It might also significantly ratchet up the death toll on the island.
"The population density around Merapi and Java in general is kind of frightening," Williams said.
The volcano is about 280 miles (450 kilometers) east of the Indonesian capital, Jakarta.
(See a map of Indonesia.)
While the volcano has a history of unpredictable eruptions, Williams said it has been studied for several decades and may help scientists better predict future eruptions.
Though Merapi is a dangerous place these days, "at least it is a dangerous place where you can learn," he said.
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