Centre for Volcanic Research, Cheltenham & Gloucester College of Higher
Education, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ
Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT
Following the first extrusion of juvenile material during Autumn 1995, activity at the Soufriere Hills Volcano has involved the construction of a lava dome complex within English's Crater in the vicinity of the old Chances Peak dome. Since the end of March 1996, and prior to the events of September 17th, instability of the growing dome has resulted in pyroclastic flows (PFs) entering the Tar River valley which drains the breached eastern side of Englishs Crater. Four major dome-collapse episodes occurred on 29-31 July, 11 August, 12 August and 2 September. Both the average and maximum sizes of PFs and their length has progressively increased, and on May 12th flows reached the sea for the first time. Prior to the September 17th event, PF production has been non-explosive, and driven purely by gravitational collapse of oversteepened dome flanks. The estimated dome volume at July 17th was 25 x 106m3, with an estimated 4.2 x 106m3 of dome-collapse derived material deposited in the Tar River valley. The most recent (August 25th) estimate of the dome volume, prior to the September 17th collapse, is 27-28 x 106m3, the largest since dome growth began.
After the episode of dome collapse and PF production on September 2nd, rock falls and PFs continued to be generated although at a relatively lower level. Few PFs were produced over the period September 7th to 15th, over which time the seismic record was dominated by shallow (<2 km bmsl) volcano-tectonic (VT) earthquakes and rockfalls, probably associated, respectively, with the entry of fresh magma into the high-level magma system and dome growth. On September 15th, vigorous steam and ash venting was noted at the dome and small PFs were generated. Activity reached a higher level on the 16th, with the near-continuous generation of rock falls and/or PFs during the middle of the day.
The activity increased further on the 17th with rock-fall formation near-continuous at times, and a phase of dome collapse and large PF formation began at 11.30, which can be viewed as the start of the eruptive episode. This consisted of four phases which can be distinguished in the RSAM record:
The start of the explosive phase at 23.42 resulted in saturation of all seismometers for the first time since reactivation of the volcano in July 1995, and appears to have been initially associated with further dome collapse and PF production. This is envisaged as unroofing the pressurised deeper levels of the dome and exposing the vent, thereby triggering a short-lived (around 48 minutes duration), but intense, explosive eruption. Residents of the nearby settlement of Long Ground present at the time, report feeling as if their houses were being repeatedly sucked towards the volcano, and this may have been the result of strong thermals associated with pyroclastic surges which destroyed the Tar River Estate House only a few hundred metres to the south. This activity was followed by the start of the explosion proper, which preferentially showered decimetre- to metre-sized blocks to the northeast of the dome, including over Long Ground, fifty percent of the buildings of which suffered impact damage. Falling blocks knocked out the Hermitage seismometer at 23.53, and the Chances Peak instrument suffered a power failure a few minutes later. Inhabitants of Long Ground report that large blocks were falling for several minutes, arguing against a single, discrete, directed explosion.
The eruption column generated produced tephra-fall over much of the southern half of the island, and although observations were difficult due to darkness, the rapid obscuration of the stars at Old Towne, together with intense thunder and lightning indicated significant column growth within minutes of the start of the explosive episode. Dispersal of coarse ejecta appears to have taken place over a time period comparable to the duration of the explosive event, while ash had ceased by 06.00 on the morning of the 18th. The finer component of the column formed an ash plume which spread rapidly beyond the island, and the Satellite Analysis Branch (SAB) of NOAA/NESDIS issued a Volcanic Hazards Alert (VHA) during the early hours of the 18th. This followed an ash encounter at 11 km altitude, within three hours of the explosive episode, by a civilian aircraft between 30 and 80 miles south of Antigua. By 07.30 on the 18th (all times local), SAB were reporting two separate ash clouds; the higher one moving east at around 40 knots and the lower travelling more slowly west at 15 knots. A later report timed at 14.07 relates an encounter between a civilian aircraft and volcanic ash at 3 km altitude between 60 and 80 miles west of Montserrat, and reports the closure of Guadeloupe airport due to ash covering the runway markings. At this time, satellite imagery revealed the plume width to be between 60 and 100 km, extending about 525 km to the east of Montserrat and 270 km to the west. By early evening on the 18th the SAB reported no visible plume on the satellite imagery, and the Alert was ended at mid-morning on September 19th.
A helicopter overflight on the morning of the 18th revealed that around 25 to 30 percent of the dome had been lost during the eruption, although how much of this occurred during the explosive phase - as opposed to the earlier collapse and PF formation - is unknown. Following around two weeks of inactivity, fresh magma appeared at the surface in early October, heralding the growth of a new dome, and growth continues at the time of writing (November 3rd).