D. M. Considine et al. (eds.), Van Nostrand's Scientific Encyclopedia ABERRATION (Optical). The failure of an optical system to form an image of a point as a point, of a straight line as a straight line, and of an angle as an equal angle. See also Astigmatism; Chromatic Aberration; Coma (Optics); Curvature of Field (Optics); Spherical Aberration. ABLATION (Geomorphology). Essentially, the wasting away of rocks; the separation of rock material and formation of residual deposits, as caused by wind action or the washing away of loose and soluble materials.ABLATION (Glaciology). The combined processes (sublimation, melting, evaporation) by which snow or ice is removed from the surface of a glacier or snowfield. In this sense, the opposite of alimentation. Ablation also refers to the amount of snow or ice removed by the aforementioned processes (the opposite of accumulation). The term may be applied to reduction of the entire snow-ice mass, and may also include losses by wind action and by calving (the breaking off of ice masses). Air temperature is the dominant factor in controlling ablation. During the ablation season, an ablation rate of about two millimeters/hour is typical of most glaciers. An ablatograph is an instrument that measures the distance through which the surface of snow, ice, or firn charges, as caused by ablation, during a specific period.ABLATION (Meteorite). The direct vaporization of molten surface layers of meteorites and tektites during flight.ABLATION (Spacecraft). In the interest of cooling space vehicles upon re-entry into the earth's atmosphere, ablation is used to control the temperature of strongly heated surfaces, such as parts of combustion chambers or nose cones. The process usually consists of the use of surface layers of materials which by their fusion, followed often by evaporation, absorb heat.The heat of ablation is a measure of the effective heat capacity of an ablating material. Numerically, this is the heating rate input divided by the mass loss rate which results from ablation. In the most general case, heat of ablation is given by ABYSSAL HILLS. Small hills averaging 100-200 meters (330-660 feet) in height that occupy the ocean floor. These may be nearly isolated or may occupy virtually the whole floor. See Abyssal Plain.ABYSSAL PLAIN. An area on the ocean floor having a flat bottom and a very slight slope of less than 1 part in I 000. It is believed that these very flat surfaces arise from the continued deposition of mud and silt from turbidity currents. See Ocean. Seismographic studies support that these surfaces consist of such deposits. Mid-ocean canyons may be found on these abyssal plains; these are flat-bottomed depressions in the plains, varying from one to several kilometers (miles) in width and varying in depth up to several hundred meters (feet). These, too, are believed to be the product of certain turbidity currents. ABYSSAL ROCKS. Proposed by Bragger as a general term for deep-seated igneous rocks, or those which have crystallized from magmas far below the surfac...