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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