Many active volcanoes have long dormant periods. Mount St. Helens in Washington, for example, erupted between 1800 and 1857, then went quiet before dramatically blowing its top in 1980.

Active vs. dormant volcanoes. Obviously, visible lava spewing from a volcano’s caldera, like Hawai’i’s Mount Kīlauea or Grimsvötn in Iceland, indicates that a volcano is active. However ...

Although the dormant Uturuncu hasn’t erupted in 250,000 years, the volcano continually burps up gasses and regularly rumbles with earthquakes—now scientists know why.