Montserrat Island is a British Overseas Territory in the Lesser Antilles, part of the British West Indies in the Caribbean. It is composed of three volcanic edifices, Silver Hills in the north, Centre Hills in the center and South Soufriere Hills in the south. Activity on the island has moved south over the period of its growth. It is 480 km ESE of Puerto Rico and 48 km SW of Antigua. It had a population of 13,000 before the eruptions began. Over 8,000 left the island primarily for Great Britain leaving a current population just under 5,900 British Subjects.
Montserrat was settled in modern times by a group of Irish in 1642. Ownership fluctuated briefly before being firmly claimed by the British.
The Soufriere Hills, an andesitic basaltic volcano started erupting in 1995. Pyroclastic flows destroyed the capital of Plymouth in 1997 and it was abandoned shortly afterwards.
The Montserrat Volcano Observatory has a significant web presence with at least three different web sites including a Facebook page. You can find them at the following links:
There are multiple web camera feeds from cameras looking at Soufiere Hills. Most of them appear to be from the same camera or group of cameras. The following is a representative link. https://www.webcamgalore.com/EN/webcam/Montserrat/Garibaldi-Hill/16671.html
The British Geological Society also monitors Soufriere Hills. Their home page for the volcano can be found here: http://www.bgs.ac.uk/discoveringGeology/hazards/volcanoes/montserrat/home.html
Bobbi wrote about Soufriere Hills and Montserrat in January 2014. https://volcanocafe.wordpress.com/2014/01/27/montserrat-island-soufriere-hills-volcano/
Soufriere Hills is a complex andesitic volcano. Eruptive products are typically andesite / dacite and basaltic andesite. Eruptive style during the current sequence is typically dome building followed by collapse, pyroclastic flows down the volcano slopes. There are also significant explosions sending columns of ash over 12 km into the atmosphere.
The 1995 series of eruptions forced the evacuation of most of the southern half of the island, eventually burying the capital city of Plymouth under pyroclastic debris. The eruptions also increased the size of the island as they built upon earlier emplaced debris. All of this makes the island and the volcano unstable, meaning flank collapses above and below water level have taken place in the historic past.
There is an active exclusion zone around the southern part of the island.
The eruptions began with a series of phreatic explosions in August 1995. This was followed some 18 weeks later with initial dome growth confined to the sector collapse scar. This coverage expanded as the volcano cycled through its dome growth / collapse / destruction / pyroclastic flow cycle.
Some 18 settlements were destroyed by lahars and pyroclastic flows. Another 15 or so were evacuated. Settlements inside the exclusion zone will not be resettled any time soon.
The last eruption sequence from Soufriere Hills ended in 2013. This marked a mostly active sequence starting in 1995 and lasting through 2010 or so. There were at least three VEI 3 eruptions during the period in 1995, 2004 and 2005. A 1997 eruption destroyed a newly emplaced dome and left a scar on the flank of the volcano.
The buildup to the 1995 eruption was long and clearly signaled the eruption. At least one paper documents this period at 100 years. Frequent warnings about potential volcanic eruptions and hazards to close communities started being published in scientific and informal literature in the late 1930s. The first seismograph was installed in 1937 and operated through 1946.
A series of premonitory earthquakes took place from 1966 – 1967. At the time, geologists believed there was a very real danger of a short notice eruption. They made the appropriate warnings and were ordered by the Administrator (equivalent to the governor of the island) not to discuss their findings and concerns with anyone outside the geology team.
Fast forward to 1988 and the geologists were still concerned. The Government of Montserrat stopped paying contributions to the Seismic Research Unit from 1990 – 1995. Those payments resumed shortly after the eruptions started.
There appear to be two sets of earlier eruptions centered around 1540 and 1624. These produced pyroclastic flows based on analysis of carbon from incinerated trees. The initial 1667 date was suspect and the presence of carbon blamed on forest fires. But deep gullies cut in pyroclastic flows deposited during the 1995 eruptions allowed sampling of older materials leading to reasonable dating of the two sets of eruptions above.
Since that time, the mountain was densely overgrown and mostly quiet other than a series of earthquakes in the 1890s. These were accompanied by an increase in Soufriere (sulphur spring) activity. This sequence ended around 1901.
After a period of quiet, earthquakes began in 1933 and increased in number and intensity through 1937. As the earthquakes increased so did the hot spring activity around Soufriere Hills. There were almost 500 felt quakes in November 1935. One of these was a 6.5 Richter tectonic quake offshore north of the island. The number of quakes fell off rapidly the next two years.
By the 1940s, the British Royal Society determined that Soufriere Hills was a live volcano in a pre-eruptive condition. They deployed seismic instruments to monitor it. These were the first in the English speaking portion of the Caribbean.
1966 – 1967 is referred to as a crisis with earthquakes starting again. 87 quakes were recorded in a 40-day period starting March 1966. The depth of the earthquakes decreased steadily and the volcano showed measurable inflation. Initial evacuation maps were prepared at this time. After October 1967, activity decreased steadily and the mountain was thought to be inactive, though it was never completely silent. There were two flurries of quakes in August 1977 and March 1978.
A pair of large earthquakes took place north of Montserrat in 1985. The first was a 6.4Richter and the second was a 5.6. Both were followed by a large number of aftershocks. Following the second quake it became apparent that the aftershocks felt on the island were not aftershocks at all but increasing activity from Soufriere Hills. Activity started ramping up in 1992 with a rapidly increasing number of earthquakes directly below a live volcano with a known history of unrest.
There was a lack of preparation for the 1995 eruptions by the local government. Part of it was due to inexperience by the geologists involved. Part of it was due to the intentional withdrawal of support by the Administrator for the Seismic Research Unit in 1990. This severely restricted the number of visits the unit could make to Montserrat and in turn significantly decreased warning for the impending eruptions.
There were large flank collapse events that littered the surrounding sea floor with debris of the volcano. The most recent of these is thought to be 460 BC. There were two other large eruptions dated at 2050 BC and 6050 BC. An expedition carrying scientists from the University of Rhode Island explored the collapse scar and tried to determine if recently emplaced pyroclastic materials harmed the sea life inhabiting it. The intervening sea water appears to have protected the sea life from the heat of the flow. http://www.nautiluslive.org/video/2013/10/30/big-rocks-big-waves-uncovering-montserrats-geological-history
The Lesser Antilles are subduction volcanoes formed by the collision of the Caribbean Plate and the oceanic portions of the North American and South American Plates.
Montserrat itself dates back to 11 – 25 MA for the Silver Hills in the north. Centre Hills date 5.5 – 9.5 MA. Soufriere Hills dates from 1.7 MA to the present.
Soufriere Hills is characterized by long periods of dormancy and activity, each measuring some 10,000 years long. A 2002 paper out of the Geologic Society of London suggests that during periods of increased activity, several major dome-forming eruptions are separated by periods of quiet lasting less than 1,000 years and that the new eruptions may be the start of a fourth period of volcanic activity.
Given the suggestion that Soufriere Hills is entering a fourth period of unrest, this remains a dangerous volcano and should continue to be so for the foreseeable future. It gives every appearance of an open system, able to go from inactive to full dome building / collapse in a very short period of time. We already see evidence of partial cone collapse on the existing mountain and a full-fledged one causing the debris on the sea floor.
Soufriere Hills is and will remain a danger to inhabitants of Montserrat and those living on surrounding islands.