Volcanic activity where fresh magma AND external water are involved.

Phreatomagmatic activity means that erupting magma reacts with external water, e.g. ground water, lake water, sea water etc. In contrast, if only magma is erupted and driven only by gasses originally contained in the magma, it is called magmatic activity. If no magma itself erupts, but heated ground water drives explosions and eruptions of older material, the activity is called phreatic.

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The most explosive and largest type of volcanic eruptions. Plinian eruptions erupt more than 1 cubic kilometer of magma often within less than a few days and produce ash columns that can reach 20-50 km height.

Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall. Many thousands of people evacuate... -> See whole entry

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explosive - vulcanian eruption

Pumice is a very light, porous volcanic rock that forms during explosive eruptions. During the eruption, volcanic gases dissolved in the liquid portion of verz viscous magma expand very rapidly to create a foam or froth; the liquid part of the froth then quickly solidifies to glass around the gas bubbles. The volume of gas bubbles is usually so large that pumice is lighter than water and floats. Pumice is an important industrial mineral used to produce high-quality cement and lightweight, isolating building materials.

Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. It is commonly, but not exclusively of silicic or felsic to intermediate in composition (e.g. rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but occurrences of basaltic and other com... -> See whole entry

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explosive - obsidian - perlite - photoglossary.html - photoglossary.html

Fluid avalanche of turbulently mixed ash, lava and or rock fragments, and air, that flows down the flanks of a volcano, driven by gravity. Pyroclastic flows are usually very hot and highly destructive.

A pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. The temperature within a pyroclastic flow may be greater than 500°C, sufficient to burn wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight... -> See whole entry

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ash - base surge - debris avalanche - explosive

A type of highly viscous magma with high silica content; it is found as pumice (in airfall deposits or ignimbrites), lava or obsidian. Rhyolite is also the name given to the volcanic rock formed from rhyolitic magma.

Shield volcanoes are volcanoes that mainly erupt fluid (usually basaltic) lava flows that are able to travel over long distances and thus construct over time broad, gentle slopes. They are called shield volcanoes, because they resemble the shape of a warriors' shield.

Shield volcanoes are volcanoes that mainly erupt fluid (usually basaltic) lava flows that are able to travel over long distances and thus construct over time broad, gentle slopes. They are called shield volcanoes, because they resemble the shape of a warriors' shield.

While stratovolcanoes, the other major morphological type of volcanoes, are representative for most subduction-type volcanoes, sh... -> See whole entry

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Skylights are openings in the roof above a lava tube, from where the flowing lava flow can be seen. Usually, these holes are caused by the simple collapse of the roof of the tube.

A rare phenomenon, where a visible vortex ring of steam and gas is expelled from a volcanic vent.

Under special conditions, gas and steam expelled from a vent can form gas rings. It probably requires a particular geolometric configuration of a circular vent exit, as well as expulsion of gas in individual puffs with just the right velocity.

This phenomenon is quite rare, but has been witnessed at several volcanoes, including Stromboli and Etna. At Etna, a spectacular period lasting several mo... -> See whole entry

Strombolian eruptions are the smallest type of explosive eruptions. Strombolian eruptions consist of intermittent, generally relatively small explosions or weak pulsating fountains of fluid (usually basaltic) lava from a single vent or crater. They are called so after the type locality of Stromboli volcano (Eolian Islands, Italy), which has been in strombolian activity for probably more than 2000 years.

An individual strombolian explosion is the result of sudden release of volcanic gasses. The typical rhythmic occurrence is caused by gradual accumulation of gas bubbles beneath a weakly solidified plug at the top of the magma column at the vent surface until the gas pressure is high enough to erupt through it, ejecting with it both solid and liquid spatter from the magma. -> See whole entry

Tephrite is an type of volcanic (extrusive) rock with low silica content, similar to basalt, but containing foid minerals (e.g. nephelinite, leucite) along with plagioclase. The composition of tephrite is called tephritic.

Tephritic refers to the mineral composition of tephrite, an type of volcanic (extrusive) rock with low silica content, similar to basalt, but containing foid minerals (e.g. nephelinite, leucite) along with plagioclase.

From Latin "tumulus" = "little hill", tumuli (pl.) are uplifted sections of pahoehoe lava crust caused by pressure from still fluid lava accumulating beneath the hardened crust.

Tumuli are a characteristic feature of all pahoehoe lava flow fields, such as prominent on Kilauea volcano, Hawaii, but also many other basaltic volcanoes including Etna. -> See whole entry

Volcanic Ash Advisory Centers (VAAC) are research centres who monitor volcanic ash clouds in real time. Each time, a volcano erupts a significant ash cloud, short reports are issued and transmitted directly to air control centres. The 9 VAAC are located in London, Toulouse, Tokyo, Darwin, Anchorage, Washington, Montreal and Buenos Aireas and collectively cover most of the globe.

The Volcanic Ash Advisory Centers (VAAC) are groups of experts and government-funded bodies to montor volcanic ash clouds all over the planet in real time. As Europe experienced in April 2010, volcanic ash clouds are hazardous for aircraft and must be avoided to fly through, even if that means cancellation of flights.



As of 2010, there are 9 VAAC located in different areas and each focussing m... -> See whole entry

The ability of a liquid to flow. Basalt magma has a relatively low viscosity making it runny, whereas rhyolite magma has a high viscosity making the magma thick and sticky.

Pathways of rising magma inside vertical fissures.

Dikes are imaginable as the veins of a volcano, the pathways of rising magma. A dike is called a -usually more or less vertical- flat, sheet-like magma body that cuts unconformingly through older rocks or sediments.



Most dikes can be described as fractures into which magma intrudes or from which they might erupt. The fracture can be caused by the intrusion of pressurized magma, or vice versa, ... -> See whole entry

Volcanic tremor is a continuous seismic signal with regular or irregular sine wave appearance and low frequencies (0.5-5 Hz).

Volcanic tremor is usually in the form of harmonic tremor which has a very uniform appearance, whereas spasmodic tremor is pulsating and consists of higher frequencies with a more irregular appearance.

Tremor can be caused by different processes inside the volcano, including resonance triggered by magma flowing through cracks and vents, or continuous low-frequency earthquakes so closely spaced th... -> See whole entry

A volcaniclastic rock or loose deposit made of (older) volcanic fragments.

The science of studying volcanoes.

Volcanology comprises the study of volcanoes and volcanic phenomena and is mostly regarded as a sub-part of geology, but is interwoven with other science disciplines as well: chemistry, physics, but also sociology, history, archaeology. -> See whole entry

A vulcanian (note the different term from "volcanic") eruption is an intermediately violent type of explosive eruption, stronger than strombolian explosions but much weaker than Plinian eruptions. During vulcanian eruptions, a relatively large solid plug is ejected when magmatic gas pressure that had build up beneath it overcomes the strength of the plug. In the same way as Stromboli island for strombolian eruptions, the termvulcanian was coined after the neighboring island of Vulcano in the Eolian Islands, where such eruptions had been observed during the eruption in 1888.

Vulcanian eruptions can follow longer or shorter intervals of repose during which gasses in the magma beneath the plug slowly build up. The smallest vulcanian explosions typically produce eruption columns of approx 1-2 km height, while larger ones approaching sub-plinian size can erupt columns reaching several to more than 10 kilometers height.

Often, the eruptions are accompanied by violent gun... -> See whole entry

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Plinian eruption - strombolian eruption

An inclusion of foreign rock in an igneous rock.

Xenoliths in volcanic rocks are often pieces of the volcano's (non-volcanic) substratum picked up by, engulfed within and erupted with the rising magma. -> See whole entry