Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 26 July
to 4 pm 27 July 1997

The alert level system has been revised by zones

Rockfalls and small pyroclastic flows have again dominated activity at the Soufriere Hills Volcano. Although no views were obtained of the dome, a few small ash clouds have been seen as a result of rockfall activity on the northern and western flanks of the volcano.

The number of rockfalls is slightly less than yesterday with 101 signals being recorded today compared to 131 yesterday. Most of these occurred in the upper parts of Mosquito Ghaut and the Gages valley. A small pyroclastic flow occurred in Gages valley at 9:07 this morning, and generated a considerable amount of ash, but did not generate a sizeable seismic signal. The only other seismic signals recorded were seven long period events, three of which triggered rockfalls.

The volume of the dome has been calculated by measuring the positions of points on the dome from photographs taken on 17th July, and the results show that the dome had a volume of 77 million cubic metres on this date. The last time that a volume could be calculated was 31st May when it measured 65 million cubic metres. Adding the volume of deposits to the amount of new material on the dome in the intervening period, this gives an average extrusion rate of over 5 cubic metres per second. This is a very fast sustained rate of growth, although it is likely that, at times during this period, the extrusion rate was even higher.

EDM measurements were conducted today on the new north-western triangle between the MVO, Garibaldi Hill and Lees Yard. Measurements were also on the Waterworks to Lees Yard radial line. The results of measurements made yesterday on this line indicate that no significant changes are yet visible.

The dome continues to grow and the potential for large pyroclastic flows onto the northern and western flanks of the volcano remains significant. The top of the Gages valley and Mosquito Ghaut have become substantially filled with debris from the growing dome. Residents should note that pyroclastic flows and ash eruptions have occurred during the past few weeks with no direct association with seismicity. This means that people entering the designated exclusion zone put themselves at extreme risk. In addition, people in the northern and central zones should stay alert and listen to Radio Montserrat.

The Belham River valley provides a pathway for mudflows which may develop rapidly following a period of prolonged rainfall. It therefore remains an area for anxiety over safety since it is also a potential pathway for pyroclastic surges. Mudflows can travel very fast and may be quite close to boiling point. In addition, they may extend much further along the river valley than the pyroclastic surges. As recent temperature measurements in the area just north of Farm's River indicate, pyroclastic flows and surges retain their high temperatures for several weeks after they were deposited. Residents are therefore urged not to approach, attempt to handle or walk on these deposits since they could sink in, due to the uncompacted nature of the deposit, and become severely burnt. Dust masks should always be worn when there is ash in the air.

Montserrat Volcano Observatory