Summary
Activity throughout December was generally low. There were several small dome collapses that generated pyroclastic flows and were followed by vigorous ash venting. Some of the dome collapses had an explosive component at the onset of the event. Episodes of ash venting occurred almost daily and were of varying vigour. Seismicity was dominated by volcano-tectonic earthquakes and rockfalls. The number of volcano-tectonic earthquakes declined towards the end of the month whereas the number of long period signals increased slightly. Long period signals appeared to correspond to ash venting episodes with a mildly explosive onset.
Visual Observations
Periods of volcanic tremor were recorded almost daily throughout December and were observed to coincide with episodes of steam and ash venting of varying vigour. A spectrum of activity was observed throughout the month from brief episodes of weak steam and ash venting to extremely vigorous pulse-like ash venting commonly following dome collapses or small explosions.
A pyroclastic flow generated by a dome collapse at 06:52 on 14 December reached the sea at the Tar River delta. A large ash cloud was generated that rose rapidly to about 20,000 ft. A short period of tremor and ash venting followed the collapse. The deposits consisted of fluidised and fine-grained material with very few blocks. A maximum temperature of 309 *C was recorded later the same day at a depth of 15 cm, 1.5 m from the margin of the deposit. Three days later the temperature of the deposit was measured again and a maximum of 203 *C was recorded (Table 1). Ash fall from this event occurred to the west and north-west of the volcano, attaining a thickness of 2 mm in Salem, and containing accretionary lapilli up to 2 mm in diameter.
On 19 December at 10:34, a pyroclastic flow again reached the Tar River delta. The flow reached the coast in less than 5 minutes. Powerful black jets of ash and rock burst from the dome at the onset of the event but it is unclear if this explosive activity preceded or followed the dome collapse. The deposit was small in volume and was almost entirely confined to the incised channel in the Tar River valley where it was emplaced on top of the 14 December deposits. There was little surge deposit on the slopes above the channel. The maximum temperature of this deposit was 297 *C measured 6 m from the margin of the deposit at a depth of 50 cm (Table 1).
On 21 December at 15:13, at the onset of a sudden, large seismic signal, dense black jets of ash and vigorously convecting ash clouds were observed coming from the main vent in the July 3rd scar. Ballistic blocks were carried to heights of about 80 m above the vent. Very vigorous ash venting continued for over 30 minutes after the initial explosion. No pyroclastic flow deposits were observed following this event. The seismic signal had a long period component.
A minor dome collapse occurred on 27 December at 17:14, and a small volume pyroclastic flow reached the Tar River delta. Poor visibility hampered observations but a significant ash cloud was generated and there was ash fall mixed with rain in Salem, Olveston and Woodlands.
On 4 December temperature measurements were made in the pyroclastic flow and mud flow deposits in the Trants area. Extensive mudflows have impacted strongly on this area. Mudflow deposits typically have maximum temperatures of around 100 *C; the maximum temperature may be reached just a few tens of centimetres below the surface. The original pyroclastic flow temperature measuring sites are difficult to locate having been buried by mudflow deposits.
On 7 December temperature measurements of the pyroclastic flow deposits emplaced in the centre of Plymouth in mid-November 1998 were made. Results are shown in Table 1.
Table 1. Temperature measurements, December 1998.
| Deposit date | Measrmnt Date | Location | Depth/cm | Temperature/*C |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 0 | 36 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 25 | 78 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 50 | 93 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 75 | 101 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 100 | 104 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 125 | 156 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 2 | 150 | 193 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 3 | 161 | 108 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 3 | 183 | 109 |
| 12 Nov. 1998 | 7 Dec. 1998 | War memorial, site 3 | 187 | 103 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 0.7 m from the edge of the deposit | 5 | 200 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 0.7 m from the edge of the deposit | 15 | 226 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 0.7 m from the edge of the deposit | 30 | 147 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 1 m from the edge of the nose of the deposit | 10 | 264 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 1 m from the edge of the nose of the deposit | 30 | 274 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 1 m from the edge of the main section of the deposit | 20 | 306 |
| 14 Dec. 1998 | 14 Dec. 1998 | Tar River delta, 1.5 m from the edge of the main section of the deposit | 15 | 309 |
| 14 Dec. 1998 | 17 Dec. 1998 | Tar River delta, 1 m from the edge of the main section of the deposit | 10 | 137 |
| 14 Dec. 1998 | 17 Dec. 1998 | Tar River delta, 1 m from the edge of the main section of the deposit | 25 | 189 |
| 14 Dec. 1998 | 17 Dec. 1998 | Tar River delta, 1 m from the edge of the main section of the deposit | 50 | 203 |
| 14 Dec. 1998 | 17 Dec. 1998 | Tar River delta, 1 m from the edge of the main section of the deposit | 75 | 178 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 1.5 m from the edge of the main section of the deposit | 25 | 219 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 1.5 m from the edge of the main section of the deposit | 50 | 272 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 1.5 m from the edge of the main section of the deposit | 75 | 240 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 6 m from the edge of the main section of the deposit | 10 | 183 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 6 m from the edge of the main section of the deposit | 25 | 237 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 6 m from the edge of the main section of the deposit | 50 | 297 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 14 m from the edge of the main section of the deposit | 10 | 186 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 14 m from the edge of the main section of the deposit | 25 | 226 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 14 m from the edge of the main section of the deposit | 50 | 280 |
| 19 Dec. 1998 | 21 Dec. 1998 | Tar River delta, 14 m from the edge of the main section of the deposit | 70 | 282* |
War memorial sites are about 20 m south of the Plymouth war memorial. Site 2 is identical to that measured on 16 and 21 November 1998. *: Measurement aborted before equilibrium reached.
On 8 December mudflows occurred all around the volcano; the delta at Old Road Bay was extended as far as the jetty.
Seismicity
Seismicity continued to consist almost completely of volcano-tectonic earthquakes and rockfall signals. Many of the latter were associated with small pyroclastic flows and/or episodes of venting at the dome. Volcano-tectonic earthquakes, the majority of which were recorded towards the start of the month, were located over a large area. Small clusters of earthquakes were located under George's Hill to the north-west of the dome, under Roaches Yard to the south-east and under Hermitage Estate to the north-east.
Table 2. Earthquake types These earthquake counts are of events that triggered the broadband network's event recording system between 00:00 and 00:00 each day (local time).
Date Hybrid LP Dome RF VT 01 December 98 0 0 7 8 02 December 98 0 0 11 13 03 December 98 0 0 14 8 04 December 98 0 0 22 38 05 December 98 0 0 29 44 06 December 98 0 0 20 2 07 December 98 0 2 30 3 08 December 98 0 1 31 0 09 December 98 0 0 15 17 10 December 98 0 1 17 1 11 December 98 0 0 18 2 12 December 98 0 0 9 0 13 December 98 0 0 22 4 14 December 98 3 0 25 1 15 December 98 0 0 18 5 16 December 98 0 0 0 0 17 December 98 0 0 0 0 18 December 98 2 0 0 0 19 December 98 1 0 4 3 20 December 98 0 0 12 1 21 December 98 0 0 28 1 22 December 98 4 0 11 2 23 December 98 0 0 3 1 24 December 98 0 0 2 0 25 December 98 0 0 6 0 26 December 98 1 0 3 0 27 December 98 1 4 9 0 28 December 98 3 0 1 10 29 December 98 2 2 10 5 30 December 98 2 1 5 5 31 December 98 0 2 9 1
Table 3. Swarms
Start Date/Time Duration Hybrid LP VT 4 December 98 16:03 10.2 0 0 27 5 December 98 04:36 7.5 0 0 34
Ground Deformation
All the GPS sites on the volcano and in the north of the island were occupied this month. The only area where significant deformation is taking place is on the eastern flank. The vectors for Long Ground shows eastward movement of these two sites amounting to 5 cm since lava stopped erupting (March 1998). Most of this movement has occurred in the last three months during the time of increased surface activity. The differential movement between Whites and Long Ground since June 1996 is over 10 cm and if Whites is held fixed would represent Long Ground moving towards 076*. The two sites are 733 m apart; thus the movement between them cannot be accommodated elastically. A ground inspection on 30 December revealed a possible fault lying between the two sites. The only surface expression is a linear break, trending 068*, in the road and it is not currently known whether this is related to volcanic deformation or to surficial movements disrupting the road surface. The Tar River GPS pin was excavated and occupied. It has followed a similar movement to Long Ground throughout the eruption. Perches, until it was destroyed in July, also followed a similar path. One possible interpretation is that a sector of the volcano including Long Ground, Perches and Tar River is moving as a coherent block along faults in a north-east direction.
Environmental Monitoring The miniCOSPEC was used several times in December although poor weather prevented its use on a number of occasions. The sulphur dioxide flux was elevated and on 22 December reached a peak average flux of 1700 tonnes per day (Table 4).
Table 4. Average daily sulphur dioxide fluxes measured by miniCOSPEC, December 1998
Date Flux (tonnes/day) Comments 9 December 1998 900 Helicopter 16 December 1998 430 Helicopter 18 December 1998 800 Helicopter 22 December 1998 1700 Helicopter 24 December 1998 1310 Helicopter 29 December 1998 1500 Helicopter 30 December 1998 860 Helicopter
Sulphur dioxide concentrations were also measured at ground level by using several sets of diffusion tubes located around the island. The results are shown in Table 5.
Table 5. Sulphur dioxide diffusion tube results, December 1998. Levels in ppb.
16-Nov to 3-Dec 1998 3 to 19-Dec 1998 Police HQ, Plymouth 39.0 38.3 St. George's Hill 1.9 Damaged Weekes 1.9 4.8 Vue Pointe Hotel 1.0 6.3 Lawyers 1.2 2.7
MVO Staff Changes
Arrivals:
Sue Loughlin (British Geological Survey)
David 'Patch' Silcott (University of the West Indies)
Thomas Christopher (University of the West Indies)
Brian Baptie (British Geological Survey)
Chloe Harford (Bristol University)
Departures:
Gill Norton (British Geological Survey)
Anne-Marie Lejeune (Bristol University)
Richard Luckett (British Geological Survey)