Animation of a volcanic eruption, cut away to show the internal structures. Volcanoes are vents onto the Earth's surface through which molten rock can erupt. The molten rock accumulates in a magma chamber below the visible volcano, but is prevented from reaching the surface by solid rock formed from the cooled liquid lava from the previous eruption. As more magma flows into the chamber from below, the pressure increases, until it reaches a level at which it can overcome the rock above it. This can lead to an explosive eruption that can devastate the surroundings. Tall conical volcanoes such as this are called stratovolcanoes, and have steep sides formed from cooled lava, built up in layers during previous eruptions.