to the surface from the earth's hot core. Technically, everything above that line is lithosphere, the rigid, brittle shell of the earth. All similar lines in the cross section mark the bottom of the plates. Note the line under the craton that is the lower boundary of the lithosphere, and is the bottom boundary of the plate. Note the continental craton (stable continent) in the middle of the drawing. The cross section should really be curved to correspond to the earth's curvature, but it has been straightened out here. The drawing above is a cross section of the earth showing the components that lie within plate tectonic theory (click for larger version). Plate Boundaries and Interplate Relationships Plate Tectonic Theory: Plates and Interplate Relationships Home | Synopsis | Glossary | Ig.
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