Tectonic movements

The upper part of the earth crust is SIAL.

Crust

A volcano is a vent or an opening in the earth's crust through which hot magma erupts from deep below the surface. The opening is usually circular in form

The layer of the earth beneath the crust is called the mantle. It is separated from the crust by a boundary called Mohorovicic discontinuity.

On the basis of shape, there are three types of volcanoes. They are:
(i) Shield volcano
(ii) Cinder-cone volcano
(iii) Composite volcano

Causes of Volcanic Activity:
(i) The temperature increases as the. depth increases at the rate of 1°c for every 35 metres.
(ii) There is also great pressure. At a depth of about 15 Ian the pressure is about 5 tonnes per em of rock.
(iii) Under these circumstances, the interior of the earth is in a semi-molten state called magma.
(iv) The magma, under great pressure has the capacity to dissolve great volume of gas; some gases are also combustible.
(v) This makes volcanic material burst forth through the weak spots in the earth's crust.

There are three major zones of volcanic activities in the world. They are:
1. The Circum - Pacific belt
2. The Mid continental belt
3. The Mid Atlantic belt
1. Circum Belt: This is the volcanic zone of the convergent oceanic plate boundary. It includes the volcanoes of the eastern and western coastal areas of Pacific Ocean.
2. Mid continental belt:
(i) This is the volcanic zone of convergent continental plate boundaries that includes the volcanoes of Alpine mountain chains, the Mediterranean Sea and the fault zone of eastern Africa.
(ii) The important volcanoes are Vesuvius, Stromboli, Etna, Kilimanjaro and Kenya.
3. Mid Atlantic Belt:
(i) This belt represents the divergent boundary of plates located along the mid- Atlantic ridges. Volcanoes of this area are mainly of fissure eruption type.
(ii) St. Helena and Azores Island are other examples.