Where volcanoes form
Earth's mantle is mostly solid, but when pressure drops or water is added, part of it melts into magma. Being lighter than surrounding rock, magma rises and collects in chambers beneath the surface, erupting when pressure builds. Volcanoes form in three main settings: where plates pull apart, at mid-ocean ridges and the East African Rift; at subduction zones where one plate dives beneath another, as in Japan, Indonesia and the Andes around the Pacific Ring of Fire; and at hotspots where hot plumes rise from deep in the mantle beneath the middle of a plate, as in Hawaii and Yellowstone. Plate tectonics explains this pattern, and the same regions are prone to earthquakes.
Shapes and eruption styles
Runny, gas-poor basaltic lava flows long distances and builds broad, gentle shield volcanoes such as Hawaii's Mauna Loa. Sticky andesitic and rhyolitic magma traps gas and erupts explosively, blasting out ash and pumice and building steep stratovolcanoes of alternating lava and ash layers, such as Mount Fuji and Mount St. Helens. The deadliest hazard is the pyroclastic flow, an avalanche of hot gas and ash moving faster than 100 km/h; it buried Pompeii during Vesuvius's eruption in 79 CE. Eruption size is ranked on the Volcanic Explosivity Index (VEI), from 0 to 8, based mainly on the volume of material ejected.
Volcanoes and climate
Large explosive eruptions inject sulfur dioxide into the stratosphere, where it forms sulfate aerosols that reflect sunlight and cool the planet. After Indonesia's Tambora erupted in 1815 (VEI 7), Europe and North America endured the 1816 "Year Without a Summer." The Philippines' Pinatubo eruption in 1991 lowered global average temperature by about 0.5 °C for one to two years and temporarily thinned the ozone layer. Volcanic gases can also cause local acid rain. Volcanoes emit roughly 1% as much CO₂ as human activity each year, so they are not a significant driver of modern warming. Mount Paektu on the China–North Korea border produced one of the largest eruptions of the past 2,000 years around 946 CE and is still monitored as active.
FAQ
- What is a supervolcano?
- A volcano capable of a VEI 8 eruption, ejecting more than 1,000 cubic kilometres of material, such as Yellowstone or Toba. Such eruptions are extremely rare, occurring perhaps once every tens of thousands of years.
- Do volcanoes cause global warming?
- Not significantly. Their CO₂ output is about 1% of human emissions, and big eruptions actually cool the planet for a few years through sulfate aerosols.
Sources reviewed: 2026-10-26
References
- Newhall, C. G., & Self, S. (1982). The volcanic explosivity index (VEI). Journal of Geophysical Research, 87(C2), 1231–1238.
- Self, S., et al. (1996). The atmospheric impact of the 1991 Mount Pinatubo eruption. In Fire and Mud. PHIVOLCS & University of Washington Press.
- Oppenheimer, C., et al. (2017). Multi-proxy dating the 'Millennium Eruption' of Changbaishan to late 946 CE. Quaternary Science Reviews, 158, 164–171.
- Smithsonian Institution Global Volcanism Program. (2024). Volcanoes of the World, v.5.