Earth's Interior

Underlying the crust is the mantle, which comprises about 82% of Earth's volume and 65% of its mass. The uppermost section of the mantle, which is solid, is called the lithospheric mantle. This section extends from the Figure 1. The interior of the earth. Illustration by Hans & Cassidy. Courtesy of Gale Group. Moho down to an average depth of 40 mi (70 km), fluctuating between 30 and 60 mi (50–100 km). The density of this layer is greater than that of the crust, averaging 3.3 g/cm³. Like the crust, this section is solid, and is cool relative to the material below. The lithospheric mantle, combined with the overlying solid crust, is termed the lithosphere, a word derived from the Greek lithos, meaning rock. At the base of the lithosphere is another seismic transition, the Gutenberg low velocity zone. At this level, the velocity of S waves decreases dramatically, and seismic waves appear to be absorbed more strongly than elsewhere within the earth. Scientists interpret this to mean that the layer below the lithosphere is a "weak" or "soft" zone of partially melted material (1–10% molten material). This zone is termed the asthenosphere, from the Greek asthenes, meaning "weak." This transition between the lithosphere and the asthenosphere is named after German geologist Beno Gutenberg (1889–1960), who made several important contributions to our understanding of Earth's interior. It is at this level that some important Earth dynamics occur, affecting those of us here at the earth's surface. At the Gutenberg low velocity zone, the lithosphere is carried on top of the weaker, less-rigid asthenosphere, which seems to be in continual circulation. This circulatory motion creates stress in the rigid rock layers above it, and the slabs or plates of the lithosphere are forced to jostle against each other like ice cubes floating in a bowl of swirling water. This motion of the lithospheric plates is known as plate tectonics (from the Greek tektonikos, meaning construction), and is responsible for many surface phenomena, including earthquakes, volcanism, mountain-building, and continental drift.

The asthenosphere extends to a depth of about 155 mi (250 km). Below that depth, seismic wave velocity increases, suggesting an underlying denser, solid phase.

The rest of the mantle, from the base of the asthenosphere at 155 mi (250 km) to the core at 1,800 mi (2,900 km), is called the mesosphere ("middle sphere"). Mineralogical and compositional changes are suggested by sharp velocity changes in the mesosphere. Notably, there is a seismic discontinuity at about 250 mi (410 km) of depth, attributed to a possible mineralogical change (presumably from an abundance of the mineral olivine to the mineral spinel), and another at about 400 mi (660 km), attributed to a possible increase in the ratio of iron to magnesium in mantle rocks. Except for these variations, down to 560 mi (900 km) the mesosphere seems to consist of predominantly solid material that displays a relatively consistent pattern of gradually increasing density and seismic wave velocity with increasing depth and pressure. Below the 560 mi (900 km) depth, the P and S wave velocities continue to increase, but the rate of increase declines with depth.

Although much of the mantle is "solid," the entire mantle actually convects or circulates like a pot of boiling water. Images produced by analysis of seismic waves show that dense slabs of oceanic crust plunge all the way through the mantle to the outer surface of the core, which indicates that the entire mantle is in motion, mixing thoroughly with itself over geological time.