Viscous lava began erupting in May 1991 at the summit of Unzen and quickly built a mound-shaped dome atop an older lava dome. The new dome spread over volcano's steep east flank (right profile). As lava moved over the edge of this steep slope, hot lava blocks unpredictably broke away or "collapse" downslope to form pyroclastic flows many times a day. Each collapse event added new fragmented material to the dome's east side and the upper Mizunashi River valley.

During collapse events of the Unzen dome, an avalanche of hot lava blocks crashed downslope. The avalanche quickly became a fast-moving pyroclastic flow of shattered lava fragments, volcanic gas, and air. Within seconds, a faster moving "cloud" of smaller ash-sized fragments, called an ash-cloud surge, formed above and in front of the pyroclastic flow. Finally, as the flow spread away from the volcano, ash and hot gas rose to build an eruption column; when detached from the volcano, the volcanic ash and gas became an eruption cloud.

Nakao River Valley. Two years after the lava dome began erupting atop Unzen, pyrocalstic flows cascaded down the northeast side of the volcano into the populated Nakao River valley. In anticipation of these new flows, Japanese officials had already ordered several thousand residents to evacuate the valley. The pyroclastic flows and associated ash-cloud surges spread across the entire valley as far as 4.5 km from the volcano, and built a thick apron of debris at the mouth of the river canyon (note large blocks of lava on apron). Lava dome is in upper right.

Nakao River Valley. Buildings and vegetation in the central valley were burned by the intense heat and knocked over by the momentum of ash-cloud surges that moved beyond the pyroclastic flows (see illustration above).