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the uppermost of the earth’s solid shells. The lower boundary of the earth’s crust is considered to be the division surface where longitudinal seismic waves passing from above to below undergo a sudden increase in velocity from 6.7-7.6 km/sec to 7.9-8.2 km/sec; this surface is also known as the Mohorovicic discontinuity. This is an indicatioryhat less elastic material has given way to more elastic ami dense material. The layer of the upper mantle which underlies the earth’s crust is often called the substratum. Together with the earth’s crust it constitutes the lithosphere. The earth’s crust on continents differs from the crust under the ocean. Continental crust is usually 35-45 km thick, attaining a thickness of up to 70 km in mountain regions. The upper part of the continental crust is made up of a fragmented sedimentary layer consisting of unchanged or slightly changed sedimentary and igneous rocks of different ages. The layers are often folded, broken, and displaced along faults. In some places (on shields) the sedimentary shell is absent. The entire remaining thickness of the continental crust is divided into two parts according to the velocity of seismic waves; the upper part is conventionally called the “granite” layer (velocity of longitudinal waves up to 6.4 km/sec) and the lower part, the “basalt” layer (6.4-7.6 km/sec). It appears that the granite layer is made up of granites and gneisses; the basalt layer is composed of basalts, gabbro, and very strongly metamorphized sedimentary rocks in varying ratios. These two layers are often divided by the Conrad discontinuity, where seismic waves undergo a sudden increase in velocity as they pass through the discontinuity. It appears that the silica content of the earth’s crust de-creases with depth and the content of oxides of iron and magnesium increases; this occurs to an even greater degree with the transition from the crust to the substratum.
The oceanic crust is 5-10 km thick (together with the water depth it amounts to 9-12 km). It is divided into three layers: under a thin (less than 1 km) layer of marine sediments lies the “second” layer where longitudinal seismic waves have velocities of 4-6 km/sec. It is 1-2.5 km thick and is probably composed of serpentinite and basalt, possibly with sedimentary interlayers. The lower “oceanic” layer has an average thickness of about 5 km, and seismic waves pass through it at a velocity of 6.4-7.0 km/sec; it is probably composed of gabbro. The thickness of the layer of sediments on the ocean floor varies, and in some places there are none at all. In the transitional zone between the continent and the ocean an intermediate type of crust may be observed.
The earth’s crust is subject to constant movements and changes. In its irreversible development the mobile areas (geosynclines) are transformed by prolonged transformation into relatively stable areas (platforms). There are a number of tectonic hypotheses that explain the process of development of geosynclines, platforms, continents, and oceans, and the causes of the development of the earth’s crust as a whole. There is no doubt that the primary reasons for the development of the earth’s crust lie deeper within the earth’s interior; this fact renders the study of the interaction between the earth’s crust and the upper mantle of special interest.
The earth’s crust is close to a state of isostasy (equilibrium): the heavier (which is to say the thicker and the more dense) a particular sector of the crust is, the deeper it sinks into the substratum. Tectonic forces disrupt isostasy, but when they weaken, the earth’s crust returns to a state of equilibrium.
E. N. LIUSTIKH