Volcanoes are formed by tectonic plates colliding with each other. The heavier plate is usually forced down below the lighter one, where part of it is melted by the heat of the crash. The melting plate forms magma or molten rock that collects below the surface of the Earth in magma chambers. As the amount of magma increases, the pressure inside the chamber rises. This creates a mountain, or volcano. The volcano has a cone, from which gases and lava may trickle out. When the pressure gets too high, the whole chamber explodes, ejecting the magma. This is a volcanic eruption.