In the Earth's mantle temperatures are hot enough to melt rock , which forms magma. It is much lighter than the solid rock that surrounds it. It is like a cork in a bottle of water.... it rises.

     As the magma rises, some of it collects in  magma chambers that, over time, collect and make the  volcanoe erupt. As the
     rising magma nears the Earth's surface, pressure decreases, which causes the gases in the magma to expand.
    Once magma is
     erupted, it is called lava.