radiocarbon dating

The age of organic material determined by the amounts of carbon isotopes 12, 13 and 14. The ratio of 12 to 14 is about the same in all living things but when a plant or animal dies, no more carbon is taken on. Carbon 12 and 13 are stable isotopes and the amounts remain the same even in dead material. Carbon 14 is an radioactive isotope that decays radioactively until none is left; . Thus, the ratio records the time elapsed since death. Since carbon 14 decays relatively rapidly, the method is only reliable for the last 40,000 years. See radiometric age.

radiolarian chert

A rock made up of the spherical siliceous shells of radiolarians which are single-celled planktonic animals (protozoans).
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radiometric age

The approximate age of a geologic event, feature, fossil, or rock in years. Radiometric ages, sometimes termed 'absolute' ages, are determined by using natural radioactive 'clocks'. See radiocarbon dating.
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radiometric dating

A dating method that uses measurements of certain radioactive isotopes to calculate the ages in years (absolute age) of rocks and minerals.
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regional metamorphism

Metamorphism affecting a large region that is associated with mountain building events.

relief

Refers to differences in elevation of different points in a region.

relative dating

The process of placing rocks and geologic structures in the correct chronological order. This process does not yield ages in number of years. See radiometric dating.

rhyolite

A volcanic rock chemically equivalent to granite. Usually light colored, very fine-grained or glassy-looking. May have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.

ribbon chert

Chert and shale in thin alternating beds. The beds resemble parallel ribbons stretched over an outcrop.

rift zone

A region of Earth’s crust along which divergence is taking place. A linear zone of volcanic activity and faulting usually associated with diverging plates or crustal stretching.

ring of fire

A zone of volcanoes, earthquakes, and mountain-building encircling the Pacific Ocean formed where plates collide.

rocks

Rocks are made of different kinds of minerals, or broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants. Rocks are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed.
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rockfall

Falling, bouncing, and rolling of debris down slope.

root of a volcano

Plutonic igneous rock formed from magma that crystallized beneath the volcano it once fed.