Before 1980, experts considered Mount St. Helens in Washington a dormant volcano, since its last period of volcanic activity was in the mid-1800s. Then, on a fateful day in May, the volcano erupted and left fifty-seven people and billions of dollars in damage in its wake. A similar situation occurred in southern Chile after the Chaitén erupted in 2008, following a nine-thousand-year period of dormancy. And in 2010, Iceland’s Eyjafjallajökull volcano, one of the biggest in the country, erupted for the first time in two hundred years.
Clearly, the line between dormant and active can change at any moment. The definitions of these terms vary, even among volcanologists: some say “active” means eruption within the past ten thousand years, while others define it as activity within historic time. At its most basic, a dormant volcano is one that hasn’t erupted in a considerable amount of time, but is expected to at some point. Numerous volcanoes all over the world fit into this category, though, judging by the previous examples, that classification is clearly subject to change.
Photo source: Spencer Critchley (cc)
Hawaiian for “White Mountain,” it’s the tallest of the five volcanoes located on the Big Island of Hawaii. Mauna Kea last erupted around 2460 BC, but that doesn’t mean it won’t happen again in the future, especially if multiple earthquakes happen in the area.
Photo source: Rei-artur (Wikimedia Commons)
With an elevation of 2,808 feet and a depth of 1,500 feet, this stratovolcano—the term for tall volcanoes with multiple layers of ash and solidified lava—is found on São Miguel Island in Portugal. It’s erupted twenty-two times since its inception date, estimated to have been about twenty-two thousand years ago; its last known eruption was in 1880.
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The highest point of elevation in Spain belongs to the Canary Islands’ Mount Teide, also the third-biggest volcano in the world. It’s been fairly stable since 1909, though there were reports of some seismic activity in 2003. The International Association of Volcanology and Chemistry of the Earth’s Interior deemed it one of sixteen Decade Volcanoes, which are monitored extra-closely because of their eruption history and proximity to areas with large populations.
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The last eruption of this snowy, two-peaked volcano (the highest in Turkey) occurred in 1840, when a huge earthquake also caused a landslide. In the Bible, Ararat is where Noah’s Ark landed.
Photo source: Überraschungsbilder (Wikimedia Commons)
In the south of Italy, near Naples, lies Campi Flegrei, a volcanic area that’s home to Solfatara, a volcano that last erupted in 1158. Though considered dormant, the volcano does regularly release sulfurous gases through the many fumaroles on its floor.
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In 2006, a volcanologist with the U.S. Geological Survey (USGS) named Mount Hood the fourth-most-dangerous volcano in the country. Some in the industry believe it’s the most likely of Oregon’s volcanoes to erupt; its last period of eruptive activity was 170 to 220 years ago.
Agua de Pau
Photo source: Rei-artur (Wikimedia Commons)
This volcano is also located on São Paulo Island and has been relatively dormant since 1564. Like Sete Cidades, it’s a stratovolcano, but its origins are older (somewhere between thirty thousand and forty-five thousand years ago).
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One of the most beautiful backdrops in Washington is also one of the most dangerous. USGS calls it a dormant volcano because its last eruption was back in 1894, but two of its characteristics are worrisome: many people live near it, and it contains more glacier ice than any other mountain in the forty-eight states. That’s why, like Mount Teide, it’s a Decade Volcano.
Photo source: Arenamontanus (cc)
Also known as the fifth-highest peak in California, this volcano actually has four cones that’ve been created over almost six hundred thousand years. During the past 4,500 years, Mount Shasta has erupted an average of once every six hundred years; the most recent eruption was about 224 years ago.
Photo source: blhphotography (cc)
Three volcanic cones make up the highest mountain in Africa: Mawenzi and Shira, which are both extinct (meaning it’s unlikely they’ll ever erupt again), and Kibo, which is dormant and has the tallest peak of the three. The most recent significant eruption took place 360,000 years ago, so there’s not much concern about future eruptions, but some in the industry are worried about its collapsing over time.
As we’ve seen in relatively recent history and over the past few years, volcanoes we think of as dormant can suddenly become active and cause a great deal of damage. Volcanologists can watch volcanoes closely, but there’s no way to determine exactly when eruptions will occur. The best thing we can do is educate ourselves about surrounding volcanoes and their activity histories. If you live in the United States, the USGS has a helpful map with alert levels. But bear in mind that for every major eruption you hear about, there are countless volcanoes all over the world that aren’t doing anything, or ones making such small eruptions that they have almost no effect. It’s good to be in the know, but if people who spend their lives studying volcanoes can’t pinpoint exactly when the next “big one” is going to strike, there’s no sense in the rest of us spending our lives worrying about it.