~Structure of the Earth~
And Its Origin and Fate...
[Note: 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 - see Brief Primer]
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 and is very thin (relatively). It is divided into the crust (the outermost layer) and the very upper part of the mantle (explored under Lithosphere Structure).
Asthenosphere - the layer below the lithosphere, about 2000 km thick. It is composed mostly of Ultramafic rocks (Igneous Primer) such as peridotite and Dunite that are weak and plastic and flow slowly under stress.
Mantle - the layer 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. It is composed mostly of Ultramafic rocks 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 pressure.
Contributed by Lynn Fichter