The eruption of Mount Pinatubo in the Philippines in 1991 spewed almost 20 million tons of sulfur dioxide into the atmosphere, causing worldwide temperatures to drop half a degree on average. Arlan Naeg/AFP/Getty Images hide caption

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Mount Etna erupted on March 28, 1983. Lava flows destroyed homes and tourist destinations causing millions of dollars' worth of damage. Teams scrambled to divert the massive flow. Courtesy of John Lockwood hide caption

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Security personnel guard the entrance of a road leading to a trail on Mount Ontake in Nagano prefecture Tuesday, three days after the volcano erupted in central Japan. Jiji Press/AFP/Getty Images hide caption

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A photo taken from a Kyodo News helicopter shows Japan's Self-Defense Forces personnel rescuing climbers who were in critical condition near the top of Mt. Ontake Monday, before rescue operations were suspended. Kyodo /Landov hide caption

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Shinichi Kuramoto of the Center for Deep Earth Exploration in Japan displays a replica of earthquake fault rock. TOSHIFUMI KITAMURA/AFP/Getty Images hide caption

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