How Do Volcanoes Erupt?

Volcanic eruptions occur when molten rock (magma), dissolved gases, and solid fragments are expelled through an opening in the Earth crust. Magma forms in the upper mantle where temperatures reach 700-1,300 °C, melting rock. Because magma is less dense than the surrounding solid rock, it rises through cracks and chambers. As it ascends, decreasing pressure allows dissolved gases (mainly water vapour, CO₂, and SO₂) to expand rapidly — like opening a fizzy drink. The violence of an eruption depends largely on the magma viscosity: runny basaltic magma (as in Hawaii) produces gentle lava flows, while thick, silica-rich magma (as at Mount St. Helens) traps gas and can explode catastrophically.