Answer:
When rock inside the Earth becomes hot enough it melts.This molten rock, or magma, is less dense than the surrounding solid rock. Just as an object that is less dense than water will float on water, the relatively low density of the magma causes it to rise to the surface of the Earth.
If the magma contains water and dissolved gasses, when the magma reaches the surface the water and dissolved gasses will suddenly expand into steam and gas due to a desrease in pressure, causing a violent eruption. This is like shaking a coke can and then popping the tab.