Montserrat Volcano Observatory

Daily Report
Report for the period 16:00 16 December
to 16:00 17 December 1996
The current alert level is ORANGE

The level of activity at the volcano has increased slightly during the last 24 hours, with continued growth of the December 11 dome. This has resulted in many moderate-sized rockfalls from the dome down into the upper Tar River valley, and some minor ash columns that drifted to the north and east. The fresh dome growth means that the Tar River and Long Ground areas are at risk from pyroclastic flows. Although the deformation of the Galway's Wall has slowed over the last few days, a sudden failure of the wall could still occur and this would have serious consequences for areas in zone A/B. Nobody should be in this zone. Zone E remains safe at this time.

Visibility was excellent this morning and a helicopter survey of the dome was carried out. The December 11 dome has started to overflow the September explosion scar, and there have been small pyroclastic flows into the Tar River valley to the south of Castle Peak. The flows have travelled to about 250 metres from the dome. The height of the December 11 dome is 909 m, or almost 3000 ft. The dome is conical with a flat top and two spines on top. The surface is very rubbly with coarse blocks.

Comparison of recent dome surveys with previous results shows that there has been uplift of the older material near the December 11 dome. Some parts of the older dome have lifted by about 80 m, involving a volume change of perhaps 7 million cubic metres since the beginning of December. There has been very little change in the height of this part of the dome during the past few days, indicating that the change happened earlier in the month.

This afternoon several large steam clouds containing small amounts of ash were seen above the volcano. Poor viewing conditions prevented the source of these clouds from being determined, but they are probably caused by steam venting from the new dome.

The level of seismicity was low during this period, and was dominated by rockfalls from the growing dome. There were 17 dome rockfalls. The largest, at 3:24 pm and 3:53 pm today were associated with moderate ash clouds. The number and size of the rockfalls has increased since yesterday, and some of the rockfalls have been associated with low-amplitude signals that have lasted for several minutes. Eighteen small, long-period earthquakes were recorded today which is an increase since yesterday.

EDM measurements were attempted today on the eastern triangle, which measures lines to a reflector on Castle Peak. However steam and ash clouds prevented the measurements from being made.

Dr Mark Stasiuk arrived on Sunday to join the MVO team. He will be involved in GPS and dome volume surveys.

The siren system will be tested tomorrow afternoon between 3 and 6 pm.

Montserrat Volcano Observatory