Report on Soufriere Hills (United Kingdom) — July 1999
Bulletin of the Global Volcanism Network, vol. 24, no. 7 (July 1999)
Managing Editor: Richard Wunderman.
Soufriere Hills (United Kingdom) Dome collapses, pyroclastic flows, and ash eruptions in April-June
Please cite this report as:
Global Volcanism Program, 1999. Report on Soufriere Hills (United Kingdom). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 24:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199907-360050.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
Small explosive blasts with ash venting occurred with regularity during April to July, sending some plumes as high as 5 km, sometimes accompanied by thunder and small pyroclastic flows down the Tar River valley. Vigorous fumarolic activity was observed frequently in the dome above the Tar River as was steaming on the N flanks of the dome. Measurements of SO2 flux typically ranged from ~200 to 500 tons/day, all substantially lower than the peak levels of ~2,000 tons/day.
During April-July there were frequent volcano-tectonic (VT) earthquakes and occasional long period (LP) events. The VT events typically ranged from ~10 to 70/ week; the LP events took place up to 9/week.
Activity increased in May and several events of greater intensity occurred in June and July, the latter with large pyroclastic flows and ash plumes rising to heights of 10 km. Explosions lasting as long as 10 minutes occurred on 10 May and again on 20 May. These were accompanied by ash plumes that rose to altitudes of several kilometers above the summit. Pyroclastic flows were seen traveling down the Tar and Gages rivers and in Tyers Ghaut after the event on 10 May. Observations of the dome on 13 May indicated that a large slab of rock had collapsed from the Galways lobe into the gully between the top of Gages and Tar river valleys.
A distinct increase of activity occurred on 22 May when a VT earthquake swarm was recorded, the first since December 1998. The swarm comprised 141 VT events and lasted ~10 hours. A large pyroclastic flow occurred the next morning, mainly traveling down the E flanks of the volcano before reaching the sea at the Tar River delta. Activity increased on the evening of 5 June when a portion of the dome on the NE flank above Tuitt Ghaut collapsed. About 1.5 x 106 m3 of material were removed from the dome, forming pyroclastic flows that traveled down the Tuitt Ghaut to about 1.5 km from the dome; down the White Ghaut to about 2 km from the dome; and down the Tar River to the sea. A large, dark ash cloud that reached an altitude of ~4 km moved W and NW from the volcano, traveling as far N as Salem with associated thunder and lightning. There was complete darkness in Salem for a short period as ash started to fall. Up to 1 cm of ash was deposited in Salem, Old Towne, and areas S of Belham. Heavy rains on 9 June produced mud flows down all flanks of the volcano depositing new material in the Belham valley.
During the rest of June and most of July, a continuing sequence of small collapse events occurred each week, some with pyroclastic flows that traveled down the Tar River and other valleys. Ash clouds reached 3 km above the summit, causing additional ash deposition in Salem and Old Towne. A 17 June collapse produced a pyroclastic flow that reached the sea at the Tar river delta.
On 20 July a major collapse occurred without any precursory signals; the resulting large pyroclastic flow traveled down the Tar River valley and covered the entire fan. On the N face of Roches Mountain the pyroclastic surge stripped off the remaining vegetation, and the event generated a mobile surge cloud that swept over the mountain and collapsed on its S side forming a secondary flow in the valley leading into Dry Ghaut. Ballistic rocks were thrown over Roches Mountain setting fire to vegetation on its E side. Subsequent visual observations confirmed the large magnitude of the collapse.
On both 25 and 26 July there occurred earthquake swarms, each of 2-hour duration and composed of 22 and 26 events, respectively. Most of the earthquakes were of small magnitude, located at depths of ~1 to 3 km directly beneath the volcano. The first and largest of three explosions that week occurred ~2.5 hours after the second swarm. Heavy meteorological cloud cover precluded visual observation of the effects of these explosions throughout the remainder of the month. However, prolonged steam venting and ash emission, roaring sounds, lightning, and thunder were associated with all three explosions.
Measurements of the volcano deformation rate during the period indicated that it remained low, fluctuating in response to changes in internal pressure. The most recent period of rapid deformation occurred during April and May, coincident with the increase in activity during that time. Electronic distance meter (EDM) measurements reported in late June indicated that the line between Chances Peak Steps and Lower Amersham showed a shortening of 18 cm over the 2-year period since the last measurement, an average rate of about 2 mm/week.
Montserrat Volcano Observatory (MVO) reported that events of the type and size recently observed can occur at any time without any build-up in activity beforehand. MVO warned that the flanks of the volcano remain extremely dangerous and advised residents to keep listening to local radio for updates on the state of the volcano.
Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.
Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).