quartzite
An extremely durable, nonfoliated metamorphic rock derived from pure sandstone and consisting primarily of quartz.
quaternary ice age
An ice age that began approximately 1.6 million years ago and continues to the present time.
quick clay
Sediment that sets off a sudden mudflow by changing rapidly from solid to liquid form, as after an earthquake, an explosion, or thunder.
rain shadow effect
The result of the process by which moist air on the windward side of a mountain rises and cools, causing precipitation and leaving the leeward side of the mountain dry.
recrystallization
The diagenetic process by which unstable minerals in buried sediment are transformed into stable ones.
regional metamorphism
Metamorphism that affects rocks over vast geographic areas stretching for thousands of square kilometers.
relative dating
The fixing of a geologic structure or event in a chronological sequence relative to other geologic structures or events. See also numerical dating.
reserve
A known resource that can be exploited for profit with available technology under existing political and economic conditions.
reservoir rock
A permeable rock containing oil or gas.
resource
A mineral or fuel deposit, known or not yet discovered, that may be or become available for human exploitation.
reverse fault
A dip-slip fault marked by a hanging wall that has moved upward relative to the footwall. Reverse faults are often caused by the convergence of lithospheric plates.
rhyolite
Any of a group of felsic igneous rocks that are the extrusive equivalents of granite.
richter scale
A logarithmic scale that measures the amount of energy released during an earthquake on the basis of the amplitude of the highest peak recorded on a seismogram. Each unit increase in the Richter scale represents a 10-fold increase in the amplitude recorded on the seismogram and a 30-fold increase in energy released by the earthquake. Theoretically the Richter scale has no upper limit, but the yield point of the Earth's rocks imposes an effective limit between 9.0 and 9.5.
rifting
The tearing apart of a plate to form a depression in the Earth's crust and often eventually separating the plate into two or more smaller plates.
rip current
A strong, rapid, and brief current that flows out to sea, moving perpendicular to the shoreline.
ripple marks
A pattern of wavy lines formed along the top of a bed by wind, water currents, or waves.
riprap
A pile of large, angular boulders built seaward of the shoreline to prevent erosion by waves or currents. See also seawall.
rock
A naturally formed aggregate of usually inorganic materials from within the Earth.
rock cycle
A series of events through which a rock changes, over time, between igneous, sedimentary, and metamorphic forms.
rock-forming mineral
One of the 20 or so minerals contained in the rock that composes the Earth's crust and mantle.
rubidium-strontium dating
A form of radiometric dating that relies on the 47-billion-year half-life of radioactive isotopes of rubidium, which decay into isotopes of strontium, to determine the age of rocks in which strontium is present. Rubidium-strontium dating is used for rocks that are at least 10 million years old, deep-Earth plutonic rocks, and Moon rocks.
