Plate Tectonics- Vulcanicity Case Study

Soufriere Hills Volcano, Montserrat 1995/7+

Location

• 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 Antigua. 
• The Island at its largest is no more that 12km by 8km and before the eruption had a population of 11,000.

When?

• 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 the volcano. 

Causes

• The Soufriere Hills volcano is situated above a destructive plate margin, or subduction zone which forms the Puerto 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. 
• 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 Primary Effects of the Eruption

• 2/3 of the island was covered in ash
• 50% of the population were evacuated to the north of the island to live in makeshift shelters
• 23 people died in 1997
• Plymouth - the capital became a ghost town
• The airport and port were closed
• Farmland was destroyed
• Many schools and hospitals were destroyed

The Secondary effects of the Eruption

• As most of the southern area was destroyed any remaining inhabitants have had to endure harsh living conditions in the North.
• Transport remains a problem for people travelling to the island as the port and airport remain closed.
• The tourist industry is still suffering with few visitors except for cruise ships looking at the volcano
• Over half the population left the island and have not returned
• Floods as valleys were blocked with ash
• Forest fires caused by pyroclastic flows

Response

• £41 million was given in aid by the British Government.
• Money was given to individuals to help them move to other countries.
• Riots occurred as locals complained that the British were not doing enough to help the island
• The MVO (Montserrat Volcano Observatory) was set up to study the volcano and provide warnings for the future
• A Risk assessment was done to help islanders understand which areas are at risk and reduce problems for the future.

Hazard Management

Once the volcano was deemed dangerous to life, evacuations plans and Hazard maps were put into place. As the eruption progressed the zones were modified until the southern two thirds of the island were declared an exclusion zone.

Monitoring

• An extensive seismograph network was established around the volcano to measure earthquake strength and depth. 
• Earth deformation meters and tilt meters 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.