Glossary of Geologic Terms

Appalachian Basin*

A long-lasting sedimentary basin that existed in the area of the present-day mid-Atlantic states. Rocks deposited in the basin range form early Cambrian through early Permian age (540-295 million years old). The Appalachian basin is known for its abundant coal, oil and natural gas deposits.

Basin

A topographic or structural low area that generally receives thicker deposits of sediments than adjacent areas; the low areas tend to sink more readily, partly because of the weight of the thicker sediments; the term also denotes an area of relatively deep water adjacent to shallow-water shelf areas.

Bedding

Sedimentary layers in a rock. The beds are distinguished from each other by grain size and composition, such as in shale and sandstone. Subtle changes, such as beds richer in iron-oxide, help distinguish bedding. Most beds are deposited essentially horizontally.

Black shale*

Variety of shale relatively rich in organic material and most likely formed in an oxygen-depleted environment.

Clay

A family of platy silicate minerals that commonly form as a product of rock weathering. Also, any particle smaller than 1/256 of a millimeter in diameter.

Compaction

Occurs when the weight of overlying material compresses more deeply buried sediment. Along with cementation, this process converts sediments to solid rock.

Crust

The rocky, relatively low density, outermost layer of the Earth.

Devonian*

A geologic period within the Paleozoic Era. The Devonian spans from 416-359.2 million years ago and was the period of Earth’s history when land plants first appeared, fish species became numerous and diverse, and the first animals crawled from the sea to walk on dry land.

Diagenesis

A group of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. Diagenesis may culminate in lithification of sediment, turning it into solid rock.

Earthquake

A sudden ground motion or vibration of the Earth. Produced by a rapid release of stored-up energy along an active fault.

Epicenter

The point on the Earth’s surface located directly above the focus of an earthquake.

Erosion

Removal of material by water, wind, or ice.

Fault

A fracture in the Earth along which one side has moved in relative to the other. Sudden movements on faults cause earthquakes.

Formation

A rock formation is a body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it.

Fracture

Any break in rock along which no significant movement has occurred.

Impermeable

Rock or sediment that does not allow passage of water or other fluids.

Induced seismicity*

Earthquakes that occur due to human activity.

Intensity

One way to measure the strength of an earthquake. Intensity measures of the affects of an earthquake on buildings and the reactions of people. Compare with magnitude.

Isotope

Different forms of a single element that have the same number of protons but different numbers of neutrons in their nuclei. Some radioactive isotopes are unstable and shed nuclear particles over time until they become stable. For instance, unstable isotopes of uranium break down to become lead.

Joint

A narrow crack in rock along which there has been no significant movement of either side. Joints commonly form in parallel sets.

Limestone

A sedimentary rock made mostly of the mineral calcite (calcium carbonate). Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

Lithification

The conversion of loose sediment into solid sedimentary rock. Several processes, including compaction of grains, filling of spaces between grains with mineral cement, and crystallization act to solidify sediment.

Magnitude

A measure of the total amount of energy released by an earthquake.

Marcellus Shale*

Organic rich black shale that formed from mud deposited in the Appalachian Basin 384 million years ago. The Marcellus contains relatively abundant organic material derived from the remains of plankton living in the waters of the shallow sea that filled the Appalachian Basin at that time. Over time, some of the organic material has been converted to natural gas.

Mud

Wet clay and silt-rich sediment.

Mudstone

A very fine-grained sedimentary rock formed from mud.

Normal fault

A fault that drops rock on one side of the fault down relative to the other side.

Permeability

The ability of a rock or other material to allow water or other fluids to flow through its interconnected spaces. Permeable bedrock makes a good aquifer, a rock layer that yields water to wells. See porosity.

Plankton

Generally tiny animals or plants that live floating in water.

Porosity

The percentage of open spaces (pores) in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable.

Reservoir rock*

A highly porous and permeable rock, such as sandstone, that collects hydrocarbons produced by source rocks.

Sand

Loose particles of rock or mineral (sediment) that range in size from 0.0625 - 2.0 millimeters in diameter.

Sandstone

Sedimentary rock made mostly of sand-sized grains.

Seismic

Refers to earthquakes.

Sedimentary

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding.

Shale

Sedimentary rock derived from mud. Commonly finely laminated (bedded). Particles in shale are commonly clay minerals mixed with tiny grains of quartz eroded from pre-existing rocks.

Silt

Loose particles of rock or mineral (sediment) that range in size from 0.002 - 0.0625 millimeters in diameter. Silt is finer than sand, but coarser than clay.

Siltstone

A sedimentary rock made mostly of silt-sized grains.

Source rock*

A rock that has generated hydrocarbons from organic material deposited along with the sediment comprising the rock.

Weathering

Weathering includes two surface or near-surface processes that work in concert to decompose rocks. Both processes occur in place. No movement is involved in weathering. Chemical weathering involves a chemical change in at least some of the minerals within a rock. Mechanical weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within it. Mechanical weathering includes processes such as water in cracks freezing and expanding, or changes in temperature that expand and shrink individual minerals enough to break them apart.

Resources

* definition by Chuck Anderson, Penn State University

All other terms from the USGS / NPS Geologic Glossary