Soufriére hills volcano (Montserrat): eruption update

[Fredfredson]

Soufriére hills volcano (Montserrat): eruption update

23-05-06 19:32
Age: 5 hrs

BY: TOBIAS

LINK

Interim report for the period midday 19 May to midday 22 May 2006

A major lava dome collapse took place at the Soufriere Hills Volcano on the morning of 20 May 2006.

A helicopter flight in the afternoon confirmed that most of the lava dome has gone, together with some remnants of the 2003 lava dome, leaving a broad, deep, eastward-sloping crater at the summit of the volcano.

The volume of the lava dome was believed to be about 90 million cubic metres and most of this collapsed over a period of less than three hours.
Views of the western part of the crater where ash venting is continuing were not possible but it is unlikely that there is significant dome material remaining there.

There was a single precursor long period seismic event 3km below the dome at 2:22am local time on 20 May. A brief episode of heightened seismic amplitude corresponding to ash venting occurred at 3-3:30am. During heavy rain, another episode of increased seismic amplitude, interpreted as ash venting, began at 5:52am and this gradually increased and developed into a high amplitude seismic signal.

The heavy rain was causing mudflows in Belham River valley at this time. By 06:32 low-level ash clouds were drifting to the northwest of the volcano from the crater area and a steam-rich plume was rising to 6,000ft. Unconfirmed reports suggest that pyroclastic flows first reached the sea at about 06:45.
Regular pulses of pyroclastic flows were reaching the sea down the Tar River valley by 07:20 with major pulses recorded in seismic amplitude at 07:36, 07:43 and between 08:01 and 08:04. Also between 07:30 and 08:10 a number of long-period seismic events were detected.

At 07:40 an ash cloud was reported at nearly 17km (55,000ft) the highest reported ash cloud during the ten years of the eruption. At 07:43 pyroclastic surges were observed spreading across the northeastern flanks of the volcano reaching the Spanish Point area. It was also estimated that surges had spread 3km offshore from Tar River valley, across the surface of the ocean.
By 07:50 lithics (pieces of lava dome) were falling in areas northwest of the volcano, most were less than 3.5 cm across, the largest found in the inhabited area was 6cm across. Six car windscreens were reported broken. The deepest ash fall in inhabited areas was about 3 cm. Activity began to reduce in intensity after 08:15am and a high amplitude seismic signal remained until about 9am. During and following the collapse, residents in the Old Towne and Salem area were subjected to high levels of volcanic gases particularly hydrogen chloride causing some to move north to get out of the volcanic plume. Widespread and noisy mudflows were reported in the Trants area to the northeast of the volcano. Vigorous ash venting from a vent in the western part of the crater continued until about 5:00pm when it began to decline in vigour. A 1m high tsunami was reported from Deshaies beach in Guadeloupe and swells were detected in Little Bay, Montserrat and at Jolly Harbour and English Harbour,Antigua.

Relatively light ash and steam venting has been continuous since then. Wind direction shifted towards the north late on 21 May causing ash fall and raining mud on most parts of the island.

Scientists remain alert to the possibility of further explosive activity but seismic activity has been low since the event on Saturday.


The observatory's website: http://www.mvo.ms


Links:

Montserrat Tourist Office: www.visitmontserrat.com/index.php&

[Fredfredson]

Soufrière Hills volcano (Montserrat): New volcano dome growing

26-05-06 23:13
Age: 49 min

BY: TOBIAS

http://www.volcanodiscovery.com

New lava dome is growing

Report for the period midday 19 May to midday 26 May 2006:

A major lava dome collapse took place at the Soufriere Hills Volcano on the morning of 20 May 2006. The dome collapse involved an explosion that sent ash clouds to 17km (55,000ft) high.
Ballistic blocks up to 35 cm (14 inches) across and weighing 15kg (32 pounds) were found in craters on the northeastern flanks of the volcano.
When large-volume, hot pyroclastic flows hit the sea they generated a blast that sent surges in a northwesterly direction across the Spanish Point area setting vegetation alight. Extensive mudflows affected all flanks of the volcano and the northern slopes of the Centre Hills. A helicopterflight confirmed that all of the lava dome had gone, together with some remnants of the 2003 lava dome, leaving a broad, deep, eastward-sloping crater at the summit of the volcano.
The volume of the lava dome before the collapse was believed to be about 90 million cubic metres and most of this collapsed over aperiod of less than three hours. '
A large part of the dome remnant above Gages valley has also disintegrated leaving two pillars of rock. A further episode of ash venting late on 21 May coincided with winds to the north so much of the island was affected by ash fall.

On Tuesday 23 May, the new lava dome was seen for the first time.
The new dome lava is darker than the older residual material left behind by the collapse onthe 20th. Rockfalls have been seen on the northeastern and southwestern sides ofthe new lava dome. There are several active vents in the crater area, the largest is on the west side of the lava dome and this is responsible for the episodes of ash venting. Clear weather at noon on the 25th of May allowed a survey of the southern and eastern sections of the dome to be carried out using a terrestrial laser scanner(LiDAR on loan from the British Geological Survey).


This showed that the summit of the new dome had reached a height of 767m (2516ft), this is 148m(486ft) lower than Chances Peak. During the reporting period, seismicity was dominated by an increase in the number of hybrid and volcano tectonic earthquakes and a decrease in long period rockfall signals relative to the previous reporting period. The seismic network recorded 373 rockfall signals, 29 long period rockfall signals and 229 longperiod earthquakes, 89 hybrid earthquakes and 11 volcanotectonic earthquakes during the reporting period.


No measurements of the sulphur dioxide flux were possible during the reporting period due to the loss of instrumentation as a result of the collapse on the20th.
One hydrogen chloride to sulphur dioxide ratio was obtained on Tuesday23rd of May and yielded a value of 1.96.
This is lower than the values obtained for the previous reporting period and most likely suggests a drop in the extrusion rate between these two periods.

Residents of Montserrat and visitors to the island are advised to tune into ZJBRadio for up-to-date information on the volcano.

Access to all areas south of Richmond Hill, and south of Jack Boy Hill to Bramble airport and beyond is prohibited at all times. The daytime entry zone, comprising the top part of St.Georges Hill, is open from 6 am to 6 pm.

The maritime exclusion zone around the southern part of the island extends 3 km off shore from Whites Yard to Roches Yard on the east side of the volcano, 2 km offshore from Garras to Gingoes on the south-west, and 200 m offshore from Plymouth.



The new lava dome in the Soufriére hills volcano