What is Plate Tectonics?

The theory of plate tectonics has revolutionized the thinking of geologists. This is a unifying theory that explains many seemingly unrelated geologic processes. Plate tectonics was first seriously proposed as a theory in the early 1960's although the related idea of continental drift was proposed much earlier.

The Plates
The outer part of the earth is broken into rigid plates approximately 62 miles thick. These outer plates are called lithosphere and include rocks of the earth's crust and upper mantle. Below the rigid lithosphere is the asthenosphere, a zone around the earth that is approximately 90 miles thick and behaves like a plastic because of high temperature and pressure. The lithosphere plates move over the plastic asthenosphere at a rate of an inch or more per year. Eight large plates and a few dozen smaller plates make up the outer shell of the earth.

The internal heat of the earth is the most likely cause of plate movement; this heat is probably generated by the decay of radioactive minerals.

The entire surface of the earth is moving, and each plate is moving in a different direction than any other. We now believe that plate movement is responsible for the highest parts of the continents and the deepest trenches in the oceans. Such movements also cause catastrophic events like earthquakes, volcanoes and tsunamis.

Plate Boundaries
Plate boundaries are of three types: a diverging plate boundary is a boundary between plates that are moving apart; a converging plate boundary is one where plates are moving towards each other; and a transform plate boundary is one at which two plates move past each other.

Diverging Boundaries
Diverging boundaries occur where plates are moving apart. Most of these boundaries coincide with the crests of the submarine mid-oceanic ridges. These ridges form by ascending hot mantle material pushing the lithosphere upward. When heat rises, molten rock moves upward and the expansion from the heart and pressure causes the ridge plate to bow upward and break apart at the spreading centers. Tension cracks form parallel to the ridge crest and molten rock from magma chambers in the mantle is intruded through the fractures. Magma errupts into submarine volcanoes and some of it solidifies in the fissure. New crust forms in rifts at the spreading centers. As new magma is extruded, it accretes to both sides of the plates as they are pushed or pulled apart. As the plates continue to pull apart, new tension fractures form and fill with magma. This cycle repeats itself again and again.

Transform Boundaries
The transform boundary occurs where two plates slide past one another. The San Andreas Fault is one of the best known land exposures of a transform boundary.

Converging Boundary
A converging boundary where plates move toward each other is responsible for the origin of most of Idaho's igneous rock as well as most of the major structural features of the state. Where one plate is covered by oceanic crust and the other by continental crust, the less dense continental plate will override the denser oceanic plate. The older the oceanic plate, the colder and more dense it is. Where two plates collide, the dense plate is subducted below the younger and less dense plate margin. At this boundary, a subduetion zone forms where the oceanic plate descends into the mantle beneath an overriding plate. As the oceanic plate descends deeper into the earth it is heated progressively hotter. Also the friction caused by the two plates grinding past each other leads to greater temperatures.

At the subduction zones, submarine trenches form, representing the deepest parts of the ocean basins. Earthquakes continuously occur at the plate margins where the overriding plate is grinding and abrading the subducted laver. The subducted plate causes earthquakes all along its downward path as it slowly moves into the earth's mantle. By measuring the depth and position of the earthquakes, geologists are able to determine almost exactly the position and orientation of the subducting plate.

Age of Ocean Basins
The youngest rocks are found at the spreading centers, and become progressively older in both directions away from the spreading centers. The oldest rocks in the ocean basins are approximately 150 million years old as compared to the oldest rocks on the continents of 3.8 billion years.

Physical Properties of Oceanic Rocks
Rocks forming the ocean basins are dark, iron-rich and have a higher specific gravity than those forming the continents. Rocks of the continents tend to be low in density, light colored and rich in silica and aluminum. Because of the lower density of continental rocks, they are floating on the denser oceanic-type rocks.

Generation of Magma, Volcanoes and Batholiths
When the plate reaches a certain depth, the heat and pressure melts the lighter minerals within it. This light molten rock or magma coalesces at depth and floats upward through the more dense rock towards the surface of the earth. Where these globs of molten rock break through the oceanic crust, they form chains of volcanic islands. The Aleutian Islands are a well-known example.

The portion of the magma that manages to break through the surface forms volcanoes like Mount St. Helens and is classified as volcanic or extrusive rock. The portion that does not break through the earth's surface, but instead solidifies within the earth's crust, is classified as intrusive igneous or plutonic rock.

Where an oceanic plate is subducted below a continent, the rising globs of magma melt and absorb portions of the silica-rich, low specific gravity continental rocks. Where magma manages to break through the continental crust, the extrusive products are much more siliceous than their oceanic counterparts.

Geologists have found that rocks intruded through continental rocks have strontium isotope values much different than those intruded through oceanic rocks. Consequently, we are able to determine, on the basis of strontium isotope ratios, whether or not a particular intrusion passed through continental rocks.

Rocks in the vicinity of a subduction zone are drastically changed by the intense heat and pressure. If these rocks do not melt, they become metamorphic rocks. Also rocks in the continental crust in the path of a rising magma chamber are metamorphosed by the heat and pressure exerted by the upward-moving molten rock.

Mantle Plumes
Mantle plumes are believed to form where convection currents in the earth's mantle cause narrow columns of hot mantle rock to rise and spread radially outward. One of the most convincing theories for the origin of the Snake River Plain proposes that a hot mantle plume tracked across the plain from west to east and was the source for most of the volcanism in the Snake River Plain. This hot mantle plume is now thought to underlie the caldera at Yellowstone Park. Many of the volcanic islands of the Pacific Ocean may have originated from a mantle plume. The best known examples are the Hawaiian Islands.

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