A selection of terms taken from the Volcanic Hazards Glossary Andesite Volcanic rock (or lava) characteristically medium dark in colour and containing 54 to 62 per cent silica and moderate amounts of iron and magnesium. Ash Fine fragments (less than 2-4 mm in diameter) of volcanic rock formed by a volcanic explosion or ejection from a volcanic vent. Basalt Volcanic rock (or lava) that characteristically is dark in colour (gray to black), contains 45 to 53 percent silica, and is rich in iron and magnesium. Basaltic lavas are more fluid than andesites or dacites, which contain more silica. Caldera A large basin-shaped volcanic depression with a diameter many times larger than included volcanic vents; may range from 2 to 50 km (1 to 30 mi) across. Cinder cone A conical hill, often steep, formed by accumulation of solidified fragments of lava that fall around the vent of a single basaltic or andesitic eruption. Composite volcano Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows. They are usually constructed over tens to hundreds of thousands of years and may erupt a variety of magma types (basalt to rhyolite). They typically consist of many separate vents. Synonym is stratovolcano. Crust The crust is the outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes. Debris avalanche Moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope. Deformation Changes to the surface of a volcano that occur due to magma movement underneath the surface. Most volcano deformation can only be detected and measured with precise surveying techniques such as with a Global Positioning System (GPS), tiltmeter, Interferometric Synthetic Aperture Radar (InSAR), or an Electronic Distance Meter (EDM). Dome A steep-sided mass of viscous and often blocky lava extruded from a vent. Effusive eruption An eruption that produces mainly lava flows and domes (as opposed to an explosive eruption). Ejecta Material explosively ejected from a volcano. Emission The release of volcanic gases from the earth, which escape into the atmosphere. Eruption column The ascending, vertical part of the mass of erupting debris and volcanic gas that rises directly above a volcanic vent. Higher in the atmosphere, columns usually spread laterally into plumes or umbrella clouds. Explosive eruption An energetic eruption that produces mainly ash, pumice, and fragmental ballistic debris (as opposed to an effusive eruption). Fallout A general term for all the ash and debris that falls to Volcanic gas haze over devastated town of Plymouth, Montserrat 2014. (Photo by Franklyn Michael) earth (also known as ashfall) from an eruption cloud. Fumarole A vent from which steam and volcanic gasses issue. Igneous Refers to rocks formed by solidification from magma. Lahar Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. Lateral Blast A lateral (sideways) explosion with a significant lowangle component that is directed towards an area that can cover as much as 180 degrees. Because they carry rock debris at high speeds, lateral blasts can devastate areas tens to hundreds of square kilometres within a few minutes, and they can destroy manmade structures and kill all living things by abrasion, impact, burial, and heat. Lava General term for magma (molten rock) that has been erupted onto the surface of the Earth and maintains its integrity as a fluid or viscous mass, rather than exploding into fragments. Lava flow Streams of molten rock that erupt relatively nonexplosively from a volcano, then move downslope until they stop, cool, and solidify. Lava fountain A jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock is called a lava fountain. Lava fountains typically range from about 10 to 100 m in height, but occasionally reach more than 500 m. Magma Molten rock beneath the surface of the Earth. Magma Chamber The location beneath the vent of a volcano where molten rock (magma) is stored prior to eruption. Also known as a magma storage zone or magma reservoir. Obsidian Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in colour. Obsidian forms in lava flows where the lava cools so fast that crystals do not have time to grow. Petrologic Petrology is the study of rocks, including their occurrence, composition, and origin. Phreatic eruption An eruption that primarily involves steam explosions, usually ground water flashed into steam by the heat of subsurface magma. Pumice Highly vesicular volcanic ejecta, typically silicic in composition. It is essentially magma that has been frothed up by escaping gases and then cooled and solidified during eruption. Rhyolitic pumice is typically of low enough density that it floats on water. Near a vent, hot pumice can accumulate and form a pumice cone. Pyroclastic flow A hot (typically >800°C), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent or collapsing flow front. Pyroclastic surge Ground-hugging clouds of ash, rock, and volcanic gas that move at hurricane velocities and have temperatures of several hundred degrees Celsius. Seismicity The phenomenon of earthquakes caused by the brittle fracturing of rocks in the earth's crust. Synonymous with seismic activity. Seismic swarm A series of minor earthquakes occurring in the same area and time, none of which may be identified as the main shock or with the same fault. Silica Silicon dioxide, the most abundant rock-forming compound on Earth and the predominant molecular constituent of volcanic rocks and magmas. It tends to polymerize into molecular chains, increasing the viscosity of the magma. Basaltic magma, having lower SiO2, is fairly fluid, but with increasing contents of SiO2, andesite, dacite, and rhyolite magmas become progressively more viscous. Because it is more difficult for dissolved gas to escape from more viscous magma, higher silica magmas generally erupt more explosively. Submarine volcano A volcano located below sea level on the ocean floor, commonly composed of basalt. Tectonic Relating to structural deformation of the Earth's crust Tephra Any type and size of rock fragment that is forcibly ejected from the volcano and travels an airborne path during an eruption (including ash, bombs, and scoria). Tsunami A gravitational sea wave produced by a large-scale, short-duration disturbance of the ocean floor, such as a submarine earthquake, slump, subsidence, or volcanic eruption. Vent Any opening at the Earth's surface through which magma erupts or volcanic gases are emitted. Volcanic Bomb bombs are lava fragments that were ejected while viscous (partially molten), and blocks are solid rock fragments; both are larger than 64 mm in diameter and are ejected during an explosive eruption. Source: United States Geological survey – USGS https://www.usgs.gov/volcanoes/ glossaryTerms/ 17
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