Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 20 March
to 16:00 21 March 1997
The current alert level is AMBER

The level of activity at the volcano has been slightly higher than yesterday. A low intensity earthquake swarm occurred this morning, but the main activity throughout the day has been moderate sized rockfalls and pyroclastic flows from the growing dome.

A further hybrid and volcano-tectonic earthquake swarm has occurred in the current reporting period. This started at 10:19 pm last night and continued to 11:56 am this morning, with 16 hybrid earthquakes and 8 volcano-tectonic earthquakes recorded. One particularly large hybrid earthquake at 4.15 am was followed by a rockfall signal and it is likely that this indicated a small pyroclastic flow. The swarm was not as intense as other, most recent swarms. All of these earthquakes were located at shallow depths beneath the crater. 19 rockfall signals were recorded by the network today, which is about the same number as yesterday. Two (2) long period events have been recorded today.

The volcano was extremely clear for all of today, and good views were obtained from various points. The main visual observations today were of small pyroclastic flows down the north-east face of the dome in the direction of Long Ground and southern face of the dome towards Perches Mountain. Some small rockfalls and pyroclastic flows were also observed over Galway's Wall in the direction of St. Patrick's. None of these pyroclastic flows travelled further than 1 km from the dome. The morphology of the summit dome has changed considerably in the last two days. This includes the collapse of the blunt spine, and growth of a new smooth-sided spine in the western part of the summit area. Frequent rockfall activity from the summit region has led to the development of a small chute to the north-east of the summit. At 11:30 am there was also a period of approximately 30 minutes of ash-venting from the rock debris on the lower north-east slope of the dome.

Static photographs of the dome were taken today from Whites, Windy Hill and Harris Lookout and will be used in an assessment of the movement of the pre-September dome. Photographic evidence and other observations indicate that this northern part of the dome has moved outwards slightly towards Tuitts' Ghaut and Gages, and may be a result of the weight of the dome pushing the wall northwards. This same region displays vigorous steaming.

The height of the dome was measured using a theodolite from Whites: these results show that the top of the dome is now at a height of 958 m or 3144 feet; this is 144 feet higher than Chances Peak. The volume of the dome, as measured in a helicopter survey two days ago, has grown by 0.8 million cubic metres over the previous 5 days. This gives an average extrusion rate of 1.9 cubic metres per second which is double the previous extrusion rate, but still below the extrusion rates of the dome in January and February.

The EDM lines between St. George's Hill to Farrell's and Windy Hill to Farrell's were occupied today. A GPS survey of the depth of water around the pyroclastic fan deposits at the mouth of the Tar River was carried out today. Results from these measurements will be released in a later report. MVO would like to thank the Police Commissioner and Inspector Morson and the crew of the police launch for their valuable help today.

The dome is now continuing to grow freely in the summit region. The dome is now very large, and a large collapse and pyroclastic flows could happen with little warning. It is dangerous to spend the night in evacuated areas, because the situation could worsen rapidly over a period of a few hours. The night-time visits by scientific teams into zones B and C are part of the routine monitoring and should not be taken as an indication that it is safe to spend the night in such areas. People entering Zone C are reminded to remain alert at all times, and spend the minimum possible time in the evacuated zone. The Tar River Valley and the upper Galway's area are very dangerous and should not be entered at any time. People visiting the north-eastern flanks in zone C should remain attentive while there is increased activity of the dome in the north adjacent to the Farrells wall

Montserrat Volcano Observatory