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Report on Bulusan (Philippines) — February 2017


Bulletin of the Global Volcanism Network, vol. 42, no. 2 (February 2017)
Managing Editor: Edward Venzke. Research and preparation by Paul Berger.

Bulusan (Philippines) Phreatic explosions with minor ashfall continue during June-December 2016

Please cite this report as:

Global Volcanism Program, 2017. Report on Bulusan (Philippines) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 42:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN201702-273010



12.769°N, 124.056°E; summit elev. 1535 m

All times are local (unless otherwise noted)

Recent eruptive activity at Bulusan included episodes during 6 November 2010-16 May 2011, 1 May-17 July 2015, and 22 February 2016; activity typically included phreatic explosions from the summit crater and flank vents, ash-and-steam plumes, and minor ashfall in nearby villages (BGVN 41:03). The most recent eruption began 10 June 2016 and continued through the end of the year. Information was provided by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the Tokyo Volcanic Ash Advisory Center (VAAC).

During the reporting period of June-December 2016, the Alert Level remained at 1 (on a scale of 0-5), indicating abnormal conditions and a 4-km radius Permanent Danger Zone (PDZ). Activity consisted of intermittent phreatic explosions generating emissions of ash and steam that typically rose 70-2,500 m above the summit crater (table 7). Minor ashfall in nearby municipalities often accompanied the explosions.

In October 2016, PHIVOLCS extended the danger zone an additional 2 km as a result of a fissure that extended 2 km down the upper S flank; PHIVOLCS was concerned that active vents along the upper part of the SE flank could pose a greater risk to the populated barangays (neighborhoods) of Mapaso (Irosin), Patag (Irosin), and San Roque (Bulusan). The municipalities of Irosin and Bulusan are about 8 km SSW and 7 km ESE, respectively, of the volcano.

Table 7. Summary of volcanic activity at Bulusan, June-December 2016.

Date(s) Max. Plume height (m) Plume drift Remarks
10 Jun 2016 2,000 NW 5-min long phreatic explosion began at 1135 and generated ash plume.
19 Jun 2016 300 NW 7-min long phreatic explosion from NW summit vent began at 1303 and generated dirty-white ash plume.
23 Jun 2016 -- -- Dirty-white steam plumes from summit vent drifted down WNW flank. Minor ashfall on nearby municipalities NW. Sulfur odor.
25-26 Jun 2016 200 NW Steam plumes.
28 Jun 2016 -- -- Steam plumes drifted down flank.
05 Jul 2016 250 SSE, SSW Copious emissions of white-to-grayish steam plumes.
06 Jul 2016 700 -- Copious emissions of steam.
10-12 Jul 2016 70 -- Diffuse steam plumes.
20 Jul 2016 -- WNW White-to-light gray plumes at low levels.
21-25 Jul 2016 250 NW, SW Diffuse white plumes.
16 Sep 2016 1,500 NE 4-min long phreatic explosion began at 1654, and generated a dark gray ash plume. Ashfall in nearby municipalities NNW, NNE, and NE.
01 Oct 2016 200 SE White-to-grayish emissions during 0650-1240 rose from vents on SE flank. Minor ashfall in nearby communities.
06 Oct 2016 -- -- 15-min long minor phreatic explosion with ashfall on nearby municipalities.
12-16 Oct 2016 500 SE, SSE Steam plumes. 2-6 volcanic earthquakes per day.
17 Oct 2016 1,000 -- 24-min long phreatic explosion at the SE vent at 0736. 24 volcanic earthquakes 16-17 October.
19 Oct 2016 1,000 -- Phreatic explosion at 0458 from upper SE flank. Explosion-type earthquake lasted 9 min.
21 Oct 2016 -- -- 20-min long phreatic explosion from summit crater began at 1234. Minor ashfall in nearby municipalities.
23 Oct 2016 2,500 WSW 15-min long phreatic explosion from summit vent began at 1531 and generated an ash plume. Small pyroclastic flows traveled 2 km down flank. Trace ashfall in nearby municipalities. Another, much smaller, explosion at 1539 from SE vent generated ash plume that rose 500 m. Rumbling and sulfur odor noted in several nearby areas.
29 Dec 2016 2,000 WSW Phreatic explosion at 1440 from vent on upper SE flank generated grayish ash plume. Explosion-type earthquake lasted about 16 min. Minor ashfall on nearby municipalities. Sulfur odor noted.

Ashfall. On 23 June 2016, minor amounts of ash fell in the barangays (neighborhoods) of Poblacion (11 km NW), Añog (12 km NW), and Bacolod (13 km NW), all in the municipality of Juban (about 12 km NW), and the municipality of Mabini (12 km NNW). A sulfur odor was detected in the neighborhoods of Mabini, Bacolod (Irosin), Añog (Juban), and Puting Sapa (Juban).

On 16 September there was ashfall in the municipalities of Casiguran (11 km NNW), Gubat (18 km NNE), and Barcelona (14 km NE). Minor amounts of ash fell during 1 October in the barangays of San Rafael, San Roque, and San Jose, all in the municipality of Bulusan. A minor explosion on 6 October caused ashfall in some areas of the municipality of Gubat, and rumbling was noted in San Roque.

A phreatic explosion on 21 October generated a plume that resulted in a thin layer of ash in Casiguran and Gubat, and trace amounts in barangays in Barcelona, Casiguran, and Gubat. On 23 October, a phreatic explosion produced trace ashfall in multiple barangays in Irosin; the most ash, 1 mm-thick deposits, were found in Puting Sapa (Juban).

On 29 December, a phreatic explosion generated an ash plume that resulted in minor amounts of ashfall in areas downwind, including several Irosin barangays (Cogon, Tinampo, Bolos, Umagom, Gulang-gulang, and Monbon) and two Juban barangays (Caladgao and Guruyan). Residents of Guruyan, Monbon, and Tinampo noted a sulfur odor.

Geological Summary. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit 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 since the mid-19th century.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).