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Involcanology, anexplosive eruption is avolcanic eruption of the most violent type. A notable example is the1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within aviscousmagma such that expelledlava violently froths intovolcanic ash when pressure is suddenly lowered at thevent. Sometimes alava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can expel as much as 1,000 kg (2,200 lb) per second[1] of rocks, dust,gas andpyroclastic material, averaged over the duration of eruption, that travels at several hundred meters per second as high as 20 km (12 mi) into the atmosphere. Thiscloud may subsequently collapse, creating a fast-movingpyroclastic flow of hot volcanic matter.
Viscous magmas cool beneath the surface before they erupt. As they do this, bubbles exsolve from the magma. Because the magma is viscous, the bubbles remain trapped in the magma.[2] As the magma nears the surface, the bubbles and thus the magma increase in volume. 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 ofMount St. Helens, the pressure was released on the side of the volcano, rather than the crater.[3]). The release of pressure causes more gas to exsolve, doing so explosively. The gas may expand at hundreds of metres per second, expanding upward and outward. As the eruption progresses, a chain reaction causes the magma to be ejected at higher and higher speeds.[2]
The violently expanding gas disperses and breaks up magma, forming anmixture of gas and magma calledvolcanic ash. The cooling of the gas in the ash as it expands chills the magma fragments, often forming tiny glass shards recognisable as portions of the walls of former liquid bubbles. In more fluid magmas the bubble walls may have time to reform into spherical liquid droplets. The final state of the emulsions depends strongly on the ratio of liquid to gas. Gas-poor magmas end up cooling into rocks with small cavities, becomingvesicular lava. Gas-rich magmas cool to form rocks with cavities that nearly touch, with an average density less than that of water, formingpumice. Meanwhile, other material can be accelerated with the gas, becoming volcanic bombs. These can travel with so much energy that large ones can create craters when they hit the ground.[2]
When an emulsion of volcanic gas and magma falls back to the ground, it can create a density current called a pyroclastic flow. The emulsion is somewhat fluidised by the gas, allowing it to spread. These can often climb over obstacles, and devastate human life.[2] Earthly pyroclastic flows can travel at up to 80 km (50 mi) per hour and reach temperatures of 200 to 700 °C (392 to 1,292 °F). 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 createlahars,[4] which pose significantknown risks worldwide.
An explosive eruption is usually triggered by exsolution ofvolatiles but there are other ways to create an explosive eruption.
Aphreatic eruption can occur when hot water under pressure is depressurised. Depressurisation reduces the boiling point of the water, so when depressurised the water suddenly boils.[5] Or it may happen when groundwater is suddenly heated, flashing to steam suddenly.[6] When the water turns into steam, it expands at supersonic speeds, up to 1,700 times its original volume. This can be enough to shatter solid rock, and hurl rock fragments hundreds of metres.[7]
Aphreatomagmatic eruption contains magmatic material, in contrast to a phreatic eruption which does not.[8]
One mechanism for explosivecryovolcanism is cryomagma making contact withclathrate hydrates. Clathrate hydrates, if exposed to warm temperatures, readily decompose. A 1982 article pointed out the possibility that the production of pressurised gas upon destabilisation of clathrate hydrates making contact with warm rising magma could produce an explosion that breaks through the surface, resulting in explosive cryovolcanism.[9]
If a fracture reaches the surface of an icy body and the column of rising water is exposed to the near-vacuum of the surface of most icy bodies, it will immediately start to boil, because its vapor pressure is much more than the ambient pressure. Not only that, but any volatiles in the water will exsolve. The combination of these processes will release droplets and vapor, which can rise up the fracture, creating a plume. This is thought to be partially responsible forEnceladus's ice plumes.[9]