Aftershocks Small earthquakes that occur after a larger earthquake.
Andesite A type of volcanic rock composed of about 54 to 62 percent silica (SiO2). Normally gray 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.
Avalanche A large mass of falling and/or sliding material (rock, soil etc.). A volcanic eruption can trigger a rock avalanche.
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.
Base Surge A type of pyroclastic surge. A cloud of rock debris, water and steam that moves along the ground at high speeds.
Block or bomb A piece of rock (more than 64 mm) that is ejected (thrown) from a volcano. A block is one size of tephra.
Caldera A large depression formed when a volcano collapses into a partially emptied magma chamber. Transfer of magma from the chamber to the surface causes the edifice to sink below the surface or collapse as a jumble of rocks. Some calderas form due to prolonged eruption of basaltic lava, like on Hawaii. Caldera formation on Montserrat is highly unlikely.
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.
Cristobalite A pure silicate mineral that formas at temperatures above 1470┬░C.
Crystallization The process in which magma solidifies into solid, crystalline rock.
Debris flow See Lahar.
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.
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).
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.
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. A navigation system which allows the location of a point on the earth's surface to be determined precisely. A receiver obtains signals from satellites which allow the location to be determined. MVO uses GPS to monitor long term deformation of the volcano's flanks.
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.
Hornblende A complex hydrous silicate of calcium, magnesium, and iron which appears green to black in colour. It is formed in the late stages of cooling in igneous rock and is observed as crystals in the lavas of Montserrat.
Hybrid [earthquake] A hybrid earthquake is characterized by seismic signals containing long and short frequencies. This type of earthquake occurs atshallow 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.
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 ininternal 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).
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.
Lapilli Pieces of rock or lava that range from 2 to 64 mm across. Lapilli can be thrown into the air by volcanic explosions.
Lateral blasts 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.
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.
Nuees ardente French term for a pyroclastic flow. Meaning ΓÇÿglowing cloud'.
Outgassing The release of a gases that were trapped, frozen, or absorbed in some material.
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.
Plagioclase A mineral with the chemical composition calcium sodium silicate formed by the crystallization of all magmas across the range basalt to rhyodacite.
Plinian eruption A sustained, explosive eruption which forms a high, jet-like column of pumice and ash in the atmosphere. Plinian eruptions commonlylast several hours and lay down thick layers of pumice as a pyroclastic falldeposit.
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. Pyroclasticsurges can form on their own or associated with pyroclastic flows.
Pyroxene A family of minerals with the general chemical composition calcium, magnesium, iron silicate.
Repose The time between volcanic eruptions or eruptive activity.
Rock An aggregation ofmore than one mineral. The earth's crust is made of rock. There are three types of rock: igneous, sedimentary, and metamorphic.
Rockfalls These are small-scale rock avalanches and a common occurrence on the growing lava dome as the continued expansion disrupts the cooled and solidified outer layers. These generate characteristic signals on the MVO seismic monitoring network.
Seisomograph A display of Seismic measurements.
Seismometer A device that records and measures seismic waves (vibrations in the Earth), such as those from earthquakes.
Scoria Porous, glassy rock formed by the rapid chilling of frothy, relatively silica-poor magma such as basalt or andesite. Scoria is typically dark-coloured; it is denser than pumice and does not float on water.
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.
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.
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.