Glossary

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A

Andesite – A type of volcanic (igneous) rock composed of about 54 to 62 percent silica (SiO2). Normally grey to black in colour. The Soufriere Hills Volcano erupts andesite lava.

Ash – Pulverised rock and volcanic glass less than 2 millimeters (0.1 inch) in diameter.

Ash Cloud – Airborne cloud of ash caused by eruptions or disturbance of ash on the ground.

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B

Ballistic Fragment – A large piece of rock explosively ejected from a volcano at great speed.

Basalt – A type of volcanic (igneous) rock composed of 45 to 54 percent silica (SiO2). This is the most common type of rock found in the earth’s crust.

BGS – British Geological Survey.

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C

Composite Volcano – A volcano that has a steep volcanic cone, built up by lava flows and pyroclastic debris. Also called a stratovolcano.

Conduit – A channel created inside a volcano by molten magma and through which magma travels to the surface.

Crater – A circular depression in the ground. It has steep sides and is usually formed by explosive activity.

Crystallization – The process in which magma solidifies into solid, crystalline rock.

 

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D

Debris Avalanche – [text]

Debris Flow – See Lahar

DFID – Department for International Development. This is the ministry of the UK Government which is responsible for overseas aid and development.

DMCA – Disaster Management and Co-ordination Agency. This body was set up by the Government of Montserrat to prepare for and deal with all disasters that affect Montserrat, both natural and man-made.

Dome (Lava) – When lava that is very viscous or semi solid erupts, it is unable to flow away from the vent and forms a thick pile. This pile solidifies around the volcano’s vent in a mound shape to become its dome. Lava formed in this way is called dome rock and is much denser than pumice.

Dome Collapse – The destruction of a lava dome, either partial or complete, by gravitational collapse, typically forming abundant pyroclastic flows and surges.

DOAS – Differential Optical Absorption Spectroscopy. A method of using the spectral characteristics of UV sunlight to measure the amount of Sulphur Dioxide (SO2) in the air.

Dormant Volcano – A volcano that is presently inactive. A dormant volcano may erupt in the future.

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E

EDM – Electronic Distance Measurement. [some explanation.]

Ejecta – Projectiles from an erupting volcano, for example bombs.

Epicenter – The point on the Earth’s surface directly above the place that an earthquake occured.

Eruption – Volcanic activity in which lava, tephra, or gases are released.

Eruption Column – A mixture of gas and tephra formed during explosive eruptions that may rise rapidly to 10’s of kilometres into the atmosphere

Extinct Volcano – One that is dead and will no longer erupt.

Extrusive Rocks – Igneous rocks that have been erupted and cooled (relatively quickly) at the surface of the Earth. They usually have a fine-grained texture (small crystals).

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F

FCO – The Foreign and Commonwealth Office, a ministry of the UK Government.

Fissure – Crack in the Earth from which lava can emerge.

FTIR – Fourier Transform Infra-Red spectroscopy. A method used to measure variations in the ratio of hydrogen chloride and sulphur dioxide gases emitted by the volcano. It utilizes the differing light absorptions in the infra-red region of the electromagnetic spectrum.

Fumarole – An opening in Earth’s crust, often in the neighbourhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. It is often accompanied by sulphur deposition around the opening of the vent.

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G

Gas (volcanic) – Volcanic gases are dissolved in the magma at depth in the chamber and are released in the low-pressure environment of the earth’s surface. The main volcanic gas is usually water with minor amounts of sulphur dioxide, carbon dioxide and gases such as chlorine and fluorine.

Glass (volcanic) – If cooling is sufficiently rapid, magma may convert to “glass” (without crystals). This typically occurs when hot magma is chilled as it mixes with air or water.

GPS – Global Positioning System. GPS uses signals from a constellation of satellites to determine your location. On modern smartphones, this is accurate to about five metres. The accuracy can be greatly increased by using frequent measurements (once per second) at permanent, cemented, receivers and custom processing software – in MVO’s network, the relative location between two stations can have an accuracy of a few millimetres. [check]

Ground Deformation – One of the principal phenomena monitored at volcanoes. The surface of the volcano responds to changes within the interior of the volcano or deeper in the magma chamber. The volcano can swell (inflate) or subside (deflate) to allow inferences to be made about the magma pressure.

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H

Hybrid (earthquake) – A hybrid earthquake is characterized by seismic signals containing long and short frequencies. This type of earthquake occurs at shallow depth (usually less than 2 km) and is interpreted as fractures forming under high gas pressures. Hybrid earthquakes are often indicative of magma motion in the upper part of the conduit or the dome. They have a mixture of the characteristics of volcano-tectonic and long-period earthquakes.

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I

IASPEI – The International Association of Seismology and Physics of the Earth’s Interior.

IAVCEI – The International Association of Volcanology and Chemistry of the Earth’s Interior.

Igneous Rock – Rock formed by solidification of magma or lava. Igneous rock may be intrusive or extrusive.

Inflation – This describes the swelling of a volcano due to an increase in internal pressure. Ground deformation measures inflation.

Intrusive Rocks – Igneous rocks that have cooled slowly beneath the surface of the Earth. These rocks usually have a coarse-grain (large crystals).

IRIS – [text].

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J

Juvenile – Water, gas, or  mineral-rich fluid believed to have originated from magma and to have come to the earth’s surface for the first time.

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K

Kipuka – An area of land ranging from several square metres to several square kilometres where existing rock of either volcanic or nonvolcanic origin has been completely surrounded, but not covered, by later lava flows.

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L

Lahar – An Indonesian word for a rapidly flowing mixture of ash, rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form during and after heavy rainfall on the upper slopes of the volcano where loose deposits of volcanic ash and fragments of dome-rock lie.

Lateral Blast – Powerful volcanic explosion with a significant directed horizontal component which can generate devastating, high-energy pyroclastic flows and surges.

Lava – Once the magma has erupted, or extruded, at the Earth’s surface it is called lava.

Lava flow – Moving mass of molten lava or its solidified remains. Lava at SHV is too viscous to form flows.

LP (Long-Period earthquake) – These have low frequencies and are thought to be caused by the movement of magma in the upper conduit causing resonance.

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M

Magma – Molten rock beneath the earth’s surface. When magma is pushed above the earth’s surface it is called lava. Due to underground pressure exerted on magma, it can contain many more gases (volatiles) than lava.

Magma Chamber – A large underground body of molten rock (magma) situated beneath the volcano.

MVO – Montserrat Volcano Observatory.

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N

NDPRAC – National Disaster Preparedness and Advisory Committee. This Government of Montserrat committee discusses and makes decision on all disasters that affect Montserrat, both natural and man-made. It is chaired by the Governor and its membership includes the Premier, the Directors of DMCA and MVO, and the Police Commissioner. It meets as and when required.

Nuees ardente – French term for a pyroclastic flow meaning ‘glowing cloud’.

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O

Outgassing – The release of a gases that were trapped, frozen, or absorbed in some material.

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P

Petrology – The study of rocks and rock chemistry.

Phenocrysts – Crystals in a lava, pumice, or scoria which form by slow cooling of the magma in the magma chamber. When the magma rises to the surface it cools more rapidly and the remaining melt either chills to a glass (as in pumice or scoria) or crystallizes to a fine mesh of microscopic crystal called the groundmass (as in many lavas). Phenocrysts in Montserrat rocks may reach several millimetres or more in size.

Phreatic (explosions) – Caused by ground water being heated by rising magma. The phase change from superheated liquid to vapour (steam) close to the earth’s surface causes explosive activity.

Plinian Eruption – A sustained, explosive eruption which forms a high, jet-like column of pumice and ash in the atmosphere. Plinian eruptions commonly last several hours and lay down thick layers of pumice as a pyroclastic fall deposit.

Pumice – A porous rock formed during explosive eruptions, generally of silica-rich magma. Gas dissolved in magma at high pressure comes out of solution as the magma ascends towards the earth’s surface. This forms a froth that then fragments violently. Pumice clasts are solidified pieces of the magmatic froth. Bubbles occupy up to 80% of the volume of pumice, allowing some to float on water.

Pyroclastic – A general term for volcanic material composed chiefly of rock fragments, especially those associated with explosive volcanic eruptions.

Pyroclastic Flow – A high-particle concentration avalanche (flow) of high-temperature pyroclastic material and gases that moves rapidly, typically in response to gravity. In many cases pyroclastic flows follow the course of valleys. Pyroclastic flows can form in different ways: as the result of gravitational dome collapse and during explosive eruptions when an eruption column collapses. Velocities as high as 60 m/s have been measured on historic pyroclastic flows, and some are believed to travel as fast as 250 m/s. Temperatures can exceed 600 degrees Celsius. Pyroclastic flows may travel many tens of kilometres from the source vent, and may even travel large distances across the sea.

Pyroclastic Surge – A flow with a low-particle concentration of pyroclastic debris. Pyroclastic surges are not strongly confined by topography and can move easily out of valleys and flow over ridges. Pyroclastic surges can form on their own or associated with pyroclastic flows.

 

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Q

Q – something beginning with Q.

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R

Repose – The time between volcanic eruptions or eruptive activity.

RMPS – Royal Montserrat Police Service.

Rock – An aggregation of more than one mineral. The Earth’s crust is made of rock. There are three types of rock: igneous, sedimentary, and metamorphic.

Rockfall – This is a small-scale rock avalanche and a common occurrence on the growing lava dome as the continued expansion disrupts the cooled and solidified outer layers. It generates a characteristic signal on the MVO seismic monitoring network.

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S

SAC – Scientific Advisory Committee.

Seisomograph – A display of Seismic measurements.

Seismometer – A device that records and measures seismic waves (vibrations in the Earth), such as those from earthquakes.

SHVO – Soufrière Hills Volcano Observatory. The name was changed to MVO after a few months’ operation.

Spine – This is a protrusion of semi-solid lava that forms at the surface of many lava flows. In some volcanoes the lava dome is in fact a single large spine.

SRC – Seismic Research Centre, University of the West Indies, Trinidad. The name was changed from Seismic Research Unit in [year].

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T

Tectonic – This term is used to describe the deformation and movements of the earth’s surface, to a large extent as a consequence of the movements of tectonic plates. Volcano-tectonic earthquakes or VT’s are generated by rock breaking as magma pushing its way to the surface.

Tephra – Airborne volcanic materials including projectiles like bombs as well as ash.

Tsunami – An ocean wave caused by displacement of water due to undersea earthquakes, volcanic eruptions etc.

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U

USGS – United States Geological Survey.

UWI – University of the West Indies. UWI has campuses in Trinidad, Barbados and Jamaica.

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V

Vent – An opening in the earth’s surface through which volcanic materials (like lava, gases, and pyroclastic debris) erupt.

Viscosity – A measure of the resistance to flow of a liquid. Viscous liquids flow very slowly (like thick syrup); less viscous fluids flow more quickly (like rum). Andesite lava has a very high viscosity.

Volcano – A place where molten rock, gases and pyroclastic debris erupt through the earth’s crust to the surface.

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W

WOVO – The World Organisation of Volcano Observatories.

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X

X – something beginning with X.

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Y

Y – something beginning with Y.

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Z

ZJB – Radio Montserrat.

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