Paleomagnetism as a means of dating geological events
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.aeolian process - A geological modification, such as sand dunes or erosion, caused by wind activity. algal mat - A layered communal growth of algae observed in fossils and in present day tidal zones associated with carbonate sedimentation.alkali metal - A strongly basic metal like potassium or sodium.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.
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.
arroyo - A steep-sided and flat-bottomed gully in an arid region that is occupied by a stream only intermittently, after rains.
artesian well - A well that penetrates an impermeable layer of rock to reach an aquifer containing water under pressure.
barrier island - A long, narrow island parallel to the shore, composed of sand and built by wave action.
basalt - A fine-grained, dark, mafic igneous rock (~50% Si O) composed largely of plagioclase feldspar and pyroxene; rough volcanic equivalent of gabbro.