On the morning of August 27, 1883, the island of Krakatoa in the Sunda Strait exploded with a force estimated at 200 megatons of TNT, the loudest natural sound in recorded history. The blast was heard 4,800 kilometres away. Pressure waves circled the Earth seven times before damping out. Some 36,000 people died, most from pyroclastic flows and tsunamis as the volcano partially collapsed into the sea. The eruption's stratospheric sulfate aerosols lowered global average temperature by about 1.2 °C the following year. Sixty-eight years earlier, the April 1815 Tambora eruption had been even larger — VEI 7 — and caused 1816 to become the year without a summer across the Northern Hemisphere: crops failed, food riots erupted in Europe, Mary Shelley wrote Frankenstein in a darkened Lake Geneva villa during what should have been a normal June.
Volcanism is the surface expression of Earth's internal heat engine — primordial heat plus radiogenic heat from uranium, thorium, and potassium decay in the mantle — driving mantle convection, which drives brief 169 plate tectonics. Three settings produce magma. Divergent boundaries (mid-ocean ridges, the East African Rift): as plates pull apart, mantle rises and decompression melting produces basaltic magma; most volcanism by volume happens here, mostly underwater. Convergent boundaries (subduction zones): water released from the descending slab lowers the mantle wedge's melting temperature, producing the andesitic and rhyolitic magmas that build the Pacific Ring of Fire — Vesuvius (Pompeii AD 79), Krakatoa 1883, Pinatubo 1991. Hotspot plumes: narrow upwellings that punch through plates and produce island chains as the plate moves (Hawaii, Yellowstone, Iceland). Magma chemistry controls eruption style. Basaltic magmas (low silica, low viscosity) erupt effusively as lava flows; andesitic and rhyolitic magmas (high silica, high viscosity, high gas) trap gas under pressure and erupt explosively. The Volcanic Explosivity Index classifies eruptions logarithmically: Mount St. Helens 1980 was VEI 5; Krakatoa 1883 and Pinatubo 1991 were VEI 6; Tambora 1815 was VEI 7. Supervolcanic eruptions (VEI 8) — Toba ~74,000 years ago, Yellowstone, Taupo — eject thousands of cubic kilometres and force global climate cooling on multi-year scales. Large Igneous Provinces — the Siberian Traps (~252 Mya), Deccan Traps (~66 Mya) — are flood-basalt provinces tied closely to mass extinctions through climate disruption.
Roughly 50-60 volcanoes erupt globally each year, with ~20 in eruption at any given moment. Hunga Tonga-Hunga Haʻapai (January 2022) produced the largest atmospheric explosion since Krakatoa and triggered globally observed pressure waves. Fagradalsfjall on Iceland's Reykjanes Peninsula reawakened in 2021 after 800 years of dormancy, and the Svartsengi magma system has displaced the town of Grindavík since 2023. Hazards split into immediate (pyroclastic flows, lahars, tephra fall, ballistic projectiles) and delayed (tsunamis from flank collapse, aviation hazards from ash clouds — the 2010 Eyjafjallajökull eruption cost European aviation $1.7 billion in six days). Astrobiological volcanism is now a subfield: Io is the most volcanically active body in the solar system, Enceladus and Europa show cryovolcanism, Venus may currently be active.