Dome Collapses Generate Pyroclastic Flows, Unzen Volcano, Japan
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
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
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
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. 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).
Photograph by K. Scott in 1994
Other examples: pyroclastic flows generated by collapsing lava domes
Mount
Hood, Oregon, an active volcano in the Cascade Range of the western United
States, erupted a lava dome only 200 years ago and formed many pyroclastic flows
by collapse events.
-
Soufrierre Hills Volcano, Monsterrat began erupting a lava dome in 1995.
Hundreds of subsequent pyroclastic flows forced several thousand people
to leave the Caribbean Island. See
Website of the Monsterrat Volcano Observatory.
