A plug dome volcano, also known as a lava dome or volcanic dome, is a roughly circular mound-shaped mountain formed from the eruption of viscous lava. Unlike the explosive eruptions of stratovolcanoes or the expansive lava flows of shield volcanoes, plug dome volcanoes are characterized by their thick lava, which cannot travel far from the vent, causing it to pile up around the vent and solidify.
How plug dome volcanoes form
The formation process of a plug dome volcano is incremental. It begins with 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. Due to this high viscosity, gases that are trapped in the magma don’t easily escape, leading to a buildup of pressure.
When this magma finally reaches the surface, it extrudes slowly, typically in the form of thick lobes or spines, because it’s too viscous to flow freely like the lava of other volcano types. As it piles up, the exterior of the lava cools down rapidly and solidifies, while the interior remains hot and malleable. Over time, this results in the creation of a steep-sided, rounded dome. This dome can grow over time with additional lava extrusions.
Plug dome volcano pyroclastic flows
While plug dome volcanoes are not known for explosive eruptions like their stratovolcano counterparts, they are not without hazards. As the dome grows and becomes unstable, parts of it can collapse, leading to pyroclastic flows.
A pyroclastic 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 most lethal volcanic hazards.
Additionally, the growth of the dome can lead to explosive eruptions if the built-up pressure becomes too great. The release of this pressure can blast apart the solidified lava, sending rock fragments and ash into the air.
The world’s largest plug dome volcano
Notable examples of plug dome volcanoes can be found worldwide. In the United States, Lassen Peak in California is a classic example. Lassen Peak is the world’s largest dome volcano with a height of 10,457 feet (3,187 meters).
Lassen Peak originated as a volcanic vent on the northern side of Brokeoff Volcano. Photo: Amanda Sweeney, USGS, public domain.
Pacific ring of fire
Lassen Peak is located in the Pacific Ring of Fire in in northeastern California and is part of Lassen Volcanic National Park. Lassen Peak is part of the Cascade Range, a volcanic arc that stretches from northern California to southern British Columbia in Canada. This arc results 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.
Lassen Peak was formed from an eruption from a vent from Brokeoff Volcano’s northern flank about 27,000 years ago. The volcano lay dormant until the eruptions that started on May 30, 1914.
The May 22, 1915 eruption of Lassen Peak 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 in Eureka, 150 miles away as volcanic debris and gases ascended 30,000 feet up into the atmosphere.
Up until the 1980 eruption of Mount Saint Helens, Lassen Peak’s 1914-1917 eruptions were the only recorded volcanic eruptions in the 20th century in the 48 lower states.
