The Tectonic System
The tectonic system operates from the earth's internal heat. The asthenosphere is more plastic than both the overlaying lithosphere and the underlying lower mantle because of an optimum balance of temperature and pressure. Above the plastic asthenosphere, relatively cool and rigid lithospheric plates split and move apart as single mechanical units. Molten rock from the asthenosphere wells up to fill the void and creates new lithosphere. Convection circulation occurs in the asthenosphere where material fills the gap between the spreading plates and moves away to areas where plates descend into the lower mantle at oceanic trenches. Some plates contain blocks of thick, low-density continental crust, which cannot sink into the mantle. As a result, where a plate carrying continental crust collides with another plate, the continental margins are deformed into mountain ranges. The plate margins are the most active areas on the Earth--the sites of the most intense volcanism, seismic activity, and crustal deformation.
Major Concepts
- The theory of plate
tectonics explains the major structural features of the Earth as a result
of a series of moving lithospheric plates.
- Where plates move apart, hot material from the mantle wells up to fill the void and creates new lithosphere. The major features formed where plates spread apart are continental rifts, oceanic ridges, basaltic volcanism, and new ocean basins.
- Where plates converge, one slides beneath the other and plunges down into the mantle. The major features formed at converging plate margins are (a) folded mountain ranges, (b) volcanic arcs, and (c) deep-sea trenches.
- The Earth's lithosphere floats on the denser, plastic, asthenosphere beneath, and it rises and sinks in attempts to maintain isostatic equilibrium.
- The crust and upper part of the mantle form the lithosphere, about 100 km thick, that overlies hotter and more plastic mantle of the asthenosphere, above which horizontal lithospheric displacement can take place.
Copyright Ó 1999, Pat Hayhurst
Home Page URL: http://www.joewalker.com/teachers/hayhurst/
Last Updated: Saturday, March 13, 1999