Buzz
When a volcano grabs headlines for its eruption, other types like dormant or extinct volcanoes often come up. But what exactly do these terms mean, and how do experts distinguish them?
To understand how volcanoes operate: Magma accumulates in chambers beneath the earth’s surface. As the pressure builds, it can eventually rupture the surrounding rocks, causing an eruption. The magma's silica content dictates the type of volcano: low silica magma forms shield volcanoes, like Kīlauea in Hawaii, and cinder cones, such as Paricutín in Mexico, which produce smooth-flowing lava. High silica magma, on the other hand, results in stratovolcanoes like Mount St. Helens or calderas like Yellowstone, producing more viscous lava that moves slowly, almost like taffy.
The definitions of dormant and extinct volcanoes aren't always clear-cut and may vary depending on the scientist or volcano in question. Generally, explains Jim Webster, Curator of the Department of Earth and Planetary Sciences at the American Museum of Natural History, experts examine both recent eruptions and geological history. If a volcano has erupted within the past 10,000 years and continues to show signs of activity like lava, ash flows, or gas emissions, it is considered active. If there’s been no eruption for over 10,000 years but scientists believe an eruption is still possible, the volcano is deemed dormant. If the volcano has been inactive for over 10,000 years and is no longer connected to its magma source, it is classified as extinct, according to Webster.
Another sign to watch for is seismic activity—or the lack of it. “Magma chambers, which supply eruptions, are typically located three to eight miles beneath the surface, and we can monitor seismic activity in that region,” explains Webster. “If seismic signals keep appearing right beneath the volcano, it's likely that active, hot magma is present. However, if the seismic activity quiets down and only background shifts in the rock or weathering are observed—for years even—that might suggest that any magma present isn’t moving. It’s not building pressure to cause an eruption, possibly signaling the volcano is heading toward extinction.”
Even a volcano considered extinct might erupt again. “Magma chambers can cool and solidify underground,” Webster notes. “But the magma could eventually find a crack and travel to a different location, possibly erupting miles away from where it originally was.”
Because of this, researchers continuously monitor volcanoes, whether they are classified as active, dormant, or extinct. “They perform thermal monitoring to detect any signs of heat,” Webster says. Tilt monitors are also placed on the volcanoes to detect any swelling of the ground, which might indicate an impending eruption. (Before Mount St. Helens erupted in 1980, Webster recalls, “there was a huge bulge visible to the naked eye.”) Additionally, they use space-based tools to observe surface changes: “InSAR,” says Webster, “is a type of radar that’s conducted via satellites. It can measure shifts on the ground in centimeters.”
