~Structure of the Earth's Lithosphere~
The image to the right shows a small portion of the earth's outer layers.The outermost brown and black layer above the Moho Discontinuity is the crust. 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 crust has two major divisions; 1) ocean basins (black layer) composed of Mafic Rocks and 2) continents (brown layer) composed of Felsic rocks. 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 center of the figure 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 in isostatic equilibrium and float above the mantle. Conversely, mountains of any kind must be held up by something, like heat or a root zone, or they will sink.
The outer layers of the earth are subdivided by two criteria;
1. Composition: 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.
2. Behavior: the outer earth layers are divided into the lithosphere and asthenosphere (see right side of Enlarged Drawing). The lithosphere is the cold, rigid outer layer, 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.
Contributed by Lynn Fichter
Wednesday, November 19, 2014