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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.
angstrom - A length of 10 meter or one hundred millionth of a centimeter. angular unconformity - An unconformity in which the bedding planes of the rocks above and below are not parallel.amber - A fossilized resin from a coniferous tree that sometimes contains entrapped and preserved insects.amphibole - group of generally dark-colored rock-forming inosilicate minerals, composed of double chain Si O), less mafic than basalt, but more mafic than dacite; rough volcanic equivalent of diorite.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.abyssal plain - The ocean floor offshore from the continental margin, usually very flat with a slight slope.accrete - To add terranes (small land masses or pieces of crust) to another, usually larger, land mass. age - A unit of geological time shorter than an epoch, usually lasting several million years.atoll - A continuous or broken circle of coral reef and low coral islands surrounding a central lagoon.autochthonous - Refers to something formed in its present location. backwash - The return flow of water down a beach after a wave has broken.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.