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anorthosite - An igneous rock predominantly composed of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%).
Anorthosites constitute the light-colored areas of the Moon's surface.
anthracite - The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. anticline - A fold of rock layers with a convex shape pointing upwards. aphanitic - An igneous rock in which the matrix grains are too small to be seen with the unaided eye. aquifer - A permeable formation that stores and transmits groundwater in sufficient quantity to supply wells.
archipelago - A group of islands; an expanse of water with scattered islands.
Geological analysis relies mainly on chemistry, physics and mathematics. a'a' lava - basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker.
A-horizon - The uppermost layer of a soil, containing organic material and leached minerals.
archean, archaean - An eon of geologic time extending from about 3800 to 2500 million years ago. The Archean eon is divided into four eras: Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean.
arkose - A variety of sandstone containing abundant feldspar and quartz, frequently in angular, poorly sorted grains.
bank - The land alongside or sloping down to a river or lake.bankfull stage - The height of water in a stream that just corresponds to the level of the surrounding floodplain.bar - A unit of pressure equal to 10 to the sixth dynes/square centimeter; approximately one atmosphere.allochthonous - Refers to something formed elsewhere than its present location. alluvial fan - An alluvial fan is a triangular-shaped deposit of sediment at the point where a river emerges from a mountainous area and flows onto a more gently sloping plain.alluvium - Sediment of sorted or unconsolidated sand, gravel, and clay that has been deposited by flowing water.
atoll - A continuous or broken circle of coral reef and low coral islands surrounding a central lagoon.