Mantle of the Earth

the shell of the “solid” earth, located between the earth’s crust and its core. It occupies 83 percent of the earth (excluding the atmosphere) by volume and 67 percent by mass. It is separated from the earth’s crust by the Mohorovidic discontinuity at which the velocity of longitudinal seismic waves passing from the crust to the mantle increases suddenly from 6.7-7.6 to 7.9-8.2 km/sec; the mantle is separated from the earth’s core by a discontinuity (at a depth of about 2,900 km) at which the velocity of seismic waves decreases from 13.6 to 8.1 km/sec. The mantle is subdivided into the lower and upper mantles. The latter in its turn is divided (from the top toward the bottom) into the substratum, the Gutenberg layer (the layer in which seismic waves have diminished velocities), and the Golitsyn layer (sometimes called the middle mantle). At the base of the mantle is a layer less than 100 km thick in which the velocities of seismic waves do not increase with depth or even decrease slightly.

It is thought that the earth’s mantle is composed of those chemical elements which during the formation of the earth were in a solid state or were part of solid chemical compounds. Among these the predominant elements are oxygen, silicon, magnesium, and iron. According to present-day ideas the composition of the mantle is close to that of stony meteorites. Of the stony meteorites, chondrites have a composition that is closest to the compositon of the mantle. It is presumed that fragments of rock carried to the surface of the earth in basaltic lava are samples of the matter of which the mantle is composed. Such rock fragments are also found together with diamonds in volcanic pipes (or diatremes). It is also thought that rock fragments dredged up from the bottom of the rifts of midoceanic ridges are material from the mantle.

Samples of the very top part of the earth’s mantle consist primarily of rocks of ultrabasic (peridotite and pyroxenite) and basic (eclogite) composition. It is usually assumed that the mantle is almost entirely composed of olivine, (Mg, Fe)₂ SiO₄, in which the magnesium component (forsterite) is strongly prevalent, but it is possible that the proportion of the iron component (fayalite) increases with depth. The Australian petrologist Ring-wood believes that the earth’s mantle is composed of a hypothetical rock that he calls pyrolite, which corresponds in composition to a mixture of three parts of peridotite and one part basalt. Theoretical calculations show that minerals should break down into oxides in the lower mantle. By the early 1970’s data had also appeared which indicated the presence of horizontal nonuniformities in the earth’s mantle.

Phase transitions are evidently a characteristic feature of the mantle. It has been established experimentally that under high pressure the structure of the crystalline lattice in olivine changes and the atoms are packed more closely together so that the volume of the mineral decreases noticeably. Such a phase transition is observed twice in quartz as pressure increases; the most dense modification is 65 percent more dense than ordinary quartz. Such phase transitions are considered to be the principal reason why the velocities of seismic waves increase very rapidly with depth in the Golitsyn layer.

There is no question that the earth’s crust originated from the mantle; the process of differentiation of the mantle is still continuing today. There is a hypothesis that the core of the earth is also growing at the expense of the mantle. Processes in the earth’s crust and mantle are closely connected; specifically, energy for the tectonic movements of the earth’s crust apparently comes from the mantle.