Plate Tectonics

Plate tectonics is a theory to explain the slow movement of sections of the earth's crust (continents and oceans).   The theory has the earth divided into about 15 sections (plates).   Most of the plates contain portions of continents and portions of ocean (Stein, 2003 and Oreskes, 2003).   The plate tectonics theory has become widely accepted and has replaced earlier theories including continental drift, paleomagnetic reconstructions, and  sea floor spreading among geoscientists.

Structure of the Earth

It is difficult to measure the structure of the earth directly, because of extremely high temperature and pressure that make exploration by conventional means such as deep drilling or excavation impossible.   Therefore, seismic studies have been used as the basis of theories about the structure and composition of the interior.

According to plate tectonics theory, the earth's outer shell consists of about 15 rigid plates.   The plates make up what is known as the earth's lithosphere.   The plates are about 60 miles (100 km) thick.   Little or no deformation (e.g. faulting, volcanism, and folding) takes place within the plates.   Considerable deformation does take place at the boundaries of the plates and in boundary zones (as shown in the illustration).

The drawing on the right illustrates some of the elements of the earth's structure at the boundary of two plates.   Rocks that make up the upper part of the continental plate (brown layer on the left side of the drawing) have been shown to consist primarily of a granitic or sialic composition with a relatively low density.   The crust under the oceans, based largely on seismic studies, consists mostly of dense, simatic rocks.   The plates are units that move over the hotter and weaker sublithospheric mantle (shown as pink in the drawing).

Temperature of the Earth's Interior

Thermal convection in the sublithospheric mantle is the driving force of plate tectonics.   Hot material is less dense and tends to rise.   Cool material is more dense and tends to sink.    Indirect evidence indicates that the center of the earth is very hot.

Estimates of the temperature of the center of the earth vary from 5,000 to 7,000 degrees K (8,500 to 12,000 degrees F).   The calculated temperature of the earth at a depth of 60 miles (100 km) is 1,300 degrees C (2,4000 degrees F).   The average thermal gradient in the crust is 13 degrees C (24 degrees F) per kilometer or 22 degrees C (40 degrees F) per mile.

Plate Tectonics Movements

Arrows on the drawing above illustrate typical movement directions of units at plate boundaries.   Plate movement is very slow, measured at the rate of a few centimeters (approx. 1 inch) per year.   The continental plate (brown) is shown colliding with a oceanic plate (gray).   Between the continental and oceanic plates is a plate boundary zone (reddish-brown) in which considerable deformation takes place.   The dense oceanic plate is shown sinking (subduction zone) into the sublithospheric mantle (pink).

Our discussion is quite brief and may raise more questions than it answers.   We suggest that you explore the resources of the internet further, or consult some of the excellent publications that are available.

Some Useful Links

Following are some hyperlinks that can further assist with the evaluation and description of plate tectonics:

• This Dynamic Earth:  The Story of Plate Tectonics (US Geological Survey website):  Illustrated online version of a publication by W. Jacquelyne Kious and Robert I. Tilling printed by the U.S. Printing Office in 1996.

• Plate Tectonics Animations (U. of California, Berkley, Natural History Museum, USA):   Animations illustrating plate tectonics movements and other related plate tectonics topics.

• Plate Tectonics:  The Cause of Earthquakes (Nevada Seismological Laboratory, USA website):  Website about plate tectonics and their relationship to earthquake activity with illustrations.

• USGS maps, products, and publications - Online information resource on the U.S. Geological Survey website.