volcanology Volcanology (also spelled vulcanology) is the study of volcanoes, lava, magma and related geological, geophysical and geochemical phenomena (volcanism). The term ''volcanology'' is derived from the Latin word '' vulcan''. Vulcan was the an ...

, an explosive eruption is a

volcanic eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are oft ...

of the most violent type. A notable example is the

1980 eruption of Mount St. Helens On March 27, 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. A series of phreatic blasts occurred from the summit and escalated until a major explosive er ...

. Such eruptions result when sufficient gas has dissolved under pressure within a

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

such that expelled lava violently froths into

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

when pressure is suddenly lowered at the vent. Sometimes a

lava plug A volcanic plug, also called a volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if rising volatile-charged mag ...

will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can expel as much as per second of rocks, dust, gas and pyroclastic material, averaged over the duration of eruption, that travels at several hundred meters per second as high as into the atmosphere. This cloud may subsequently collapse, creating a fast-moving

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

of hot volcanic matter.

Stages of an explosive eruption

An explosive eruption begins with some form of blockage in the crater of a volcano that prevents the release of gases trapped in highly

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

magma. The pressure of the magma builds until the blockage is blasted out in an explosive eruption through the weakest point in the cone, usually the crater. (However, in the case of the eruption of

Mount St. Helens Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...

, the pressure was released on the side of the volcano, rather than the crater.) The sudden release of pressure causes the gases in the magma to suddenly froth and create

and

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...

, which is then ejected through the

volcanic vent A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

to create the

eruption column An eruption column or eruption plume is a cloud of super-heated Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or Plume (fluid dynamics), ...

commonly associated with explosive eruptions. The size and duration of the column depend on the volume of magma released and how much pressure it was under.

Types of explosive eruptions

Vulcanian eruption A Vulcanian eruption is a type of volcanic eruption characterized by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. They usually commence with phreatomagmatic eruptions which can be extremely noisy due t ...

Peléan eruption Peléan eruptions are a type of volcanic eruption. They can occur when viscous magma, typically of rhyolitic or andesitic type, is involved, and share some similarities with Vulcanian eruptions. The most important characteristic of a Peléan erupt ...

# Plinian eruption # Phreatic eruption #

Phreatomagmatic eruption Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions cont ...

Surtseyan eruption A Surtseyan eruption is an explosive style of volcanic eruption that takes place in shallow seas or lakes when rapidly rising and fragmenting hot magma interacts explosively with water and with water-steam-tephra slurries. The eruption style is ...

* Consequences: **

Eruption column An eruption column or eruption plume is a cloud of super-heated Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or Plume (fluid dynamics), ...

Pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

Pyroclastic fall A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic air fall deposits are a result of: # Ballistic transport of ejecta such as volcanic blocks, vol ...

Pyroclastic surge A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock, which makes ...

Pyroclastic flows

Pyroclastic flows A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

occur toward the end of explosive eruptions, as volcanic gases are depleted and the gas pressure that supports the eruption column declines. When the pressure falls, the eruption column begins to collapse in on itself, and ash and rock fall back to the ground and flow down the slopes of the volcano. These flows can travel at up to per hour and reach temperatures of . The high temperatures can burn flammable materials in the flow's path, including wood, vegetation, and buildings. Alternately, when an eruption has contact with snow, crater lakes, or wet soil in large amounts, water mixing into the flow can create

lahars A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extremel ...

, which pose significant known risks worldwide.

Supervolcanoes

The eruptions of supervolcanoes are the rarest of volcanic eruptions but also the most destructive. The timescale between these eruptions is generally marked by hundreds of thousands of years. This type of eruption generally causes destruction on a continental scale, and can also result in the lowering of temperatures worldwide.Oppenheimer, C. (2011): ''Eruptions that shook the world.'' Cambridge University Press.

References

External links