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Terri Mathews


GEOL 302

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Plate Tectonics

The Earth is composed of several lithospheric plates that float atop the asthenosphere and are in constant motion. Plate Tectonics is essentially the study of the movement of the lithospheric plates and the consequences of that movement.

The interior of the Earth is composed of different materials. Geologists know the composition of the Earth through study of seismic waves. The composition and density of the materials in the Earth cause energy waves to either slow down or speed up. By measuring the speed of seismic waves, scientists are able to determine the approximate location and composition of material within the Earth.

The traditional model of the Earth has been refined:

  1. Lithosphere: includes the crust and upper mantle.  Is composed of a rigid solid.
  2. Asthenosphere: lower mantle, composed of "plastic solid" akin to playdoh.
  3. Outer core: liquid
  4. Inner core: solid



The term Lithosphere is Greek for "rock layer."   Comprised of the crust and uppermost part of the mantle, the lithosphere consists of cool, rigid and brittle materials. Most earthquakes originate in the lithosphere.

Because it is close to the surface, both temperatures and pressures are relatively low in comparison to the other layers.

Two different types of crustal material are found in the lithosphere:

Continental Crust Crust is thicker and composed of light materials; both in color and density.
Oceanic Crust Crust is thin and composed of more dense materials.



Mantle & Asthenosphere

The Mantle lies below the Lithosphere. The mantle makes up 80% of the Earth�s material and is composed of an upper Mantle and a lower Mantle. The upper Mantle ranges from a depth of approximately 100 kilometers below the Earth�s surface to a depth of approximately 670 kilometers.

The portion of the upper Mantle from a depth of 100 to approximately 350 kilometers below the surface is known as the Asthenosphere. The Asthenosphere is made up of semi-plastic rock. Since the Lithosphere has a lower density, it floats on top of the Asthenosphere similar to the way in which an iceberg or a block of wood floats on water.

The lower mantle below the Asthenosphere is more rigid and less plastic.  


Outer and Inner Core

Below the Mantle is the outer core. The outer core is composed of a liquid. Within the liquid outer core sits the solid inner core. It is believed that the inner core is composed of iron and nickel. The inner core is spinning within the liquid outer core. It spins faster than the crust and this spin is believed responsible for the Earth's magnetic field.

The table below shows the depths and properties of the earth's layers as well as the composition of those layers. 




Continue on to "History of Plate Tectonics