YELLOWSTONE NATURE NOTES

GLOSSARY

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Brief, simplified definitions of words that may not be familiar to some readers. More precise definitions can be found in any standard geology text book.

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Agglomerate — explosive volcanic fragmental rock composed mostly of rounded or slightly angular particles greater than 32 mm. in diameter.

Algae — simple forms of plant life that are the predominant source of brilliant coloration seen in run-off channels of many hot springs and geysers.

Alluvium — stream deposits of sand and gravel.

Andesite — fine-grained igneous rock intermediate in composition between granite and basalt.

Aquifer — a stratum of rock or deposit of sand and gravel capable of transmitting water.

Basalt — a fine-grained, dark colored extrusive igneous rock.

Batholith — a large intrusive mass of igneous rock that increases in size as it extends downward and has no determinable floor. They have surface exposures greater than 40 square miles.

Block faulting — where large blocks of rock, bordered by faults on opposite sides, are either elevated or depressed.

Breccia — a fragmental rock consisting of larger particles than ash or tuff, but formed similar to them by showers of volcanic debris. There are other types of breccia.

Dolomite — a sedimentary rock consisting of calcium magnesium carbonate and usually formed from limestone by replacement processes.

Fault scarp — the cliff formed by a fault.

Geyserite — common deposit of the geyser basins, usually opaline silica.

Glacial drift — sediment deposited by glaciers.

Gneiss — a coarse-grained metamorphic rock that is distinctly banded.

Graben — depression formed by subsidence of a block of rock between normal faults.

Granite — a coarse-crystalline, light-colored, intrusive igneous rock.

Gravimeter — an instrument for measuring variations in magnitude of earth's gravitational field.

Igneous — formed by solidification from a molten state.

Kaolin — a rock composed essentially of clay minerals. Formed in Yellowstone by chemical alteration of feldspars in rhyolite.

Lopolith — a large floored intrusive that is centrally sunken into the form of a basin.

Magma — molten rock beneath earth's surface. When magma flows out on surface, it is called lava.

Normal fault — high angle fault formed by tension rather than compression.

Obsidian — a natural volcanic glass, usually a variety of rhyolite that has cooled rapidly.

Overthrust fault — a low angle fault formed by compression.

Perlite — a volcanic glass with numerous concentric cracks which give rise to perlitic structure.

Petrography — a branch of geology concerned with the systematic description and classification of rocks.

Pumice — cellular glassy lava usually composed of rhyolite.

Rhyolite — an extrusive igneous rock, fine grained, light in color, with same chemical composition as granite.

Schist — a metamorphic rock with foliated structure, split in thin irregular plates.

Tillite — unsorted and unstratified glacial sediment that has been cemented into a solid mass.

Travertine — a form of calcium carbonate deposited from solution in ground water.

Tuff — a deposit of volcanic ash compacted and cemented to form a solid rock mass.

Welded tuff — a dense form of rhyolite composed mainly of fragmented glass shards that have been welded and fused into solid rock. Their mode of emplacement is unknown. Some believe they are explosive and the result of settling; others believe they are formed as froths or avalanches. Thin section analysis under a petrographic microscope reveals their welded structure.