R

RADIAL DRAINAGE: A system of streams running in a radial pattern away from the center of a circular elevation, such as a volcano or dome.

RADIOACTIVITY: The emission of energetic particles and/or radiation during radioactive decay.

RADIOMETRIC DATING: The method of obtaining ages of geological materials by measuring the relative abundances of radioactive parent and daughter isotopes in them.

RAIN SHADOW: An area of low rainfall on the leeward slope of a mountain range.

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.

RECHARGE: In hydrology, the replenishment of groundwater, usually 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, usually following sets of joints.

RECURRENCE INTERVAL: The average time interval between occurrences of a geologic event, such as a flood or earthquake, of a given or greater magnitude.

REEF: A mound or ridge-shaped organic structure that is built by calcareous organisms, is wave resistant, and stands in relief above the surrounding seafloor.

REGIONAL METAMORPHISM: Metamorphism caused by deep burial or strong tectonic forces that impose high temperatures and high pressures over large belts or regions of the crust.

REGOLITH: The layer of loose, heterogeneous material lying on top of bedrock; includes soil, unweathered fragments of parent rock, and rock fragments weathered from the bedrock.

REJUVENATION (OF MOUNTAINS): Renewed uplift in a mountain chain on the site of earlier uplifts, returning the area to a more youthful stage of the cycle of erosion.

RELATIVE AGE: The age of a geologic event or feature relative to other geologic events or features and expressed in terms of the geologic time scale. (Compare Absolute age.)

RELATIVE HUMIDITY: The amount of water vapor in the air, expressed as a percentage of the total amount of water vapor that the air could hold at that temperature if saturated.

RELIEF: The maximum regional difference in elevation.

RESERVES: Deposits of minerals, coal, or oil and gas that have been shown to be extractable profitably with existing technology. "Proven reserves" are those for which good estimates of the quantity and quality have been made. (See also Resources.)

RESERVOIR: A source or place of residence for elements in a chemical cycle or hydrologic cycle.

RESIDENCE TIME: The average length of time that an atom of a particular element spends in a reservoir before leaving.

RESOURCES: Discovered and undiscovered deposits of minerals, coal, or oil and gas that are or may become available for use in the future; includes reserves, plus discovered deposits not now commercially or technologically extractable, plus undiscovered deposits that may be inferred to exist. (See also Reserves.)

RESPIRATION: The process by which carbohydrates combine with oxygen to produce carbon dioxide and water and to release energy. (See also Photosynthesis.)

RETROGRADE METAMORPHISM: Metamorphism in which a rock that has been metamorphosed to a fairly high grade is later remetamorphosed at lower temperature and pressure to a lower grade.

RHYOLITE: The fine-grained volcanic or extrusive equivalent of granite, light-brown to gray and compact.

RICHTER MAGNITUDE: A measure of earthquake size, determined by taking the common logarithm (base 10) of the largest ground motion observed during the arrival of a P wave or seismic surface wave and applying a standard correction for distance to the epicenter. (See also Moment magnitude.)

RIFT VALLEY: A fault trough formed at a divergent plate boundary or other area of tension.

RIPPLE: A very small dune of sand or silt whose long dimension is formed at right angles to the current.

RIVER: A general term for a relatively large stream, or the main branches of a stream system.

ROCK AVALANCHE: The rapid, downhill-flowing mass movement of broken rock material, during which further breakage of the material may occur.

ROCK CYCLE: The set of geologic processes by which each of the three great groups of rocks is produced from the other two: 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.

ROCKFALL: The relatively free falling of a newly detached segment of bedrock from a cliff or other steep slope.

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.

ROCKSLIDE: The mass movement of large blocks of detached bedrock sliding more or less as a unit.

RUNOFF: The amount of rainwater that does not infiltrate the ground but leaves an area in surface drainage.