Magma is predominantly a molten silicate saturated with gases that are dissolved in it. It has a marked quantity of easily voltiling compounds ( O vapour, C , HF, HCI, etc).
Owing to the high pressure existing in deeper part of the earth where volatile compounds are in a dissolved state within magma, diminishing its viscosity and increasing the degree of its mobility and chemical activity.
Formation of magmatic sources under the earth is in general a continuous process. They are accumulated in the upper part of the asthenosphere (33-140 km deep from the surface of the earth) which then ascends into the upper levels of the earth’s crust.
The movement of the magma towards the earth surface is conditioned by hydrostatic pressure along with considerable increase in the volume, which accompanies the transition of solid rocks into the molten state.
Some magma melts penetrate and break through the horizons of earth surface and some invasive magma on its way to earth surface and solidifies at certain depth within the earth.
Volcanism unites all the processes connected with the outflow on the earth’s surface. The volatile components, which, in the deeper regions owing to high pressure and temperature remains in the magma in dissolved state, are released it on the way to the earth’s surface. The products of volcanic eruptions include liquid, solid and gaseous materials (varitia).
Liquid products of volcanic eruptions are represented by lava. And are classified as acid, medium, basic (or) ultra basic depending upon its chemical composition especially (silica) content.
Lavas of ultra basic or basic are poor in silica and rich in Ferro-magnesium compounds with temperature existing at the surface at the time of outflow being C and characterized by low viscosity and high mobility. So they easily move and spread themselves over a considerable distances and form sheets and streams of undulating surface. Thus the lava flowing out of a volcano is hot.
Acid and medium lavas rich in silica with surface temperature of C – C possess high degree of viscosity and little mobility and more for short distance and quickly solidify forming small streams and blocks.
Solid particles also emerge during volcanic explosion as a result of an ejection into the atmosphere and dispersion of huge masses of lava as well as fragments of rocks.
Depending on size they are classified as volcanic bombs, Lapilli, volcanic sand and volcanic ash. Volcanic ash is the main product of eruption. Gaseous products released are made up of water vapour (60-90 percent followed by S, , Co, , HCI,HF, etc.)