Geology Terms and Definitions

Some of the words and phrases that Geologists use to describe the Earth can be new or confusing to non-geologists. Once you know what these words mean, you can gain a better understanding of how the thermal water system works.

This is a guide to reading the list of terms and definitions below:

term. The technical definition of the term from the Dictionary of Geological Terms (1984). Simplified explanation of the definition, or clarification of terms within the technical definition.

Carboniferous. The Mississippian and Pennsylvanian periods combined, ranging from about 345 to about 280 million years ago. Sometimes referred to as the age of amphibians or the age of coal.

chert. An extremely hard sedimentary rock with conchoidal (smooth, curved surface) fracturing. It consists chiefly of interlocking crystals of quartz. Also called “flint.”

cross section. A graphic interpretation of geology, structure, and/or stratigraphy based on mapped and measured geologic extents and attitudes, depicted in a vertical plane (i.e., a cut or profile view).

cryptocrystalline. Describes a rock texture in which very small individual crystals cannot be recognized or distinguished with an ordinary microscope. Compare with “microcrystalline.” The crystals in this rock texture are so small that only a very high-powered microscope can allow us to see them.

deformation. A general term for the processes of rock faulting, folding, and shearing as a result of various Earth forces, such as compression (pushing together) and extension (pulling apart).

fault. A break in rock characterized by displacement (movement) of one side relative to the other.

fold. A bend in bedding (rock layers), foliation (flattened, layered minerals within a rock), cleavage (plane of weakness in a mineral), or other planar features in rocks. A fold is usually a product of deformation, but the definition does not specify the manner of origin.

geologic time. The time extending from the end of the formative period of the earth to the beginning of human history; the part of the earth's history that is recorded in the succession of rocks. The term implies extrememly long duration or remoteness in the past, although no precise limits can be set. Geologists around the world have determined that the earth was formed about 4.6 billion years ago.

geologic time unit. A span of continuous time in geologic history, during which a corresponding chronostratigraphic unit (rocks that were formed during a specific interval of time) was formed; a division of time distinguished on the basis of the rock record. Geologic-time units in order of decreasing magnitude are eon, era, period, epoch, and age.

geothermal gradient. The rate of increase of temperature in the earth with depth. The gradient differs from place to place depending on the heat flow in the region and the thermal conductivity of the rocks (how well heat travels through the rocks). The average geothermal gradient approximates 25o C/km (77o F/km) of depth.

microcrystalline. Describes a rock texture consisting of crystals visible only with a microscope. Compare with “cryptocrystalline.” A standard microscope is strong enough to see the crystals.

mineral. A naturally occuring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properites. There are hundreds of minerals, a few examples are: quartz, calcite, hematite, feldspar, gold, copper, and diamond. The most common ones found in the rocks around Hot Springs are quartz, calcite, and clay minerals like smectite and kaolinite.

novaculite. A dense hard even-textured light-colored cryptocrystalline siliceous (made of silica dioxide; SiO2) sedimentary rock, similar to chert but richer in microcrystalline quartz (quartz is also made of silica dioxide).

Pangaea. A supercontinent that existed during the Permian and Triassic periods (about 300 to 200 million years ago) and included much of Earth’s continental crust. Split into Gondwana and Laurasia.

recharge. Infiltration (movement of water into soil or porous rock) process that replenishes groundwater.

sandstone. Clastic (made from fragments of pre-existing rocks) sedimentary rock of predominantly sand-sized grains.

sediment. Solid material that has settled down from a state of suspension (floating) in a liquid. More generally, solid, fragmental material transported and deposited by wind, water, or ice, chemically precipitated from solution, or secreted by organisms, and that forms in layers in loose unconsolidated form, e.g. sand, mud, till. When sediments are compacted and cemented together, they can become sedimentary rocks.

shale. A clastic (made from fragments of pre-existing rocks) sedimentary rock made of clay-sized particles that exhibit parallel splitting properties.

spring. A site where water issues from the surface due to the intersection of the water table with the ground surface. Springs can be hot or cold.

travertine. A limestone deposit or crust, often banded (looks like layers), formed from precipitation of calcium carbonate (CaCO3) from saturated waters, especially near hot springs and in caves.

tufa. A chemical sedimentary rock composed of calcium carbonate, formed by evaporation as an incrustation around the mouth of a spring, along a stream, or, exceptionally, as a thick, concretionary deposit in a lake or along its shore. It may also be precipitated by algae or bacteria. A hard, dense variety of travertine. This is what is formed around the hot springs in Hot Springs National Park.

Bates, R. L., & Jackson, J. A. (1984). Dictionary of Geological Terms (Third ed.). New York: American Geological Institute, Anchor Books.

Last updated: September 4, 2019

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