active layer The seasonally thawed zone above permafrost. aftershock An earthquake that follows and has its epicenter near a larger earthquake.agate A siliceous rock with alternating bands of chalcedony and variously colored chert. An assemblage of sediments marking place where a stream moves from a steep gradient to a flatter gradient and suddenly loses transporting power.
14C method A method for determining the age in years of organic matter by calculating the amount of radioactive carbon still remaining, as compared to the stable isotope, 12C. Where possible, definitions conform generally, and in some cases specifically, to definitions given in Robert L Bates and Julia A Jackson (editors), Glossary of Geology, 3rd ed., American Geological Institute, Alexandria, Virginia, 1987. Richardson (Englewood Cliffs, NJ, Prentice Hall, 1995).40K/40Ar method A method used for the dating of potassium-bearing rocks by using the ratio of radioactive 40K to its daughter, 40Ar.abyssal plain Large area of extremely flat ocean floor lying near a continent and generally over 4 km in depth.acceleration The rate at which velocity changes, either by increasing or decreasing.
accretion The process by which the terrestrial planets grew, increasing their mass by gradually accumulating smaller bodies, called planetesimals.
acid mine drainage Water contamination by sulfuric acid produced by seepage through sulfur-bearing spoil and tailings from coal and metal mining acid rain The acidity in rain due to gases from internal combustion engines and coal- and oil-burning power plants.
aa A Hawaiian term for a lava flow that has a rough, jagged surface. ablation As applied to glacier ice, the process by which ice below the snow line is wasted by evaporation and melting.
absolute time Geologic time expressed in years before the present.
abundant metal Iron, aluminum, magnesium, manganese, and titanium.
Ores of the abundant metals only need to be 3 – 5 times as metal-rich as average rock.