Volcano

Volcanic features

A volcano requires a magma chamber, a conduit for magma to rise, and a vent for lava to reach the surface. Erupted material accumulates around the vent to form a volcanic edifice, often a cone or mountain. While many envision a conical peak with a summit crater, volcano shapes vary widely: some have rugged lava domes, others form massive plateaus, and vents can appear anywhere on the landform, sometimes creating smaller cones like Puu Ōō on Kīlauea.

Craters are not always at the summit and may host lakes, as seen at Lake Taupō in New Zealand. Some volcanoes have low relief and are difficult to recognize. Other types include mud volcanoes, which usually lack magmatic activity, and cryovolcanoes found on moons of Jupiter, Saturn, and Neptune. Fissure vents, common at divergent plate boundaries, are flat linear fractures that emit low-viscosity basaltic lava, forming gently sloping plateaus or shield volcanoes.

Erupted material

Volcanic eruptions expel three types of material: volcanic gases, lava, and tephra. Volcanic gases are mostly steam, carbon dioxide, and sulfur compounds (sulfur dioxide or hydrogen sulfide), with lesser amounts of hydrogen sulfide, hydrogen chloride, hydrogen fluoride, and trace gases like hydrogen, carbon monoxide, halocarbons, and volatile metal chlorides.

Lava is classified by silica content: felsic lava (over 63% silica) is highly viscous, forming domes or short flows, as at Lassen Peak in California. Such viscous magmas trap gases, leading to explosive eruptions and pyroclastic flows that can reach 850°C and incinerate everything in their path. Tephra includes solid particles of all sizes ejected into the air; fine ash can rise high into the stratosphere and travel hundreds of kilometers before falling as fallout tuff.

Volcanic eruptions

The Smithsonian Institution's Global Volcanism Program lists 10,652 confirmed Holocene eruptions from 1,214 volcanoes as of August 2026, plus 1,113 uncertain and 168 discredited eruptions. Eruption styles are magmatic, phreatomagmatic, or phreatic, with intensity measured by the volcanic explosivity index (VEI) from 0 to 8. Hawaiian eruptions involve low-viscosity mafic lava with little gas, producing effusive lava fountains and flows with columns under 2 km.

Strombolian eruptions have moderate viscosity and gas, yielding frequent short-lived explosions and scoria. Vulcanian eruptions feature higher viscosity and partially crystallized magma, with short explosions destroying a central dome, followed by effusive rebuilding. Peléan eruptions involve dome growth and collapse generating pyroclastic flows. Plinian eruptions produce sustained high columns whose collapse causes catastrophic pyroclastic flows, named after Pliny the Younger's account of Vesuvius in 79 AD.

Volcanoes and humans

Volcanic eruptions pose hazards including phreatic explosions, explosive high-silica eruptions, effusive low-silica flows, sector collapses, pyroclastic flows, lahars, and gas emissions. Volcanic gases reaching the stratosphere form sulfuric acid aerosols that reflect solar radiation, cooling the surface; sulfur dioxide from Huaynaputina may have caused the Russian famine of 1601–1603. These aerosols can also damage the ozone layer, and acids like hydrogen chloride and hydrogen fluoride fall as acid rain. Fluoride salts from eruptions have poisoned Icelandic livestock.

Explosive eruptions release carbon dioxide, contributing to biogeochemical cycles. Ash can damage jet aircraft by melting on turbine blades, disrupting air travel. The 1815 Tambora eruption led to the 'Year Without a Summer,' and the 1740–41 famine in northern Europe may also stem from a volcanic eruption.

Did You Know?

• The word 'volcano' comes from the Italian name Vulcano, a volcanic island in the Aeolian Islands, derived from the Latin name for Vulcan, the Roman god of fire.
• The study of volcanoes is called volcanology, sometimes spelled vulcanology.
• The Decade Volcanoes are 16 volcanoes identified by the International Association of Volcanology and Chemistry of the Earth's Interior for study due to their history of large eruptions and proximity to populated areas.