The dome continues to grow in the south-eastern part of English's Crater, near Castle Peak. There was another collapse of the dome on 20 January, which resulted in another pyroclastic flow to the sea and an ash cloud to about 30,000 ft. This collapse was preceded by a volcano-tectonic earthquake swarm, in a similar fashion to recent collapses. Several other swarms occurred during the period, and for 4 days were repeating regularly. A new extrusion was seen on 22 January, in the scar left by this collapse. Visibility was poor for the remainder of the week.
There was rapid growth of new extrusions within scars formed by collapses of the south-eastern flank of the composite October-December dome. This was a continuation of the previous pattern of dome growth. Some activity was also noted on the north-eastern and northern flanks of the dome. Progressive dome growth was seen on 19 and 20 January, with marked changes from each previous day visible from Whites. The visibility was generally poor on 19 and 20 January.
A period of quite intense pyroclastic flow activity began at 6:30 pm on 20 January, and lasted for about one hour. Pyroclastic flows originated from the south-eastern part of the October-December composite dome, and travelled down the southern Tar River to the sea. The volume of material reaching the sea was not as high as that during the activity of 16 January. The flows ignited some trees on the lower slopes of Perches Mountain, on the south side of the Tar River valley. A scoop-shaped excavation was carved into the south-eastern flank of the composite dome, in the same place as the 16 January excavation. No detailed estimates of volume lost are available, but the scar was estimated to be slightly larger than that of 16 January.
The ash cloud resulting from the pyroclastic flow rose to about 30,000 ft. At low levels, the ash was carried to the south-west. The upper level winds carried ash to the north-east, where it mixed with rain and fell as wet ash over most of northern Montserrat. The higher amount of ash production on 20 January, compared with 16 January, is possibly because the 20 January lava was hotter.
New growth in the scar formed by the pyroclastic flow activity was seen on 22 January, about 40 hours after the activity. A large flower-shaped extrusion appeared within the scar, and the highest point of the extrusion was just above the height of the scar rim. The extrusion comprised large, angular, grey blocks, with radial cracks separating the blocks.
Visibility was poor for the rest of the week, and no further views of the new growth were possible.
Table 1: Earthquake types: 19 to 25 January 1996
These earthquake counts are of events that triggered the short-period seismic network event recording system between 0000 and 2400 each day.
Date VT LP Hybrid Dome RF 19 JAN 97 29 4 28 41 20 JAN 97 24 3 21 35 21 JAN 97 19 3 12 25 22 JAN 97 83 8 6 21 23 JAN 97 38 3 7 31 24 JAN 97 36 3 6 38 25 JAN 97 27 2 1 19
Apart from the pyroclastic flow on 20 January, the seismic activity has mostly been dominated by swarms of volcano-tectonic earthquakes (table 2). Hybrid earthquakes became less common. As usual, all the locateable earthquakes in the VT swarms were at shallow depths beneath the crater. The pyroclastic flows on 20 January were preceded by a VT swarm, in a similar way to the pyroclastic flows of the previous week. From 21 January, the swarms became fairly regular, occurring twice per day at about 12 hour intervals. This pattern was similar to that observed in the two weeks prior to the explosion of September 1996, when the time interval between the swarms gradually decreased to 4 hours immediately prior to the explosion. Almost all the VT swarms were accompanied by several long-period earthquakes, with dominant frequency about 1 Hz or lower. The LP earthquakes tended to occur at the end of the swarms, and there were between two and five LPs with each swarm.
Table 2: VT earthquake swarms
Date Start Stop Durn No VTs PF time Gap (hrs) 190197 0513 0645 1.53 21 200197 1544 1718 1.57 22 1824 1.10 210197 2120 2227 1.12 19 220197 0751 0947 1.93 34 220197 1749 2113 3.40 48 230197 0540 0723 1.72 16 230197 1654 1908 2.23 15 240197 0306 0525 2.32 36 250197 1102 1325 2.38 26
Ground Deformation And Gravity Measurements
The northern EDM triangle was measured with the MVO total station on 22 January. No changes in line length were detected that were greater than the measurement error.
The GPS network BIGNET was occupied this week, including the Tar River EDM site. No baseline shows any significant length changes.
Static gravity measurements were made on the 22 January and 24 January. Two radial lines were read. On the west flank of the volcano from Brodericks dry tilt station to the shoreline and on the east flank from Whites Yard to Spanish Point.
The two prominent cracks on Chances Peak were measured on three occasions this week. Both Cracks are located at the Northern intersection of Galway's Wall and Chances Peak. Crack 1, the most southerly of the cracks orientated NE-SW, has shown an acceleration in opening. From the 12 to 22 January, extension was 0.69 cm/day and shear 0.31 cm/day. From the 22 to 26 January extension increased to 1.6 cm/day and shear 0.7 cm/day. From 26 to 28 January extension increased to 1.75 cm/day and shear remained at 0.7 cm/day. Crack 2, the most northerly of the two cracks is transgressive but has a general trend of NE-SW. The extensometer showed steady movement of 0.7 cm/day over the last week. This was confirmed by measurements on the 22, 26, and 28 January. Shear increased slightly from 0.06 cm/day to 0.25 cm/day between the 26 and 28 January.
The tiltmeter on Chances Peak is out of action because of a power failure.
Dome Volume Measurements
No dome volume measurements were made in this period, because of mechanical problems with the helicopter and poor conditions
Measurements of sulphur dioxide flux were made with the COSPEC instrument from the road to the south-west of the volcano on 21 (1260 tonnes per day) and 24 January (590 t/d). The 21 January measurement is high, and typical of recent measurements made immediately after dome collapses. The 24 January measurement is similar to the background level of about 400 t/d made in recent weeks.
Results from dust samples collected in early October have been received, and show that some samples, especially those collected in Plymouth, have high numbers of particles less than 0.7 microns in size. The level of silica in the samples averages at 8-9 %. The rate of air sample collecting has been increased, with samples collected twice per week.
Water sampling continues, with acidity levels noticeably higher (pH 2-3) in the Amersham area, as a result of acid rain forming in the plume. This effect decays with distance. More rigorous collection of rainwater samples, and further interpretation of the recent results are planned.
The deployment of sulphur dioxide diffusion tubes continues downwind of the volcano. No new results have been received.
Arrivals John Shepherd, Lancaster University / BGS
John Power, USGS
Richard Luckett, Leeds University / BGS
Maggie Mangan, USGS
Josclyn Knight, UWI