Aa - Hawaiian word used to describe a lava flow whose surface is broken into angular, jagged fragments.
Agglutinate - A volcanic deposit formed by the accumulation of flattened and welded fragments, typically derived from showers of still-molten rock particles ejected in magma fountains. The liquid fragments may accumulate to form a stream of lava.
Andesite - A lava of intermediate composition, usually light gray or brown in color. Andesite has a silica content ranging from about 54 to 62 percent.
Andesite line - An imaginary line drawn around the boundary of the Pacific Ocean basin, seperating continental and oceanic lava rocks according to their chemical composition. On the Pacific side of the line lavas are basaltic. On the continental side, lavas with a higher silica content, such as andesites, commonly occur.
Ar - the element argon
Ash - fine particles of pulverized blown from a volcano. Measuring less than about 0.1 in diamer, ash may be either solid or molten when first erupted.
By far the most common variety is vitric ash, glassy particles formed by gas bubbles bursting through liquid magma.
Lithic ash is formed of older rock pulverized during an explosive eruption, while in:
crystal ash each grain is composed of a single crystal or group of crystals with only traces of glass adhering to them. Many volcanic ash deposits contain mixtures of all three kinds in various proportions.
Ash fall -a rain of ash from an eruption cloud
Ash flow - An avalanche of hot volcanic ash and gases that can travel great distances at high speeds from an erupting vent. Large-volume ash flow deposits commonly solidify to form Ignimbrites. (Also called a pyroclastic flow).
Asthenosphere - A zone of the earth's outer shell beneath the lithosphere. Of undetermined thickness, this is a region of weakness where plastic movements occur.
Bibliography
Fire Mountains of the West: The Cascade and Mono Lake Volcanoes, Stephen L. Harris. Mountain Press Publishing Company. 1988
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