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Geological Terms Beginning With "R"

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Radial Drainage:

A drainage pattern in which stream channels run away from a central high point such as a volcano or dome.


A group of one-celled marine animals with a siliceous skeleton that occupies shallow portions of the water column. Radiolarians have a range from Cambrian to present.

Radiolarian Ooze:

A deep-sea pelagic sediment that contains at least 30% siliceous radiolarian remains.

Rating Curve:

A plot that shows the relationship between the stage and discharge (streamflow) of a specific stream at a specific location. It is customary to plot stream stage on the y-axis of the plot and discharge on the x-axis. The resulting relationship is normally a curve. Rating curves can be used to estimate discharge (which is time consuming and expensive to measure) using a single stage measurement (which can be collected with automatic equipment). The principle of a rating curve enables hydrologists to monitor the discharge of many streams simultaneously once gages have been placed to collect and report the stage of the stream.

Reaction Series:

A series of interactions between a melt and mineral crystals in contact with the melt. In a reaction series the first formed crystals (highest temperature minerals) react with the melt to produce a new mineral.


Water added to an aquifer or other water body. An aquifer is recharged by precipitation in an area where the aquifer has a porous connection to the surface.

Recharge Area:

The geographic area where water infiltrates into the ground and enters an aquifer.


A solid state reaction in which the atoms of existing crystals within a rock are reorganized in response to heat and/or pressure. The recrystallized mineral grains are typically larger in size than the original crystals.

Rectangular Drainage:

A drainage pattern in which stream channels develop within a large-scale network of intersecting joints. This drainage pattern is characterized by right-angle bends in the channels of streams and streams that intersect at right angles.

Recumbent Fold:

An overturned fold that has two limbs which are nearly horizontal.

Redrilled Well:

A previously drilled hole that is reentered and deepened by additional drilling.


The bending of a seismic wave as it enters a material of different density, or, the bending of a beam of light as it enters a material of different refractive index.

Regional Metamorphism:

Metamorphism across a broad area caused by the elevated temperatures and pressures of plate collision or deep burial.


A general term used in reference to unconsolidated rock, alluvium or soil material on top of the bedrock. Regolith may be formed in place or transported in from adjacent lands.


A retreat of the sea from land areas. Possible causes include a drop in sea level or uplift.


Variations in the height and slope of Earth's surface. Also used in reference to the vertical difference between the highest and lowest elevations of an area.

Remote Sensing:

The collection of information about an object or area from a distance. Methods employed include photography, radar, spectroscopy and magnetism.


The dissolving or disintegration of one material followed by precipitation of a new material in its place.


A subsurface rock unit that is porous and permeable, and that contains an accumulation of oil and/or natural gas.

Retrograde Metamorphism:

Mineral changes within a rock that are caused by adjustments to conditions of reduced temperature and pressure.

Reverse Fault:

A fault with vertical movement and an inclined fault plane. The block above the fault has moved upwards relative to the block below the fault.


The fine-grained volcanic or extrusive rocks that are equivalent in composition to granite. Normally white, pink or gray in color. Picture of Rhyolite.

Richter Magnitude Scale:

A scale that is used to compare the strength of earthquakes based upon the amount of energy released. The scale is logarithmic and an arbitrary earthquake was used as a starting point for creating the scale. As a result it is a continuous scale with no upper limit and negative numbers possible for very small earthquakes. An upper limit of approximately 9.0 is suspected as Earth materials will most likely fail before storing enough energy for a larger magnitude earthquake.

Ridge (Mid-Ocean):

An elevated area of the sea floor in the center of an ocean basin with rugged topography, a central rift-valley and recurring seismic activity. Ridges generally stand about 1000 meters to 3000 meters above the adjacent ocean floor and are about 1500 kilometers in width.

Right-Lateral Fault:

A fault with horizontal movement. If you are standing on one side of the fault and look across it, the block on the opposite side of the fault has moved to the right. (Also see Left-Lateral Fault.)

Rip Current:

A strong, narrow current of high velocity and short duration that flows seaward through the breaker zone. Caused when a build up of water pushed onto the beach by winds and waves returns seaward.

Ripple Marks:

A series of parallel or sub-parallel ridges in sand or sediment that is caused by the rhythmic or directional movement of wind or water.

Rock Cycle:

All rock at or near Earth's surface is being modified by the processes of metamorphism, melting, crystallization, lithification and weathering. These processes move rock material through the states of metamorphic rock, igneous rock, sedimentary rock, melts and sediment. The natural and continuous cycling of rock materials through these states is known as the rock cycle.

Rock Flour:

Finely pulverized rock material of silt or smaller size produced by abrasion at the base of a glacier.

Rock Glacier:

A mass of rock material, cemented together by ice, that flows down a slope under the force of gravity much like the motion of a glacier.

Rock Salt:

A chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters. It is also known by the mineral name "halite". It is rarely found at Earth's surface, except in areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter highway treatment. Some halite is processed for use as a seasoning for food. Picture of Rock Salt.


A type of mass wasting in which a large volume of rock debris slides down a slope under the influence of gravity.


Liquid water moving over the land surface as a sheet or channelized flow. The portion of precipitation that moves over the ground instead of evaporating or infiltrating.

Rupture Strength:

The maximum amount of stress that a material can sustain without failure.

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