******************************************************* MVO weekly report: New dome at Soufriere Hills Volcano ******************************************************* From: Lee D. Jones <ldjon@xxxxxxxxx> Montserrat Volcano Observatory Montserrat, W.I. 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 helicopter flight 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 a period 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 on the 20th. Rockfalls have been seen on the northeastern and southwestern sides of the 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 â?? 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 long period 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 the 20th. One hydrogen chloride to sulphur dioxide ratio was obtained on Tuesday 23rd 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 ZJB Radio 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. Georgeâ??s 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 Oâ??Garras to Gingoes on the south-west, and 200 m offshore from Plymouth. For further explanations of the volcanic phenomena described, and information about the Soufrière Hills Volcano and the ongoing eruption, please look at the observatoryâ??s website: http://www.mvo.ms. Noon, Friday, 26 May 2006 ============================================================== 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. ==============================================================
