Photo of a huge plume of volcanic ash on a bright sunny day.

The largest dome volcano in the world

A plug dome volcano, also known as a lava dome or volcanic dome, is a roughly circular hill-shaped mountain formed by erupting viscous lava. Unlike the explosive eruptions of stratovolcanoes or the expansive lava flows of shield volcanoes, plug-domed volcanoes are characterized by thick lava, which cannot migrate far from the vent, causing it to accumulate around the vent and solidify.

How are dome volcanoes formed?

The process of forming a plug-dome volcano is gradual. It starts with the high-viscosity magma being pushed towards the Earth’s surface. Viscosity, in this context, refers to the resistance of a fluid (magma, in this case) to flow. The higher the viscosity, the slower the magma moves. Because of this high viscosity, gases trapped in the magma do not escape easily, leading to a buildup of pressure.

When this magma finally reaches the surface, it extrudes slowly, usually in the form of thick lobes or spikes, because it is too viscous to flow freely like lava in other types of volcanoes. As it accumulates, the outer part of the lava quickly cools and solidifies, while the inner part remains hot and elastic. Over time, this creates a steep, rounded dome. This dome can grow over time as additional lava is withdrawn.

Lava flows blocked dome

While plug-domed volcanoes are not known for explosive eruptions like their stratovolcano counterparts, they are not without risk. As the dome grows and becomes unstable, parts of it can collapse, resulting in lava flows.

Lava flow from Mount St. Helens on August 7, 1980. Photograph: Peter Lipman, USGS, public domain.

A lava flow is a fast-moving, ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas. These flows are extremely hot, often exceeding 700 °C (1,300 °F), and can move at alarming speeds, making them one of the deadliest volcanic hazards.

In addition, dome growth can lead to volcanic eruptions if the built-up pressure becomes too great. Releasing this pressure can explode hardened lava, sending rock fragments and ash into the air.

The largest dam dome volcano in the world

Outstanding examples of plug-dome volcanoes can be found throughout the world. In the United States, Lassen Peak in California is a classic example. Lassen Peak is the largest domed volcano in the world It has an elevation of 10,457 feet (3,187 metres).

View across the Dome Volcano Valley on a bright sunny day with blue skies and some clouds.
View of Lassen Peak from Brokov Volcano, Lassen Volcanic National Park.
Lassen Peak arose as a volcanic vent on the north side of Brokeoff Volcano. Image: Amanda Sweeney, USGS, public domain.

Pacific Ring of Fire

Lassen Peak is located on the Pacific Ring of Fire in northeastern California and is part of Lassen Volcanic National Park. Lassen Peak is part of the Cascade Mountains, a volcanic arc that extends from northern California to southern British Columbia in Canada. This arc arises from the subduction of the Juan de Fuca plate beneath the North American plate. Lassen Peak is the southernmost active volcano in the Cascade Range.

Black and white photo of a volcanic dome with ash spewing from the summit.
A photo of Lassen Peak taken four hours before the volcano erupted in 1915. Image: USGS, public domain.

Lassen Peak was formed by a volcanic eruption from a vent on the north side of Brokov Volcano about 27,000 years ago. The volcano remained dormant until Explosions that began on May 30, 1914.

Black and white photo from 1915 showing an erupting volcano with trees in the foreground and a lake.
Lassen Peak eruption captured by photographer B.F. Loomis from Manzanita Lake in 1915. Source: NPS, public domain.

the May 22, 1915, Lassen Peak volcano erupts The 1915 eruption was the most powerful in a series of eruptions that lasted from 1914 to 1917. The 1915 eruption devastated nearby areas, creating a new crater and sending volcanic ash as far as 200 miles (320 km) to the east. Ash could be seen at Eureka, 150 miles away, where volcanic debris and gases rose 30,000 feet into the atmosphere.

A photographic postcard of Lassen Peak during an eruption from downtown Red Bluff taken in 1915.
Postcard showing the eruption of Lassen Peak on May 22, 1915, taken from 37 miles away in Red Bluff, California. Source: NPS, public domain.

Until the 1980 eruption of Mount St. Helens, the Lassen Peak eruptions of 1914-1917 were the only volcanic eruptions recorded in the 20th century in the lower 48 states.

More geographical articles about volcanoes


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