Plates

Plate tectonics deals with the study of the motion and deformation of the Earth's crust. This type of study is based on the theory that the lithosphere is divided into seven major and several minor plates that are all moving in relation to each other as well as in relation to fixed "hot spots," which are areas of upwelling mantle material. Plate tectonic theory tries to account for the movement of the crust throughout geologic time. Motion of the crust is that of a rigid body, which accounts for the style of deformation we see.

Plate tectonic theory is based on several assumptions about tectonic processes: 1) that new material is generated by sea-floor spreading at the mid-ocean ridges, which once formed become part of a plate, 2) that surface area is conserved, therefore plate material must be destroyed through another process, and 3) motion of plates is accomodated only along plate boundaries (Fowler,1990).

Plates are rigid bodies of rock that essentially float atop a region of partial melt called the asthenosphere. The plates comprise the lithosphere, composed of the crust (which is the rigid, outermost layer of the Earth) and the solid portion of the upper mantle. The crust is of two basic types, continental and oceanic, that differ on the basis of composition.

Continental crust is mostly of granitic composition. This means that the rocks contain an abundance of quartz and feldspars, which are called felsic (meaning light-colored) minerals. Oceanic crust, on the other hand, is of basaltic composition. Basalts contain minerals such as olivine and plagioclase feldspar, which are called mafic (meaning dark- colored) minerals. The two different types of crust differ in density and thickness as well as in composition-- continental crust (avg. density = 2.8 gcm^-3) is much less dense than oceanic crust (avg. density = 3.3 gcm^-3). Oceanic crust is 7-10 km thick, while continental crust is 35-70 km thick.

Structure of the Earth History of plate tectonics Plates Plate boundaries Forces in the Earth Faults Hypercard Resources

Plate tectonics Activities

Last modified on 8/14/98 by Maggi Glasscoe (scignedu@jpl.nasa.gov)