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.
　　Mauna Kea
　　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.
　　Sete Cidades
　　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.
　　Mount Teide
　　Photo source: Wolfiewolf (cc)
　　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.
　　Mount Ararat
　　Photo source: martijn.munneke (cc)
　　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.
　　Solfatara
　　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.
　　Mount Hood
　　Photo source: Tony the Misfit (cc)来源：考试大
　　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).
　　Mount Rainier
　　Photo source: Amenokami (cc)
　　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.