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Bulusan Volcano, Philippines

One of the most prominent among the active volcanic arcs in the Philippine Mobile Belt is the Bicol Arc, a ~260 km long volcanic arc located in the central-eastern margin of the Philippine Mobile Belt. The Bicol Arc hosts three of the active volcanoes (Mayon, Iriga and Bulusan) in the country, including the Mayon Volcano, which has erupted repeatedly within the last decade. The Bicol Arc is named after the Bicol Region (see map). Aside from the three active volcanoes, there are at least nine other Pleistocene major volcanic centers that make up the volcanic arc. Although the rocks from the arc range in composition from basalt to rhyolite, they are mostly basaltic andesites and andesites. Mayon, Iriga and Bulusan are respectively marked as M, I and B on the Google map below.

A volcanic arc is a chain of volcanoes formed as an oceanic tectonic plate (in this case the Pacific Plate) subducts under another tectonic plate (here the Philippine Plate) and produces magma at depth under the over-riding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.

The volcano Bulusan, was constructed within an 11 km diameter dacitic caldera, which was formed more than 36,000 years ago. Bulusan is flanked by several other large intracaldera lava domes and cones. The summit of Bulusan volcano is unvegetated and contains a 300 m wide, 50 m deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded at Bulusan since the mid-19th century.

On 7 November 2010 increased seismic activity was recorded at Bulusan Volcano, 24 hours after the active volcano spewed a 600 m ash column and grayish steam on 5 November 2010. Volcanologists noted that at least 10 volcanic earthquakes and four explosion-type events have been documented subsequent to the emission of ash and steam Saturday morning 5 November 2010. It was also noted that steaming activity was characterized by strong emission of a white steam column that reached a maximum height of 200 m above the crater rim. The public was warned not to enter the four-kilometer permanent danger zone from the volcano. People residing near the valleys and streams were also advised to be extra alert against sediment-laden stream flows in the event of heavy rains. More ash ejection were reported on 8 and 9 November 2010 with ash fall on areas in Juban and Irosin towns in Sorsogon province. A bulletin on 9 November 2010 stated that eight volcanic earthquakes were recorded from the volcano during the past 24 hours and weak to moderate emission of ash explotion was observed. Residents near valleys and streams were also warned against sediment-laden stream flows in the event of heavy and continuous rainfall.

During 10-11 November 2010 18 volcano tectonic earthquakes from Bulusan were detected by the seismic network. On 11 November white steam plumes rose 150 m above the Bulusan crater. During an aerial survey, scientists saw that recent explosions resulted in the joining of two craters from 2006-2007 into one. They also noted that deposits from an explosion on 9 November did not contain juvenile material. On 12 November an ash explosion produced an ash-and-steam plume that rose 700 m above the crater. Multiple neighbourhoods reported light ashfall. Four volcano-tectonic earthquakes were detected during the previous 24 hours. Steam plumes rose to a maximum height of 100 m above the crater. Some steaming was seen during 13-14 November. White steam rose from the NW vent, but no steaming was observed from the crater and SE vent on 14 and 15 November. An explosion late at night on 15 November was followed by ashfall in nearby neighbourhoods. Cloud cover prevented observations of the summit area the next morning.

For more details on the current activity see Erik Klemetti’s Eruptions blog a.o. at http://bigthink.com/ideas/25127

The latest news I have seen at the time of writing is that the National Risk Reduction and Management Council in its latest bulletin released 5 p.m. Wednesday 24 November, reported an ash explosion in Mt. Bulusan at 1:24 p.m., with ash reaching up to 1,000 m high above the volcano’s crater. The ash ejected by the volcano drifted in the southwest direction, although state volcanologists said there were no major emissions of steam and sulfure dioxide gas observed around the volcano. Because of this, Alert Level 1 remains in effect over Mt. Bulusan. Residents around the area were also asked to continue avoiding the four-kilometer permanent danger zone around the volcano. 2,827 families have already been forced to leave their homes due to the volcano’s heightened activity.

Stratigraphy.net | Impressum

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