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Tectonic plates

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Tectonic Plates

The  theory of plate tectonics proposes that the Earth's crust is broken up into eight major and and about twenty smaller tectonic plates that move over the surface of the Earth.  These tectonic plates are massive, irregularly shaped slabs of solid rock.  The smaller plates are only a few hundred km in diameter, while the major plates are thousands of km in diameter.  Plate thickness is also highly variable, ranging from less than 15 km at the oceanic ridges to over 200 km beneath the Himalayan Mountains.  With the exception of the dominantly oceanic Pacific Plate and some of the smaller plates, these plates consist of both oceanic and continental crust and upper mantle material.  For example, the North American plate consists of the North American continental mass and the oceanic crust extending out form the eastern edge of the North American continent to the spreading ridge of the Mid-Atlantic ocean ridge.  

Lithospheric plates

According to plate tectonic theory, the outer 400 km of the Earth is is broken into two zones, the upper, thinner, rigidly deforming lithosphere 100 km or less in thickness and the lower ductilely deforming asthenosphere.  The tectonic plates are part of the upper lithospheric layer that rigidly deforms.  Each of these lithospheric plate acts as a coherent unit that moves relative to the other tectonic plates.   

The lithospheric plates "float" on the lower ductile hotter asthenosphere.  This hot plastic asthenosphere convects in the same way water in beaker over a Bunsen burner convects.  Hot, partially molten peridotite (a dense rock composed of 60% or more olivine plus pyroxene and plagioclase, spinel, or garnet, depending upon pressure) upwells at the oceanic ridge and continental rift zones.  Cold, dense oceanic slabs sink back down into the mantle in subduction zones.  The forces generated by these gravitational movement are great enough to move continental masses a 2 - 15 cm a year.  Over tens and hundreds of millions of years, large tectonic movements can occur.

There are three types of plate boundaries: 1) those composed of the tectonic plates that are moving apart, 2) those composed of tectonic plates that are slipping sideways, one past the other, and 3) those composed of tectonic  plates that are colliding or being pushed together.  Regions where plates are diverging are commonly called spreading centers, and coincide with large oceanic ridges.  Plate boundaries where crustal plates are slipping past each other horizontally are marked by transform faults.  The San Andreas Fault is a transform fault.  Regions where two plates are converging are called subduction zones or continent-continent collision zones. 



Frequently used abbreviations: NPL  Non-vertebrate Paleontology Laboratory | TNSC Texas Natural Science Center | UTDGS Department of Geological Sciences | BEG  Bureau of Economic Geology | VPL Vertebrate Paleontology Laboratory | JSG  Jackson School of Geosciences | SUPPORT | VOLUNTEER | GLOSSARY

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