Montserrat Volcano Observatory

Weekly Report
Report for the period 5 p.m. 2 July 1999
to 5 p.m. 9 July 1999

The level of activity at the volcano remained low this week, although there were two small explosions and several other ash venting and collapse episodes.

The seismic networks recorded a total of 83 rockfall signals, 36 volcano-tectonic earthquakes and 1 long-period earthquake over the week.

The most significant event during the week was an explosion on Saturday 3 July at 8:50 pm. Clear conditions enabled good observations of this event, which was preceded by rockfall activity in the Gages valley and produced a rapidly rising ash cloud which reached at least 12,000 feet. The initial explosion was followed by a period of ash venting. Abundant lightning was produced within the ash cloud, which was distributed to the west. A similar though smaller explosive event occurred at 1:45 pm on Wednesday 7 July, with ash reaching about 8,000 ft and again being distributed to the west. Both of these events appeared to be explosions driven by rapid expansion of gas pockets within the dome.

Other events of note during the week were 3 small collapse events which generated ash clouds and were followed by ash venting. No significant pyroclastic flows were noted from these events, although the one at 1:35 on Saturday 3 July did produce a small flow deposit in the Tar River Valley.

Heavy rain produced mudflow signals at 4 and 8:30 am on Thursday morning, 8 July; some rocks and mud were deposited on the Belham Bridge, temporarily blocking it.

Excellent visibility on Monday 5 July enabled good observations of the dome to be made. There are unstable areas on the northeast, west and southwest flanks of the dome which could collapse at any time producing pyroclastic flows and ash clouds.

No COSPEC measurements were taken during the week.

Residents of Montserrat are advised to keep listening to ZJB Radio Montserrat for information in case of any changes in the state of the volcano.

Montserrat Volcano Observatory