Glossary

A volcano that is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. The Smithsonian Global Volcanism Program's definition of "active" is having erupted within the last 10,000 years.

A giant volcanic crater (notionally larger than 1 km across) formed by collapse or explosion, collapse being more important among larger calderas.

A volcanic feature formed by the ejection of scoria (cinder) which accumulates around a vent. Relatively short-lived eruptions may produce cones ranging in height from a few tens of metres to in excess of 1km. Cinder cones may form above vents on larger volcanoes, or occur in isolation.

Linear zones within which one tectonic plate plunges beneath another.

Linear zones along which two tectonic plates move apart.

A volcano that is not currently active but could become restless or erupt again.

A volcano that is considered unlikely to erupt again.

Break in the surface allowing the escape of volcanic gases and heated groundwater.

A within-plate magma source driven by upwelling convection in the mantle (e.g. Hawaii and Yellowstone).

A volcanic mudflow composed of fragmented material and water, that may be triggered by torrential rain, snow melt or crater lake breaching or overspill.

Magma extruded at the surface as a fluid and able to flow away from its source.

A broad, low relief crater that is caused by a water-driven eruption or explosion caused by groundwater contact with hot lava or magma.

Collections of cinder cones and/or Maar vents associated with lava flows and pyroclastic deposits.

Formations created by geologically excreted liquids and gases. Ejected materials are often a slurry of fine solids (mud) suspended in liquids such as water (frequently acidic or salty) or hydrocarbon fluids. Sometimes the gases can ignite and flames can reach several hundred metres in height.

An eruption of pre-existing rock driven by non-volcanic water that has been vaporised to steam by the heat from ascending magma. If new magma is also expelled, the eruption is termed hydromagmatic or phreatomagmatic.

An eruption producing a buoyant cloud of ash and hot gas (eruption column) that may rise tens of kilometres into the atmosphere before spreading outwards. Column collapse may generate potentially lethal and destructive pyroclastic flows.

Highly vesicular magma normally produced during Plinian eruptions.

A ground hugging cloud of hot gas and incandescent ash characterised by high temperatures (100-800°C) and velocities (> 10's km h).

A volcano that extrudes huge quantities of low viscosity basaltic lava that gradually builds a wide mountain with a shield-like profile.

A hot spring or fumarole formed in high-temperature geothermal areas where water is in short supply. The little water that is available rises to the surface at a spot where the soil is rich in volcanic ash, clay and other fine particulates.

Tall conical mountains composed of higher viscosity lava flows and fragmental ejecta in layers.

The mild and intermittent expulsion of fragmented magma up to metres across.

Smaller scale versions of Plinian eruptions that give rise to extensive ash-fall, pyroclastic flows and eruption columns up to 20km high.

A volcano that develops beneath an icecap.

Volcanoes that form under a body of water (typically on the ocean floor at divergent plate boundaries).

The popular term for large caldera volcanoes capable of hosting explosive eruptions with the potential to cause devastation on an enormous, sometimes continental, scale. Such eruptions may cause severe cooling of global temperatures for many years afterwards (volcanic winter) because of the huge volumes of sulphur and gases emplaced in the stratosphere.

Generic term for all volcanic fragments that are explosively ejected.

An index devised to provide a relative measure of the explosiveness of volcanic eruptions. The scale is logarithmic and open-ended with values ranging from 0, for non-explosive eruptions (less than 104 cubic metres of tephra ejected) to 8, for super-eruptions that can eject 1012 cubic metres of tephra and have a cloud column height of over 25 km.

¹ http://en.wikipedia.org/wiki/Volcanic_Explosivity_Index