Illustrated Volcano Glossary

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magma

Volcanology
Molten rock within a volcano or underneath the earth's surface. Once magma reaches the surface it is called lava.

Related keywords (2):

explosive - lave

mud volcano

Volcanology
Sidoarjo mud volcano (East Java)
Sidoarjo mud volcano (East Java)
Mud volcanoes are no true volcanoes, but vents that erupt mud, as fine sediemtn is squeezed upwards by prezzurized water, steam and gas escaping from deeper deposits.
A mud volcano is a vent on the surface erupting mud and gas or steam, but no lava. Mud volcanoes are usually not the result of volcanic processes, but more generally related to environments where pressurized deposits at depth occur that release gas and steam, which mixes with fine-grained sediments to form mud. Temperatures are much cooler than at volcanic processes. The largest structures are 10 ... Read all

Related keywords (1):

Sidoarjo

obsidian

Volcanology
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of cr... Read all

Related keywords (3):

perlite - Pumice - photoglossary.html

phreatomagmatic

Volcanology: phreatomagmatic activity
Lava fountain at Etna during the 2001 eruption. The activity is phreatomagmatic in origin, which explains the large amount of ash involved in the fountain: the rising water on its way meets wet layers where the contact between water and magma produces violent fragmentation.
Lava fountain at Etna during the 2001 eruption. The activity is phreatomagmatic in origin, which explains the large amount of ash involved in the fountain: the rising water on its way meets wet layers where the contact between water and magma produces violent fragmentation.
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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Plinian eruption

Volcanology
Plinian eruption of Mt. St. Helens on May 18, 1980 (USGS Photograph taken on May 18, 1980, by Donald A. Swanson)
Plinian eruption of Mt. St. Helens on May 18, 1980 (USGS Photograph taken on May 18, 1980, by Donald A. Swanson)
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... Read all

Related keywords (2):

explosive - vulcanian eruption

Pumice

Volcanology: pumice stone
Our tour guide Marta posing in Lipari's pumice...
Our tour guide Marta posing in Lipari's pumice...
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... Read all

Related keywords (5):

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

pyroclastic flow

Volcanology
Pyroclastic flow travelling down the Krasak ravine at Merapi volcano on 27 May 2006.
Pyroclastic flow travelling down the Krasak ravine at Merapi volcano on 27 May 2006.
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... Read all

Related keywords (4):

ash - base surge - debris avalanche - explosive

rhyolite

Volcanology
Rhyolite from Milos island (Greece)
Rhyolite from Milos island (Greece)
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 volcano

Volcanology
Mauna Loa shield volcano (Big Island, Hawaii)
Mauna Loa shield volcano (Big Island, Hawaii)
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... Read all

Related keywords (1):

photoglossary.html

skylight

Volcanology
Skylight on Etna volcano (Italy)
Skylight on Etna volcano (Italy)
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.

Related keywords (1):

Coulée de lave

smoke ring

Volcanology: volcanic smoke ring, gas ring, steam ring
Etna's Bocca Nuova crater in 2000, emitting smoke rings
Etna's Bocca Nuova crater in 2000, emitting smoke rings
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... Read all

strombolian eruption

Volcanology
Strombolian eruptions from vents of Stromboli volcano, Eolian Islands, Italy
Strombolian eruptions from vents of Stromboli volcano, Eolian Islands, Italy
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. Read all

Related keywords (1):

vulcanian eruption

tephrite

Volcanology
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

Volcanology
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.

tremorSynonym of: volcanic tremor

Volcanology
Seismogram signal examples from volcanic earthquakes: Volcano Tectonic (VT) Low Frequency (LF), hybrid (mix of VT and LF), Very Low Frequency (VLF), and Tremor. Volcano name/date in lower left. (image: USGS)
Seismogram signal examples from volcanic earthquakes: Volcano Tectonic (VT) Low Frequency (LF), hybrid (mix of VT and LF), Very Low Frequency (VLF), and Tremor. Volcano name/date in lower left. (image: USGS)

tumulus

Volcanology
Tumulus with strange shapes, on the lava flow field of the 1614-24 eruption of Mt. Etna, Italy.
Tumulus with strange shapes, on the lava flow field of the 1614-24 eruption of Mt. Etna, Italy.
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. Read all

VAAC

Volcanology: Volcanic Ash Advisory Centre
Coverage of the globe by the 9 VAAC.
Coverage of the globe by the 9 VAAC.
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... Read all

Related keywords (1):

ash

viscosity

Volcanology
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.

Related keywords (2):

explosive - lava dome

volcanic dike

Volcanology: volcanic dike
Exposed volcanic dike on Santorini (Greece)
Exposed volcanic dike on Santorini (Greece)
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, ... Read all

volcanic tremor

Volcanology
Seismogram signal examples from volcanic earthquakes: Volcano Tectonic (VT) Low Frequency (LF), hybrid (mix of VT and LF), Very Low Frequency (VLF), and Tremor. Volcano name/date in lower left. (image: USGS)
Seismogram signal examples from volcanic earthquakes: Volcano Tectonic (VT) Low Frequency (LF), hybrid (mix of VT and LF), Very Low Frequency (VLF), and Tremor. Volcano name/date in lower left. (image: USGS)
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... Read all

volcaniclastic

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

Volcanology

Volcanology
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. Read all

vulcanian eruption

Volcanology
Eruption column from a vulcanian eruption on Krakatau
Eruption column from a vulcanian eruption on Krakatau
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 term vulcanian 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... Read all

Related keywords (2):

Plinian eruption - strombolian eruption

Xenolith

Volcanology
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. Read all

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