What are the hazards from volcanoes?

Volcanoes produce a variety of hazards, depending on the chemical composition and gas content of the lava (as well as on other factors):

Lava

Lava (molten rock) can erupt as fire fountains or lava flows (when it is runny) or as steep-sided domes (when it is viscous). Lava may destroy buildings and infrastructure, but it moves relatively slowly so is rarely a direct threat to people.

a lava flow
Lava flow at Kilauea volcano in Hawaii, January 2003. United States Geological Survey image.
Pyroclastic flows

Pyroclastic flows are hot avalanches of rock, ash, and gas that travel down volcano slopes at high speeds. They may be very dangerous close to a volcano.

a pyroclastic flow
Pyroclastic flow at Mount St. Helens, Washington, August 7, 1980. United States Geological Survey image by Peter Lipman.
Phreatic explosions

Phreatic explosions are explosions caused by the interaction of water with hot rock or magma (lava).

a phreatic flow
Steam blast (phreatic) eruption from the summit crater of Mount St. Helens, Washington, on April 6, 1980. United States Geological Survey image by James Moore.
Lahars

Lahars are hot or cold mixtures of water and volcanic debris that form when volcanic materials interact with water, ice, snow, or loose wet sediments. Lahars are most dangerous close to a volcano, but large lahars may rapidly travel many tens of kilometres from a volcano, along river valleys, so they can pose a threat to people and infrastructure far beyond the volcano’s slopes.

a large boulder
Large boulder carried downstream by lahars from the May 18, 1980 Mount St. Helens eruption. United States Geological Survey image by Lyn Topinka.
Jökulhlaups

Jökulhlaups are large abrupt floods of water from glacial lakes or from beneath glaciers. They may be triggered when a volcano erupts under a glacier.

Landslides

Landslides and other collapses of large parts of a volcano may occur with or without accompanying eruptions, and may transform into lahars if significant water is available. Because many volcanoes are steep and unstable, landslides frequently pose a hazard even when a volcano is dormant (not erupting).

a deposit of avalanche debris
Debris avalanche deposit from the May 18, 1980 eruption of Mount St. Helens, Washington. United States Geological Survey image by Lyn Topinka.
Volcanic earthquakes

Volcanic earthquakes are caused by the movement of magma or fluids underground, and are often too small to detect without instruments. Because significant seismic activity precedes most eruptions, especially when a volcano has not erupted for hundreds or thousands of years, volcanic earthquakes are a valuable tool for monitoring volcanoes and forecasting eruptions.

a seismic recording
Seismic record showing a swarm of volcano-tectonic earthquakes at Little Sitkin volcano, Alaska, August 30, 2012. United States Geological Survey image by Christina Neal.
Tsunamis

Tsunamis are water waves that can result from submarine eruptions or from volcanic landslides into large bodies of water, although submarine earthquakes are a more frequent cause of tsunamis.

Volcanic gases

Volcanic gases bubble out of lava or escape through soil or vents in the ground. The most common volcanic gases are water vapour, carbon dioxide, sulphur dioxide, hydrogen, hydrogen sulphide, and carbon monoxide. Some of these gases are irritating or poisonous, or cause breathing problems, and the release of sulphur dioxide may cause acid rain to form. Over long time periods, volcanic gases may affect climate.

a gas plume
Gas plume at Kilauea volcano, Hawaii, June 2013.
Tephra

Tephra consists of rock fragments ejected from a volcano. Volcanic ash is tephra smaller than 2 mm (and may be as small as thousandths of a millimetre), while larger fragments are called lapilli or bombs and blocks. Blocks and bombs fall to earth within a few kilometres of a volcano, but ash may be ejected high into the atmosphere and carried hundreds or thousands of kilometres downwind

an example of tephra
Ash erupted from Veniaminof volcano, Alaska, August 2013. 500 μm equals 0.5 mm. United States Geological Survey/ Alaska Volcano Observatory image by Kristi Wallace.
an ash plume
Ash plume at Redoubt volcano in Alaska, March 2009. United States Geological Survey/ Alaska Volcano Observatory image by Kristi Wallace.
some stratovolcanic hazards
Volcanic hazards from a stratovolcano (a large, long-lived volcano like Mount Garibaldi, British Columbia, or Mount St. Helens, Washington).
some cinder cone hazards
Volcanic hazards from a cinder cone (a relatively small and short-lived volcano like Eve cone or Tseax cone, both of which are located in northern British Columbia).