Smithsonian Institution Global Volcanism Network Bulletin v. 20, no. 6, June 1995 Soufriere Hills (Montserrat) Small phreatic eruptions-the first in recorded history Soufriere Hills Montserrat, West Indies 16.72N, 62.18W; summit elev. 915 m All times are local (= GMT - 4 hours) The following is based on information as of 24 July from the Seismic Research Unit (SRU) team at the University of the West Indies and Volcanic Alert News Releases from the Montserrat Emergency Operations Center. The SRU maintains a seismic network on Montserrat (figure 1), currently composed of seven instruments. On 18 July, villagers around Soufriere Hills volcano reported unusually loud rumbling noises coming from the fumarolic areas, light ashfall, and a strong sulfur odor. Following confirmation of these reports, an Emergency Operations Center, located in the capitol city of Plymouth (on the coast ~4 km W of the summit), was activated and fully operational by 1830 that night. The Emergency Operations Center identified two schools as potential refugee centers, but no evacuation was ordered. As of the morning of 19 July, based on conversations with Montserrat residents, SRU inferred that the initial explosion was small, phreatic, and only spread minor ashfall around the island. In accord with a small explosion size, the Synoptic Analysis Branch of NOAA saw no evidence of a plume on satellite imagery. Seismicity has been elevated since August 1992, and an earthquake swarm began on 14 July. However, no additional increase in seismicity was associated with the 18 July explosions. An explosion earthquake at 0924 on 19 July was centered close to the top of Chance's Peak, the summit located on the W side of the crater rim. A field team led by Lloyd Lynch (SRU) trekked in from the N to make an initial inspection just after 1300. They reported minor explosions from an area SW of Tar River Soufriere (a fumarolic area ~1.5 km NE of the summit), explosions discharging from a vent within the summit crater between Chance's Peak and the Tar River area. The explosions took place at intervals of ~20 minutes, sending ash and steam ~40 m high. Based on these observations, no evacuations were recommended. Explosions continued that afternoon (figure 2). William Ambeh (SRU) led another observation team on the morning of 20 July to the Paradise Estate area (~2 km N of the summit), and additional monitoring equipment was installed in the Long Ground area (~2.5 km NE of the summit). Reconnaissance photographs taken from a Royal Air Force aircraft confirmed the early field reports. Later photographs taken from a Royal Navy helicopter indicated no increased activity in the Long Ground area. The shallow earthquake swarm that began on 14 July ended on the 21st; depths were 2-4 km, and the largest event was M 3.5. Volcanic earthquakes were concentrated along the ENE and WSW areas of Lang's Soufriere. Phreatic activity continued on 22 July. Early morning ashfall was reported in Plymouth (~4 km W of the summit) and the SW-sector villages of Gages, Parsons, and Amersham. A small steam-and-ash eruption around 0800 lasted ~10 minutes. As of 1030 on 23 July, there was no new volcanic activity. At the request of Montserrat, France sent two scientists (arriving on 25 July) to provide the SRU with technical assistance and additional equipment. They were joined on 26 July by five geologists from the U.S. Geological Survey's Volcanic Crisis Assistance Team. Soufriere Hills volcano sits on the N flank of the older South Soufriere Hills volcano, located at the S end of Montserrat Island (13 x 8 km). The summit area consists primarily of a series of lava domes emplaced along a ESE-trending zone. Block-and-ash flow and surge units associated with dome growth predominate in flank deposits. The most recent pyroclastic-flow deposits, associated with the formation of English's Crater, have been dated at around 19,000 years BP (before present) (Baker, 1985). Wadge and Isaacs (1988) dated a series of eruptions at 16,000-24,000 years BP, and noted that Castle Peak dome in English's Crater post-dated this by an unknown period of time. English's Crater is breached to the E. There have been no reported historical eruptions, but some undated deposits and the cone have a young appearance. A radiocarbon date of ~320 +- 54 years BP from a NE-flank pyroclastic-flow deposit is significantly younger than other radiocarbon dates from the volcano, but could have resulted from the latest activity of Castle Peak. Because the sampling site has not been relocated for confirmation, this date is considered somewhat uncertain. Periods of increased seismicity below Soufriere Hills were reported in 1897-98, 1933-37, and again in 1966-67. Shepherd and others (1971) concluded that the 1966-67 seismicity was caused by a relatively small volume of magma injected from >10 km depth into a zone of fractured rocks below the volcano, and not from a shallow magma body. References: Baker, P.E., 1985, Volcanic hazards on St. Kitts and Montserrat, West Indies: Journal of the Geological Society, London, v. 142, p. 279-295. Shepherd, J.B, Tomblin, J.F., and Woo, D.A., 1971, Volcano-seismic crisis in Montserrat, West Indies, 1966-67: Bulletin of Volcanology, v. 35, p. 143-163. Wadge, G., and Isaacs, M.C., 1988, Mapping the volcanic hazards from Soufriere Hills volcano, Montserrat, West Indies using an image processor: Journal of the Geological Society, London, v. 145, p. 541-551. Information Contacts: Richard Robertson, Seismic Research Unit, University of the West Indies, St. Augustine, Trinidad; Montserrat Emergency Operations Center, Plymouth, Montserrat; Adam Dennis, 144 1/2 F Street SE, Washington, DC 20003 USA. Figure 1. Index map showing Montserrat, the island where Soufrierre Hills is located. Figure 2. Photograph of Soufriere Hills volcano after a phreatic explosion between 1400 and 1500 on 19 July. View is from the center of Plymouth, ~4 km SW of the summit. Courtesy of Nicole and Adam Dennis.