When a volcano makes the news for erupting, you’ll often hear mention of other kinds of volcanoes that are dormant or extinct. But what do those terms mean, exactly, and how do scientists figure that out?
First, a little bit about how volcanoes work: Magma collects in reservoirs beneath the earth’s surface, and as it accumulates, pressure in the chamber increases; if it gets high enough, the rocks over it will break, and an eruption will ensue. The silica content of the magma determines what kind of volcano you’ll have. Low silica magma makes shield volcanoes, like Kīlauea on Hawaii’s big island, and cinder cones, like Mexico’s Paricutín, which have lava that flows easily, like molasses. High silica magma creates stratovolcanoes, like Mount St. Helens in Washington, and calderas like the one under Yellowstone National Park, which have lava that is more viscous and flows like taffy.
The definitions of what constitutes dormant and extinct volcanoes aren’t exact, and can differ depending on the volcano or the scientist. Typically, says Jim Webster, Curator of Department of Earth and Planetary Sciences, Division of Physical Sciences at the American Museum of Natural History, scientists will look at recent history and the geologic record. If a volcano has erupted since the last Ice Age—in the last 10,000 years or so—and is still showing activity like lava and ash flows or gas emissions, it’s considered to be active. If a volcano hasn’t erupted in the last 10,000 years, but scientists think it will erupt again, it’s considered dormant. “If it has been more than 10 thousand years [since the volcano has erupted],” and the volcano is cut off from its magma supply, “it’s considered to be extinct,” Webster says.
Another indicator is seismicity—or lack thereof. “Often times the magma bodies, or chambers—the source that feeds eruptions—are three to eight miles below the surface, and you can monitor seismic behavior there,” Webster says. “If there’s a repeated concentration of seismic activity immediately below the opening, there’s probably hot, active magma down there. If that goes quiet and dormant, and its just background kind of seismic activity—shifting of the rock or weathering, how the planet’s always kind of creaking and moving—for an extended period, perhaps years, that would indicate that, if there’s magma there, it’s not moving. It’s not working toward surface, it’s not generating pressure to try to break the rocks, and perhaps the system is going extinct.”
But even a volcano that’s been deemed extinct could erupt again. “Magma bodies can cool and crystallize below the surface,” Webster says. “The magma may find a fracture and move off someplace else, and come up miles away or a long distance away.”
Because of this, scientists keep tabs on volcanoes whether they’re considered active, dormant, or extinct. “They do thermal monitoring, just to see if there’s heat,” Webster says. They also place tilt monitors on the volcano, which notify scientists if the ground starts to swell—a sign that an eruption could be imminent. (Before Mount St. Helens erupted in 1980, there was “a huge bulge visible to the eye,” Webster says.) And they also use instruments in space to monitor how things are going on the ground: “InSAR,” Webster says, “is a special type of radar that’s done from satellites. It can detect centimeter motions from space, on a land surface.”