Bulletin of the Global Volcanism Network
Volume 25, Number 11, November 2000
Rabaul (Papua New Guinea) Ashfall during August-October most abundant since
1995
Ulawun (Papua New Guinea) An eruption during 28 September-2 October 2000
sends a plume to 1012 km
Langila (Papua New Guinea) Mild Vulcanian eruptions during July-October 2000
Karkar (Papua New Guinea) Rumored eruption on 29 September believed to stem
from Ulawun's ash fall
Manam (Papua New Guinea) Low August-October 2000 activity; increased
seismicity 18 September
Karangetang (Indonesia) February-December included explosions, ash falls,
lava flows, and debris flows
Tengger Caldera (Indonesia) Sudden explosion 29 November; eruption
continues as of 18 December
Merapi (Indonesia) Consistent gas plume; lava avalanches and landslide; new
lava dome and fractures
Slamet (Indonesia) During May-October 2000, continuous tremor and abundant
explosion earthquakes
Kaba (Indonesia) Explosions and light gray ash
Marapi (Indonesia) Large explosions in March 2000 eject ash
Heard Island (Indian Ocean) Discovery of a distinct vent below Mawson Peak;
brown fumes in November
Shiveluch (Kamchatka) Frequent steam plumes, weak tremor, and possible
gasandash explosions
Rabaul
New Britain Island, Papua New Guinea
4.271 S, 152.203 E; summit elev. 688 m
All times are local (= GMT + 10 hours)
This report, which covers the period from August through October 2000,
notes that Vulcanian eruptions prevailed at Tavurvur during 29 August
through October 2000. These eruptions bore far more ash than typical.
Substantial amounts of ash blew towards the city of Rabaul and other
inhabited areas. During the second and third weeks of September the amount
of ash falling reached a maximum since the current eruption began on 28
November 1995.
On 2 and 9 August eruptive plumes contained ash. At other times during
August vent activity was relatively low, consisting only of small volumes
of thin white vapor. The 29 August eruption changed this pattern.
Although the main phase of the 29 August eruption began at 1158, it was
preceded by a series of weak explosions yielding dark gray ash plumes.
After these less-forceful explosive events ended, moderate volumes of dark
gray ash clouds erupted. Subsequently, and through 8 September, the
eruption pattern was comprised of interchanging periods of either
continuous, moderate-volume ash emissions or relatively subdued ash-cloud
emissions typically white to pale gray in color.
At about 2200 on 8 September, the eruption became more intense, frequent,
and discrete explosions producing thick billowing dark ash clouds.
Throughout the rest of September, periods of sub-continuous ash emissions
occurred. After 21 September, however, discrete explosions and
sub-continuous ash emissions declined and continued at a subdued level.
Throughout September, observers saw incandescent lava fragments
occasionally ejected. Residents saw these ejected fragments more frequently
after the onset of explosive activity on 8 September, and around this time
the residents regularly noted audible noises associated with venting.
Explosions every few minutes produced sub-continuous moderate to heavy ash
emissions that continued in October. Some of the larger explosions showered
the N and NE flanks of the volcano with bombs. At night these were seen to
be incandescent (red to dull orange). On inspection most projectiles were
lithic blocks, but a minority were in a plastic state with some bread-crust
surfaces. Jet-aircraft-like roaring from the vent was common and some of
the larger explosions could be heard at distances of over 15 km. Toward the
end of October, the explosions were less forceful, but contained increased
volumes of solid fragments.
All activity during the reporting period appears to have been confined to
the 1941 vent; the 1995 crater issued only white vapor. The current phase
enlarged the vent as a result of explosive activity. The N crater was
breached, forming a saddle-like depression in the rim reaching about 15-20
m lower than the originally symmetrical and level line comprising the ridge.
During the first 2-3 weeks of September ash clouds rose as high as 2 km
above the summit. As previously noted, many plumes blew N and NW to drop
ash on inhabited areas. As a result, many Rabaul businesses were adversely
affected and, by the end of the September, some had moved ~40 km SE of
Rabaul to the settlement of Kokopo. The major health center in Rabaul town
was also affected and patients with respiratory problems were moved to
another health center near Kokopo. During early October the ashfalls caused
patients to undergo evacuation from Nonga Hospital (5 km NNW of Rabaul).
Toward the end of October the annual change in the trade winds caused less
ashfall in Rabaul town; frequent heavy rains around this time also provided
relief from suspended dust.
Seismicity correlated fairly directly with behavior observed at the summit,
and accordingly, seismic activity remained relatively low until the onset
of the eruption. The total number of low-frequency earthquakes recorded
during August was 208, with the largest daily number of 35 on 30 August.
This monthly total was slightly higher than that in July. During the main
episodes of ash eruption between 29 August and 8 September, seismicity was
characterized by bands of non-harmonic volcanic tremor. Bands of volcanic
tremor with reduced amplitude were recorded again during 17-19 September
and, until the end of the month, seismicity included sporadic
short-duration tremor associated with sub-continuous ash ejections.
For August and September, respective high-frequency earthquakes totaled 17
and 8; all occurred NE of Rabaul caldera. The time difference between S-
and P-wave arrivals for these events (S minus P) was 3-6 seconds. Since 28
November 1995, strings or significant numbers of high-frequency NE
earthquakes have correlated with greater summit activity at Tavurvur. The
time interval from the onset of the high-frequency earthquakes to greater
summit activity ranged between one week to a few months. Rabaul's last
major NE earthquake sequence occurred during May-June 2000 (see Bulletin v.
25, no. 7).
During September a total of 3,661 low-frequency earthquakes were recorded.
This total includes explosion earthquakes (with air phases). When like
earthquake records were compared, the September record attained the highest
total since May 1996 (when 3,993 such earthquakes were recorded).
September's highest daily total, 228, was recorded on 11 September; the
lowest total, 3, was recorded on 4 and 7 September, occurring on days when
tremor dominated the seismicity. October seismicity included 2,544
low-frequency earthquakes.
Ground deformation measurements from electronic and wet tiltmeters showed
some caldera inflation from July 1999 to April 2000. After April, the trend
changed to show deflation until about August 2000. Tiltmeters registered
slow inflation during the first few days of September, followed by
subsidence, and then stable conditions on 8 September. Some
deformation-monitoring sites in the area of Tavurvur seemed to indicate a
deflationary trend starting in late September and continuing through October.
Background. The last of two Holocene caldera-forming eruptions of Rabaul
took place 1,400 years ago. Several large historical eruptions have formed
intra-caldera cones. The latest significant eruptive episode, in 1994,
included venting at both the Tavurvur and Vulcan cones. The low-lying
Rabaul caldera forms a sheltered harbor once utilized by New Britain's
largest city. The 8 x 14 km caldera is widely breached on the east, where
its floor is flooded by Blanche Bay. Two major Holocene caldera-forming
eruptions took place as recently as 3,500 and 1,400 years ago. Three small
stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera
eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor
near the NE and western caldera walls. Several of these, including Vulcan
cone, which was formed during a large eruption in 1878, have produced major
explosive activity during historical time. A powerful explosive eruption in
1994 forced abandonment of Rabaul city.
Information Contacts: Ima Itikarai, David Lolok, Herman Patia, and Steve
Saunders, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New
Guinea (Email:
rvo@global.net.pg).
Ulawun
New Britain, Papua New Guinea
5.04 S, 151.34 E; summit elev. 2,334 m
All times are local (= GMT + 10 hours)
An eruption from Ulawun that started on 28 September continued until 2
October 2000 (Bulletin v. 25, no. 8). Preliminary information was based on
news media reports and aviation sources. The Rabaul Volcano Observatory
(RVO) has since provided more accurate and detailed reports for the
August-October 2000 period.
Activity during August. During August, summit activity remained low,
chiefly consisting of weak releases of thin white vapor. While the summit
activity remained quiet, some obvious changes were recorded in seismicity
and ground deformation (electronic tiltmeter).
High-frequency earthquakes had occurred sporadically since mid-1998 in very
low numbers (~1/day). A brief period of recording in April 2000 registered
~ 2/day. In mid-June 2000, following the resumption of seismic recording,
high-frequency earthquakes were first seen to have increased to 15-20/day.
This comparatively high number of earthquakes continued through July and
August. Deployment of a portable seismograph during 13-15 August indicated
these earthquakes had epicenters at Ulawun.
During August, low-frequency earthquakes occurred as usual. On about 26
August, both amplitudes and event counts dropped by a factor of three. Also
beginning in August, an electronic tiltmeter located on the flanks ~2.5 km
WSW of the crater vent began to show some inflation on the NS component. By
month-end about 7 mrad of inflationary tilt was recorded. The EW component
(radial to the vent) lacked changes.
Eruption of 28 September-2 October. A relatively small-to-moderate eruption
began on 28 September 2000. Several long-term precursors to the eruption
were unambiguous. High-frequency earthquakes increased dramatically after
June 2000. Electronic tiltmeter trends showed inflation beginning in early
August 2000. Short-term, immediate precursors included an increase in both
the number of low-frequency earthquakes and the real-time seismic amplitude
measurements (RSAM). The latter began to escalate on 27 September 2000.
The late-stage build-up towards the eruption began at 2200 on 27 September
with an increase in low-frequency earthquakes and their amplitudes. A
further increase in seismicity took place at about 0200 on the 28th. There
began a series of small nonharmonic volcanic tremors with durations up to a
few minutes. These increased between 1200 and 1800 on the 28th.
The late-stage buildup also appeared in RSAM readings. RSAM values changed
at 2200 on the 27th from a background level of 2 units, rising at 1200 on
the 28th to ~10 units, then by 1800 to 30 units. After 1800, RSAM values
continued to rise and the instruments registered intense continuous
volcanic tremor. Around this time the RSAM counts exhibited an artificial
decline attributed to an overwhelmed event-counting system.
Other notable changes in the character of the RSAM plot occurred at 2240 on
the 28th, and at 0115 and 0240 on the 29th. The first two times correlated
with behavior noted by the observer watching the summit crater.
Specifically, at 2240 on the 28th the observer saw the first glow
reflecting off an ash-cloud emission. RVO scientists mark this as the
beginning of the eruption. At 0120 on the 29th, production of incandescent
lava fragments intensified, indicating the onset of a Strombolian eruption.
At 0240 on 29 November, the RSAM peaked at 8045 units. Thus, scientists
inferred this as the time when the Strombolian phase of the eruption
peaked. During the cover of darkness, ash clouds were just visible,
initially illuminated by the glow and gradually by incandescent fragments.
The latter became common by about 1200 on the 29th when ash clouds were
seen blowing NW. The ash clouds began to become voluminous from 0100 on the
29th, coinciding with the intensification of the Strombolian phase of the
eruption. The ash clouds were emitted forcefully, accompanied by loud
roaring and rumbling noises.
By first light on 29 September people some distance away from Ulawun were
able to see a thick vertical eruption column that rose ~12-15 km above the
summit (figure 1). In relatively clear morning weather, the eruption column
was visible from Rabaul, ~130 km NE of Ulawun. By this time ash clouds had
blown WSW to NW. Roaring and rumbling noises ceased at about 1000 and
resumed again at about 1300. At about this time also, there was a slight
and brief change in wind direction resulting in the eruption's ash clouds
being redirected to the N and NE.
The eruption produced a moderate amount of ashfall. Ashfall was heavy 10 km
downwind of the vent. Beyond that, the amount of ashfall was much reduced,
lessening still further away from the vent. The ashfall destroyed gardens
and cash-crop plantations within 10 km of its main path. Satellite images
of the ash distribution showed that the ash was blown downwind 80-100 km
from the vent.
The eruption produced three pyroclastic flows. They traveled down
pre-existing gullies on the N, NW, and SE flanks. During past eruptions,
pyroclastic flows also followed the same gullies down from the cone's 2,300
m summit elevation. The N-directed pyroclastic flow was the biggest. It
descended to 580 m elevation. The NW-directed pyroclastic flow was the
second biggest. It divided into two arms at 900 m elevation and its
terminus reached an elevation similar to the one on the N flank. The SE
flow was relatively small. The eruption lacked lava flows.
Scientists inspected the crater area from the air on 7 October and found
two vents on the summit (labeled Vents A and B on figure 2). Comparing Vent
A to its appearance during 1993, the overall depth of the crater floor
appeared to have risen, becoming about 100-150 m shallower. The fill
consisted of older material that collapsed from the inner crater wall and
possibly new ejecta from the current eruption. A prominent breach on the N
crater rim was evident and may have been created by the N pyroclastic flow.
Judging by its close proximity, this flow originated from Vent B.
En-echelon cracks on the E end of the crater suggested inward sagging of
that side of the crater rim. At the time of the inspection, few emissions
escaped the vents; however, traces of white and blue vapor wafted from
other areas inside the crater.
Figure 2. An oblique aerial photo showing Ulawun's summit area and Vents A
and B between 0800 and 0900 on 7 October 2000. Other visible features
include a breach on the NNE crater rim caused by the avalanche of
pyroclastic flow material from Vent B. Photographed by Ima Itikarai;
courtesy of RVO.
There were reports of multiple vents during past eruptions. However, aerial
inspections in 1985 and 1993 only revealed evidence of Vent A. Ulawun had a
flank eruption on the SE side during the 1978 eruption.
RVO staff in Rabaul established communication links with the volcano
observer based near Ulawun at 1606 on 27 September after noting increased
RSAM values. Data from Ulawun were transmitted to RVO every 20 minutes. The
RSAM values led to discussions with civil authorities and directions to
local residents throughout the night at two-hour intervals. During the
process, alert stages one and two were declared. This resulted in
evacuating the local population, an effort accomplished with the help of a
local timber company. Assistance was later provided by the West New Britain
Provincial Government. This time-line of events is contrary to preliminary
information (Bulletin v. 25, no. 8). that relied on local news media.
Activity during October. By 3 October the volcano produced only thin white
vapor with no noise or night glow. Various ancillary observations occurred
in the next weeks: during 6, 8, 10, 14, 15, 16, 17, 18 October and 1
November observers noted low rumbling or banging or both; when under cover
of darkness they usually saw accompanying weak, steady glows. In at least
one case they smelled sulfurous odors.
The noises at 1000 on 1 November accompanied a thick dark ash cloud that
rose 100-200 m above the summit. By 1100 silent ash clouds were seen
forcefully rising ~200 m above the summit. Activity then declined for a few
hours until 1430-1700, an interval when observers saw thick dark gray ash
clouds and occasionally heard weak rumblings. Activity quieted after that
and by 2 November emissions had returned to white vapor without noise or
night glow.
On 13 and 17 October small mudflows occurred, originating on the upper NW
flanks and sweeping down a dry creek through Ubili village and then to the
sea. At certain places the flows spread laterally. No reported damage or
casualties were caused by either mudflow.
October seismicity included tremor and numerous low-frequency earthquakes.
Volcanic tremors were dominant during 1-4, 8-11, and 30-31 October. The
low-frequency earthquakes can be characterized by RSAM data. On 1 October
RSAM stood at ~30-40 units; steady decline brought the 7 October RSAM to ~5
units. On the 8th and 11th there followed peaks of over 20 units. After
that RSAM declined steadily until it reached background levels on 20
October. On 30 October, RSAM underwent a sudden increase to ~20-30 units
heralding the brief 1 November eruption.
The single electronic tiltmeter located on the high WSW flank showed a
steady change throughout October. The behavior could possibly be related to
edifice inflation.
Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is
the highest volcano of the Bismarck arc, and one of Papua New Guinea's most
frequently active. Ulawun rises above the N coast of New Britain opposite
Bamus volcano. The upper 1,000 m of the 2,334-m-high volcano is
unvegetated. A prominent E-W-trending escarpment on the S may be the result
of large-scale slumping. Satellitic cones occupy the NW and E flanks. A
steep-walled valley cuts the NW side of the volcano, and a flank lava-flow
complex lies to the S of this valley. Historical eruptions date back to the
beginning of the 18th century. Twentieth-century eruptions were mildly
explosive until 1967, but after 1970 several larger eruptions produced lava
flows and basaltic pyroclastic flows, greatly modifying the summit crater.
Correction: In Bulletin v. 25, no. 8, it was erroneously stated that Ulawun
is also known as the North Son. Ulawun (or Ulavun) is, in fact, sometimes
referred to as The Father. North Son is Mount Likuruanga, a well eroded
cone abutting Ulawun to the NE. This group is completed by Bamus volcano to
Ulawun's SW. Bamus, a 2,000-m-high cone that last erupted in 1886, is
sometimes referred to as South Son.
Information Contacts: RVO (see Rabaul).
Langila
New Britain Island, Papua New Guinea
5.53 S, 148.42 E; summit elev. 1,330 m
All times are local (= GMT + 10 hours)
The mild Vulcanian eruptive activity that occurred at Langila's Crater 2
through June 2000 (Bulletin v. 25, no. 7) continued during July-October
2000. In addition, low-level volcanic activity continued at Crater 3. No
reports of unusual activity were submitted during July and August.
During September, intermittent, mild Vulcanian activity occurred at Crater
2. The activity consisted of moderate emissions of thin-to-thick white
vapor, which were occasionally accompanied by gray ash clouds. On 21, 25,
and 30 September thick, dark gray, convoluting ash clouds were forcefully
released, rose 200 m above the summit, blew to the N and NW, and deposited
fine ash. On 7, 9-11, and 27 September wisps of blue vapor accompanied the
emissions. During the month volcanic activity was low at Crater 3, with
only thin white vapor sporadically visible.
Through October intermittent, mild Vulcanian eruptions continued at Crater
2. The vent usually emitted white vapor, which was sometimes accompanied by
a blue tinge and occasionally by a light ash component. On 8 October a
forceful emission of thick ash rose to 1 km above the crater rim. This
heralded a few days of increased ash emissions, with some forcefully
expelled light gray/brown clouds on the 15th. During 16-24 October
continuous white vapor emissions with a small ash component were common. At
0801 on 24 October a dark gray-to-black ash column rose 1 km above the
crater rim. On 25 October an ash cloud that rose to 2 km above the crater
deposited ash toward the N. Likewise, at 0655 on 26 October a thick, white
vapor plume was accompanied by an ash column that rose to 1 km above the
crater rim. The ash emissions continued throughout the day, and similar
activity occurred the next day. For the rest of the month activity was
confined to white vapor with an occasional ash component. During October
varying amounts of white fume were emitted from Crater 3. Throughout the
period there were no reports of noises or night glow at the volcano; the
seismograph remained out of operation.
Background. Langila, one of the most active volcanoes of New Britain,
consists of a group of four small overlapping composite cones on the lower
eastern flank of the extinct Talawe volcano. Talawe is the highest volcano
in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long
crater is breached widely to the SE; Langila volcano was constructed NE of
the breached crater of Talawe. An extensive lava field reaches the coast on
the N and NE sides of Langila. Frequent mild-to-moderate explosive
eruptions, sometimes accompanied by lava flows, have been recorded since
the 19th century from three active craters at the summit of Langila. The
youngest and smallest crater (Crater 3) was formed in 1960 and has a
diameter of 150 m. The Cape Gloucester observation post, airstrip, and
seismometer are 9 km N of the volcano.
Information Contacts: RVO (see Rabaul).
Karkar
offshore New Guinea, Papua New Guinea
4.65 S, 145.96 E; summit elev. 1,839 m
On 29 September there were reports of light ash fall on the NE coast of
Karkar island, and fine sand deposits were found at the bottom of a
swimming pool. The ash fall led to the rumor that Karkar volcano had
erupted. However, an aerial inspection of the central caldera and Bagiai
cone on 1 October showed that no eruptive activity had taken place.
Volcanologists believe that the ash most likely originated from an eruption
at Ulawun, which, at its peak on 29 September, had an eruption column to
12-15 km altitude. Ash erupted to those heights would have been taken well
into the jet stream and could have been transported to Karkar, which is
located 600 km to the W (downwind at high altitudes) of Ulawun. There have
been no reports of anomalous activity at Karkar since fumarolic gases
killed vegetation in September 1997 (Bulletin v. 22, no. 9).
Background. Karkar is a 19 x 25 km wide, forest-covered island that is
truncated by two nested summit calderas. The 5.5-km-wide outer caldera was
formed during one or more eruptions, the last of which occurred 9,000 years
ago. The excentric 3.2-km-wide inner caldera was formed sometime between
1,500 and 800 years ago. Parasitic cones are present on the northern and
southern flanks of Karkar; a linear array of small cones extends from the
northern rim of the outer caldera nearly to the coast. Most historical
eruptions, which date back to 1643, have originated from Bagiai cone, a
pyroclastic cone constructed within the steep-walled, 300-m-deep inner
caldera. The floor of the caldera is covered by young, mostly unvegetated
lava flows.
Information Contacts: RVO (see Rabaul).
Manam
offshore New Guinea, Papua New Guinea
4.10 S, 145.06 E; summit elev. 1,807 m
Following the 4 June 2000 eruption at Southern Crater (Bulletin v. 25, no.
7), volcanic activity was low at both summit craters through October 2000.
Seismicity remained relatively stable except for a slight increase in
amplitudes beginning on 18 September.
During August, activity at the two summit craters was low. Main Crater
gently emitted small-to-moderate volumes of white vapor, while Southern
Crater weakly emitted white vapor. Seismic-event amplitudes steadily
increased throughout the month, though the overall trend remained within
background levels. Daily average event counts were about 1,200, with some
fluctuations. No significant movements were recorded by the water-tube
tiltmeter 4 km SW of the summit.
Activity remained low throughout September, with vapor emissions from both
craters similar to those in August. Seismic amplitudes were steady until 18
September when a slight increase was observed that continued through the
end of the month. Daily average seismic event counts remained steady, with
about 1,300 events/day.
Throughout October both craters emitted varying amounts of white vapor, and
there were no reports of noise or night glow. Seismicity and tilt
measurements appeared to remain at background levels, although consistent
measurements could not be made because scientists did not have access to
the Tabele Observatory during 5-21 October.
Background. The 10-km-wide island of Manam is one of Papua New Guinea's
most active volcanoes. Four large radial valleys extend from the
unvegetated summit of the conical stratovolcano to its lower flanks. These
"avalanche valleys," regularly spaced 90 apart, channel lava flows and
pyroclastic avalanches that have sometimes reached the coast. Five
satellitic centers are located near the island's shoreline. Two summit
craters are present; both are active, although most historical eruptions
have originated from the southern crater, concentrating eruptive products
during the past century into the SE avalanche valley. Frequent historical
eruptions have been recorded since 1616.
Information Contacts: RVO (see Rabaul).
