Batu Tara, Indonesia, spectacular photo

Batu Tara, August 18, 2001
Photo by Thorsten Bockel-http://www.tboeckel.de

VPOW featured this beautiful image this week. I was fascinated by the features at the lower left and center bottom--they look like fireworks on July 4, but come, instead, from the impact of volcanic bombs on the slopes near the vent.

Batu Tara is a small stratovolcano that forms an isolated island in the Flores Sea. Normally covered with vegetation, its first historical eruption occurred from 1847-1852, and the current eruption cycle, starting in 2006, is only the second. A pilot reported an ash cloud that year, but there was no other confirmation.  In 2007, MODIS infrared satellite data showed thermal anomalies.  A continuous low-level plume developed on March 15, 2007, and residents on an island about 50 km south reported a 500-1500 m plume. Lava flows were observed in April of that year, and ash plumes were fairly continuous. Here is the Smithsonian compilation of monthly reports on the activity.

Cold Lahars Displacing People at Mount Merapi, Indonesia

Dredging in the Code River, Yogyakarta
Photo from here.

Eruptions at Mount Merapi, discussed here, here, and here before in this blog, produced ash and other debris that sits atop the mountain awaiting the monsoon rains. Those rains have arrived; the wet monsoon season in Indonesia lasts from November through March. For weeks muddy torrents have been flowing down from the mountain. On November 30 an apparently big one, reported to be up to 2 meters deep, arrived. According to the Nov. 30 edition of the JakartaGlobe, thousands of residents were forced to evacuate. These muddy flows of volcanic materials are termed "lahars", and they can be either hot or cold depending on the timing of the volcanic activity and the weather.

The lahars flow down the channel of the Code River, which flows through the center of Yogyakarta.  The river has been channelized to control the path of the lahars, a historic danger from Merapi. Sandbags up to 1 meter high help control the flows, but authorities have warned residents that these temporary levees will not contain the large flows that are likely to occur.  Surono, the head of the Volcanology and Geological Disaster Mitigation Agency (PVMBG) has warned that people should stay 300 meters away from the river.  Evacuation has been hindered by damage to roads and bridges, as well as by heavy traffic from people coming in to see the disaster.

An interesting side-bar mentioned in the DredgingToday.com article linked in the figure caption is that the dredging of the Code River has been hampered by the lack of heavy machinery.  However, people living near the river have been helping by manually carting material away.  Apparently it is well known that the volcanic mud is rich in nutrients, and a truck of this material is fetching 50-60 thousand Rp! (50,000 rupiahs convert to about $5.50 US.)

Merapi--again! Disrupts Obama visit this week

A village covered with ash from an eruption on
Thursday, November 11, 2010.  Photographer unknown,
source

According to some reports, the death toll from the on-going Merapi eruptions has exceeded 200, and nearly 400,000 people have been displaced from their homes and villages. President Obama visited Indonesia earlier this week, but cut his visit short be several hours so that his plane would not be caught by changing winds.  To date, the eruptions have released about 140 million cubic meters, exceeding the 100 million cubic meters estimated to have erupted in 1872.

In a paper in press for Geophysical Research Letters (now online), Anchukaitis et al. have examined the influence of volcanic eruptions on climate, and arrive at conclusions that challenge the major results from the Global Circulation Models currently used to model climate.  Those models predict that large volcanic eruptions should result in unusually dry conditions through the regions of Asia that experience monsoons. Anchukaitis et al. used two long tree ring-based proxy's to reconstruct moisture patterns. The first was a tree-ring chronology from a long-lived ccypress species in southern Vietnam, and the second is the "Monsoon Asia Drough Atlas", which also includes tree-ring proxies.  These reconstructed conditions indicate that the response to volcanic eruptions is an anomalously wet southeast Asia and dry conditions over central Asia, a conclusion that is the exact opposite of effects predicted by three widely used climate models: CSM1.4, CCSM3, and GISS ModelE. The authors also conclude that strong El Nino and La Nina weather conditions could be important.  Anchukaitis concludes "...that some GCMs do not correctly capture the balance of important coupled ocean-atmosphere processes involved in the response of Asian climate to radiative forcing.  In a press release accompanying the article the authors also caution that the study suggests that proposed geoengineering schemes to counteract manmade climate change with huge artificial release of sulphates similar to those emitted by volcanoes might have complex unintended consequences.

As of this time, volcanologists do not think that the eruption at Merapi is big enough, or sulfurous enough, to affect the climate.

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.

Mount Merapi erupts again

Mount Merapi on a clear day,
photo from
http://uktodaynews.com/9910/indonesias-volcano-mount-merapi-erupts-again/

Mount Merapi in Indonesia began erupting on October 25 (2010), causing 38 deaths including the "gatekeeper" of 220 years.  Merapi is a dangerous and heavily monitored volcano, with a long history of eruptions. It builds domes which collapse, effuses lava flows, pyroclastic flows and lahars.  It has been argued that a major eruption in 1006 AD weakened the Mataram civilization of Central Java, causing it to move from Central to East Java (see summary of 10,000 years of Merapi history in Newhall et al., Journal of Volcanology and Geothermal Research, v. 100, Issues 1-4, pp. 9-50, 2000). A distinguishing feature of Merapi, shown in the photo to the left, is a "somma", an opening in the summit to the southwest.  Such "sommas" usually indicate that a flank of the volcano has collapsed; the "amphitheater" at Mount St. Helens that opens to the north is such a feature. Hard evidence for the collapse such as obvious avalanche deposits has not been found and so the somma itself is the best evidence for this process. Eruptions in the 20th-21st century have been rather mild, and less explosive than those for which the field evidence suggests occurred between the 7-19th centuries A.D.  Dome collapse producing pyroclastic flows is so characteristic of this volcano that this style of eruption has been called "Merapi-type" at volcanoes over the world.  Volcanologists suspect that the 20th century "mild" activity of Merapi is just an interlude between larger events typical of the past record.  Precursors of such an event are not known, resulting in a dangerous situation for the hundreds of thousands of people who now live around the volcano.  80,000-100,000 live inside"The Forbidden Zone", an area of about 10 km radius on the south west side.  Several hundred thousand more live just outside this zone.  Volcanologists work with the populace to come to an understanding that prediction is not an exact science, that there will be false alarms, but that risk management is a necessary public good.  A documentation of historical eruptions between 1768-1998 is available in Voight et al., Journal of Volcanology and Geothermal Research, volume 100, issues 1-4, pp. 69-138, 2000.

From the U.S. Geological Survey