Why do volcanoes erupt?

Deep beneath the surface, intense heat melts rock into magma, which is less dense than the solid rock around it and therefore tends to rise. As magma ascends, dissolved gases such as water vapour and carbon dioxide come out of solution and form bubbles, much like opening a shaken bottle of fizzy drink, dramatically increasing the pressure. When that pressure overcomes the strength of the overlying rock, the magma bursts out as an eruption, emerging as lava if it reaches the surface. The style of eruption depends on the magma: runny, low-gas magma produces gentle lava flows, while thick, gas-rich magma traps pressure until it explodes violently. Most volcanoes occur at the boundaries of tectonic plates — where plates pull apart or one dives beneath another — though some, like Hawaii, sit over hotspots where plumes of hot material rise from deep in the mantle.