soufriere hills volcano eruption 1995 case study

Soufrière Hills 1995-present

Soufrière Hills in Montserrat has been erupting since 1995.

Chronic Medical Aspects

Crystalline silica in volcanic ash, when inhaled, adversely affects health..

The extended eruption of a lava dome at Soufrière Hills Volcano that began in 1995 generated large amounts of fine ash by (1) explosive events from the dome; and (2) frequent collapse of unstable parts of the growing dome that generated pyroclastic flows and associated plumes of ash. A detailed study of ash from both types of events determined that the sub-10 micron fraction of ash from the pyroclastic flows consisted of 10-24 percent crystalline silica , the highest yet documented for a historical eruption (Baxter and others, 1999). In contrast, the sub-10 micron fraction of ash from the explosive events consisted of 3-6 percent crystalline silica. The free silica minerals are produced within the lava dome over a period of many days or weeks.

Monitoring of the concentration of airborne respirable dust and ash around the volcano beginning in August 1997 showed that concentrations of ash have regularly exceeded 50 micrograms/m3 per 24-hour rolling average in areas subject to frequent ashfall. The exposures to cristobalite sometimes reached the 0.05 mg/m3 averaged over an 8-hour workday. Also, the monitoring consistently showed increased concentrations of airborne dust whenever there was human activity.

This study raises concern that exposure to long-lived eruptions of lava domes that produce persistent ashfall over many years may result in adverse health effects in affected communities.

Water Supply

The eruptions of Soufrière Hills during 1997 produced chemical contamination of rainwater and surface water. Water sampling in January 1997 indicated highly acidic water with high concentrations of sulphates, chloride and fluorides. Similar results were recorded until June 1997 although all fell within World Health Organization recommended levels for all measured components (see Smithsonian Institution Global Volcanism Program ).

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Soufrière Hills Volcano, Montserrat, West Indies.

Soufrière Hills Volcano , Montserrat, West Indies. Synopsis of events by former Montserrat resident, photographer and Author Lally Brown. 

Where is Montserrat? Montserrat is a small tropical island of approximately 40 sq. miles in the Caribbean, fifteen minutes flying time from Antigua. It is a British Overseas Territory and relies on UK Government aid money to survive. It is of volcanic origin with the Soufrière Hills above the capital of Plymouth the highest point of the island.

How and when did the volcano erupt? Prior to 1995 the volcano in the Soufrière Hills had been dormant for 350 years but on the morning of 18th July 1995 steam and fine ash could be seen coming from the flanks of the Soufrière Hills accompanied by a roaring sound, described as being like a jet engine. In the capital of Plymouth there was a strong smell of ‘bad eggs’ the hydrogen sulphide being emitted by the awakening volcano.

Montserrat was totally unprepared. No-one had ever imagined the dormant volcano would erupt. The Soufrière Hills was the breadbasket of the island where farmers worked the fertile agricultural land, while the busy capital and island port of Plymouth nestled at the foot of the hills.

Scientists arrived from the University of the West Indies to assess the situation. They said the volcano was producing ‘acoustic energy explosions’ at approximately half-hour intervals sending ash and vapour three to four hundred metres into the air.

What happened next? Before July 1995 Montserrat was a thriving tourist destination with a population of 10,000 people but over several weeks there was a mass exodus from the island and a run on the banks with people withdrawing cash.

Several areas near the vent that had opened up in the hillside were declared exclusion zones and residents were evacuated to the safe north of the island into schools and churches.

It was evident the volcano was becoming more active when a series of small earthquakes shook the island. Heavy rain from passing hurricanes brought mudflows down the hillsides into Plymouth. Sulphide dioxide emissions increased, a sure sign of heightened activity.

The scientists hoped to be able to give a six hour warning of any eruptive activity but when they discovered the magma was less than 1 km below the dome they said this could not be guaranteed, saying there was a 50% chance of an imminent eruption. An emergency order was signed by the Governor and new exclusion zones were drawn with people evacuated north.

The years 1995 to 1997 The Soufrière Hills volcano became increasingly active and more dangerous.

Montserrat Volcano Observatory (MVO) was established to monitor activity and advise the Government.

December 1995 saw the first pyroclastic flow from the volcano.

The capital of Plymouth was evacuated for the last time in April 1996.

Acid rain damaged plants.

Two-thirds of Montserrat became the new exclusion zone , including the fertile agricultural land.

Population dropped to 4,000 with residents leaving for UK or other Caribbean islands.

Frequent heavy ashfalls covered the island with blankets of thick ash.

On the seismic drums at the MVO swarms of small hybrid earthquakes frequently registered. Also volcano-tectonic earthquakes (indicating fracture or slippage of rock) and ‘Broadband’ tremors (indicating movement of magma).

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MVO Seismograph printout Dec 1997

‘Spines’ grew rapidly out of the lava dome to heights of up to 15 metres before collapsing back.

Rainfall caused dangerous mudflows down the flanks of the Soufrière Hills.

Temporary accommodation was built to house evacuees living in churches and schools.

25th June 1997 Black Wednesday For a period of twenty minutes at 12.59 pm the volcano erupted without warning with devastating consequences. A massive pyroclastic flow swept across the landscape and boulders up to 4 metres in diameter were thrown out of the volcano. Over 4 sq.km was destroyed including nine villages and two churches. The top 300ft had been blown off the lava dome. Tragically nineteen people were caught in the pyroclastic flow and died.

Post Office and War Memorial 1997

Lateral blast December 1997 Midnight on Christmas Day 1997 the MVO reported that hybrid earthquakes had merged into a near-continuous signal clipping the sides of the seismic drum. At 3am on Boxing Day there was a massive collapse of the dome. Approximately 55 million cubic metres of dome material shot down the flanks of the volcano into the sea. Travelling at speeds of 250-300 km per hour it took less than a minute to slice a 7 km wide arc of devastation across southern Montserrat. The evacuated villages of Patrick’s and O’Garros were blasted out of existence. A delta 2 km wide spilled into the sea causing a small tsunami .

Police checkpoint Montserrat

March 1999 After a year of apparent inactivity at the volcano the Scientists declared the risk to populated areas had fallen to levels of other Caribbean islands with dormant volcanoes. Arrangements were made to encourage overseas residents to return. Plans were put in place to reopen the abandoned airport.

2000 to 2003 One year after the volcano had been declared dormant there was a massive collapse of the dome, blamed on heavy rainfall.

In July 2001 another massive collapse of the dome described as ‘a significant eruption’ caused airports on neighbouring Caribbean islands to close temporarily due to the heavy ashfall they experienced. A Maritime Exclusion Zone was introduced around Montserrat and access to Plymouth and the airport prohibited.

Soufrière Hills volcano was now described as a ‘persistently active volcano’ that could continue for 10, 20 or 30 years. (ie possibly to 2032).

In July 2003 ‘the worst eruption to date’ took place, starting at 8 pm 12th July and continuing without pause until 4 am morning of 13th July. Over 100 metres in height disappeared from the mountain overnight. It was the largest historical dome collapse since activity began in July 1995.

A period of relative quiet followed.

2006 The second largest dome collapse took place with an ash cloud reaching a record 55,000 metres into the air. Mudflows down the flanks of the Soufrière Hills was extensive and tsunamis were reported on the islands of Guadeloupe and Antigua.

Another period of relative quiet followed.

Soufriere Hills volcano 2007

2010 Another partial dome collapse with pyroclastic flows reaching 400 metres into the sea and burying the old abandoned airport. There was extensive ashfall on neighbouring islands.

Again followed by a period of relative quiet.

2018 Although the Soufrière Hills volcano is described as ‘active’ it is currently relatively quiet. It is closely monitored by a team at the Montserrat Volcano Observatory (MVO). They advise the Government and residents on the state of the volcano.

Negative effects of the volcano:

·       Approximately two-thirds of Montserrat now inaccessible (exclusion zone);

·       Capital of Plymouth including hospital, government buildings, businesses, schools etc. buried under ash;

·       Fertile farming land in the south in exclusion zone and buried under ash;

·       Population reduced from 10,000 to 4,000;

·       Businesses left Montserrat;

·       Tourism badly affected;

·       Concern over long term health problems due to ash;

·       Volcano Stress Syndrome diagnosed;

·       Huge financial cost to British Tax Payer (£400 million in aid);

·       Loss of houses, often not insured;

·       Relocation to the north of Montserrat by residents from the south.

Positive effects:

·       Tourists visiting Montserrat to see the volcano, MVO and Plymouth, now described as ‘Caribbean Pompeii’;

·       Geothermal energy being investigated;

·       Sand mining for export;

·       Plans for a new town and port in north;

·       New housing for displaced residents built;

·       New airport built (but can only accommodate small planes);

·       New Government Headquarters built;

·       Businesses opening up in the north of the island;

·       Ferry to Antigua operating.

Lally Brown

You can follow Lally Brown on Twitter.

If you are interested in reading a dramatic eyewitness account of life with this unpredictable and dangerous volcano then the book ‘THE VOLCANO , MONTSERRAT AND ME’ by Lally Brown is highly recommended. You can order a paper back or Kindle version on Amazon .

“As time moves on and memories fade, this unique, compelling book will serve as an important and accurate first-hand record of traumatic events, faithfully and sensitively recounted by Lally Brown.”

Prof. Willy Aspinall Cabot Professor in Natural Hazards and Risk Science, Bristol University.

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Soufrière Hills eruption, Montserrat, 1995 - 1997: volcanic earthquake locations and fault plane solutions

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A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel-time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE-SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano-tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late-1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth - the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.

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Report on Soufriere Hills (United Kingdom) — September 1995

Bulletin of the Global Volcanism Network, vol. 20, no. 9 (September 1995) Managing Editor: Richard Wunderman. Soufriere Hills (United Kingdom) Phreatic eruptions continue; new lava dome in summit crater

Please cite this report as: Global Volcanism Program, 1995. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network , 20:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199509-360050

Soufriere Hills

United kingdom, 16.72°n, 62.18°w; summit elev. 915 m, all times are local (unless otherwise noted).

Following the formation of Vent 3 and significant ashfall on 22 August (20:8), more than 6,000 residents of southern Montserrat were evacuated to safe areas in the N part of the island. Press sources estimated that by late August ~3,000 people had left for neighboring islands. Vent 4 opened on 27 August and produced mainly steam emissions with some minor ash through 30 August. Although seismicity was high from 30 August through 1 September, steam and ash emissions remained low (20:8).

From 0500 on 1 September through 0500 on the 3rd, only 19 shallow earthquakes occurred beneath the volcano. During that same period, 17 episodes of gas venting were recorded; at least six of those episodes produced some ash, and the two events on 2 September each decayed into a long-period signal of ~10 minutes duration. Venting continued to enlarge vents 2 and 3, but emissions from Vent 4 remained low. A helicopter observation flight on the afternoon of 2 September was in progress when an emission episode began at 1606 with increased steaming that developed rapidly into a small steam-and-ash plume. The emission occurred from a narrow part of the main group of vents that extend SE from Vent 1. Mud on the floor of the vent was expelled during the episode, forming a small mudflow that moved down the S side of the moat and over the area of Vent 2. A gas-and-ash emission at 1912 on 2 September, similar in size and duration to emissions in recent days, was widely observed because of clear conditions. Lightning associated with this activity lasted ~1.5 hours, and an SO 2 odor was detected. Installation of a hardened EDM (electronic distance meter) station in the Tar River area was completed on 2 September.

During 3-4 September there were four gas-venting episodes, twelve volcano-tectonic (VT) earthquakes, and four long-period earthquakes. Aerial observations on the morning of 3 September revealed that the area around the S end of the main group of vents had been enlarged. The moat pond in the NW corner was still present, and fragmental material had collapsed into Vent 1. Afternoon observations showed no new mudflows, and the S moat appeared dry.

On the afternoon of 3 September, scientists at the volcano observatory completed an assessment of the current volcanism since 21 August and prospects for future activity. The rate of eruption signals increased slightly after 21 August, but the size of the eruptions did not. No change in the style of eruptions was anticipated, but areas downwind could be subject to ashfall and temporary darkness. Eruptions were thought likely to be concentrated along the linear vent chain on the W side of Castle Peak dome. The amount of shallow seismicity decreased below that prior to 21 August. SO 2 flux remained near detection limits since 21 August. The rate of long-period seismic events showed no clear pattern, although a slight decrease may have occurred. Initial EDM results indicated no movement of the SE flank of Castle Peak dome or at a site in Upper Gages. Electronic tiltmeters have detected no large-scale deformation since they stabilized on 5 August. Ash samples analyzed through 27 August revealed no juvenile material.

The scientists concluded the following: ". . . eruptions to date have been entirely phreatic, with no direct evidence of magmatic involvement. So long as this behavior pattern persists, it only constitutes a significant hazard to areas within 1.5 km of Castle Peak dome and the areas S of White's Bottom ghaut. All ghauts [ephemeral watercourses] that originate on the flanks of the Soufriere Hills volcano are subject to flooding and should be avoided." Based on this advice, the government approved re-occupation of the areas immediately S of the Belham Valley River from which residents were evacuated on 23 August. All other residents from areas closer to the crater, evacuated since 21 August, were required to stay in the northern third of the island. Controlled entry restrictions were relaxed in most areas to allow residents to prepare for an approaching hurricane. Following passage of the hurricane, on 6 September the remaining evacuation orders were lifted.

Activity during 4-8 September was consistent at a low and generally declining level. At about 1530 on 8 September there was a significant steam explosion. Two hours later, at about 1730, two large ash eruptions produced a vertical plume that formed a mushroom cloud, which drifted to St. Peters (~30 km NNW) and to the N. Soufriere Hills continued to have intermittent swarms of earthquakes from the summit and nearby areas, including three events felt in Woodlands on 11 September. Occasional steam eruptions produced falls of fine ash in communities around the volcano, and morphological changes were continuing in the summit area. These developments suggested to volcanologists that magma was close to the surface under the volcano and that a magmatic eruption was still a possibility.

Two weeks later, on 25 September, a lava dome began growing in the W part of the moat near the linear chain of vents. An explosion between 1100 and 1200 on 27 September caused ashfall on the S part of the island, with minor ashfall also reported in the St. Georges area. Minor explosive activity continued through the end of September.

Geological Summary. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Soufriere Hills Volcano Observatory , Plymouth; Seismic Research Unit , UWI; UNDHA ; AP ; Caribbean News Agency (CANA), Barbados.

• The 1995 Soufrière Hills Eruption

In 1995, the Soufrière Hills volcano on the Caribbean island of Montserrat became active. As a result, half of Montserrat became uninhabitable. As the volcano had been dormant for over 3 centuries and had been deemed inactive, this came as a devastating blow to the small island and its inhabitants.

Location And Geography

The Soufriere Hills volcano is situated in the Caribbean Island of Montserrat. The Island is a British Overseas Territory and is a part of the Leeward Islands which is a chain of islands known as the Lower Antilles. The total land area is about 100 square kilometers. The Soufrière Hills Volcano is part of the Lesser Antilles Volcanic Arc and is situated to the south of the island. The capital city was called Plymouth before it was buried in debris after the eruption.

History And Timeline

The early history of the volcano is relatively unknown due to inconsistent record keeping. The first explosive eruption is estimated to have been around 2,500 years ago. The last known eruption was in the 16th century where anywhere between 25 to 65 million cubic meters of lava erupted at Castle Peak. The 1995 eruption was preceded by seismic activity recorded in 1897, 1933 and lastly in 1966. The eruption in 1995 was also preceded by seismic activity but what ensued was mostly unexpected. Earthquake swarms had first been detected in 1992 and again in 1994.

Eruption Of The Soufrière Hills Volcano

The eruption of ash in July 1995 prompted an evacuation of almost 5,000 residents. The volcano grew a new dome on November 1995. By January 1996, the old dome was rapidly buried and between March and September of the same year, the first pyroclastic flows poured down the Tar river valley. This created a new delta and in April the south of the island was evacuated. The capital city of Plymouth was also abandoned. Pyroclastic flows and eruption columns are the main features of this volcano. They occur when the dome collapses or explodes. Tonnes of hot rock, lava and ash explode from the crater in a cloud moving at speeds of up to 100 miles per hour with temperatures reaching over 400°C. The fast moving cloud annihilates and incinerates everything in its way.

Aftermath Of The Eruption

The eruption left the southern two-thirds of the islands completely inhabitable. Pyroclastic flows still pour down the slopes of the volcano. The eruptions continued after the volcano became active. The disaster resulted in the collapse of the tourism and also the local rice processing industries. Unemployment shot up from a manageable 7% to over 50%. Agricultural activities became nearly impossible and living conditions were further worsened by respiratory problems caused by the spewing ash. The aid and relief activities were spearheaded by both British and Montserrat governments.

The 1995 eruption changed the landscape and living conditions of the Montserrat Island completely. It destroyed the economy and forced most residents to abandon the city. As a result of this eruption, several monitoring initiatives were undertaken like the establishment of an extensive seismograph network. The volcano is still active and subject to eruptions from time to time. It remains to be seen how long it will take until the island is habitable again.

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The University of The West Indies

Oriens ex occidente lux.

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

Source: https://goo.gl/ibBKmS , https://goo.gl/NCXeRD , https://goo.gl/cNGbof

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Montserrat Volcano 1995-1997 Case Study

Subject: Geography

Age range: 14-16

Resource type: Lesson (complete)

Last updated

26 November 2018

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Soufrière Hills eruption, Montserrat, 1995 - 1997: volcanic earthquake locations and fault plane solutions

A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel-time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE-SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano-tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late-1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth - the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.

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Volcanic Eruption- Soufriere Hills, 1997

The Soufriere Hills Volcano erupted in 1997.

Date of eruption: June 25th 1997

Size of eruption: Large- 4.5 million m cubed of rocks and gas was released

Death toll: 19 killed

• Created by: Lottie_Anderson
• Created on: 18-03-13 16:54

• Rock landscapes and processes

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