Montserrat Volcano Observatory

Daily Report
Report for the period 16.00 26 May
to 16:00 27 May 1997
The current alert level is ORANGE

There has been a change in the nature of the volcanic activity, with signs of a switch in the area of dome growth to the east and south-east. There was one moderate pyroclastic flow this afternoon, which flowed down the Tar River at high speed.

The level of seismic activity has been moderate, with a swarm of hybrid earthquakes which began at 6:28 am and was still ongoing at the end of the reporting period. A total of 62 hybrid earthquakes were recorded, along with 3 VTs and only two long-period earthquakes. The level of rockfall activity was generally low during the period, although several discrete pyroclastic flows resulted in ash clouds.

The largest pyroclastic flow, at 3:18 pm, was of the biggest recorded since 15 May. It was of moderate size, and originated from near the top of the dome and went down the Tar River, reaching onto the delta and about 200 m from the sea. The ash cloud rose to at least 9,000 ft, and was blown to the north-west, resulting in light ash fall in Corkhill, Old Towne and Salem. The flow was observed very clearly from Harris, and was preceded by a large block of lava rolling down the slope of the dome. The most impressive feature of this flow was its speed- it is estimated to have reached the Tar River Estate House in less than 15 seconds, and the delta in less than one minute.

Visual observations from the helicopter this afternoon showed that the area of dome growth had changed, with a larger part of the dome being active. There were fresh rockfall deposits seen on the south-east and south flanks of the dome, that is in the south side of the Tar River valley and on the talus slope where the Galway's Wall used to be. The scar formed by the pyroclastic flows on 13 May showed some changes, and was probably the source of this afternoon's pyroclastic flow which occurred after the helicopter flight. Observers at Whites and Perches Mountain this afternoon saw many small pyroclastic flows on the eastern and south-eastern sides of the dome. The northern side of the crater was inspected from the helicopter, and the dome is now flush with the top of the wall at Tuitt's Ghaut. The crater wall is still protecting the top of Mosquito Ghaut from small pyroclastic flows, with the dome talus several metres below the top of the wall.

Measurements of the dome were made from Perches Mountain, Windy Hill, Harris and Whites today. These measurements and photographs will be used to determine the current volume of the dome.

There was a significant mudflow in Paradise River this morning, at around 8:40 am. The mudflow came from Mosquito Ghaut and Tuitt's Ghaut, and resulted from ash and soil being washed into the ghauts by heavy rain. The maximum thickness of the flow at Trants was reported to be 20 inches. Mudflows were also recorded in Fort Ghaut.

Rainwater samples were collected from several points around the volcano, and these will be chemically analysed as usual. COSPEC measurements made yesterday and today showed a level of sulphur dioxide emission from the volcano of about 950 tonnes per day, about the same as the values measured on 24 May. This is quite a high rate, and similar to values recorded during previous periods of enhanced pyroclastic flow activity.

The volcano has become more active, and the area of dome growth seems to have changed and increased during the last 24 hours. Significant pyroclastic flows are possible in the Tar River and White River valleys, and there may be little or no warning that these events are about to occur. The northern side of the dome is still active, and pyroclastic flows down the northern ghauts are possible. Nobody should enter zones A and B, and only essential visits should be made to the evacuated zone. Ash masks should be worn when in the ashy areas.

Montserrat Volcano Observatory