To search for terms starting with a certain letter, click a letter in the bar below.



The emission of radiation (energy) by an unstable atom.

radiocarbon dating

A method of determining the age of a biological object by measuring the ratio of carbon isotopes 14C and 12C. Because the decay rate of 14C is 5000 years, it is useful for numerical dating as far back as 50,000 years. Beyond this point, nearly all of the 14C has decayed.


A naturally occurring radioactive, colorless, odorless gas. It is one of the products of decay from the breakdown of radioactive elements in soil, rock, and water, released by weathering.

rare earth elements

A set of 17 heavy, lustrous elements with similar properties, some of which have technological applications. Although they are relatively common in the crust, these metals are not usually found concentrated in economically viable ore deposits.


The change in structure of mineral crystals that make up rocks, or the formation of new mineral crystals within the rock.
Recrystallization commonly occurs during metamorphism. When rocks are metamorphosed, individual grains that make up the original rock are melted slightly and recrystallize. The pressure allows crystals to grow into a tighter, interlocking arrangement than in an unmetamorphosed rock.

recurrence interval

The time elapsed between major events, such as floods.


A feature lying beneath the surface of the water, which is a buildup of sediment or other material built by organisms, and which has positive relief from the sea floor.
While some reefs result from abiotic processes such as deposition or wave action, the best-known reefs are built by corals and other marine organisms.

regional metamorphism

A metamorphic rock that has been altered due to deep burial and great pressure. This type of metamorphic rock tends to occur in long belts at the center of mountain ranges. Different types of metamorphic rock are created depending on the gradients of heat and pressure applied.


A drop in sea level.

relief (topography)

The change in elevation over a distance.

renewable energy, renewable resource

Energy obtained from sources that are virtually inexhaustible (defined in terms of comparison to the lifetime of the Sun) and replenish naturally over small time scales relative to human life spans.


A fossilization method by which the original material is chemically replaced by a more stable mineral.

residual weathering deposit

A mineral deposit formed through the concentration of a weathering -resistant mineral, in which the other minerals around it have been eroded away.

reworked sediment

Sediment that has been further eroded (after initial deposition) by wave, current, or wind action.

rhyolite, rhyolitic

A felsic volcanic rock high in abundance of quartz and feldspar.


A break or crack in the crust that can be caused by tensional stress as a landmass breaks apart into separate plates.

rift basin

A topographic depression caused by subsidence within a rift; the basin, since it is at a relatively low evelation, usually contains freshwater bodies such as rivers and lakes.


Rock and rubble used to fortify shorelines, streambeds, pilings, and other structures against erosion.
See also: erosion

ripple marks

Surface features created when sediment deposits are agitated, typically by water currents or wind. The crests and troughs formed by this agitation are occasionally lithified and preserved, providing information about the flow of water or wind in the paleoenvironment.

rock flour

Very fine sediments and clay resulting from the grinding action of glaciers.


Spontaneous, violent fracturing of rock occurring in deep mines.


A supercontinent that contained most or all of Earth’s landmass, between 1.1 billion and 750 million years ago, during the Precambrian. Geologists are not sure of the exact size and shape of Rodinia. It was analagous to but not the same supercontinent as Pangaea, which formed was assembled several hundred million years later during the Permian.

roof pendant

A downward projection of metamorphosed basement rock that hangs exposed above an uplifted igneous intrusion.
See also: basement rock, igneous rock, intrusive, metamorphic rock


An extinct group of box- or tube-shaped bivalves that arose during the Jurassic. They were major reef formers, but went extinct at the end of the Cretaceous.

rugose coral

An extinct group of corals that were prevalent from the Ordovician through the Permian. Solitary forms were most common; these were horn-shaped, leading to their common name, "horn corals."


A typically reddish brown mineral formed of TiO2. It is an ore of titanium.