Soufriere Hills

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Soufriere Hills
The Soufriere Hills volcano

The Soufriere Hills volcano

height 1050  m
location Montserrat
Coordinates 16 ° 42 '56 "  N , 62 ° 11' 8"  W Coordinates: 16 ° 42 '56 "  N , 62 ° 11' 8"  W.
Map of Soufriere Hills
Type Stratovolcano
Last eruption February 11, 2010

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The stratovolcano Soufrière Hills is currently the largest volcano on the Caribbean island of Montserrat with a height of around 1050  m . Geoscientists assume that there were several eruption phases before the island was settled by Europeans , for example around 400 and around 4500 years ago. In historical times the volcano was not active, but recurring light earthquakes and fumaroles indicated that it was by no means extinguished. Due to the volcanic activity from 1995 (growth and subsequent collapse of lava domes ), the height of the mountain has since varied considerably between 915  m (summit of Chances Peak) and 1150  m (height of the lava dome before the eruption of 2010).

General

Summary of volcanic activity since 1995

Increased seismic activity near Montserrat was registered as early as April 1989. The first volcanically initiated swarms of earthquakes occurred in 1992 and intensified in 1994 in the Soufrière Hills, before the volcanic eruption began on July 18, 1995 with a phreatic explosion in the north-west crater . As a result, there were numerous other swarms of earthquakes, phreatic explosions and the onset of ash rain in the island's capital Plymouth . The south of Montserrat was evacuated for the first time on August 21, 1995 . Since then, volcanic activity has followed a cyclical pattern with extrusion phases, which are characterized by increased surface activity (escape of magma and growth of the lava dome, dome collapse, volcanic eruptions , pyroclastic flows ) alternating with periods of rest in which the eruptive activity paused. In the geodetic measurement data, extrusion phases correlate with a subsidence of the soil due to the rise of magma from the earth's crust to the earth's surface (loss of volume / pressure decrease in the magma chambers ). In contrast to this, a bulging of the surface is observed during the eruptive rest phases, which testifies to a renewed pressure build-up in the magma chambers. Since the beginning of the 1995 eruption, the volcano has gone through five phases of extrusion:

Phase 1: November 15, 1995 to March 10, 1998

Phase 2: November 27, 1999 to July 28, 2003

Phase 3: August 1, 2005 to April 20, 2007

Phase 4: July 29, 2008 to January 3, 2009

Phase 5: October 9, 2009 to February 11, 2010

Between 1995 and 2010 an estimated 1000 million m³ of volcanic rock was erupted. In the first three extrusion phases, around 300 million m³ of material were ejected and in the subsequent phases 39 million m³ (phase 4) and 74 million m³ of material (phase 5). The average extrusion rate of magma fluctuated between 2.9 and 6.8 m³ / s depending on the phase. A partial cathedral collapse on February 11, 2010 marks the last eruptive activity to date (as of June 2020). Continuing deformation of the ground (bulging of the earth's surface) has since shown a continuous increase in pressure in the magma chambers, so that, despite the long period of rest, the eruption cannot be considered to have ended.

The magmatic system of the Soufrière Hills

Changes in volume and pressure in magma chambers cause deformation of the earth's surface, which can be recorded by geodetic measurements. The deformation patterns allow conclusions to be drawn about the properties of the magma chambers in the earth's interior. The depth of the magma chamber in the earth's crust correlates with the wavelength of the deformation signal at the earth's surface, while the ratio of the vertical to horizontal offsets allows statements about the geometry of the magma chamber. In principle, however , models are based on the assumption of an idealized ellipsoidal body as the source of the volume / pressure changes.

The analysis of geodetic measurement data ( GNSS , inclination, strain) from Montserrat, which were recorded during various rest and extrusion phases of the volcano, has shown that pressure changes in the subsurface originate from different sources. The most widely accepted model to date was derived from this, which describes the magmatic system in the subsurface of the Soufrière Hills consisting of two magma chambers that are arranged vertically under the crater. The lower magma chamber is approx. 12 km deep and the upper magma chamber is 5.5 km deep (depth data refer to the center of a magma chamber). The relative volume of the magma chambers to one another could be determined from events with coupled magma chamber activity (e.g. extrusion phase 4b) to 1: 3, the volume of the upper, smaller magma chamber being estimated at 8 km 3 . The connection between the two magma chambers cannot be resolved with geodetic measurements, since the signal is too weak and the deformation signals of the large magma chambers overprint it. However, the near-surface connection of the upper magma chamber to the earth's surface could be identified as a north-west-south-east facing volcanic dike , which narrows into a cylindrical vent about 1 km below the crater .

The assumption that the magmatic system consists of two superposed, interconnected magma chambers is also proven on the basis of petrological data . The rock erupted on the Soufrière Hills volcano is andesitic (low SiO 2 content) and the analysis of the pressure / temperature-dependent mineral composition suggests extraction from a magma chamber at a depth of approx. 5 km. In the ejecta there are, however, trapped basaltic (high SiO 2 content) parts, which presumably originate from a deeper magma chamber. Gas geochemical measurements of volcanic emissions document increased sulfur dioxide and carbon dioxide proportions. The high sulfur dioxide content measured, however, contradicts the low sulfur content in andesitic magma and is therefore attributed to the degassing of basaltic melts at greater depths.

Eruption dynamics

The initial trigger of the 1995 eruption is assumed to be the penetration of hot, basaltic magma into the upper andesitic (cooler) magma chamber. Petrological studies of the ejected eruption products have shown that crystals are mixed in the magma that were exposed to significantly different temperature conditions during their growth (upper vs. lower magma chamber). The penetration and mixing of the hotter, basaltic magma with the andesitic magma led to (re) heating and thus a remobilization of the existing magma and ultimately to the eruption of the volcano.

The processes that trigger the beginning of the cyclically recurring extrusion phases and control the change in the character of the eruption since the 4th eruption phase are still not fully understood.

High-resolution soil deformation measurements (strainmeter data) enabled detailed analyzes of the Vulcanian eruptions and explosions. The results showed that sudden explosive events can be triggered by two different mechanisms:

  1. sudden degassing from the magma chamber, with gases rising from the 5 km deep magma chamber to the surface of the earth within 1–2 min
  2. explosive bursting and tearing of magma close to the surface due to increasing internal pressure from expanding gas bubbles trapped in the magma

Chronology of the eruption

Extrusion phase 1 (November 15, 1995 to March 10, 1998)

On November 14th, magma appeared on the surface of the earth for the first time. From then on, a lava dome of rising magma grew above the volcanic vent. With increasing steepness and the associated instability of the dome, parts of the flanks broke off again and again, creating glowing clouds, so-called pyroclastic currents, which move down the slope at over 100 km / h. The phase of increasing cathedral instability culminated in a first cathedral collapse on September 17, 1996 shortly before midnight local time, during which large quantities of pumice and rocks were erupted. In Long Ground (2.1 km from the cathedral) houses were destroyed by rock bombs the size of a football. However, there were no injuries as the area had already been evacuated on a large scale. A new cathedral was then formed and activity continued to increase. On June 25, 1997, pyroclastic currents also reached previously unaffected areas. 19 farmers who refused to leave their fields died. Speculation about an imminent explosion across the island caused a large part of the population to leave the island. They were picked up from other Caribbean islands as well as from metropolitan Britain. By September 1997, the capital Plymouth and all settlements on the southern half of the island as well as Bramble Airport were destroyed and covered by a layer of ash up to 12 m thick. The heaviest eruption of this phase occurred on December 26, 1997, during which 35 - 45 million m³ of cathedral material was removed and pyroclastic flows destroyed 10 km 2 of the south of Montserrat. The cathedral began to grow again, but stagnated in March 1998 after the cathedral had reached almost the same size as it was on December 24, 1997. At the beginning of the rest phase that followed, the appearance of the lava dome was characterized by a prominent rock spur on the summit. A partial cathedral collapse in June 1998, however, eroded a large part of the cathedral.

Extrusion phase 2 (November 27, 1999 to July 28, 2003)

The second extrusion phase is the longest to date with almost continuous lava extrusion. During this phase there were three cathedral collapses (March 2000, July 2001 and July 2003) in which the majority of the material removed was transported eastwards into the Tar River Valley and beyond the coast into the sea. On July 9, 2003, at the time the cathedral had reached its largest volume, earthquake activity began to increase significantly. On July 12, this turned into seismic tremor, which is indicative of increased gastric pressure in the vent. At the same time, heavy rain set in, which is believed to have also had a destabilizing effect on the cathedral. On 12./13. In July 2003 the cathedral was dismantled for over 18 hours. It was the largest cathedral collapse in the history of the eruption. Over 210 million m³ of material was distributed over the already devastated area and the vertical eruption column was over 15 km high. The largest pyroclastic currents produced tsunamis and hydrovolcanic explosions in the sea. The northern part of the island was also covered in ash, causing damage to the infrastructure and buildings. People were not harmed. The cathedral collapse and with it the activity phase ended with a sequence of Vulcanian explosions.

Extrusion phase 3 (August 1, 2005 to April 20, 2007)

The growth of a new lava dome from the beginning of August 2005 led to another major dome collapse on May 20, 2006 with the material leaving to the east via the Tar River Valley. Due to a high extrusion rate, the lava at the time of the dome collapse was hotter and richer in gas than in other collapses, which meant that this event took place in a shorter time (3 h) but more intense. New magma extrusion started as early as 8 hours after the dome collapse, but this slowed down as the activity phase continued. However, at the end of 2006, the cathedral grew to the northwest for the first time, which posed the risk that a collapse could reach populated areas that were previously considered safe. It was the first time since the 1990s that there were temporary evacuations on Montserrat (areas immediately north of the volcano). In fact, in January 2007, a pyroclastic flow reached Belham Valley on the northwest flank of the volcano. The growth of the dome then continued at a slower rate on the northeast side of the volcano until it completely stagnated in April 2007.

Extrusion phases 4 and 5 (July 29, 2008 to February 11, 2010)

The EO-1 satellite image shows the volcano on December 29, 2009.

With extrusion phase 4, the activity pattern on Soufrière Hills changed. Activity phases 4 and 5 were significantly shorter and more explosive than the previous ones. Phase 4 is divided into 4a and 4b. The former began at the end of July 2008 with increased seismic activity and culminated between July 29 and August 25, 2008 in a series of volcanic explosions, dome growth, ejection of pumice and volcanic ash, and the discharge of debris and mud flows . After a short break in activity, phase 4b set in on December 3, 2008, which was initiated and ended by Vulcanian explosions and in between was characterized by rapid magma extrusion. After the abrupt end of the magma discharge on January 3, 2009, a pause began again until the fifth and, for the time being, last phase of activity began on October 9, 2009. Explosions, Vulcanian eruptions and pyroclastic currents occurred at high frequencies. The latter reached, due to the orientation and the enormous size of the cathedral, all the surrounding valleys around the volcano. Finally, on February 11, 2010, after almost 5 years, there was another major collapse of the cathedral, this time facing north. The high-energy pyroclastic currents destroyed many of the ruins of the old settlements of Harris and Streatham and buried the northeast flank of the volcano by another 2–10 m of sediment. Following this cathedral collapse, the eruptive activity stopped. However, persistent gas emission and floor bulging are used as indicators for future recurring activity.

monitoring

At the end of July 1995, the was Volcano Observatory Montserrat Volcano Observatory (MVO) was founded. The MVO is responsible for the routine monitoring of the volcano. The network of seismic, geodetic and geochemical measuring devices required for observation has been continuously expanded since 1995 and data acquisition has been partially automated. In addition, further instruments were added in the course of research projects. The Soufrière Hills volcano is therefore one of the most intensively researched andesitic, domed volcanoes in the world. Fundamental insights gained from research on Soufrière Hills are potentially transferable to other volcanoes.

literature

  • TH Druitt, BP Kokelaar (eds.): The Eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999 . In: The Geological Society of London (Ed.): Geological Society Memoir . tape 21 , 2002, ISBN 978-1-86239-396-7 , pp. 1–639 , doi : 10.1144 / GSL.MEM.2002.021.01.32 (English).
  • G. Wadge, REA Robertson, B. Voight (eds.): The Eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010 . In: The Geological Society of London (Ed.): Geological Society Memoir . tape 39 , 2014, ISBN 978-1-86239-630-2 , pp. 1-501 , doi : 10.1144 / M39.0 (English).

Web links

Commons : Soufrière Hills  - Collection of images, videos and audio files

Individual evidence

  1. ↑ a b c B. P. Kokelaar: Setting, chronology and consequences of the eruption of Soufrière Hills Volcano, Montserrat (1995-1999) . In: Geological Society, London, Memoirs . tape 21 , no. 1 , 2002, ISSN  0435-4052 , p. 1–43 , doi : 10.1144 / GSL.MEM.2002.021.01.02 .
  2. ↑ a b c d e G. Wadge, B. Voight, RSJ Sparks, PD Cole, SC Loughlin: Chapter 1 An overview of the eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010 . In: Geological Society, London, Memoirs . tape 39 , no. 1 , 2014, ISSN  0435-4052 , p. 1.1-40 , doi : 10.1144 / M39.1 .
  3. ^ Henry M. Odbert, Roderick C. Stewart, Geoffrey Wadge: Chapter 2 Cyclic phenomena at the Soufrière Hills Volcano, Montserrat . In: Geological Society, London, Memoirs . tape 39 , no. 1 , 2014, ISSN  0435-4052 , p. 41-60 , doi : 10.1144 / M39.2 .
  4. ^ A b Henry M. Odbert, Graham A. Ryan, Glen S. Mattioli, Stefanie Hautmann, Joachim Gottsmann: Chapter 11 Volcano geodesy at the Soufrière Hills Volcano, Montserrat: a review . In: Geological Society, London, Memoirs . tape 39 , no. 1 , 2014, ISSN  0435-4052 , p. 195-217 , doi : 10.1144 / M39.11 .
  5. ↑ G. Wadge, R. Herd, G. Ryan, ES Calder, J.-C. Komorowski: Lava production at Soufrière Hills Volcano, Montserrat: 1995-2009 . In: Geophysical Research Letters . tape 37 , no. October 19 , 2010, doi : 10.1029 / 2009GL041466 .
  6. ^ Montserrat Volcano Observatory: MVO Open File Report. March 31, 2020, accessed June 10, 2020 .
  7. ↑ Stefanie Hautmann, Joachim Gottsmann, R. Stephen J. Sparks, Glen S. Mattioli, I. Selwyn Sacks: Effect of mechanical heterogeneity in arc crust on volcano deformation with application to Soufrière Hills Volcano, Montserrat, West Indies . In: Journal of Geophysical Research . tape 115 , B9, September 9, 2010, ISSN  0148-0227 , p. B09203 , doi : 10.1029 / 2009JB006909 .
  8. ^ D. Elsworth, G. Mattioli, J. Taron, B. Voight, R. Herd: Implications of Magma Transfer Between Multiple Reservoirs on Eruption Cycling . In: Science . tape 322 , no. 5899 , October 10, 2008, ISSN  0036-8075 , p. 246-248 , doi : 10.1126 / science.1161297 .
  9. ↑ M. Paulatto, C. Annen, TJ Henstock, E. Kiddle, TA Minshull: magma chamber properties from integrated seismic tomography and thermal modeling at Montserrat . In: Geochemistry, Geophysics, Geosystems . tape 13 , no. January 1 , 2012, doi : 10.1029 / 2011GC003892 .
  10. ^ Oleg Melnik, Antonio Costa: Chapter 3 Dual-chamber-conduit models of non-linear dynamics behavior at Soufrière Hills Volcano, Montserrat . In: Geological Society, London, Memoirs . tape 39 , no. 1 , 2014, ISSN  0435-4052 , p. 61-69 , doi : 10.1144 / M39.3 .
  11. ↑ Stefanie Hautmann, Dannie Hidayat, Nicolas Fournier, Alan T. Linde, I. Selwyn Sacks: Pressure changes in the magmatic system during the December 2008 / January 2009 extrusion event at Soufrière Hills Volcano, Montserrat (WI), derived from strain data analysis . In: Journal of Volcanology and Geothermal Research . tape 250 , January 2013, p. 34-41 , doi : 10.1016 / j.jvolgeores.2012.10.006 .
  12. ^ A. Costa, O. Melnik, RSJ Sparks, B. Voight: Control of magma flow in dykes on cyclic lava dome extrusion . In: Geophysical Research Letters . tape 34 , no. 2 , January 24, 2007, ISSN  0094-8276 , doi : 10.1029 / 2006GL027466 .
  13. ↑ Stefanie Hautmann, Joachim Gottsmann, R. Stephen J. Sparks, Antonio Costa, Oleg Melnik: Modeling ground deformation caused by oscillating overpressure in a dyke conduit at Soufrière Hills Volcano, Montserrat . In: Tectonophysics . tape 471 , no. 1-2 , June 2009, pp. 87-95 , doi : 10.1016 / j.tecto.2008.10.021 .
  14. ↑ J. Barclay, MJ Rutherford, MR Carroll, MD Murphy, JD Devine: Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufriere Hills magma . In: Geophysical Research Letters . tape 25 , no. 18 , September 15, 1998, pp. 3437-3440 , doi : 10.1029 / 98GL00856 .
  15. ^ Marie Edmonds, David Pyle, Clive Oppenheimer: A model for degassing at the Soufrière Hills Volcano, Montserrat, West Indies, based on geochemical data . In: Earth and Planetary Science Letters . tape 186 , no. 2 , March 30, 2001, p. 159-173 , doi : 10.1016 / S0012-821X (01) 00242-4 .
  16. ↑ MD Murphy, RSJ Sparks, J. Barclay, MR Carroll, TS Brewer: Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies . In: Journal of Petrology . tape 41 , no. 1 , January 1, 2000, ISSN  1460-2415 , p. 21-42 , doi : 10.1093 / petrology / 41.1.21 .
  17. ^ J. Gottsmann, S. De Angelis, N. Fournier, M. Van Camp, S. Sacks: On the geophysical fingerprint of Vulcanian explosions . In: Earth and Planetary Science Letters . tape 306 , no. 1-2 , June 2011, pp. 98-104 , doi : 10.1016 / j.epsl.2011.03.035 .
  18. ↑ Stefanie Hautmann, Fred Witham, Thomas Christopher, Paul Cole, Alan T. Linde: Strain field analysis on Montserrat (WI) as a tool for assessing permeable flow paths in the magmatic system of Soufrière Hills Volcano . In: Geochemistry, Geophysics, Geosystems . tape 15 , no. 3 , March 2014, p. 676-690 , doi : 10.1002 / 2013GC005087 .
  19. ↑ Mikhail Alidibirov, Donald B. Dingwell: Magma fragmentation by rapid decompression . In: Nature . tape 380 , no. 6570 , March 1996, ISSN  0028-0836 , p. 146-148 , doi : 10.1038 / 380146a0 .