A volcanic eruption is the discharge of molten rock (lava), fragmented material (tephra), and gases from a vent or fissure at Earth’s surface, powered by the ascent of Magma and the exsolution and expansion of dissolved volatiles. According to the U.S. Geological Survey (USGS), magma composition and gas content govern whether eruptions are comparatively gentle or violently explosive, with higher-silica, gas-rich magmas tending toward explosive behavior and lower-silica magmas favoring effusive outpourings. USGS; Britannica.

Tectonic settings and global distribution

• –Most active volcanoes cluster along convergent plate margins and arcs surrounding the Pacific Ring of Fire, as well as at rift zones and intraplate hotspots. The Smithsonian’s Global Volcanism Program (GVP) catalogs roughly 1,300 volcanoes with Holocene activity and notes that typically 40–50 eruptions continue at any given time. Smithsonian GVP; Smithsonian GVP.
• –Volcanism also occurs away from plate boundaries at hotspots such as Hawai‘i and Iceland. Britannica.

Styles of eruption

• –Effusive versus explosive: Effusive eruptions produce flowing lava, commonly basaltic and low in viscosity; explosive eruptions fragment magma into ash and larger clasts driven by rapid gas expansion. Geological Society of London; USGS.
• –Named styles: USGS describes Hawaiian (lava fountains/flows), Strombolian (discrete bursts), Vulcanian (short ash-laden blasts), Pelean (dome-collapse and associated Pyroclastic flow), and Plinian (sustained, stratosphere-reaching ash columns) styles, alongside phreatic (steam-driven) and phreatomagmatic (magma–water interaction) eruptions. USGS; USGS glossary.

Erupted materials

• –Lava: Composition ranges from basalt to rhyolite; viscosity increases with silica content, affecting flow morphology and eruption behavior. USGS.
• –Tephra and ash: Fragmental products of explosive eruptions that can be dispersed regionally to globally. Britannica.
• –Gases: Dominated by H₂O, CO₂, and SO₂ with trace H₂S and halogen acids (HF, HCl, HBr). CO₂ can accumulate in low-lying areas to lethal levels; SO₂ forms vog and, when injected into the stratosphere during major eruptions, converts to sulfate aerosols that cool the surface. USGS; USGS HVO.

Hazards

• –Pyroclastic density currents (PDCs): Ground-hugging mixtures of hot gas and particles that can exceed tens to hundreds of km/h and devastate everything in their path; historic examples include Mont Pelée (1902). USGS; Britannica.
• –Ashfall and aviation: Volcanic ash abrades aircraft and can melt onto turbine components, causing engine failure; NOAA supports Volcanic Ash Advisory Centers and modeling (e.g., HYSPLIT) to forecast plume dispersion. NOAA NESDIS; NOAA ARL.
• –Lahars: Volcanic mudflows formed by mobilized ash and debris with water (from snow/ice melt, rain, or crater lakes), capable of traveling many tens of kilometers along valleys; historically among the deadliest hazards. USGS fact sheet; USGS CVO. See also Lahar.
• –Lava flows: Typically slower-moving but destructive to infrastructure; hazard depends on effusion rate, viscosity, and topography. USGS.
• –Volcanic gases and health: SO₂ and fine aerosol (vog) aggravate respiratory conditions; ash can irritate eyes/skin and contaminate water and food. USGS; WHO.
• –Tsunamis: Though most tsunamis are earthquake-generated, eruptions can trigger tsunamis via caldera collapse, explosions, landslides, or atmospheric pressure waves, as documented during the 15 January 2022 Hunga Tonga–Hunga Ha'apai event. NOAA NCEI; NOAA PMEL. See also Tsunami.

Magnitude and atmospheric effects

• –Volcanic Explosivity Index (VEI): A semi-quantitative 0–8 scale comparing explosive eruptions using erupted tephra volume, column height, and qualitative descriptors; examples include VEI 5 for Mount St. Helens (1980) and VEI 6 for Pinatubo (1991). USGS; USGS. See also Volcanic Explosivity Index.
• –Climate forcing: NASA analyses show that Pinatubo’s 1991 eruption injected ~20 Mt SO₂ into the stratosphere, increasing aerosols and contributing to a global surface cooling on the order of ~0.5 °C in 1992. NASA Earth Observatory; NASA SAGE.

Monitoring and early warning

• –Observatories track unrest using seismic networks, deformation (GPS, tiltmeters, InSAR), gas measurements, infrasound, thermal/satellite remote sensing, and visual observations, issuing alerts and forecasts to emergency managers and the public. USGS VHP; USGS HVO monitoring; USGS; USGS.

Notable eruptions (illustrative)

• –1815 Tambora (Indonesia): VEI 7; among the largest historic eruptions. USGS.
• –1883 Krakatau (Indonesia): Explosive eruption and caldera collapse generated deadly tsunamis recorded across ocean basins. NOAA NCEI (context for tsunami mechanisms); NOAA PMEL.
• –1912 Novarupta–Katmai (Alaska, USA): Largest eruption of the 20th century by volume, producing the Valley of Ten Thousand Smokes. USGS; USGS Professional Paper 1791.
• –1980 Mount St. Helens (USA): VEI 5; lateral blast, PDCs, ashfall; a benchmark in modern hazard mitigation. USGS; USGS.
• –1991 Pinatubo (Philippines): VEI 6; substantial stratospheric aerosol loading and short-term global cooling. NASA Earth Observatory.
• –2010 Eyjafjallajökull (Iceland): Volcanic ash disrupted trans-Atlantic and European aviation, highlighting airborne ash risks. Britannica.
• –2022 Hunga Tonga–Hunga Ha'apai (Tonga): Extraordinary plume height, atmospheric shock waves, and tsunamis recorded globally; assessed at VEI 5 with unusual source mechanisms under study. NOAA NCEI; USGS.