Report on Soufriere Hills (United Kingdom) — 6 February-12 February 2013
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 6 February-12 February 2013
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2013. Report on Soufriere Hills (United Kingdom). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 6 February-12 February 2013. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
MVO reported that during 1-8 February activity at the Soufrière Hills lava dome was at a low level, although there was a slight increase during 3-6 February characterized by volcano-tectonic earthquakes, elevated gas flux, and possible light ash venting. The volcano-tectonic earthquakes occurred in four brief swarms: at 2220 on 3 February, at 0915 and 0950 on 4 February, and at 0620 on 5 February. The second swarm was the most intense, and was followed by a hybrid seismic event. Another hybrid event was not associated with a swarm.
After the second, and largest, volcano-tectonic swarm on 4 February, there were increases in the temperatures of several fumaroles inside the 11 February 2010 collapse scar, as observed using a handheld thermal infra-red camera at MVO, 5.7 km away. There was a further increase, as well as some loud roaring sounds, around the time of the third swarm. The activity likely included minor ash venting from a large fumarole in the floor of the collapse scar because fresh ash deposits were observed adjacent to this fumarole on the morning of 5 February. All fumaroles had returned to background levels of activity and temperature by later that day.
Sulfur dioxide measurements showed an average flux of 929 tonnes/day during the week, with a maximum of 2,381 tonnes/day and a minimum of 273. The flux was not steady, with peaks of 962, 1,266 and 2,381 on 1, 4 and 6 February, respectively. The last measurement is the highest daily value since the ash-venting episode that occurred during 23-25 March 2012. The Hazard Level remained at 2 (on a scale of 1-5).
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.