Why do volcanos erupt?

Answer 1

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 the water, the relatively low density of the magma causes it to rise to the surface of the Earth. In the most common form of volcanism, the type caused by plate subduction, the magma rises and pools into areas beneath the surface called magma chambers. If the pressure continues to increase on the magma within the chamber, it may eventually breach the surface in an eruption. The eruption ceases when the magma chamber is fully or partially emptied, and the process of filling the chamber will begin again (so the process of eruption may occur again). Fissure eruptions occur as a result of a process of convection in the asthenosphere. Heated rock rises in the mantle, melts from decompression, and fills in the gaps created in the crustal rock at divergent plate boundaries. The mid-ocean ridge, the world's longest mountain range, is created from fissure eruptions.

'Hot spot' volcanoes (as in the Hawaiian Island Chain) erupt from a constant stream of molten material from a specific point in the Earth's mantle which remains constant as the plates move slowly across it. Eruptions here will be slow and fairly steady as the source and pressure of the magma is constant.

The magma that flows from a chamber may harden to form a "plug" or a dome. This kind of structure may enlarge or be pushed upward when the magma cannot freely flow out. The pressure on these rocks is one factor that is monitored by vulcanologists when forecasting the possibility of another eruption from an existing volcano.
Volcanoes erupt because the magma underneath them builds upward, and eventually has enough force to shoot or flow out of the volcano.