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Volcano Term Definitions
A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the major avalanche involved in the eruption of Mount St. Helens, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material. Most of the May 18, 1980 deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche.
The surface along which a landslide disconnects from its original position.
A period of time in the Paleozoic Era that covered the time span between 400 and 345 million years.
A breccia filled volcanic pipe that was formed by a gaseous explosion.
A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes.
A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent (often circular in plane view) and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.
The area of land drained by a river system.