Ray: A linear landform of the lunar surface emanating from a large crater and extending as much as 100 kilometers outward, probably consisting of fine ejecta thrown out by the impact of a meteorite.
Reaction series: A series of chemical reactions occurring in a cooling magma by which a mineral formed at high temperature becomes unstable in the melt and reacts to form another mineral (see also Discontinuous reaction series).
Recharge: In hydrology, the replenishment of ground-water by infiltration of meteoric water through the soil.
Rectangular drainage: A system of streams in which each straight segment of each stream takes one of two characteristic perpendicular directions, with right-angle bends between. The streams are usually following two perpendicular sets of joints.
Recumbent fold: An overturned fold with both limbs nearly horizontal.
Regional metamorphism: Metamorphism occurring over a wide area and caused by deep burial and high internal temperatures of the Earth.
Regolith: Any solid material lying on top of bedrock. Includes soil, alluvium, and rock fragments weathered from the bedrock.
Relief: The maximum regional difference in elevation.
Remote sensing: The study of Earth surface conditions and materials from airplanes and satellites by means of photography, spectroscopy, or radar.
Replacement deposit: A deposit of ore minerals by hydrothermal solutions that have first dissolved the original mineral to form a small cavity.
Respiration: The chemical reaction by which carbohydrates are oxidized and by which all animals and plants convert their food into energy. Carbon dioxide is released and oxygen used up.
Reversible reaction: A chemical reaction which can proceed in either direction, depending on the concentration of reacting materials.
Rheidity: (1) The ability of a substance to yield to viscous flow under large strains. (2) One thousand times the time required for a substance to stop changing shape when stress is no longer applied.
Rhyolite: The fine-grained volcanic or extrusive equivalent of granite, light brown to gray and compact. Richter magnitude scale: See Magnitude.
Rift valley: A fault trough formed in a divergence zone or other area of tension.
Right-lateral fault: A strike-slip fault on which the displacement of the far block is to the right when viewed from either side.
Ring dike: A dike in the form of a segment of a cone or cylinder, having an arcuare outcrop.
Rip current: A current that flows strongly away from the sea shore through gaps in the surf zone at intervals along the shoreline.
Ripple: A very small dune of sand or silt whose long dimension is formed at right angles to the current. River order: See Stream order.
Rock cycle: The geologic cycle, with emphasis on the rocks produced; sedimentary rocks are metamorphosed to metamorphic rocks, or melted to create igneous rocks, and all rocks may be uplifted and eroded to make sediments, which lithify to sedimentary rocks.
Rock flour: A glacial sediment of extremely fine (silt-and clay-size) ground rock formed by abrasion of rocks at the base of the glacier.
Rock glacier: A glacier-like mass of rock fragments or talus with interstitial ice that moves downhill under the force of gravity.
Rockslide: A landslide involving mainly large blocks of detached bedrock with little or no soil or sand. Rounding: The degree to which the edges and corners of a particle become worn and rounded as a result of abrasion during transportation. Expressed as angular, subrounded, well-rounded, etc.
Runoff: The amount of rain water directly leaving an area in surface drainage, as opposed to the amount that seeps out as groundwater.
Rupture strength: The greatest stress that a material can sustain without fracturing at one atmosphere pressure.