Geology: The mantle

The upper and lower mantle. Seismic data suggest that most of the mantle is composed of solid rock. P waves travel at an average of about 8 kilometers per second through the mantle, suggesting it is composed of ultramafic rocks such as peridotite. The behavior of P waves indicates the mantle can be divided into two parts: the upper and lower mantle. The upper mantle begins at a depth of from 5 to 50 kilometers (3−30 miles) and extends to a depth of approximately 670 kilometers (400 miles) from the surface; the lower mantle extends from a depth of about 670 kilometers (400 miles) to about 2,900 kilometers (1,740 miles).

The lithosphere. Changes in P wave velocities have identified other boundaries in the mantle. Together the crust and the uppermost mantle form the lithosphere. This brittle exterior shell of the earth ranges in thickness from about 75 kilometers beneath oceans to about 175 kilometers beneath continental masses (45−105 miles). The maximum depth of the lithosphere from the surface is thought to be no more than 200 kilometers (120 miles).

The asthenosphere. P waves slow down at the boundary between the lithosphere and the underlying asthenosphere. Most geologists believe the asthenosphere, also called the low‐velocity zone, is about 200 kilometers (120 miles) thick. The rocks in the asthenosphere are thought to be partially melted and hotter and more plastic than those in the lithosphere. If this is the case, the asthenosphere could be the weaker surface on which the crustal plates move and a possible source for the generation of magma.

Other boundaries. Seismic data has identified two other concentric boundaries within the upper mantle at depths of 400 and 670 kilometers (240 and 400 miles). These are thought to represent pressure‐temperature zones in which minerals collapse to form new, more compact atomic structures. It is therefore possible that even though the mantle may be chemically homogenous, it does not have the same mineral composition throughout. The 670‐kilometer boundary is believed to represent a chemical as well as physical boundary between the upper and lower mantle. The thick lower mantle is believed to consist of plastic or partially melted ultramafic rocks that continue to the boundary with the outer core.