plate tectonics: Plate Boundary Conditions
There are numerous major plate boundary conditions. When a large continental mass breaks into smaller pieces under tensional stresses, it does so along a series of cracks or faults, which may develop into a major system of normal faults. The crust often subsides, forming a rift valley similar to what is happening today in the Great Rift Valley through the Red Sea. If rifting continues, a new plate boundary will form by the process of seafloor spreading. Mid-ocean ridges, undersea mountain chains, are the locus of seafloor spreading and are the sites where new oceanic lithosphere is created by the upwelling of mantle asthenosphere.
Individual volcanoes are found along spreading centers of the mid-ocean ridge and at isolated hot spots, or rising magma plumes, not always associated with plate boundaries. The source of most hot-spot magmas is believed to be well below the lithosphere, probably at the core-mantle boundary. Hot-spot volcanoes often form long chains that result from the relative motion of the lithosphere plate over the hot-spot source.
Subduction zones along the leading edges of the shifting plates form a second type of boundary where the edges of lithospheric plates dive steeply into the earth and are reabsorbed at depths of over 400 mi (640 km). Earthquake foci form steeply inclined planes along the subduction zones, extending to depths of about 440 mi (710 km); the world's most destructive earthquakes occur along subduction zones.
A third type of boundary occurs where two plates slide past one another in a grinding, shearing manner along great
- Development of Plate Tectonics Theory
- Plate Boundary Conditions
- Movement of the Continents
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
See more Encyclopedia articles on: Geology and Oceanography