Eruption of volcanoes is preceded by emergence of the magmatic centers. They appear in the place of the movement of plates of a lithosphere - a stone cover of Earth. Under the influence of high pressure, magma escapes outside in places where there are breaks or a cover is thinned. The volcanic eruption results.
To learn when there is a volcanic eruption, it is necessary to consider the structure of Earth. The outer sheath of the planet is called a lithosphere (from Greek "a stone cover"). Its thickness on the land reaches 80 km, and at an ocean floor - only 20-30 km. It makes about 1% of the size of radius of earth crust. The layer following bark - a cloak. It has two parts - top and lower. Temperature in these layers reaches several thousand degrees. In the center of Earth - a firm kernel.
The lower layer of a cloak which is closer to a kernel heats up more, than top. The difference of temperatures leads to the fact that layers mix up: hot substance rises up, and cold - falls. Along with this process there is a cooling of blankets and heating of internal. For this reason the cloak is in the constant movement. The consistence she reminds hot pitch, in the center of the planet very high pressure. On a surface of this viscous environment the lithosphere "floats", having plunged into it the lower part.
As the stone cover is shipped in a cloak, it involuntarily moves together with it. Its separate parts, earth's plates, can crawl at each other. The plate which appeared from below plunges into a cloak more and stronger and melts under the influence of high temperatures. Gradually it turns into magma (from Greek "dough") - dense weight from the melted breeds, with water vapor and gases. Through collision of earth's plates the magmatic centers are formed. In them the magma lifting to a surface gathers. In the centers she behaves like the test rising on yeast: increases in volume, climbs from the depths of Earth cracks and fills with itself all free space. Where bark is thinned or there are breaks, there is a volcanic eruption. It comes when there was a decontamination (an exit of gases outside) magmas. Mix is in the center under the high pressure which pushes out it from subsoil as soon as an opportunity appears. Rising up, magma loses gases and turns into a fluid lava.