Volcano: How it is formed

The earth is a sphere made of many layers, the core(innermost), the mantle, which is the layer in the centre, and the crust(top layer). The crust is where there are lives, and volcanoes. 

The layer under the crust is the mantle, made of hot rocks, and no life can stay there, much less lived there. Hot rock is also known as magma, or molten rock. The crust, which we
live on, is actually floating on this deep and dense sea of molten rock.

The crust is actually made of plates, which sail on this sea of magma. Each year, the plates move only a few centimetres. In places called rifts, the plates tear apart from one another. Sometimes, the magma is pushed out of the cracks. Over thousands or millions of years, molten rock
brimming out of the rifts hardens into a series of ridges. The ridges is like a belt, and is sometimes quoted as "a serpent" across the floor of the earth's ocean all around the globe.

In some places, the plates pull apart or collide, and the result is the same. Molten rock wells up and thus, causing volcano eruptions.