|
Six major plates and a host of smaller ones have been recoginzed through modern science. The Alpine fold belt extending from Gibraltar to the Middle East is made up of many mini-plates. Plates are either oceans (such as the Cocos and Nazca), or solely continental (for example, Iran), or both (like the North American Plate). The older continental crust of North America has been welded together with the new oceanic crust. Thus the margins of some continents, such as the western side of South America, are occupied by an active compressional plate boundary, where two plates are opposing; whereas other continental borders, such as the western side of Africa, are a part of the plate and therefore are tectonically silent.
The plates are made up of the lithosphere, which includes the oceanic and continental crust and upper part of the mantle upon which these crusts rest. The plates have a thickness of about 70 to 80 km under the oceans, of which approximately 7 km is the actual crust and the rest the upper mantle. The thickness of plates beneath the continents is about 100 to 150 km and the crust is averagely 40 km and about 70 to 80 km thick under most mountain ranges.
The plates have three known boundaries -
- The mid-oceanic ridge. This is where two plates move apart allowing magma to flow out, and therefore new oceanic lithosphere is constantly created
- The Convergent zones, where one plate bends down in a subduction zone and is consumed beneath another plate. Most commomly a oceanic trench is formed; or Sutures, where two continental plates have collided to form a upright structure, commonly mountains.
- The transform fault, where two plates slide silently pass each other with no creation or destruction of the lithosphere.
Most earthquakes are experienced at plate boundries. Earthquakes prove the presence of active plates, which constantly move, destruct
or build.
Pictures: Above-Plates and their boundries.
|
|
|
