Basalt - a dark-colored, fine-grained lava rich in iron and magnesium and relatively poor in silica (less than 54 percent.) The most common of earth's volcanic rocks, basaltic lavas compose all the ocean floors and many continental formations as well. Typically very fluid because of their low silica content, basaltic lavas can flow great distances from their source, forming broad lava plains such as the Columbia River Plateau. Shield volcanoes are typically formed exclusively of basalt.
Bergschrund - A Crevass at the back of a glacier between the glacier and its rock headwall, formed by partial melting and movement of the glacier. [A German word. Berg= mountain. Schrund = crevass.
Block - Angular fragment of lava rock measuring a minimum of 2.5 inches to several tens of feet in diameter.
Block-and-ash flow - Variety of a pyroclastic flow, a turbulent mass of hot dense rock fragments that avalanches downslopes as the result of an eruption. Block-and-ash flows are commonly triggered by the distruption or collapse of a dome while hot.
Blocky lava - A lava flow whose surface is characterized by a jumble of large angular blocks.
Bomb - a lump of plastic or molten lava thrown out during an explosive eruption. Bombs range in size from 2.5 inches to many feet in diameter. Because of their plastic condition when first ejected, bombs are commonly modified in shape during their flight through the air and/or by ther impact on the ground. As the outer crust cools and solidifies, continued expansion of the interior by gas pressure sometimnes causes cracking, which may form a bomb surface resembling the crust of freshly baked bread (breadcrust bombs).
Breccia - Rock composed of many distinct fragments, typically sharp and/or angular, embedded in a matrix of fine material. Breccias are sometimes formed when shattered lava blocks are transported by avalanches or mudflows.
Bibliography
Fire Mountains of the West: The Cascade and Mono Lake Volcanoes, Stephen L. Harris. Mountain Press Publishing Company. 1988
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment