Montserrat Volcano Observatory

Daily Report
Report for the period 10 pm 6 August
to 4 pm 7 August 1997

The 10-12 hour pattern of explosive volcanic eruptions and hybrid earthquakes has continued at the Soufriere Hills volcano. There were two intense periods of activity with vertical eruption columns at 00:34 am this morning and 12:04 pm this afternoon.

Following the early morning rainfall, the dome cleared for about 20 minutes today and allowed the first views to be obtained of the summit since the current phase of elevated activity began one week ago. A bowl-shaped crater with a lower edge facing towards Gages mountain is now present at the top of the dome. The Gages valley is deeply incised. Static photographs were taken so as to allow future calculations of the volume of the dome and the amount of material removed during the last few days.

137 hybrids, 9 rockfalls, and nine long period events were recorded during the day up to 4:00 pm. No volcano-tectonic earthquakes were recorded. Most of the hybrid earthquakes occurred in two swarms, the first started at just after 1:30 am and continued until after 4 am. The second swarm started at 10:34 am and continued until 2:34 pm.

Both periods of activity culminated in vertical eruption columns and intense pyroclastic flows at 00:34 am and 12:04 pm. The explosion columns rose rapidly to between 15000 and 20000 feet spreading at the top to form an umbrella shaped column. The eruption which occurred at 12:04 pm was the most intense of the two recorded during the and produced pumice clasts up to a maximum of 1.5 cm in St Johns. Both eruptions were accompanied by loud audible rumblings and produced pyroclastic flows which descended the valley which radiate from the volcano. Observations from the MVO, Flemmings and Jack Boy Hill indicate that the pyroclastic flows were generated by collapse of the eruption column.

During the midnight eruption an eruption column rose rapidly to in excess of 10,000 ft. Immediately following the start of the eruption incandescent debris was observed to fallback onto the northern and western flanks of the volcano feeding pyroclastic flows which descended the Gages valley, Mosquito Ghaut and possibly Tuitts Ghaut. The event saturated the seismic drum recorders for about 2.5 minutes and was followed by a period of low frequency tremor which lasted for about 80 minutes.

Pyroclastic flows from midday activity was again observed and documented by MVO field teams positioned at strategic locations around the volcano and in the helicopter. Estimates of the ascent rates of the plume indicate that it ascended rapidly up to 1000 m at a maximum rate of 47 m/s. Flows had descended Tuitts Ghaut up to Farms; Mosquito Ghaut up to Harris; Tar River to the sea and had also entered the Gages valley. Flows from both eruptions also entered the upper parts of the Belham valley but did not descend much further than Dyers.

The activity continues to follow a cyclical pattern with an approximately 10 to 12 hour periodicity. The periodicity shows up well in the earthquake data, and once a hybrid earthquake swarm starts, people are advised to move out of the central zone. People are also advised to remain outside the evacuated areas during the entire night.

The Gages valley is now filled with hot pyroclastic flow deposits and under no circumstances should people venture into this area. It is expected that as the current elevated level of activity continues further pyroclastic flows would occur in the Gages valley or other flanks of the volcano. This makes Plymouth extremely dangerous. The Belham River valley is also very dangerous and should not be entered at all. Access to the exclusion zone is completely restricted, and people should stay completely away from any of the flanks of the volcano. The central zone is evacuated overnight and people should not return to their homes until advised to do so. Everyone should continue to stay alert, and listen to Radio Montserrat for any announcements.

Montserrat Volcano Observatory