What causes volcanoes to erupt?

Magma forms where conditions allow rock in the upper mantle or crust to melt, most commonly where one tectonic plate subducts beneath another and releases water that lowers the melting point of surrounding rock, where plates pull apart and reduced pressure permits melting, or over hotspots where unusually hot mantle material rises. Because molten rock is less dense than the solid rock enclosing it, it works upward, collecting in magma chambers. Dissolved gases, principally water vapor, carbon dioxide, and sulfur dioxide, stay in solution under pressure and come out as bubbles as the magma rises, exactly as a carbonated drink fizzes when opened. That expansion is what drives an eruption.

Viscosity determines the character of the eruption more than anything else. Magma low in silica is runny, lets gas escape easily, and produces the flowing lava and relatively gentle eruptions characteristic of Hawaiian volcanoes. Magma high in silica is thick and sticky, traps gas until pressure becomes extreme, and produces violently explosive eruptions that shatter rock into ash and generate pyroclastic flows, the fast-moving avalanches of superheated gas and debris that killed the residents of Pompeii and are the deadliest volcanic hazard. Mount St. Helens in 1980 and Pinatubo in 1991 are the well-documented modern examples of this type.

Volcanoes are monitored rather than predicted precisely. Warning signs include earthquake swarms as magma forces its way upward, ground deformation measured by GPS and satellite radar, changes in gas emissions, and thermal changes, and these have enabled successful evacuations including at Pinatubo. Effects reach far beyond the immediate area: the 1991 Pinatubo eruption injected enough sulfate aerosol into the stratosphere to lower global average temperatures measurably for about two years, and the 1815 Tambora eruption produced the year without a summer in 1816. Volcanic ash is also a serious aviation hazard because it melts in jet engines and can cause flameout, which is why eruptions close airspace across large regions.