Mantle

I

Introduction

Mantle, region of the interior of the earth that lies between the outer crust and the core. The mantle is the largest of these regions and comprises the bulk of the earth’s mass and volume. Temperatures in the mantle are high, reaching about 3700° C (about 6700° F). Pressures inside the mantle are also high, reaching about 137 gigapascals (1.37 million atmospheres). The mantle is primarily solid, and its density increases with depth. Under these high temperatures and high pressures, however, material in the mantle can deform, or flow slowly like putty. Slow-moving currents, called convection currents, within the mantle are thought to play a part in the movement of the crustal plates on the earth. See also Plate Tectonics

The relative size of the mantle can be understood by likening the earth in cross-section to a soft-boiled egg. The eggshell represents the thin, hard crust of the earth, which only comprises about one half of one percent of the mass of the earth. The white of the egg represents the soft mantle, which comprises about two-thirds of the mass and about five-sixths of the volume of the earth. The yolk represents the core of the earth.

The mantle is separated from the crust by a sharp boundary known as the Mohoroviić discontinuity, or Moho. It is separated from the core by another sharp boundary known as the Gutenberg discontinuity. Both boundaries are named in honor of the men who discovered them, Croatian seismologist Andrija Mohoroviić and German-born American seismologist Beno Gutenberg. The Mohoroviić discontinuity lies at a depth of about 8 km (5 mi) under the oceans and an average depth of about 35 km (about 22 mi) under the continents, but it plunges to as deep as 80 km (50 mi) under tall mountain ranges. The Gutenberg discontinuity lies at a depth of about 2900 km (about 1800 mi). Both men discovered the boundaries by using the fact that when an earthquake, or seismic, wave reaches a sharp boundary between two materials with different densities or elastic properties, some of the wave’s energy bounces, or reflects, off of the surface. In addition, seismic waves may bend, or refract, as they cross a boundary.

The mantle is chemically distinct from both the crust and the core. It is primarily composed of the rock peridotite, which principally consists of the minerals olivine, pyroxene, and amphibole. The crust contains lighter materials and the core is composed of iron and nickel. Thus, the Mohoroviić and Gutenberg discontinuities represent chemical as well as physical boundaries.

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II

Structure of the Mantle

Seismologists have discovered that the mantle is subdivided into a number of layers. The upper mantle extends from the Moho to a depth of about 400 km (about 250 mi). The upper mantle is composed of iron and magnesium silicates, such as the minerals olivine, pyroxene, and amphibole. Within the upper mantle is a zone called the asthenosphere. The asthenosphere ranges from a depth of about 100 km (about 60 mi) to about 350 km (about 220 mi). The asthenosphere is a zone of weakness. It is thought that the asthenosphere contains a small amount of melt, or liquid fused rock, which acts as a lubricant, allowing the grains of rock to slide past each other. According to the theory of plate tectonics, large crustal plates of the zone above the asthenosphere, called the lithosphere, move about the surface of the earth, gliding on the weaker asthenosphere.

The zone from about 400 km to about 670 km (about 250 mi to about 420 mi) is known as the transition zone. In the transition zone, the minerals that make up the upper mantle undergo a process called phase transition, in which they change in structure and form other atomic arrangements. The pressures at these depths compress the minerals into more compact forms. For example, olivine is compressed into the mineral spinel, in which the atoms are packed closer together. By the bottom of the transition zone, spinel has undergone another phase transition to the mineral perovskite. With each phase transition, the rock becomes denser and seismic waves travel through it faster. This transition at 670 km also corresponds to the lowest depths at which earthquakes have been recorded.

The mantle below 670 km is called the lower mantle. The lower mantle may consist of magnesium, silicon, and iron. Unlike the upper mantle, this region does not change much in composition or phase as it gets deeper. It is more dense than the upper mantle due to the increase in pressure.

III

Mantle Flow

The flow of material within the mantle affects the earth’s crust in several ways. Geologists believe that large-scale convection currents within the mantle are part of what drives the movement of the plates in the earth’s lithosphere. These currents also result in mantle material rising beneath midocean ridges and forming new crust, leading to seafloor spreading. Descending mantle material drags down and destroys other portions of the crust at regions called subduction zones. Evidence suggests that these convection currents have been operating for hundreds of million or even billions of years. In some places, such as Hawaii, plumes of mantle material have been slowly and steadily rising for tens of millions of years, forming chains of volcanoes (see Hawaii: Formation of the Islands and Volcanoes). In other places, large bubblelike portions of gas-rich mantle material rise explosively from the mantle, forming kimberlite pipes. Kimberlite pipes are the major source of diamonds.

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