Tukuk-OSMagma: Rock That Has Melted Underground
A volcano is a vent where molten rock (magma, once it reaches the surface called lava) and gas escape from deep in the crust or upper mantle. But rock does not just melt anywhere. Three specific settings generate magma, and all three trace the edges of Earth's drifting tectonic plates.
- Subduction zones. Where one plate dives beneath another, water dragged down with it lowers the melting point of the mantle above, and that melts into magma. This makes the great volcanic arcs — the Andes, Japan, Cascades.
- Mid-ocean ridges. Where plates pull apart, the mantle below rises, depressurises and melts without any heat added. This quietly builds new seafloor, and most of Earth's volcanic output — largely unseen — happens here.
- Hotspots. A narrow plume of abnormally hot mantle can melt through the middle of a plate far from any edge — the chain of Hawaiian islands being the textbook case, each island a record of the plate drifting over a fixed plume.
What a Volcano Actually Gives Off
An eruption is not only lava. The same outgassing that drives the explosion also releases a mixture that shapes a volcano's danger. Water vapour and carbon dioxide make up most of the gas; sulphur dioxide is what cools into the bright gas clouds that can circle the globe and briefly cool the climate after a very large eruption. Solid debris ranges from fine ash — powdered rock and glass sharp enough to ground aircraft and collapse roofs under its weight — to lapilli and, on runny flows, smooth ropes of basalt. The 1815 eruption of Tambora and the 1991 eruption of Pinatubo both dropped enough sulphur aerosol into the stratosphere to measurably cool the following year. So a single volcano can threaten with heat, with falling rock, with gas, and even, indirectly, with weather.
Why Some Eruptions Ooze and Others Explode
The drama of an eruption is set by two things: how viscous (sticky) the magma is, and how much gas it holds. As magma rises, pressure drops and dissolved gases (mostly water vapour and CO₂) come out of solution into bubbles, exactly like a shaken soda.
- Runny, low-silica (basaltic) magma lets gas bubbles rise and slip out easily. Pressure never builds much, so you get effusive eruptions: fountains and long rivers of lava that creep rather than blast. Hawaii's Kīlauea is the model — spectacular but often survivable.
- Thick, high-silica (andesitic/rhyolitic) magma is so sticky that bubbles cannot escape; pressure mounts until it shatters the magma into froth and rock. That is an explosive eruption, throwing pumice, ash and fragments at great height and sending fast, deadly pyroclastic flows down the flanks. Subduction-zone volcanoes tend this way, and they are the ones that make history's catastrophes.
Viscosity, in short, decides whether a volcano behaves like a leaking faucet or a shaken bottle.
The Shape Betrays the Type
The pile a volcano builds records how it erupts. Broad, gently sloping shield volcanoes are built from many thin runny lava flows — they spread wide. Steep stratovolcanoes (composite cones) alternate layers of ash and sticky lava and climb tall; these are the classic explosive cones. Cinder cones are small piles of frothy fragments from a short burst, and vast calderas are collapse scars left when a huge eruption empties the magma chamber beneath and the roof caves in — Yellowstone and Santorini are calderas, not peaks.
Where Volcanoes Cluster
Because eruptions follow plate boundaries, they are not scattered randomly. The Ring of Fire, a horseshoe around the Pacific, hosts most of the world's active volcanoes and the largest subduction-zone quakes and eruptions, because so many plates are diving under it. The second great belt runs through the Mediterranean and along the East African Rift, where continents are slowly pulling apart. Knowing a volcano's setting already tells you roughly how explosive it is likely to be.
You can explore the world's volcanoes — where they sit, how tall and how active — on our volcanoes hub, see the giant collapse calderas in the calderas index, and track how live volcanism fits alongside earthquakes and other active events on the disasters tracker.