II.

Tectonic Plates

Tectonic plates are made of either oceanic or continental crust and the very top part of the mantle, a layer of rock inside the earth. This crust and upper mantle form what is called the lithosphere. Under the lithosphere lies a fluid rock layer called the asthenosphere. The rocks in the asthenosphere move in a fluid manner because of the high temperatures and pressures found there. Tectonic plates are able to float upon the fluid asthenosphere because they are made of rigid lithosphere. See also Earth: Plate Tectonics.

A.

Continental Crust

The earth’s solid surface is about 40 percent continental crust. Continental crust is much older, thicker and less dense than oceanic crust. The thinnest continental crust, between plates that are moving apart, is about 15 km (about 9 mi) thick. In other places, such as mountain ranges, the crust may be as much as 75 km (47 mi) thick. Near the surface, it is composed of rocks that are felsic (made up of minerals including feldspar and silica). Deeper in the continental crust, the composition is mafic (made of magnesium, iron, and other minerals).

B.

Oceanic Crust

Oceanic crust makes up the other 60 percent of the earth’s solid surface. Oceanic crust is, in general, thin and dense. It is constantly being produced at the bottom of the oceans in places called mid-ocean ridges—undersea volcanic mountain chains formed at plate boundaries where there is a build-up of ocean crust. This production of crust does not increase the physical size of the earth, so the material produced at mid-ocean ridges must be recycled, or consumed, somewhere else. Geologists believe it is recycled back into the earth in areas called subduction zones, where one plate sinks underneath another and the crust of the sinking plate melts back down into the earth. Oceanic crust is continually recycled so that its age is generally not greater than 200 million years. Oceanic crust averages between 5 and 10 km (between 3 and 6 mi) thick. It is composed of a top layer of sediment, a middle layer of rock called basalt, and a bottom layer of rock called gabbro. Both basalt and gabbro are dark-colored igneous, or volcanic, rocks.

C.

Plate Sizes

Currently, there are seven large and several small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Caribbean plate, and the Gorda plate. Plate sizes vary a great deal. The Cocos plate is 2000 km (1400 mi) wide, while the Pacific plate is the largest plate at nearly 14,000 km (nearly 9000 mi) wide.