Absaroka Sequence - A sequence of Permian-Pennsylvanian sediments bounded by both above and below by a regional unconformity and recording an episode of marine transgression over an eroded surface, full flood level of inundation, and regression from the craton.
Abyssal plains – Broad and very flat submarine plains of the ocean covered with a layer of pelagic (deep sea) sediment between the depths of 3 km to 5 km.
Acadian Orogeny - An episode of mountain building in the northern Appalachians during the Devonian Period.
Acanthodians - The earliest known vertebrates(fishes) with a movable, well-developed lower jaw, or mandible; hence, the first jawed fishes.
Accommodation zones – A region where two rift segments interact and connect containing complex deformation from strike slip, dip slip and oblique slip faulting.
Accretionary prism or accretionary wedge – the accumulation of slivers of oceanic crust and mantle from thrusting and deformation resulting from plate convergence.
Accretionary terrane - A block of continetal crust having fault boundaries that is geologically distict from surrounding terranes.
Achondrites – a class of meteorite known as stones (made up of silicate minerals) that do not contain chondrites (nearly spherical silicate inclusions between 1.0 & 3.0 in diameter.)
a b James Furman Kemp: A handbook of rocks for use without the microscope : with a glossary of the names of rocks and other lithological terms. 5. Aufl., New York: D. Van Nostrand, 1918, pp. 180 , 240 : C. E. Dutton, 4th Annual Report U.S. Geological Survey, 1883, S. 95; Bulletin of the Geological Society of America, Volume 25 / Geological Society of America. 1914, p. 639