Glossary
The following definitions are adapted from information on the website of the United States Geologic Survey: http://volcanoes.usgs.gov.
Fumarole. A vent from which volcanic gas escapes into the atmosphere. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic flows.
Lahar. An Indonesian term describing a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano. A moving lahar looks like a mass of wet concrete carrying rock debris ranging from clay to 30-foot boulders. A lahar can flow at speeds of 20 to 25 miles per hour.
Magma. Molten or partially molten rock beneath the Earth’s surface. Magma that has erupted or oozed onto the surface is called lava. Magma typically consists of a liquid portion, minerals that crystallize out of it, solid rocks incorporated into the magma from its conduit or reservoir, and dissolved gases.
Plinian eruption. The most powerful kind of volcanic eruption, in which relatively viscous lava is explosively ejected, sending ash and volcanic gas tens of miles into the air. Pyroclastic flows are also commonly associated with plinian eruptions, named after Pliny the Younger, who described the eruption of Vesuvius. Other examples are the eruptions of Mount St. Helens in 1980 and Mount Pinatubo in 1991.
Pyroclastic flow. Sometimes called nuée ardente (glowing cloud). A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano, typically at speeds of 80 to 100 km/hour. It can result from the explosive eruption of magma and rock fragments, or from the collapse of a vertical eruption column. The temperature within the flow may be as high as 700º C. A pyroclastic flow will incinerate, bury, or carry away nearly everything in its path. Once deposited, the material may flatten and weld together because of its intense heat and weight.
Pyroclastic surge. Similar to a pyroclastic flow, but lower in density, and therefore able to spread over large areas and move up and over ridge crests. By contrast, pyroclastic flows are high-density and tend to be confined in valleys. (The two terms are sometimes used interchangeably.)
Stratovolcano. A steep, conical volcano built by the eruption of viscous lava flows, tephra, and pyroclastic flows, forming many layers of deposits. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes can grow to great heights and often generate highly explosive eruptions. Mount Vesuvius and Mount St. Helens are examples.
Tephra. A general term for fragments of lava and volcanic rock—including pumice, glass shards, mineral crystals, and shattered rock—blasted into the air by explosions or carried upward by hot gases in an eruption column. The fragments can range in size from less than 2 mm (ash) to more than 1 meter in diameter. Medium-sized pieces of tephra are called lapilli, and the largest are known as blocks. Pumice is light, porous rock consisting of many trapped gas particles.
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