Montserrat Volcanic Eruptions / Soufriere Hills Volcano
Where is the volcano?
The Soufriere Hills volcano lies in the small Caribbean Island of Montserrat in the Antilles Islands. The Island is a British Protectorate and lies to the SW of Antiqua. The Island at its largets is no more that 12km by 8km and before the eruption had a population of 11000.
When was this eruption?
The current eruptive phase began on 18th July 1995 when large billowing clouds of ash and steam erupted from Soufriere Hills volcano in the south of the island. The eruption has left the southern two thirds of the island uninhabitable and it remains so to this day. Pyroclastic Flows still pour down the slopes of he volcano. Check out the latest eruptions at the MVO- Montserrat Volcanic Observatory
Why s there a volcano here?
The Soufriere Hills volcano is situated above a destructive plate margin, or subduction zone which forms the Peurto Rico Trench. Oceanic crust from the North American plate is sinking under the Caribbean plate, forming the Antilles volcanic island arc . As the slab of crust descends, sediments, water and the heat of the mantle cause the wedge of mantle above the slab to melt. The molten rock is less dense than the surrounding crust and rises to the surface . The magma formed at a depth of around 6km, with a temperature of 820–885°C, and then partially crystallised before a new injection of deep magma boosted it towards the surface.
At the surface, a thick viscous dome of lava appeared in English’s Crater in the Soufriere Hills. Most of the gas in the lava had already escaped but some remained in pockets, which caused explosive eruptions.
The volcano is a strato volcano, composed mainly of consolidated ash layers from previous eruptions. Initially pyroclastic flows flowed eastwards from the open crater down the Tar River valley, but as the dome grew, eventually flows were able to come down any side of the volcano. The flows particularly followed four main river valleys: Tar River, White River, Mosquito Ghaut and Fort Ghaut. Where the flows reached the sea they created new deltas of land e.g. Tar River valley delta.
The volcano is a strato-volcano . Pyroclastic Flows and eruption columns characterise the Soufriere Hills volcano . Such flows occur when the dome collapses or explodes. Tonnes of hot rock, lava and ash explode from the crater in a fast-moving cloud. The cloud can race straight down the mountainside, or if there is a large vertical explosion, which creates an ash column, come crashing down as the ash column collapses. Such clouds have two parts: on the ground, an avalanche of large boulders and rocks, and above this a cloud of ash and gas with temperatures of 400°C+. The cloud moves at speeds of 100mph, obliterating and incinerating everything in its path.
Sequence of Events
- 1992/1994 Earthquake swarms first detected
- 1995 July Phreatic eruption of ash and steam August 5,000 evacuated as a precaution December Plymouth evacuated as new dome grows.
- 1995 Nov New dome grows
- 1996 January English’s Crater and rapidly buries old dome.Residents allowed to return during quiet phase. March–Sept First pyroclastic flows down the Tar River valley, creating a new delta in the sea. April South of island evacuated. Plymouth abandoned.
- 1997 June 25/6 Most devastating flow yet down NE slopes of volcano. 19 people killed who had returned to their homes in the danger zone.9000m high eruption cloud.
- 1997 August Pyroclastic flows destroy Plymouth, Montserrat’s capital 3 miles from crater. Permanent relocation of inhabitants away from island is considered by British government. Exclusion zone expanded, residents advised to seek shelter, wear helmets and masks outdoors. Warning of cataclysmic eruption.
- 1997 September Montserrat’s Volcanic Observatory advises all residents to move as far north as possible immediately. 5,000 now live in shelters. Flows destroy Bramble airport 3.5 miles from crater. People may be moved forcibly from the exclusion zone.
- 1997 October Eruptions approximately every 9 hours. Ash and tephra land in safe zone. Dome is 250ft above old crater and growing at 280ft3/second.
- 1997 November Dome collapse remains a threat, causing large eruptions.
- 1997 December Boxing Day eruption. Pyroclastic flows down the White River valley. Eruption clouds to 36,000ft.
- 1999 July 20 Large eruption from dome collapse. Eruption cloud to 35,000 ft. Threat of dome collapse is still real. People largely out of danger unless in the exclusion zone. Restructuring of the north continues.
Effects and Aftermath
Montserrats population have had their lives completely changed. Their economic,administrative and cultural capital lies in ruins like a modern Pompeii. Jobs,schools and community activities have been disrupted. At one point 5,000 people were living in temporary shelters in the north, in some cases for over two years. The north of the island had few settlements of any size and found it difficult to cope with the influx from the south. Salem is now being repopulated and is being developed together with St Johns as new population centres. The following are some of the problems :
- Plymouth contained all the main services – hospitals, banks, schools, government and many private businesses.
- collapse of the tourist and riceprocessing industries
- unemployment has risen from 7% to 50%.
- agriculture is at a standstill. • respiratory problems caused by ash.
- psychological problems and discontent with relocation package
- traffic congestion
- shortage of available building land
- 70% rise in rent for accommodation
- skills shortage as people left.
Once the volcano was deemed dangerous to life. Evacuaions plans and Hazard maps were put into place. The maps contained areas of increasing risk including the highest risk area- the exclusion zone. As the eruption progressed the zones were modified until the southern two thirds of the island was delcared an excluson zone.
- An extensive seismograph network was established around the volcano to measure earthquake strength and depth.
- Earth deformation meters and tiltmeters were also put in place to show any signs of the ground swelling or deflating as magma rose within the volcano.
- A satellite location GPS (global positioning system) was also used to check ground movements.
- An instrument called COSPEC was used to measure gas emissions from the volcano, particularly sulphur dioxide which is a good indicator of magma type and movement.
- The pH of rainwater was monitored as it gives an indication of the magmatic gas content.
- Geologists regularly flew into the most dangerous areas to sample new flow deposits, measure cracks caused by the volcano swelling and check the growth of the dome.