Glossary
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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.
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 atoms to daughter atoms in a rock’s radioactive elements.
rain band
A spiraling arm of a hurricane radiating outward from the eye.
rain shadow
The inland side of a mountain range, 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
Particularly turbulent stream water that develops where water flows over a bed with clasts whose diameter approaches the water depth.
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 the 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.
reversed polarity
Polarity in which the paleomagnetic dipole points north.
reverse fault
A steeply dipping fault on which the hanging-wall block slides up.
Richter magnitude scale
A scale that defines earthquakes based on 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.
rift basins
basins that form over continental rifts where the surface of the lithosphere becomes lower, relative to its surroundings, as a consequence of thinning and stretching.
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_moutonnee
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.
rocky coast
An area of coast where bedrock rises directly from the sea, so beaches are absent.
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
Surface seismic waves that cause the ground to ripple up and down, like water waves in a pond.