The volcano, located in central Japan, erupted at 11:53 a.m. Saturday. Twitter user @setori_ posted a photo from the volcano crater just moments before the eruption.
"Seven people were lightly injured and one person suffered serious injuries as a result of the eruption," a Nagano prefecture fire department spokesperson told Reuters.
The Japanese Meteorological Agency raised the volcano alert level for Mt. Ontake to 3 on a scale of 1 to 5.
Aerial pictures from Japanese state broadcaster NHK showed nearby buildings blasted with volcanic ash.
Other people in the area posted images of smoke and ash rising over Mt. Ontake.
Jon Passantino is the Los Angeles bureau chief for BuzzFeed News.
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