What caused the Soufrire Hills Volcano to erupt?

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What caused the Soufrière Hills Volcano to erupt? 

The Caribbean island of Montserrat is situated on a destructive plate boundary. A plate boundary occurs when two of the plates that make up the surface of the earth meet. Underneath Montserrat the Atlantic plate is slowly being forced under the Caribbean plate. This happens because the oceanic plate is denser than continental plate. Convection currents pull the dense Atlantic plate into the mantle where intense heat and friction cause the rock to melt. This molten rock is lighter than the surrounding rock, forcing it to rise through cracks in the in the rock towards the earth's surface.

Molten rock or magma gathers under the volcano until the pressure is so great that it is forced up through a vent towards the surface of the earth. The Soufrière Hills Volcano produces a thick sticky lava called andesite. The lava is so thick that it builds up at the top of the volcano in a dome until it becomes too heavy and the dome collapses. When the dome collapses hot rocks, gases and ash are released from the volcano causing the following hazards. Pyroclastic flows are extremely fast-moving flows of lava, hot rocks and gas that flow down the side of the volcano. Pyroclastic Surges are fast-moving clouds of hot ash that travel with the pyroclastic flow. Tiny pieces of volcanic ash are blasted into the air forming a plume of ash. As it cools down it falls back to the earth, covering vast areas with a layer of ash.

The current eruption of the Soufrière Hills Volcano started on 18 July 1995 and is still continuing in 2003. Throughout this time, the volcano has experienced different stages of eruption from earthquake swarms to explosive eruptions. On 25 June 1997 at about 1 p.m. the Soufrière Hills Volcano erupted catastrophically. The dome of the volcano collapsed, sending 5 million cubic metres of hot rocks and gases down the side of the volcano towards the north of the island. This signalled an increased period of volcanic activity and had a significant impact on the island.

What was the impact of the eruption? 

The eruption on 25 June 1997 affected Montserrat in a number of ways. For the first time during the eruption people were killed and injured. Villages were destroyed and land previously used for farming was covered in rock and ash deposits.

Impact on the Environment

During the eruption between 4 - 5 million cubic metres of lava was unloaded in the form of pyroclastic flows.
A total of at least 4 square kilometres of land was covered by the deposits from the pyroclastic flows and surges.
The main part of the flow in Mosquito Ghaut caused intense scouring to the top of the valley walls.
Pyroclastic flow deposits completely filled Pea Ghaut.
Numerous villages were affected by deposits: Dyer's, Streatham, Riley's Yard, Farrell's Yard, Windy Hill, Harris, Bramble, Bethel, Spanish Point, Farm, Trant's.
The villages of Farms and Trant's were completely buried by block and ash flow deposits.
Houses were partially buried or burned down by the intense heat. Aluminium window shutters were melted and twisted. Everything made from wood was burned.
Other houses were destroyed by direct impact of rocks, up to 5 metres in size.
The fine grained pyroclastic surges broke and flattened trees. In some areas there was no vegetation left at all, vegetable beds were bare of plants and the soil was baked hard.
Deposits from the ash cloud were limited to areas west and north-west of the volcano. The main impact of the ash was the hazard to the search and rescue mission.

Impact on the People

The eruption on 25 June 1997 destroyed between 100 -150 houses. The houses were buried by debris from the pyroclastic flow, burned by the hot pyroclastic surges, or set alight by the intense heat in the atmosphere. Most of the houses destroyed were in the Exclusion Zone and should have been empty but an estimated 15 people were living and sleeping in the Exclusion Zone.

Some farmers regularly returned to the Exclusion Zone to care for animals or tend to crops. They were producing crops to feed evacuees in the north of the island. Most land suitable for farming was in the south of the island, close to the volcano. According to reports there was not enough food to go round. Therefore locals put pressure on officials to allow access for farming. After the eruption much of this land was destroyed by volcanic deposits.

Survivors and Victims 
The 19 people who were killed by the pyroclastic flows and surges were in the Exclusion Zone. Common injuries to the seven people injured were severe burns to the feet as a result of walking on ash deposits only about 1-2 cm deep.. Other survivors suffered burns to various parts of their bodies, including inhalation injuries and burns to the nostrils and mouth.

Debris from the pyroclastic flow came within 50 metres of W.H. Bramble Airport. The airport was undamaged but since then the airport has not re-opened. Although the capital, Plymouth, was no longer occupied the port was still in use. On the day of the eruption a fuel tanker was connected to the port to deliver fuel for vehicles. The tanker was notified and fuel lines to the port were disconnected. Although the port was not destroyed by this eruption, an emergency jetty was built in the north of Montserrat. Many of the islanders relied on cars and vans to get themselves about the island. Some people in the Exclusion Zone tried to escape using their vehicles. The ash was so thick that it was impossible to see the road. The intense heat would also have burned the tyres of a car or even set it alight.

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Was the eruption predicted ? 

The scientists at the Montserrat Volcano Observatory (MVO) play a key role monitoring the developments of the Soufrière Hills Volcano. Despite sophisticated monitoring equipment it still remains very difficult to pinpoint exactly when an eruption will happen. From an early stage the MVO predicted increased volcanic activity and the dangers associated with them. Continuous monitoring of the volcano is vital in the scientist role to obtain accurate data. The Soufrière Hills Volcano was monitored using a number of different methods.

Seismic Activity
Before an eruption, magma moves into the area beneath the volcano and collects in a magma ...

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The spelling and grammar and punctuation were fine. I found one mistake of "slighlty" instead of "slightly". This shows the lack of proof reading which is essential in limiting such silly mistakes. However, one slip up like this would not affect the mark greatly, although the impression on the examiner is not very good. The technical terms were appropriately used and well integrated into the answer. The student does not present their work in one long essay format which is interesting. However, there is no 'right' way to present geography work like this and using subtitles is a sensible way as well as presenting it as one long extended essay.

The level of analysis is excellent. The student supports each point well with evidence and evaluated the evidence they found. Perhaps a bibliography would have been good to show the sources used for this piece of work. The appropriate conclusion was made using well supported material. I would suggest that any diagrams / pictures and so on be sourced accordingly as well to avoid infringing copyright or being accused of plagiarism. The response was coherent and well developed through the use of strong and well selected evidence.

The student demonstrates a high level of understanding to the eruption right from the start as their introduction shows a clear knowledge towards the tectonic plate theory which caused the eruption. The answer is well supported by evidence and each point the student makes is clearly supported. This creates a convincing and fluent piece of written work which would score the student very high marks. Their response is very clear as the student has laid out the work under subtitles which helps guide the examiner through the work.