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Report on Soufriere Hills (United Kingdom) — 25 July-31 July 2001

Soufriere Hills

Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
25 July-31 July 2001
Managing Editor: Gari Mayberry

Please cite this report as:

Global Volcanism Program, 2001. Report on Soufriere Hills (United Kingdom). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 25 July-31 July 2001. Smithsonian Institution and US Geological Survey.

Weekly Report (25 July-31 July 2001)

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

On 29 July ash from Soufrière Hills drifted W, leaving deposits as far away as San Juan, Puerto Rico, 450 km to the WNW. The Washington VAAC issued an advisory on 27 July stating that a steady stream of ash was emitted from the volcano through the evening, rising to 800 m. There was also a persistent, strong hot spot over the volcano's summit visible in satellite imagery. According to MVO, the amount of ash emitted from the volcano increased during 27 July through 29 July, and seismic activity rose on 29 July. Beginning at 1500 on 29 July heavy rainfall mixed with ash deposits and generated lahars that flowed NW down the Belham River. The lava dome that had been growing in the summit region of the volcano during recent years partially collapsed, generating pyroclastic flows that traveled down the E flank of the volcano and entered the sea. Shortly after 1700 observers reported seeing pyroclastic flows and a continuous dense plume of ash that drifted to the W. Dense meteorological clouds, associated with a tropical wave, crossed the island and prohibited ash cloud detection in satellite imagery or ground confirmation of the height of the ash cloud. MVO reported that the large amount of ash that was being vented from the volcano rose to below 6 km. By midnight seismic and pyroclastic flow activity returned to low levels. The next day AVHRR imagery showed possible ash in an area W of Montserrat and SE of Puerto Rico. The position of the cloud correlated with ground observations of ash and haze from Christiansted, St. Croix.

There were reports of substantial ashfall and sporadic falling of "stones" in the Montserrat residential areas of Salem and Olveston in the N part of the island. Ash also fell in the US and British Virgin Islands, Roosevelt Roads (Puerto Rico), Christiansted (St. Croix), and as far as 450 km away from the volcano in San Juan (Puerto Rico). The ashfall in San Juan and the surrounding area led to the closing of the San Juan International Airport on 30 July. The airport reopened the next day.

Geological Summary. 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.

Sources: Montserrat Volcano Observatory (MVO), Washington Volcanic Ash Advisory Center (VAAC), Associated Press, Reuters