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radial network	A drainage network in which the streams flow outward from a cone-shaped mountain, and define a pattern resembling spokes on a wheel.
radioactive decay	The process by which a radioactive atom undergoes fission or releases particles thereby transforming into a new element.
radioactive isotope	An unstable isotope of a given element.
radiometric dating	The science of dating geologic events in years by measuring the ratio of parent radioactive atoms to daughter product atoms.
rain band	A spiraling arm of a hurricane radiating outward from the eye.
rain shadow	The inland side of a mountain range, which is arid because the mountains block rain clouds from reaching the area.
range (for fossils)	The interval of a sequence of strata in which a specific fossil species appears.
rapids	A reach of a stream in which water becomes particularly turbulent; as a consequence, waves develop on the surface of the stream.
reach	A specified segment of a stream’s path.
recessional moraine	The end moraine that forms when a glacier stalls for a while as it recedes.
recharge area	A location where water enters the ground and infiltrates down to the water table.
recrystallization	The process in which ions or atoms in minerals rearrange to form new minerals.
rectangular network	A drainage network in which the streams join each other at right angles because of a rectangular grid of fractures that breaks up the ground and localizes channels.
recurrence interval	The average time between successive geologic events.
red giant	A huge red star that forms when Sun-sized stars start to die and expand.
red shift	The phenomenon in which a source of light moving away from you very rapidly shifts to a lower frequency; that is, toward the red end of the spectrum.
reef bleaching	The death and loss of color of a coral reef.
reflected ray	A ray that bounces off a boundary between two different materials.
refracted ray	A ray that bends as it passes through a boundary between two different materials.
refraction	The bending of a ray as it passes through a boundary between two different materials.
reg	A vast stony plain in a desert.
regional metamorphism	Dynamothermal metamorphism; metamorphism of a broad region, usually the result of deep burial during an orogeny.
regolith	Any kind of unconsolidated debris that covers bedrock.
regression	The seaward migration of a shoreline caused by a lowering of sea level.
relative age	The age of one geologic feature with respect to another.
relative humidity	The ratio between the measured water content of air and the maximum possible amount of water the air can hold at a given condition.
relative plate velocity	The movement of one lithosphere plate with respect to another.
relief	The difference in elevation between adjacent high and low regions on the land surface.
renewable resource	A resource that can be replaced by nature within a short time span relative to a human life span.
reservoir rock	Rock with high porosity and permeability, so it can contain an abundant amount of easily accessible oil.
residence time	The average length of time that a substance stays in a particular reservoir.
residual mineral deposit	Soils in which the residuum left behind after leaching by rainwater is so concentrated in metals that the soil itself becomes an ore deposit.
resurgent dome	The new mound, or cone, of igneous rock that grows within a caldera as an eruption begins anew.
retrograde metamorphism	Metamorphism that occurs as pressures and temperatures are decreasing; for retrograde metamorphism to occur, water must be added.
return stroke	An upward-flowing electric current from the ground that carries positive charges up to a cloud during a lightning flash.
reverse fault	A steeply dipping fault on which the hanging-wall block slides up.
reversed polarity	Polarity in which the paleomagnetic dipole points north.
Richter magnitude scale	A scale that defines earthquakes on the basis of the amplitude of the largest ground motion recorded on a seismogram.
ridge axis	The crest of a mid-ocean ridge; the ridge axis defines the position of a divergent plate boundary.
right-lateral strike-slip fault	A strike-slip fault in which the block on the opposite fault plane from a fixed spot moves to the right of that spot.
rip current	A strong, localized seaward flow of water perpendicular to a beach.
riprap	Loose boulders or concrete piled together along a beach to absorb wave energy before it strikes a cliff face.
roche moutonnêe	A glacially eroded hill that becomes elongate in the direction of flow and asymmetric; glacial rasping smoothes the upstream part of the hill into a gentle slope, while glacial plucking erodes the downstream edge into a steep slope.
rock	A coherent, naturally occurring solid, consisting of an aggregate of minerals or a mass of glass.
rock burst	A sudden explosion of rock off the ceiling or wall of an underground mine.
rock cycle	The succession of events that results in the transformation of Earth materials from one rock type to another, then another, and so on.
rock flour	Fine-grained sediment produced by glacial abrasion of the substrate over which a glacier flows.
rock glacier	A slow-moving mixture of rock fragments and ice.
rock slide	A sudden downslope movement of rock.
Rodinia	A proposed Precambrian supercontinent that existed around 1 billion years ago.
rotational axis	The imaginary line through the center of the Earth around which the Earth spins.
R-waves (Rayleigh waves)	Surface seismic waves that cause the ground to ripple up and down, like water waves in a pond.