When pasty lava began extruding from the forested summit crater of Unzen in May 1991, few were prepared for the dramatic changes that were about to take place on the volcano and in the Mizunashi River valley (foreground). As the spreading lava built a lava dome and reached the steep east side of the volcano a few days later, huge blocks of lava collapsed and cascaded into the valley. Hot, fast-moving pyroclastic flows knocked down trees, buried farmland, and crushed or burned dozens of houses.

Over the next few years, hundreds of pyroclastic flows from dome collapses forced thousands of people from their homes and land.

As illustrated by the Unzen lava dome, viscous lava flows can be extremely hazardous when they collapse to form pyroclastic flows. Learn more about the collapses of Unzen's dome.