Dome Collapses Generate Pyroclastic Flows, Unzen Volcano, Japan

Unzen lava dome and Mizunashi River Valley

Unzen lava dome and Mizunashi River Valley

Gray pathways spreading from the lava dome atop Unzen Volcano are the deposits of many small pyroclastic flows that originated from dome collapses. A few pyroclastic flows swept as far as 5 km down this populated river valley, destroying several hundred homes and precious farmland. Lahars destroyed even more property downstream of the loose pyroclastic-flow deposits. The lava dome was active between 1991 and 1995.

Lava Dome Grows Above Steep Slope

Unzen lava dome and Mizunashi River Valley

Photograph by S.R. Brantley on March 22, 1993

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.


Sequence of a Lava Dome Collapse

Illustration of how a dome collapse generates a pyroclastic flow

Sketch by B. Myers

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.


Pyroclastic Flows Rush Down New River Valley

Unzen lava dome and Nakao River Valley

Nakao River Valley. Photograph by K. Scott, 1994

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.


Trees and Buildings Burned by Ash-Cloud Surges

Nakao River Valley destoyed by pyroclastic flows

Nakao River Valley. Photograph by K. Scott, 1994

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).


Unzen lava dome and Nakao River Valley

Photograph by K. Scott in 1994


Other examples: pyroclastic flows generated by collapsing lava domes