Volcanoes

Volcanoes are openings in a planet’s crust through which magma, volcanic ash, and gases escape from a magma chamber beneath the surface. On Earth they primarily form at tectonic plate boundaries and over geological hot spots, occurring most visibly at subduction zones, rift zones, and oceanic hot spots. Eruptions are driven by pressure from accumulating magma, gas exsolution, and changes in magma viscosity, producing a range of surface expressions from lava flows to explosive ash plumes.

Volcanoes come in several forms. Shield volcanoes have broad, gently sloping profiles built by low-viscosity magma that erupts lava flows. Stratovolcanoes, or composite cones, are steeper and typically erupt alternating lava, ash, and pumice with powerful explosions. Cinder cones are smaller piles of tephra formed by ballistic ejecta around vents. Lava domes are built from viscous magma that traps eruptions and can bulge near vents. Some volcanic features, such as fissure eruptions, produce linear lava flows along cracks rather than centralized cones.

Volcanic activity yields lava flows, tephra such as ash and pumice, volcanic bombs, and volcanic gases including water vapor, carbon dioxide, and sulfur dioxide. Explosive eruptions can generate pyroclastic flows, ash clouds that disrupt aviation, lahars (volcanic mudflows), and, in coastal areas, tsunamis. Hazards extend from immediate blast zones to far-reaching ash dispersal affecting air travel and climate.

Distribution mirrors plate tectonics: many active volcanoes occur around the Pacific Ring of Fire, with others at mid-ocean ridges, island arcs, and hot spots. Monitoring relies on seismology, ground deformation measurements, gas sampling, and satellite observations to forecast eruptions and inform evacuations. Volcanoes influence landscapes, climate, and ecosystems, and provide geothermal energy and fertile soils, while posing significant risks to nearby populations.