Large-scale geological structures / Крупномасштабные геологические структуры

The study of seismic wave propagation, especially of the P (longitudinal) waves, has enabled us to distinguish two components in the shallower part of the earth:

- The crust (thickness varying from 10 to 70 km, average 30 Ian);

- The underlying mantle, separated from the crust by a surface of discontinuity at which the seismic-wave velocities change suddenly (the Mohorovicic discontinuity, generally shortened to Moho).

The crust is passive in behaviour. The driving force for tectonic phenomena is situated in the mantle, mainly in the deeper part. It is in fact observed, mostly by means of seismic waves, that the surficial part of the earth is rigid over a thickness of the order of 100 km; this part is known as the lithosphere (the crust and upper mantle).

The zone below the lithosphere is the asthenosphere; here the seismicwave velocities decrease, at least in its upper part. This low-velocity zone corresponds to a readily deformable material. It is probably at this level that isostatic compensations occur, as also the upper cells of the convective motions considered to be the causative force for most of the orogenic phenomena. The surface of lithosphere consists of two types of crust, continental and oceanic. <...>