Dome growth and collapse have continued. Large pyroclastic flows in the Tar River valley reached the sea on 13 and 16 January, with the flow on 16 January estimated as the largest single pyroclastic flow during the eruption, on the basis of the amount of material that reached the delta at the bottom of the valley. Most of the pyroclastic flows have been initiated on the south-east side of the October 1 dome, and travelled to the south of Castle Peak and down the south side of the Tar River valley. The 16 January event removed a large volume of material from the October 1 and December 11 domes, and left a large scoop-shaped depression. Fresh lava started to in-fill this excavation in subsequent days.
Growth of the "Santa lobe" on top of the October 1 dome continued during the week, concentrated in the area above Castle Peak, and to the south-west and west of the peak. At the beginning of the week the rate of growth seemed to be low, and daily changes were minor, concentrated at the active face (above Castle Peak) and also at the north-west of the October1 dome. Night incandescence was limited to the eastern face, in contrast to the previous week when much of the eastern and northern flanks of the dome were active.
Large pyroclastic flows occurred on 13 January (11:30 to 13:00 and 22:15 to 23:05), and 16 January (05:30 to 06:30). In all three episodes of activity the pyroclastic flows originated from the south-eastern part of the October 1 dome, close to the December 11 dome, and travelled down the south side of the Tar River valley to the sea. The flows around midday on 13 January formed three thin lobes on the fan surface, to the NNE, east and south-east, which extended to the sea. Smaller flows occurred later the same day. The combined flows on 13 January eroded a chute to the south of Castle Peak, and created a scar in the October 1 dome behind Castle Peak.
Following the pyroclastic flow activity on 13 January, rapid growth re-started within the scar left by the collapse. This new growth was first observed on 14 January, when an incandescent face was observed at the top of the chute, and small rockfalls produced relatively large ash clouds. The growth continued to be concentrated in this area, with the Castle Peak chute remaining stable. Continuous steam and ash venting from the dome was observed on the afternoon of 14 January.
The pyroclastic activity in the morning of 16 January was short-lived, but was estimated to be the largest single flow in this eruption. The activity built up quickly, and reports from the airport described a rumbling of rocks and ash accompanying a glowing cloud down to the sea. The entire fan was covered by a thick deposit of new material, including blocks 1-2 m in diameter (up to 5 m diameter at the head of the fan). The deposit was quite narrow in the upper reaches of the Tar River valley. The southern chute cut back deeply into the dome to the south of Castle Peak, creating a large amphitheatre cut into the mass of the October 1 dome (Santa lobe) and the December 11 dome. After the pyroclastic flow activity, a prominent scar was noted, running north-east to south-west across the dome. The scar was estimated to be about 8 metres deep.
The ash cloud from this activity rose to at least 20,000 ft. The low level ash was carried to the south-west, causing heavy ash fall in Plymouth. At higher levels, the winds blew the ash to the south and east, and light ash fall was reported from Guadeloupe.
Once again, growth continued in the same area following the flows of 16 January, and there were rockfalls and small pyroclastic flows during the remainder of the period.
Table 1: Earthquake types: 12 to 18 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 12 JAN 97 6 0 4 47 13 JAN 97 63 0 21 84 14 JAN 97 44 3 1 77 15 JAN 97 17 2 7 36 16 JAN 97 33 9 4 30 17 JAN 97 3 6 8 42 18 JAN 97 37 7 27 46
Rockfall and pyroclastic flow signals continued to dominate the seismicity during this period. There were several swarms of volcano-tectonic earthquakes (table 2). As usual, all the locateable earthquakes in these swarms were at shallow depths beneath the crater. All of the major pyroclastic flows were preceded by earthquake swarms, suggesting that the earthquakes may be caused by pulses of magma moving to the surface, with these pulses then causing dome collapses a short time later. Out of 9 earthquake swarms, 3 lead to pyroclastic flows in the following 27 to 66 minutes.
Table 2: VT earthquake swarms
Date Start Stop Duration No of VTs PF time Gap (hrs) 130197 0847 1025 1.47 20 1136 1.10 130197 1856 2127 2.52 27 2217 0.80 140197 0005 0148 1.72 9 140197 0518 0630 1.20 12 140197 1413 1542 1.48 12 150197 1731 1825 0.90 13 160197 0341 0513 1.73 12 0530 0.45 160197 2226 2307 0.68 16 180197 1757 1924 1.45 21
Low-amplitude banded tremor was also observed during this period, at a much lower amplitude than in late December. The tremor was first visible on the drum recorders on 15 January, and occurred until the end of the reporting period. The typical tremor duration was about 2 hours, and the tremor episodes were about 8 hours apart. The amplitude of the tremor increased slightly during 15 January, prior to the collapse early on 16 January. Another tremor episode started before midnight on 16 January, and they continued at regular intervals until the end of the period.
Ground Deformation And Gravity Measurements
No Electronic Distance Meter (EDM) measurements of the eastern triangle have been possible, because the reflector on Castle Peak is covered by ash. No other EDM measurements were made.
The BIGNET GPS network was occupied on 17 January. There has been no significant change in length since the network was first measured in June 1996. A new network, "CLOSENET" was established on 18 January. This network has four points, at Chances Peak, Farrells Wall, Hermitage and Perches Mountain, with a base station at Harris.
No microgravity measurements on existing network sites were made. On 13 January the northern radial line (Farrells Road to Farrells Target) was further extended towards Farrells crater wall.
The extensometer across one of the cracks of Chances Peak shows continued slight extension of the crack, totalling 5 mm over the week. In contrast to the constant rate of deformation during the previous week, there were two pulses of deformation, on 13 and 18 January, when 2-3 mm of extension occurred (not 3 cm as reported in the daily report of 14 January). On 12 January, a visit was made to Chances Peak to remeasure crack widths. The rate of deformation was similar to past results, with opening over 12 days of 7 mm on the crack nearest Chances Peak, and 27 mm on the crack further to the east.
Some minor landslide activity was observed from the outside of Galway's Wall during a helicopter flight on 14 January, with new talus deposits at the western end of the wall, and smaller new deposits at the east side.
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 on 14 and 16 January, by driving the COSPEC instrument along the road to the south-west of the volcano. On 16 January, measurements were also made from offshore, along lines off the south-west coast. The boat traverse yielded average sulphur dioxide values of 344 tonnes/day at 5.5 km from the dome and 233 t/d 8.5km from the dome. On 16 January measurements were made within a few hours of dome collapse. Due to the wind direction, it was only possible for half of the plume to be measured. A value of 1017 tonnes/day for half plume width was calculated.
The routine collection of water samples and deployment of sulphur dioxide diffusion tubes continues at sites downwind of the volcano. Recently received sulphur dioxide concentration results show a gradual decrease throughout the period 1 October to 30 December down to levels similar to those prior to the September 1996 explosion event.
Willie Scott, USGS
Rick Hoblitt, USGS
Simon Young, BGS
Jenni Barclay, BGS