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. 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 send rocks, dust, gas and pyroclastic material up to 20 km (12 mi) into the atmosphere at a rate of up to 100,000 tonnes per second, traveling at several hundred meters per second. This cloud may then collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.
Stages of an explosive eruption
An explosive eruption always 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 high viscosity of these forms of magma prevents the release of trapped gases. The pressure of the flowing magma builds until eventually the blockage is blasted out in an explosive eruption. The pressure from the magma and gases are released 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 signature eruption column commonly associated with explosive eruptions. The size and duration of the column depend on the volume of magma being released and how much pressure the magma was under.
Types of explosive eruptions
- Vulcanian eruption
- Peléan eruption
- Plinian eruption
- Phreatic eruption
- Phreatomagmatic eruption
Pyroclastic flows occur towards the end of an explosive eruption, as pressure begins to decline. The eruption column of ash is supported by pressure from the gases being released, and as the gases are depleted, the pressure falls and the eruption column begins to collapse. When the column collapses in on itself, ash and rock fall back down to the ground and begin to flow down the slopes of the volcano. These flows can travel at up to 80 km per hour, and reach temperatures of 200° to 700° Celsius. The high temperatures can cause the combustion of any flammable materials in its path, including wood, vegetation, and buildings. When snow and ice melt as a part of an eruption, large amounts of water mixed in with the flow can create lahars. The risk of lahars is particularly high on volcanoes such as Mount Rainier near Seattle and Tacoma, Washington.
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.
- Effusive eruption
- Volcanic explosivity index
- Skinner, Brian J. (2004). Dynamic Earth: An Introduction to Physical Geology. John Wiley & Sons. Inc. Hoboken, NJ. ISBN 978-0-471-15228-6.
- "Pyroclastic flows move fast and destroy everything in their path".
- Oppenheimer, C. (2011): Eruptions that shook the world. Cambridge University Press.ISBN 978-0-521-64112-8
Media files used on this page
After May 18th five more explosive eruptions of Mount St. Helens occurred in 1980, including this spectacular event of July 22nd. This eruption sent pumice and ash 6 to 11 miles (10-18 kilometers) into the air, and was visible in Seattle, Washington, 100 miles (160 kilometers) to the north. The view here is from the south.
A picture of Russia's Sarychev Volcano, on Matua Island in the Kuril Islands, erupting on 12 June 2009, as seen from the International Space Station (ISS). The ISS orbits the Earth at a height of between 347 and 360 km.
Original description by NASA:
"A fortuitous orbit of the International Space Station allowed the astronauts this striking view of Sarychev volcano (Russia’s Kuril Islands, northeast of Japan) in an early stage of eruption on June 12, 2009. Sarychev Peak is one of the most active volcanoes in the Kuril Island chain and is located on the northwestern end of Matua Island.
"Prior to June 12, the last explosive eruption had occurred in 1989 with eruptions in 1986, 1976, 1954 and 1946 also producing lava flows. Commercial airline flights were diverted from the region to minimize the danger of engine failures from ash intake. This detailed photograph is exciting to volcanologists because it captures several phenomena that occur during the earliest stages of an explosive volcanic eruption."The main column is one of a series of plumes that rose above Matua Island (48.1 degrees north latitude and 153.2 degrees east longitude) on June 12. The plume appears to be a combination of brown ash and white steam. The vigorously rising plume gives the steam a bubble-like appearance; the surrounding atmosphere has been shoved up by the shock wave of the eruption. The smooth white cloud on top may be water condensation that resulted from rapid rising and cooling of the air mass above the ash column, and is probably a transient feature (the eruption plume is starting to punch through). The structure also indicates that little to no shearing winds were present at the time to disrupt the plume. By contrast, a cloud of denser, gray ash -- most probably a pyroclastic flow -- appears to be hugging the ground, descending from the volcano summit. The rising eruption plume casts a shadow to the northwest of the island (bottom center). Brown ash at a lower altitude of the atmosphere spreads out above the ground at upper right. Low-level stratus clouds approach Matua Island from the east, wrapping around the lower slopes of the volcano. Only about 1.5 kilometers of the coastline of Matua Island (upper center) can be seen beneath the clouds and ash."