When the mantle, that is the solid body of rock lying between the molten iron core and the surface crust, melts, the molten rock moves higher up on the surface of the Earth and results in volcano eruption. Extreme high temperature and pressure causes the rocks in the mantle to melt, forming magma.
When the volcanoes erupt, the magma rises via the denser rocks on to the surface of the Earth, along with several gases. The magma that reaches the Earth's surface is called lava.