Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 16 October
to 6 pm 17 October 1997

There have been four explosions during this reporting period and a number of other long-period earthquakes correlating with ash venting. The explosions were slightly more powerful than those of yesterday, producing higher ash columns although the pyroclastic flows were still relatively small. There was some evidence for a phreatic (explosive interaction with ground water) component of at least one explosion today.

Explosions occurred at 18:48 last night, 4:01 this morning ,12:35 and 16:05 this afternoon. All of the explosions produced steamy ash clouds which drifted very slowly north-eastwards no ash fell on inhabited areas of Montserrat. Ash clouds were in the range of 15,000 to 25,000 feet high. Vigorous ash venting followed each of the explosions.

The explosion which occurred at 12:35 this afternoon was preceded by approximately one hour of elevated tremor associated were associated with pyroclastic flows on all flanks of the dome. During the explosion pyroclastic flows were only generated down Tuitts Ghaut. In addition observers noted small phreatic explosions occurring in the Tar River valley prior to the 12:35 event. Following the 12:35 explosion there was a significant collapse of the eastern side of the dome, lasting 30 minutes. This produced pyroclastic flows down the Tar River valley which reached the sea. Temperature measurements of these pyroclastic flow deposits showed that these flows were comprised of relatively cold, dense dome material. This indicates that these flows were sourced from the older dome as opposed to having been generated from column collapse.

The explosion at 16:05 today had an apparent phreatic component. The initial explosion was directed northwards and an associated pyroclastic flow moved down Tuitts Ghaut. Some 30secs after the initial explosion a small flow travelled down the upper part of the Gages valley.

The seismic network recorded 13 long-period events, and 3 rockfall signals during the reporting period. There was no particular pattern to these events, although several of the lp events were quite large and were associated with ash venting - these are interpreted as minor explosions.

The wind direction to the east has kept ash levels down in inhabited areas. The amount of ash in the air both in Northern and Central Montserrat is low. Readings are being taken for Central Montserrat in the Olveston area and in Northern Montserrat in the Mongo Hill area. Readings are taken as an average over the period of the report. Peaks in ash levels still occur, and ash masks should be worn routinely in ashy environments such as close to roads, especially when it is dry.

There is still a possibility that larger explosions could occur without any changes in the seismicity or other parameters. Therefore all residents of Montserrat are reminded to be vigilant, to avoid entering the evacuated zone and to stay tuned to Radio Montserrat for further information.

Montserrat Volcano Observatory