In volcanology, an explosive eruption is a volcanic eruption 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 eru ...

. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug 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 Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

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 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 viscous andesitic or rhyolitic 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, 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 volcanic ash and pumice, which is then ejected through the volcanic vent to create the eruption column 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 # Peléan eruption # Plinian eruption #

Phreatic eruption A phreatic eruption, also called a phreatic explosion, ultravulcanian eruption or steam-blast eruption, occurs when magma heats ground water or surface water. The extreme temperature of the magma (anywhere from ) causes near-instantaneous evapo ...

# Phreatomagmatic eruption ## Surtseyan eruption * Consequences: ** Eruption column ** Pyroclastic flow ** Pyroclastic fall ** Pyroclastic surge

Pyroclastic flows

Pyroclastic flows 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, which pose significant known risks worldwide.

Supervolcanoes

The eruptions of

supervolcano A supervolcano is a volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for such an eruption is greater than 1,000 cubic kilometers (240 cubic ...

es 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.

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