Eruption 1997

[Melanie Wendelberger]

TheSoufrire Hills are an active, complex stratovolcano with many lava domes forming its summit on the Caribbean island of Montserrat. After a long period of dormancy, the Soufrire Hills volcano became active in 1995 and continued to erupt through 2010.[3] Its last eruption was in 2013.[2] Its eruptions have rendered more than half of Montserrat uninhabitable, destroying the capital city, Plymouth, and causing widespread evacuations: about two-thirds of the population have left the island.[4] Chances Peak in the Soufrire Hills was the highest summit on Montserrat until the mid-1990s, but it has since been eclipsed by various rising and falling volcanic domes during the recent volcanic activity.[5]

The volcano is andesitic in nature, and the current pattern of activity includes periods of lava dome growth, punctuated by brief episodes of dome collapse which result in pyroclastic flows, ash venting, and explosive eruption. The volcano is monitored by the Montserrat Volcano Observatory. Volcanic gas emissions from this volcano are measured by a multi-component gas analyzer system, which detects pre-eruptive degassing of rising magmas, improving prediction of volcanic activity.[6]

The Centre Hills in the central part of the island and the Silver Hills in the north are older volcanic massifs related to the subduction zone. There are three main parts of the island: the central zone, subduction and exclusion.[7]

Many volcanoes in the Caribbean are named Soufrire (French: "sulphur outlet"). These include La Soufrire or Soufrire Saint Vincent on the island of Saint Vincent, and La Grande Soufrire on Guadeloupe.[8][9]

Pyroclastic flows were first observed on 27 March 1996.[13] Although these pyroclastic flows and mudflows were initially confined to unpopulated areas, a major pyroclastic flow on 17 September 1996 caused severe damage to the village of Long Ground, near the volcano.[14] By August 1997, much of the southern part of the island, including most of Plymouth, had been buried by pyroclastic flows.

During this period, the southern portion of the island, including the capital Plymouth, was evacuated three times. The first evacuation lasted from 21 August 1995 - 3 September 1995, after the first phreatic eruption. The second evacuation began on 1 December 1995,[15] due to growth of the lava dome. Residents were allowed back to most areas, including Plymouth, on 1 January 1996,[16] but residents of some areas on the eastern flank of the volcano were not allowed back until 15 January 1996.[17] On 3 April 1996, after pyroclastic flows and mudflows began occurring regularly, southern Montserrat, including the capital, Plymouth, was permanently evacuated.[18]

Earthquakes continued to occur in three epicentre zones: beneath the Soufrire Hills volcano, in the ridge running to the northeast, and beneath St George's Hill, about 5 kilometres (3.1 mi) to the northwest.[19] A large eruption on 25 June 1997 resulted in the deaths of nineteen people. The island's airport was directly in the path of the main pyroclastic flow and was completely destroyed.[20] Montserrat's tourist industry also collapsed, although it began partially to recover within fifteen years.[21]

The governments of the United Kingdom and Montserrat led the aid effort, including a 41 million package provided to the Montserrat population; however, riots followed as the people protested that the British Government was not doing enough for aid relief.[22] The riots followed a 10 million aid offer by International Development Secretary Clare Short, prompting the resignation of Bertrand Osborne, then Chief Minister of Montserrat, after allegations that he was too pro-British and had not demanded a better offer.[23]

The British destroyer HMS Liverpool took a major role in evacuating Montserrat's population to other islands, including Antigua and Barbuda who warned they would not be able to cope with many more refugees.[23] About 7,000 people, or two-thirds of the population, left Montserrat; 4,000 went to the United Kingdom.[24]

On 24 December 2006, streaks of red from the pyroclastic flows became visible. On 8 January 2007, an evacuation order was issued for areas in the Lower Belham Valley, affecting an additional 100 people.

At 11:27 pm local time on Monday 28 July 2008, an eruption began without any precursory activity. Pyroclastic flow lobes reached Plymouth. These involved juvenile material originating in the collapse of the eruption column. In addition, a small part of the eastern side of the lava dome collapsed, generating a pyroclastic flow in Tar River Valley. Several large explosions were registered, with the largest at approximately 11:38 pm. The height of the ash column was estimated at 12,000 m (40,000 ft) above sea level.

The volcano has become one of the most closely monitored volcanoes in the world since its eruption began, with the Montserrat Volcano Observatory taking detailed measurements and reporting on its activity to the government and population of Montserrat. The observatory is operated by the British Geological Survey, under contract to the government of Montserrat.

On 5 February 2010, a vulcanian explosion simultaneously propelled pyroclastic flows down several sides of the mountain, and on 11 February 2010, a partial collapse of the lava dome sent large ash clouds over sections of several nearby islands, including Guadeloupe and Antigua. Inhabited areas of Montserrat itself received very little ash accumulation during either event.[27]

On 12 February 2010, at 1200 UTC/GMT, Meteosat SEVIRI Channel 7 shows the ash plume from the eruption was caught up within the warm sector of a frontal system heading towards western Europe exacerbating two East Atlantic winter storms. [reference] Violent storms accompanied by torrential rain and high wind passed over Madeira on 20 February and western Europe on 26-28 February. In Madeira there was at least 48 fatalities and damage to infrastructure at a total estimated cost of 4 billion Euros. The storm landfalling in western France (named 'Cyclone Xynthia') resulted in at least 63 fatalities and estimated damage within the range of 1.3-3 billion Euros.

A huge cloud of volcanic ash and gas rises above Mount Pinatubo, Philippines, on June 12, 1991. Three days later, the volcano exploded in the second-largest volcanic eruption on Earth in this century. Timely forecasts of this eruption by scientists from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey enabled people living near the volcano to evacuate to safer distances, saving at least 5,000 lives.

Precursors to the 1991 EruptionsOn July 16, 1990, a magnitude 7.8 earthquake (comparable in size to the great 1906 San Francisco, California, earthquake) struck about 60 miles (100 kilometers) northeast of Mount Pinatubo on the island of Luzon in the Philippines, shaking and squeezing the Earth's crust beneath the volcano. At Mount Pinatubo, this major earthquake caused a landslide, some local earthquakes, and a short-lived increase in steam emissions from a preexisting geothermal area, but otherwise the volcano seemed to be continuing its 500-year-old slumber undisturbed. In March and April 1991, however, molten rock (magma) rising toward the surface from more than 20 miles (32 kilometers) beneath Pinatubo triggered small earthquakes and caused powerful steam explosions that blasted three craters on the north flank of the volcano. Thousands of small earthquakes occurred beneath Pinatubo through April, May, and early June, and many thousand tons of noxious sulfur dioxide gas were also emitted by the volcano.

Following Mount Pinatubo's cataclysmic June 15, 1991, eruption, thousands of roofs collapsed under the weight of ash made wet by heavy rains (see example in photo above). Ash deposits from the eruption have also been remobilized by monsoon and typhoon rains to form giant mudflows of volcanic materials (lahars), which have caused more destruction than the eruption itself (photo at right shows village buried by lahars). (Photo above courtesy of Peter Baxter, University of Cambridge.

The EruptionsFrom June 7 to 12, the first magma reached the surface of Mount Pinatubo. Because it had lost most of the gas contained in it on the way to the surface (like a bottle of soda pop gone flat), the magma oozed out to form a lava dome but did not cause an explosive eruption. However, on June 12 (Philippine Independence Day), millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption.

When even more highly gas charged magma reached Pinatubo's surface on June 15, the volcano exploded in a cataclysmic eruption that ejected more than 1 cubic mile (5 cubic kilometers) of material. The ash cloud from this climactic eruption rose 22 miles (35 kilometers) into the air. At lower altitudes, the ash was blown in all directions by the intense cyclonic winds of a coincidentally occurring typhoon, and winds at higher altitudes blew the ash southwestward. A blanket of volcanic ash (sand- and silt-size grains of volcanic minerals and glass) and larger pumice lapilli (frothy pebbles) blanketed the countryside. Fine ash fell as far away as the Indian Ocean, and satellites tracked the ash cloud several times around the globe.

Huge avalanches of searing hot ash, gas, and pumice fragments (pyroclastic flows) roared down the flanks of Mount Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as 660 feet (200 meters) thick. The eruption removed so much magma and rock from below the volcano that the summit collapsed to form a large volcanic depression (caldera) 1.6 miles (2.5 kilometers) across.

Much weaker but still spectacular eruptions of ash occurred occassionally through early September 1991. From July to October 1992, a lava dome was built in the new caldera as fresh magma rose from deep beneath Pinatubo.

Continuing HazardsEven after more than 5 years, hazardous effects from the June 15,1991, climactic eruption of Mount Pinatubo continue. The thick, valley-filling pyroclastic-flow deposits from the eruption insulated themselves and have kept much of their heat. These deposits still had temperatures as high as 900&#176F (500&#176C) in 1996 and may retain heat for decades. When water from streams or underground seepage comes in contact with these hot deposits, they explode and spread fine ash downwind. Since the climactic 1991 eruption, ash deposits have also been remobilized by monsoon and typhoon rains to form giant mudflows of volcanic materials (lahars).

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