Smithsonian Institution
Global Volcanism Network Bulletin v. 19, no. 11, November 1994
Bulusan (Philippines)  Phreatic explosions cause ashfall in local
      villages and up to 16 km away

Luzon, Philippines (12.77N, 124.05E)
All times are local (= GMT + 8 hours)
A phreatic eruption at 2043 on 27 November sent an ash plume 1.5 km
high that drifted W and SW, causing ashfall in six villages, and
was accompanied by 14 minutes of felt tremor. Following this event,
PHIVOLCS declared the area within 4 km of the crater off-limits. A
second ash explosion on 3 December at 2348 was accompanied by
rumbling, but details are sketchy owing to heavy cloud cover. The
third ash ejection, on 4 December, deposited traces of ash ~7 km
downwind; no other observations were possible. The next day,
another explosion at 1227 sent ash 1.5 km high that caused ashfall
5 km WSW, noticed in two villages.
A phreatic explosion at 0650 on 12 December was also the strongest
so far. The cauliflower-shaped eruption column, accompanied by a
loud "pop," rose 3 km and deposited ash as far as 16 km SW. The
main eruption column, light gray in color, rose vertically, and a
smaller dark-gray surge cloud seemed to emanate from the base of
the main eruption cloud. However, the runout was still within 4 km
of the vent and no evacuation was recommended.
Five additional small explosions occurred through 28 December.
Observations of an ash explosion at 0155 on 18 December were
hampered by clouds, but it was inferred from the seismogram and ash
deposits at 5 villages, all SW of the volcano. A minor ash
explosion at 0807 on 20 December produced an ash cloud not directly
observed due to rain clouds, but ash fell ~7 km SW of the vent. A
brief cloud break enabled volcanologists to make a COSPEC
measurement of ~370 metric tons/day. At 1525 on 23 December, a
slightly stronger ash ejection lasted 4 minutes, causing light
ashfall in 6 villages, also in the SW. Light ashfall 7 km from the
summit was noted again following a 3-minute ash ejection at 2153 on
24 December. Ash output from a 7-minute eruption at 1253 on 27
December seemed to be larger than other events and spread to a
wider area, despite calmer winds, depositing small amounts of ash
in nine villages.
The onset of all ash emissions had a corresponding explosion-type
earthquake recorded on the seismogram. This became diagnostic
during heavy cloud cover when ash plumes could not be observed
directly. Based on the earthquake amplitudes, the 27 November and
12 December events were the biggest explosions, although ash
emission was greater on 27 December. In nearly each case, the ash
deposit was <=2 mm thick at ~7 km downwind. Hazard maps had been
prepared before the 27 November event. PHIVOLCS is planning to pull
the telemetered seismic network installed on Mindoro for aftershock
monitoring, and move it to Bulusan.
Luzon's southernmost volcano, Bulusan, was constructed within the
11-km-diameter Pleistocene Bulusan (Irosin) caldera. It is 280 km
NNE of Canlaon and 70 km SSE of Mayon volcano. Moderate explosive
eruptions have been recorded since 1852. Between May 1918 and May
1922 there were 17 explosive, ash-producing eruptions, a lava flow,
possible dome growth, and nuees ardentes. Intermittent explosions
during 1978-83 prompted some evacuations. The most recent activity,
February-March 1988, consisted of mild phreatic eruptions that sent
ashfall to the SW and NW (see Bulletin v. 13, nos. 2, 6, & 8).
Information Contacts: Ray Punongbayan (Director), Ernesto Corpuz
(Monitoring), and Ed Listanco (Geology), Philippine Institute of
Volcanology and Seismology (PHIVOLCS), 6th Floor, Hizon Building,
29 Quezon Avenue, Quezon City, Philippines; Reuters.