GIU Montserrat Online - MVO Weekly Report

Montserrat Volcano Observatory
Montserrat, W.I.

Report for the period midday 25 January to midday 01 February 2008

Excellent visibility of the Soufrière Hills Volcano during the reporting period allowed exceptional opportunity to view the dome from all locations around the volcanic edifice and from the air. An observation flight on Wednesday 30 January continued to maintain that there is very little change to the morphology of the dome. Erosion continues on the eastern talus flank and the horizontal scarp reported earlier in January is now less distinct as it degrades under the influence of minor rainfall associated mudflows and small rockfalls confined to the Tar River Valley.
The seismic network recorded 25 long period events and 3 volcano-tectonic earthquakes.

The recorded sulphur dioxide (SO2) flux rate for the reporting period ranged from a maximum of 852 tonnes per day (t/d) on 30 January to a minimum of 167 t/d on 28 January. The average flux rate for the period was 468 t/d. This figure is slightly below the long term average for the eruption which stands at 550 t/d. A Fourier Transform Infra Red (FTIR) HCl:SO2 flux ratio of 0.5 was recorded on 28 January.

Fumarolic activity mentioned in the last reporting period remains the same. The predominantly steam rich fumaroles on the northern and eastern flanks continue to be active as are the fumaroles in the Galway's area. The volcanic gas rich Gage's Wall fumaroles are similarly no less active. Tremor detected by the seismic network on 28 January and triggered as long period events is believed to have been caused by slightly increased fumarolic activity at this location. Because of the superb visibility on 30 January it was possible to collect thermal images from the helicopter of the Gage's fumarole which revealed fumarole temperatures which were detected as high as 480°C (896°F) and reflect the very high temperatures within the dome and conduit.

The pause in activity continues with no dome growth from lava extrusion but a possibility of collapse or explosion remains, however as the system cools without the introduction of fresh magma and the pause continues the likelihood of this is believed to decrease.

During periods of heavy precipitation it must be remembered the potential hazard from lahar (mudflow) in the drainage channels around the volcano remains.

The alert level remained at 4.

Access to all areas on the southern flanks of the Belham Valley east of the Belham Bridge and areas south are prohibited as is south of Jackboy Hill to Bramble Airport and beyond. The Maritime Exclusion Zone around of the southern part of the island extends 4 km offshore from Spanish Point to Roches Bluff on the eastern side of the volcano and 2 km offshore from Ogarra's to Sturge Park on the southwest (readers are asked to refer to the risk map).

For further explanations of the volcanic phenomena described, and information about the Soufrière Hills Volcano and the ongoing eruption, please look at the MVO website: http://www.mvo.ms.

Volcano Risk Maps - January 2007

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Glossary of Volcano Related Terms

For further information about the Soufriere Hills Volcano and the ongoing eruption, please look at the website: www.mvo.ms

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VOLCANO GLOSSARY

Andesite: The name given to the type of magma/lava erupted in Montserrat.

Basalt: The type of magma entering the magma reservoir below Montserrat.

GPS: This stands for Global Positioning System. It is a navigation system which allows the location of a point on the earth’s surface to be determined precisely. This is done when a receiver obtains signals from satellites that allow the location to be fixed. MVO uses continuous GPS to monitor long-term deformation.

Conduit: Magma flows to the earth's surface along a pathway known as a conduit. The exact size and shape of the conduit is not known.

Deformation: The surface of the volcano and surroundings may respond to changes within the the volcano or deeper in the magma chamber. The volcano can swell (inflation) or subside (deflation) suggesting variations in the magma pressure.

FTIR: Fourier Transform Infra-Red spectroscopy. A method to measure variations in the ratio of hydrogen chloride to sulphur dioxide gases emitted by the volcano.

Fumarole: A place where hot gases including steam are released and there is often deposition of sulphur.

Glass: When liquid magma is cooled very quickly, crystals do not have time to develop and it forms glass. Glass commonly forms when hot magma is ‘chilled’ as it mixes with air or water.

Hybrid earthquake A variety of earthquake signal often indicative of magma motion in the upper part of the conduit and dome. It has a mixture of the characteristics of volcano-tectonic and long-period earthquakes.

Magma: Molten rock. It is not simply a liquid, but a mixture of liquid, crystals and volcanic gases. If the liquid is cooled quickly it forms glass.

Lava: Once magma gets to earth’s surface and extrudes it is called lava.

Lava dome: A dome is formed by eruption of extremely viscous or semi-solid lava that piles up around and above the vent in the volcano’s crater. The lava dome can grow by swelling from the inside or by pushing out ‘lobes’ or ‘spines’ of lava. Commonly, only one sector of the lava dome actively grows at any one time and this is the area most prone to collapse and generation of pyroclastic flows.

Long-period earthquakes: A low frequency earthquake commonly associated with magma movement in the upper conduit.

Mudflow: Also called lahars. A concentrated slurry of volcanic debris and water that occurs on many volcanoes particularly during eruptions and after very heavy rain

Phreatic: Phreatic explosions are caused by ground water being heated by rising magma to high temperatures. The phase change from superheated liquid to vapour close to the earth’s surface causes explosive activity.

Pyroclastic flow: Hot, fast-moving avalanche of lava blocks, volcanic ash and gases. They can be formed both by explosions and by collapse of parts of an unstable lava dome.

Pyroclastic surge: These are also flows, but they are dilute clouds rather than dense avalanches. Surges are fast-moving mixtures of hot ash and gas and their behaviour can be compared to a very severe hurricane. Surges can be formed either above pyroclastic flows or directly by very violent explosions.

Rockfalls: These are small-scale rock avalanches and falls of individual rock from a growing dome that generate ‘rockfall signals’ on the MVO seismic monitoring network. There is no definitive boundary between a rockfall and a pyroclastic flow, although in general pyroclastic flows develop when avalanching rocks disintegrate to form a large amount of fine-grained ash.

Swarm: A large number of earthquakes occurring in rapid succession and coming from the same place.

Tremor: A continuous seismic disturbance lasting at least several minutes, sometimes caused by numerous overlapping earthquakes or ash venting.

Vent: A place where magma and or gases are extruded from the volcano.

Volcanic ash: Powdered rock. Ash particles are defined as less than 4 millimetres in diameter.

Volcano-tectonic earthquake: A high frequency earthquake caused by brittle fracture of rocks.

Visit the MVO website for more information on the Volcano and the weekly report
www.mvo.ms

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