Formerly it was commonly judged, that the structure of the Earth is uniform, that its state of aggregation changes only under the pressure of the adjacent layers, turning initially into liquid, then into gassy. It was related to a fact that was known long ago, that the temperature rises as you go deeper into the Earth, As a result, even at a depth of several hundred meters, the work of a miner is difficult and very onerous.
Based on the observation of the increase in temperature with depth, a supposition was made, that the outer part of the Earth's crust, called lithosphere (from the Greek litos - stone and sfair - a ball), rests on the liquid interior. However, subsequent research proved, that the fluid interior of the Earth was mistakenly assumed to exist at a shallow depth under the crust. From the course of elastic vibrations caused by earthquakes, and transmitted by seismic waves, follows, that above the depth 2900 km the earth's matter is in a solid state.
Only the outer part of the earth's crust is accessible to direct research. By comparing the depth of the drill holes, which have reached almost 10 km deep, with the radius of the Earth, having a length 6370 km, can be said, that man knew the structure of the Earth directly to a depth of just over 1/1000 parts of its radius. Information about its deeper parts is based only on hypothetical assumptions and the results of geophysical surveys.
One of the most important achievements of geophysics is the finding of discontinuities in the structure of the Earth. Observations of the behavior of earthquake-induced seismic waves showed, that the Earth is composed of several concentric zones with different elastic properties and a core located in the center.
Seismic waves consist of transverse vibrations, propagating only in solids, and from longitudinal vibrations. The speed of propagation of seismic waves varies, depending on the elastic properties and the density of the environment, in which the waves diverge. Surfaces, where there are changes in the behavior of seismic waves, are called discontinuity surfaces. The clearest of them is at depth 2900 km. At this depth, the speed of the seismic waves decreases sharply z 13 do 8 km/s. Shear waves also stop at this depth, what can testify, that the central part of the Earth - the core - is in a liquid state of aggregation.
As early as the beginning of this century, a supposition was made, that the surface part of the globe consists of two zones of different composition and density. The outer zone, composed of rocks rich in silicon oxide - Si silica)2 and aluminum oxide - Al2O3 clay was named after the symbols of silicon Si and aluminum Al sial, while the lower zone, rich in silicate compounds of magnesium Mg, was called a sim. Sial is made of rocks, the main ingredients of which are feldspars (potassium aluminosilicates, sodium and calcium), and the sima consists of rocks composed mainly of pyroxenes and olivines (magnesium and iron silicates). Because the speed of propagation of seismic waves in the shale corresponds to their speed in granites, and in simie - speeds in basalt, these zones were called the granite layer and the basalt layer.
At a depth of 80-150 km, an asthenosphere is assumed (from the Greek astenos - weak), which is formed by a glassy olivine basalt, behaving like a plastic body under the influence of prolonged pressures. Below is an intermediate zone, that is, the mesosphere (from the Greek mesos - medium, middle), also called a cloak. Its upper part consists of rocks called peridotites, the main ingredient of which is olivine. They are presumably to a depth of approx 413 km, where the sim ends. There are probably also peridotites below, but with the addition of Cr chromium and Fe iron - crofesima, and deeper and nickel Ni - nifesima. The density of the mesosphere increases to 5-6 g / cm³.
Below a distinct area of discontinuities in depth 2900 km is the core of the Earth, called from high specific gravity - barysphere (from the Greek barys - heavy), and from the main ingredients: molten iron Fe and Ni-nife nickel. The core density is approx 8 g/cm³. From the depth 5100 km the nucleus is probably in the solid state.
Cross-section of the globe.