Eruptive activity on Etna volcano where lava flows down towards the Move valley on March 27, 2017. Volcanoes erupt when molten rock (magma) rises to the surface. Magma forms when the Earth’s mantle melts under the extreme conditions caused by the planet’s tectonic plates pulling apart or being pushed against each other. As magma rises to the surface, bubbles of gas form inside it, creating pressure that forces an eruption, which flows down the volcano as lava.