Montserrat Volcano Observatory

Daily Report
Report for the period 4 pm 25 July
to 4 pm 26 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, clear conditions for most of the day allowed observation of increased rockfall activity on the northern flank of the volcano.

The number of rockfalls has increased further since yesterday with 131 signals being recorded today compared to 115 yesterday. Most of these occurred in the upper parts of Mosquito Ghaut and the Gages valley and were visible from the Observatory. A few small pyroclastic flows developed in Mosquito Ghaut but these did not extend further than about 300 metres from the dome. The only other seismic signals recorded were three long period events.

A late evening observation flight indicated that all of the rockfall activity was confined to Mosquito or Gages Ghaut. Both of these ghauts have continued to be filled with blocky scree deposits from the growing lava dome and are now significantly reduced in depth. There have been a few more rockfalls in the Galways area with several large blocks now resting near to the top of the scree slope. No new pyroclastic flow deposited were noted in this area. Steaming from the flank of the dome in the Tar River area was a bit more vigorous today than observed two days ago, but there was no evidence of any recent rockfalls in the area. The top of the dome was obscured for the entire period and no estimates were possible of the current height.

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. There has been a marked increase in rockfall and small pyroclastic flows into Mosquito Ghaut during the past 24 hours. In addition, the top of the Gages valley has become substantially filled with debris from the growing dome. These developments further illustrate the potential for large pyroclastic flows to occur in Mosquito Ghaut and the Gages valley. 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