Mount Merapi and "ash hurricanes"

This image from NASA's ASTER instrument on the Terra satellite shows
a signature of hot ash and gases flowing down the flank of Merapi volcano
as well as a hot spot on the central dome.  Image taken on November 1.

Since the eruption of Mount Merapi started on October 26, 2010,at least 156 people have died (reports are not consistent at this time).  Press reports have told of people dying from burns and ash suffocation, but no evidence had surfaced regarding the cause of the burns and suffocation because bad weather had prohibited direct observation of the volcano.  Now, the ASTER instrument on NASA's Terra satellite has documented that hot pyroclastic flows ("ash hurricanes") have been sweeping down the flanks of the volcano. (A video of pyroclastic flows on Merapi from previous eruptions can be found here by scrolling down the post to the video.) Temperatures of 450-600 degrees Celsius (842-1112 Fahrenheit have been reported by the Indonesian Volcanology Technology Development and Assessment Agency.  I have found a good blog on the Merapi eruption by Erik Klemetti here.

Merapi in eruption; images from
Clara Prima/AFP/Getty Images posted
on Washingtonpost.com.

What drives pyroclastic flows? In the early frames of the video, large boulders can be seen bouncing down the hill side-by-side with the pyroclastic flow.  This indicates that the flow, like the bouncing boulders, is being driven downhill by gravitational forces.  When gravity is the main driving force, hot material generally is confined to a valley--it's running downhill.  It can "run up" and over hills because of momentum--just like a car that loses its brakes at the top of a hill may race downhill and up the other side of a valley--but generally flows given by gravity follow the terrain. In later frames of this video, including a spectacular segment taken at night, you can see that the pyroclastic flows are being generated by collapse of individual pieces of the steep lava dome.  Section after section is peeling off and falling down and, as the sections impact the ground or other collapsing pieces, they fragment to release fragments and gasses that combine to give form the pyroclastic flow itself.  You do not see "blowouts" of gas-rich material in this video.

A more dangerous condition may arise if pressurized gases inside the collapsing dome decompress rapidly, forming blowouts. Jon Fink and I considered some of these conditions in a paper in Nature, vol. 363, pp. 612-615, 1993.  This paper was motivated by the fact that a pyroclastic flow on Mount Unzen on June 3, 1991, killed three volcanologists who were on high ground that had escaped damage from pyroclastic flows earlier in the eruption.  What happened?  We speculated that a section of the dome containing slightly more volatiles than earlier collapsed sections produced a pyroclastic flow that had higher velocities than had been produced in earlier volatile-poor eruptions.  We concluded that velocity excesses of over 100 m/sec could be caused by decompression of gases in such a section.  These flows would not be confined to valleys, such as the flows to date appear to have been at Merapi, but can have broader extent and longer runout distances. The lateral blast at Mount St.Helens was such a blast.

In spite of the inconveniences and economic consequences, it is necessary to evacuate people from areas around the volcanic summit that appear safe based on the trajectories of small, gravity-driven pyroclastic flows, but that may be devastated by more gas-rich eruptions. Reportedly, about 200,000 people have been displaced by the current eruption.