Plate Tectonics: Theory & Definition

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  • 0:01 Geology's Unifying Theory
  • 0:54 A Broken Lithosphere
  • 6:58 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Plate tectonics describes the internal workings of our planet. Using it allows geologists to explain a wide range of geologic features and processes. You will learn the basics about the theory in this lesson.

Geology's Unifying Theory

Plate tectonics is the framework in which geologists study and understand the inner workings of the earth. It provides us with a way to tie together what once was considered unrelated geologic processes and events. Consider the origin of the ocean basins, shifting position of the continents, where volcanoes erupt, where earthquakes happen, and why mountain ranges are found where they are; all of them are explained by plate tectonics.

The name itself is derived from the terms for two important components of the model. The term plate refers to the large pieces of the earth's lithosphere that are in constant motion; you may see them referred to as lithospheric plates. Tectonics is the branch of geology dealing with how continents form, change, and move over time, plus the study of mountain-building episodes.

A Broken Lithosphere

The fundamental units of plate tectonics are large pieces of Earth's lithosphere, the outermost rocky layer of the planet that is composed of the Earth's crust and the upper 50 to 80 miles of the next-lower layer, the mantle.

The lithosphere is not a uniform mass of rocks. There are differences in chemical composition (the rock types or the minerals they contain), that result in a broad categorization of the layer as either continental or seafloor lithosphere. Regardless of their composition, the rocks making up the lithosphere have a common physical property in that they are relatively rigid and brittle solids.

Just below the lithosphere is another layer of rocks called the asthenosphere. Those rocks are very hot and under a lot a pressure due to the weight of the rocks above. Those conditions allow the rocks to deform, or change shape, rather easily. In fact, even though they are solid, they can actually flow like a very viscous liquid.

Hot asthenosphere rocks, rising through the mantle and pushing up on the lithosphere, have caused it to fragment into about a dozen large plates and several small ones.

A plate boundary is what we call the edge of a plate where it is in contact with an adjacent plate. Along these boundaries, plates are in constant motion, either away from, towards, or past each other.

We know the plates are moving. The first formal description was presented by Alfred Wegener in 1912. But it was not until after World War II that geologists had collected the evidence for why they moved. It took time, because until relatively recently, we lacked the ability to actually measure the movement. All of the evidence was somewhat circumstantial: the presence of identical fossil species on widely separated continents; the age of the ocean basins and continents; the pattern of magnetism preserved in seafloor rocks. But all of it added up to one undeniable conclusion that the plates were in motion.

Exactly why the plates are moving is still a matter of investigation. One model is that plate movement is caused by very slow circulation, or flow, of rocks in the asthenosphere caused by temperature differences.

Hot rocks rise and cooler ones sink; this is known as mantle convection. In this model, plate motion is caused when the hot, rising asthenosphere rocks hit the base of the lithosphere, where they spread out and flow to either side. The overlying lithosphere rocks break and are pulled along.

But recently, a growing number of geologists have wondered whether that is really what starts plate motion. These researchers raise something of a chicken-or-egg argument about which came first, mantle convection or plate motion.

These new models propose two possible, and not mutually exclusive, mechanisms that might cause the plates to move. First, where hot mantle rocks do rise towards the surface, and before they can spread out, the lithosphere is pushing up, creating what amounts to a topographic high. The plates then start to slide down the hill in opposite directions, which pulls the upper asthenosphere rocks with them. The mechanism is called ridge push.

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