Now that Iceland's Eyjafjallajokull has done its worst, what other dangerous volcanoes should we watch out for?
Location: Central Java, Indonesia
Last major eruption: An earthquake associated with magmatic activity killed more than 5,000 people in May 2006, and an ash eruption in 2007 reached an altitude of nearly 4 miles.
Reasons to worry: Merapi is capable of producing a Plinian eruption (spewing at least 1 cubic kilometer of magma and producing 20 to 50 kilometer-high ash columns) every 1,000 years or so. The last one occurred around 1006 A.D. -- covering the island of Java in a layer of ash and possibly contributing to the demise of the Mataram Hindu Kingdom, which had dominated the area for three centuries.
Merapi is known for its large lava domes (mounds of cooling lava), pyroclastic flows (fast-moving clouds of hot gas and ash), and lahars (rivers of hot mud, rock, and lava). Pyroclastic flows can be particularly dangerous when careening down a mountain's steep sides, like those of Merapi: A large flow in 1930 killed some 1,400 people, and another in 1994 killed 43. It is also one of the most active volcanoes in Indonesia, with more than 25 eruptions in the 20th century alone. Despite the danger, the rapidly expanding metropolis of Yogyakarta lies only 20 miles to the south, putting at least half a million people at risk in the case of a massive eruption.
The trends certainly don't look promising. Geologists think that small eruptions began in the area some 400,000 years ago, but became much more violent 10,000 years ago, particularly in the last 500 years. Now, there are large eruptions every 10 to 15 years -- enough activity to make it the subject of constant monitoring by experts at the International Association of Volcanology and Chemistry of the Earth's Interior.
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Location: Goma, Democratic Republic of the Congo
Last major eruption: An eruption in 2002 produced fast-moving lava flows and is still going today.
Reasons to worry: Nyiragongo, like Merapi, has the characteristic steep sides of a stratovolcano. Whereas typical lava flows rarely kill people due to magma's slow movement, a combination of Nyiragongo's steep mountain sides and unusually low-silica-content magma makes its lava flows uniquely fast-moving. Nyiragongo is also near a major population center, the city of Goma, which is home to a quarter-million people.
When an eruption in 1977 drained the crater of its lava lake, Nyiragongo flooded 20 square kilometers in an hour and killed around 70 people. Lava lake activity resumed once again in 1994 after a fresh eruption. A 2002 eruption sent lava streaming out of fissures on the southern slopes, just north of Goma. The 6-foot-deep river of lava flowed directly through the center of town, destroying 40 percent of its buildings, while killing dozens and displacing half a million people. The volcano is one of the most active in Africa -- Nyiragongo and its sister volcano Nyamuragira are responsible for two-fifths of the African continent's volcano eruptions.
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Location: Kamchatka Peninsula, Russia
Last major eruption: 1945, expelled a quarter cubic kilometer of magma
Reasons to worry: Although Avachinsky is not the Kamchatka Peninsula's largest or most active volcano (that would be its larger cousin, Kliuchevskoi), it is still fairly active, having erupted 16 times since 1737. But, like the other volcanoes on this list, what is particularly worrisome is its proximity to the peninsula's largest city, Petropavlovsk, with a population of 200,000. Avachinsky is known for explosive eruptions accompanied by large avalanches and mudslides. As you can see on this map, major portions of the city are vulnerable to pyroclastic flows.
Although the most recent eruption in 2001 was relatively minor, another in 1945 demonstrated the volcano's potential destructive power. That eruption was categorized as a level 4 out of 8 on the Volcanic Explosivity Index (VEI) -- of which there have only been 420 in the past 10,000 years (the Eyjafjallajokull eruption is also a level 4). Ominously, Avachinsky is also classified as a "somma" volcano because its structure -- a caldera with a new cone pushing through its basin -- resembles that of Italy's Mount Vesuvius, whose eruption in 79 A.D. famously destroyed the Roman town of Pompeii.
Photo by Flickr user Robnunn. Used under creative commons license.
Location: Western highlands of Guatemala
Last major eruption: 1902, around 5,000 dead
Reasons to worry: Santa María's last big blast was one of the four largest volcanic eruptions in the 20th century, registering a VEI magnitude of 6 and leaving a 700-by-1,000 meter crater behind. Approximately 5,000 people in the nearby town of Quetzaltenango died immediately, while many others were later infected with malaria. Although scientists remain unsure of what caused the malaria outbreak, one hypothesis is that the large quantities of ash produced by the eruption killed many birds in the area, which in turn allowed Quetzaltenango's mosquito population to flourish. After a 20-year period of dormancy, the volcano began to erupt once more. These subsequent eruptions, which were smaller, ultimately formed what Guatemalans named Santiaguito, the lava dome that formed around the volcano.
Although Santa María's recent activity doesn't compete with 1902, the volcano remains continuously active and still poses various threats to those living in Quetzaltenango. One such threat is the lahars (a Javanese word for volcanic mudslides) that commonly occur during Guatemala's rainy season. The town of El Palmar, for example, which is located just 6 miles from Santiaguito, was nearly destroyed by lahars during the late 1990s.
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Location: Batangas province, Luzon island, Philippines
Last major eruption: Oct. 3, 1977, around 100 dead
Reasons to worry: Of the many volcanoes that stretch across the western side of Luzon island, Taal, measuring 311 meters, is surely the most infamous and active. Since 1572 there have been 33 recorded eruptions at Taal, including a massive one in 1754 that, according to the records of Friar Buencochillo, a Spanish Catholic priest who was stationed in the Philippines at the time of the explosion, lasted almost half a year. More recent eruptions occurred in 1911, when more than 1,000 people were killed, and between 1965 and 1977, during which time volcanic base surges and pyroclastic flows destroyed nearby villages and killed some 100 villagers.
Because of Taal's proximity to many populated areas and its active status, the volcano is closely monitored by the Philippine Institute of Volcanology and Seismology from a nearby observatory. This monitoring imperative was made all the more urgent when geologists detected nine volcanic quakes at Taal between June 13 and July 19, 2009. As a result of this increased seismic activity as well as recent increases in sulfur dioxide gas output, the Filipino government has designated Taal a "Level 1 Alert" volcano. Although this warning doesn't imply any knowledge of when the next eruption will take place, it does prohibit people from traveling anywhere within a 6-kilometer radius of the volcano.
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WILD CARD: THE YELLOWSTONE SUPERVOLCANO
Location: Yellowstone National Park, Wyoming, United States
Last major eruption: 638,000 B.C.
Reasons to worry: Huckleberry Ridge Tuff, Lava Creek Tuff, and Mesa Falls Tuff are among the many attractions for visitors to Yellowstone National Park. These rock formations -- produced by ash ejected by volcanic explosions -- were the products of major eruptions that occurred at Yellowstone approximately 2.1 million, 1.3 million, and 640,000 years ago, respectively. Fortunately, it's doubtful the Yellowstone "supervolcano" -- a term given to volcanoes whose explosions measure a magnitude 8 on the VEI -- will be erupting any time soon.
This isn't to say, however, that there hasn't been any anxiety about what the future may hold. Over the past decade, some scientists have been concerned by the movement of the Yellowstone caldera floor, which can be interpreted as a proxy for rising magma chamber pressure and in turn as an indicator of future volcanic activity. Monitoring of upward floor movement, which is caused by the progressive accumulation of magma in volcanoes' subsurface reservoirs, led to a certain level of alarm in 2007 when scientists from the University of Utah and the U.S. Geological Survey published research indicating that the Yellowstone caldera floor had experienced a total "uplift" of an unprecedented 17 centimeters between 2004 and 2007.
So what would happen if Yellowstone did erupt? Around 70,000 years ago another volcano with a VEI magnitude of 8 exploded in Lake Toba, Indonesia. The result? While scientists continue to debate the aftermath of the explosion, many hypothesize that the blast sent the world into a devastating volcanic winter that dramatically changed the course of human evolution. In other words, it will be a pretty bad day.
Photo by Flickr user Alaskan Dude. Used under creative commons license