STRUCTURE OF THE EARTH'S LITHOSPHERE

In the image to the right we see a small portion of the earth's outer layers enlarged to show more detail (click picture for a larger version).
The drawing begins at the earth's surface (sea level) and descends about 150 kilometers down. There are several layers shown, color coded brown and black, green, and reddish.
The outermost brown and black layer, above the Moho (boundary between crust and mantle) is the crust. It has two major divisions: ocean basins (black layer) composed of mafic rocks (or see primer) like basalt and gabbro, and continents (brown layer) composed of felsic rocks (or see primer) such as granite. the crust extends to a depth of about 70 km, and represents less than 0.1% of the earth's total volume.
The continent in the upper center is a continental craton. A craton is a stable continent eroded down to just above sea level. Because continental cratons are composed of light weight rock such as granite they are like a boat resting in water, neither rising nor sinking on their own. They are said to be in isostatic equilibrium. Conversely, mountains of any kind must be held up by something, like heat or a root zone, or they will sink right back into the earth.
The Moho lower boundary of the crust marks the transition from the granite and basalt of the crust to the ultramafic rocks (or see igneous primer) of the mantle below.

The outer layers of the earth are subdivided on two totally different criteria, composition and behavior, and these overlap in non-obvious ways, often leading to confusion if we don't pay attention.
Compositionally, the Moho separates the crust, composed of relatively light weight felsic and mafic rocks, from the ultramafic rocks of the mantle. The felsic and mafic rocks are analagous to the light weight slag that floated to the surface during the earth's molten stage.

Behaviorally, the outer earth layers are divided into the lithosphere and asthenosphere (see right side of enlarged drawing). The lithosphere is the cold, rigid outer layers, and is composed of the crust and the outermost portion of the mantle.
The underlying asthenosphere is all ultramafic mantle, but it is hot and plastic. The convection cells operate within the asthenosphere.

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STRUCTURE OF THE EARTH

To understand plate tectonic theory, we need to know something about the structure of the earth. To understand this structure we need to know something about igneous rocks. If you are unfamiliar with igneous rocks, there is a brief primer that in two minutes will tell you everything you need to know to get started.

The earth is approximately 12,740 kilometers in diameter, and is stratified into layers as shown in the illustration to the right. The properties of each are:

Lithosphere - the outer rigid shell of the earth's structure, extending from the surface to about 1000 km deep (outer blue line on drawing). It consists of the crust and the upper mantle. Note how very thin the lithosphere is, comparatively thinner than an egg shell. It is divided into the crust (the outermost layer) and the very upper part of the mantle (explored under lithosphere structure).

Asthenosphere - the layer or shell below the lithosphere, about 2000 km thick. It is composed mostly of ultramafic rocks (or see igneous primer) such as peridotite and dunite that are weak and plastic, and flow slowly under stress.

Mantle - layer or shell of the earth below the lithosphere and above the core; it is about 2800 km thick, although it is subdivided into many sub-layers, including the asthenosphere and lower mantle as well as others. It is composed mostly of ultramafic rocks (or see igneous primer) such as peridotite and dunite and their metamorphic equivalents (e.g. ecologite).

Core - the central portion of the earth about 7000 km in diameter. It is composed of an iron-nickle alloy. The outer core is molten, while the inner core, even though just as hot, is a solid because of the increased pressure.

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