Stratovolcano erupting, cutaway artwork. During a volcanic eruption, melted rock (magma, red/orange) moves up from a chamber through a conduit (pipe) and is forced out the top of the volcano as lava. Shattered rock (volcanic ash, grey) is also ejected. The enlarged section depicts crystals forming in cooling magma. The magma is cooling due a change in pressure as it migrates. When magma cools, minerals contained in it begin to crystallise, forming phenocrysts. The number of crystals in the magma determines it viscosity. Volcanoes with more viscous magma tend to erupt less frequently than those with less viscous magma, but with greater energy.