Montserrat is one of the Leeward Islands in the eastern Caribbean, 40 km southwest of Antigua. Sixteen km long, with an area of 102 square km, the island compromises three mountain ranges. Soufrière rises to 901 m
The eruption began on 18 July 1995 within English's Crater, which is a structure about 1 km in diameter with walls 100 to 150 m high, open to the east. The first four months of the eruption involved intense earthquake swarms and vigorous steam explosions, caused by rapid heating of the groundwater by rising magma. The magma reached the surface by mid-November 1995 and a new lava dome began to form. The lava is typical of many Caribbean volcanoes and is known as andesite. Such lava is so viscous that it piles up around the vent to form a dome; a steep-sided rubbly mound hundreds of metres high. The Soufriere Hills dome has been growing ever since.
As a lava dome grows it becomes unstable and parts of the dome can suddenly avalanche away and simultaneously disintegrate to form a flow of fragments and volcanic ash known as a pyroclastic flow. The flows vary from small avalanches down the sides of the dome to major failures of the dome in which millions of tonnes of fragmented lava move at devastating speeds of over 100 kph and temperatures up to 800°C.
In April 1996 the first major pyroclastic flows moved down the Tar River valley to the east of the volcano. By May 1996 pyroclastic flows entered the sea on the east coast and there were further large flows in July, August and early September. A major shift in the volcano's behaviour occurred around 20 July 1996 which heralded an escalation of activity in the following months. The first explosive eruption of the volcano occurred on 17 September 1996, generating an eruption column about 14 km high and ejecting 1 metre diameter rocks to about 2 km from the volcano. The escalation of activity and new explosive behaviour indicated more rapid flow of gas-rich magma to the surface. The explosive eruption was triggered by about 30% of the dome avalanching away in the previous 12 hours which decompressed gas-rich magma deeper in the volcano. This explosion is used as a reference event later in this assessment.
Dome growth recommenced two weeks after the explosive eruption on 17 September 1996. Both the rate of growth and size of the dome increased over the next several months, interrupted repeatedly by many episodes of pyroclastic flow generation. Eventually the dome became so large that it filled up English's Crater. The walls of the crater had protected the southwestern, western and northern flanks of the volcano from pyroclastic flows, but by March 1997 the southwestern wall was overwhelmed and from June 1997 onwards the northern wall was overtopped. The major pyroclastic flow eruption of 25 June 1997 killed at least 19 people and nearly reached the airport 5.5 km northeast of the volcano. About 8 million cubic metres of the dome avalanched in less than 20 minutes. In late July 1997 large pyroclastic flows went down valleys on the west, resulting in the partial destruction of Plymouth.
Following major dome collapses in early August 1997 explosive eruptions occurred at fairly regular (12 hourly) intervals over an 8 day period. These eruptions introduced an additional kind of hazard: pyroclastic flows formed by explosions rather than by avalanching of the unstable dome. Although these pyroclastic flows are similar in behaviour and consequent hazard to the dome collapse type, they are less constrained by topography, as the explosion can eject the materials in all directions around the volcano. By this time the scientists at MVO had recognized regular patterns of pressure build-up which allowed quite accurate prediction of when explosive events or pyroclastic flows would occur. This enabled emergency operations to take place in potentially dangerous areas in the periods when the internal pressure of the volcano was estimated to be low.
The largest pyroclastic flow so far occurred on 21 September 1997 and destroyed the airport terminal. A prolonged period of quite regularly spaced explosive eruptions followed. Between 22 September and 21 October 1997 there were 75 explosions spaced at a mean interval of 9.5 hours. The explosions produced eruption columns of 5 to 12 km height and the largest events were only slightly less energetic than the 17 September 1996 explosion. Since then dome growth has continued and further dome collapses have generated more pyroclastic flows.
The general trend of the eruption so far has been slow escalation (Figure 2). The mean flux of magma in the first 6 months was less than 1 m3/s; it rose to 2.3 m3/s in 1996 and 5 to 8 m3/s in the past 6 months. Superimposed on this trend are many pulsations. The dome growth rate and activity can be well below average for days to many weeks and then increase quite rapidly to well above average. The activity is punctuated by episodes of major dome collapse and pyroclastic flow generation. Each of the three major periods of explosive activity have occurred after one of the major dome collapses. Measurements of the flux of Sulphur dioxide gases from the volcano also show a slow baseline increase with time.
The crisis prompted more than half of the island's population to leave; those who stayed were evacuated to the north. The restless volcano has prevented their return.
Britain offered temporary, and later permanent, residence to all Montserratians. Others left for New York, or for other Caribbean islands including Antigua.
Montserratians living in the US have had their "temporary protected status" revoked; many may have to leave. With volcanic activity in their homeland unlikely to cease, the US Department of Homeland Security no longer considered their situation to be temporary.
Tourism was once the lifeblood of the economy. However, the destruction of the capital and the closure of the island's airport halted much economic activity. Montserrat has relied heavily upon British and EU aid to rebuild; a new airport was inaugurated in 2005
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