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Montserrat Soufriere Hills Volcano at Alert Level 4
The Alert level at Montserrat Soufriere Hills Volcano remained at Level 4 on January 9th after explosive activity was observed one day previous. The Level 4 alert signifies that "large unconfined dome actively growing towards the north or west; or, large dome with: high levels of pyroclastic flow activity in other directions and/or high rates of dome growth; or intense earthquake swarms or tremor; or with tropical storm imminent or already affecting the island." The explosive activity was described on the Montserrat Volcano Observatory as follows: 08 January 2007 - Explosive activity
Pyroclastic flows travelled down Gages Valley and Tyres Ghaut into the head of the Belham River Valley simultaneously suggesting source high on dome directly behind Gages Mountain. At 06:15 the largest pyroclastic flow to date flow entered the Belham Valley, reaching as far as the edge of Cork Hill, a run out distance of around about 5 km. At 06:25 the ash cloud was reported to have reached 30,000 ft. Between 06:20-06:25 seismic activity began to wane and by 06:30 seismicity had returned to just above background, dominated by isolated PF signals (extended duration rockfall signals, none or which was especially large, and definitely not suggestive of a collapse) and small long period earthquakes. Pyroclastic activity in Tyres Ghaut continued at higher levels than recent days over the next approx. 1.5 hours establishing an almost regular 7 minute cycle. The run out distances of the individual flows were limited to 1-1.5 km, and each flow was preceded by a minor pulse of ash venting. At about 08:20 a short period of rumbling/roaring preceding pulse ash venting. By 09:30 seismicity had returned to background levels. Pyroclastic activity is ongoing on the NW sector of the volcano with small flows in Tyres Ghaut, run out distances up to 1.5 km. Low-level ash and steam venting continus from a vent high up on the western side of the lava dome. Fortunately the prevailing wind carried ash out to the WSW at lower levels (<about 15,000 ft) and at higher level to the ESE, so northern Montserrat remains unaffected by any significant ashfall from this event.
Preliminary observations of the dome suggest that
very little material has been removed, although the dome about the head
of Tyres Ghaut appears to have been eaten back to form a chute, possibly
making it easier for large volumes of dome material to enter Tyres Ghaut
(and Belham Valley) during future events." |
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