Montserrat Volcano Observatory Weekly Report for the period 28 November 2008 - 5 December 2008 Activity at the Soufrière Hills Volcano increased with a series of four explosive events which started on the evening of Tuesday 2 December. The first and largest event occurred at approximately 9:35 pm local time on 2 December without any warning or precursory seismic activity. The event started with a small dome collapse which was followed, within 15 seconds, by a pyroclastic flow. There was then an explosion, possibly two explosions, within 90 seconds. The explosion sent large incandescent blocks up to a mile from the dome and these were seen scattered over the north-western slope of Gages Mountain. Some of these were later seen to have generated substantial impact craters. There were no reports of air-fall pumice or ballistics in inhabited areas. The explosion and pyroclastic flow both generated ash columns which were accompanied by lightning strikes. The ash is estimated to have reached a height of 40,000 feet above sea level. The ash was blown to the west over Plymouth and there was no ash fall in inhabited areas. The pyroclastic flow was on the western flank of the volcano. It travelled down Gages Valley and then into both Lee's Yard and Plymouth. It is estimated that the flow reached the sea in Plymouth within two minutes of the start of the event. Some structures in Plymouth were set on fire and could be seen burning from Salem for several hours afterwards, with one fire near the Secondary School lasting well into the next day. The pyroclastic flow generated extensive pyroclastic surges. These contain hot ash and gas but no rock fragments and are not restricted by valleys in the way that pyroclastic flows are. One pyroclastic surge rose up the south face of St. George's Hill and only just failed to go over the top of the hill. Other surges reached Aymers Ghaut in the south and Richmond in the north. The surges set fire to trees, bushes and pieces of timber throughout the affected areas. The deposits from the pyroclastic flow appear to contain very little pumice, indicating the source material came from the dome and was not fresh lava. This is consistent with a dome collapse. The subsequent explosions would have been triggered when the collapse exposed regions of hot, pressurised lava. Due to cloud cover, it has not yet been possible to estimate how much of the dome was involved in the collapse. The subsequent three explosive events were smaller than the first. The second event was an explosion at approximately 8:43 pm on 3 December which scattered incandescent blocks over the north-west side of Gages Mountain. The third event, at 4:22 pm on 4 December did not appear to eject any ballistics, but sent a forceful emission of ash to heights of up to 15,000 feet. Neither of these events generated pyroclastic flows. The fourth event was similar to the first, although smaller. It occurred at approximately 5:41 am on Friday 5 December. The explosion again scattered incandescent blocks over the north-west face of Gages Mountain and a pyroclastic flow was generated in the Gages Valley. This travelled towards Plymouth but does not appear to have reached the sea. The ash cloud from this event rose almost vertically above the volcano and drifted slowly to the north-west over Olveston and Woodlands. There were no reports of ash fall. There was a subsequent small but forceful ash emission at approximately 8:18 am. Since the first explosive event, there have also been three small pyroclastic flows in the Gages Valley. These occurred at approximately 09:01 am on 3 December, 5:32 am on 4 December and 00:06 am on 5 December. These pyroclastic flows had a different character to the two flows that were associated with the explosive events. They were relatively slow moving and did not travel more than about 2 miles from the dome. In the periods between these events, the volcano has been more active than beforehand, with near-continuous emission of ash-laden vapour. There has also been an increase in seismic activity. MVO recorded 35 volcano-tectonic, 4 hybrid and 19 long-period events as well as 34 rockfalls during the week. There were also two episodes of continuous low-level seismic tremor. Almost all of these were after the explosive event on 2 December. There were no measurements of sulphur dioxide emissions during the week due to an equipment malfunction. Measurements resumed on Friday 5 December. There have been no pyroclastic flows in the Tar River or White River valleys associated with this activity. There was one pyroclastic flow in the Tar River Valley at approximately 3:54 pm on Friday 28 November which reached the sea. This flow would have been generated by a collapse from the steep cliff on the eastern side of the dome and is not associated with the subsequent activity on the west side of the dome. Due to the presence of cloud cover, it is not yet possible to give a full description of the events of this past week. However, it is clear that the explosive event on Tuesday night has opened and enlarged the previously quiet vent on the north-west flank of the dome. All the ash emissions and explosions have come from an area around this vent. The three sluggish pyroclastic flows in the Gages Valley may be associated with extrusive activity in the Gages vent, which was opened up by the explosion on 28 July 2008. If the activity continues at about the same level, the volcanic hazards will be restricted to zones C and V of the Hazard Level System. The position of the north-western vent raises the possibility of pyroclastic flows on the northern flank of the dome, but there has been no indication of any activity yet. The Hazard Level remains at 3. The activity this week has demonstrated that explosions and pyroclastic flows do occur without any warning whatsoever. Mudflows can also occur without warning, especially when there is heavy rainfall. Additional information on the Soufrière Hills Volcano and the Hazard Level System can be found at the MVO website: http://www.mvo.ms. Roderick Stewart Acting Director, Montserrat Volcano Observatory tel: +1 664 491 5647, +1 664 491 5726 (direct line) home: +1 664 491 3139 cell: +1 664 495 0743 fax: +1 664 491 2423 email: rod@xxxxxx ============================================================== To unsubscribe from the volcano list, send the message: signoff volcano to: listserv@xxxxxxx, or write to: volcano-request@xxxxxxxx To contribute to the volcano list, send your message to: volcano@xxxxxxxx Please do not send attachments. ==============================================================
