Glossary

This page was written by Erin Klauk as part of the DLESE Community Services Project: Integrating Research in Education.

amphibolite:

A crystalloblastic rock consisting mainly of amphibole and plagioclase with little or no quartz. As the content of quartz increases, the rock grades into hornblende-plagioclase gneiss.

anticline:

A fold, generally convex upward, whose core contains the stratigraphically older rocks.

basalt:

A dark-colored igneous rock, commonly extrusive, composed primarily of calcic plagioclase and pyroxene; the fine-grained equivalent of gabbro.

batholith:

A large, generally discordant plutonic mass that has more than 40 mi² (100 km²) of surface exposure and no known floor. Its formation is believed by most investigators to involve magmatic processes.

breccia:

A course-grained clastic rock, composed of angular broken rock fragments held together by a mineral cement or a fine-grained matrix.

butte:

A conspicuous isolated flat-topped hill with steep slopes or precipitous cliffs, often capped with a resistant layer of rock and bordered by talus, and representing an erosion remnant carved from flat-lying rocks; the summit is smaller in extent than that of a mesa.

Cambrian:

The earliest period of the Paleozoic era, thought to have covered the span of time between 570 and 500 million years ago; also, the corresponding system of rocks.

Cenozoic:

The latest of the four eras into which geologic time is divided; it extends from the close of the Mesozoic era, about 65 million years ago, to the present. This era is sub divided into Tertiary and Quaternary.

Cretaceous:

The final period of the Mesozoic era, thought to have covered the span of time between 135 and 65 million years ago; also, the corresponding system of rocks.

Devonian:

A period of the Paleozoic era, thought to have covered the span of time between 400 and 345 million years ago; also, the corresponding system of rocks.

diatreme:

A breccia-filled volcanic pipe that was formed by a gaseous explosion.

dike:

A tabular body of igneous rock that cuts across the structure of adjacent rocks or cuts massive rocks.

disseminated ore:

A scattered distribution of generally fine-grained metal-bearing minerals throughout a rock body, in sufficient quantity to make the deposit an ore.

escarpment:

A long, more or less continuous cliff or relatively steep slope facing in one general direction, separating two level or gently sloping surfaces, and produced by erosion or faulting.

fault:

A fracture or fracture zone along which there has been displacement of the sides relative to one another parallel to the fracture.

fissure:

An extensive crack, break, or fracture in rocks. It may contain mineral-bearing material.

hoodoo:

A column, pinnacle, or pillar of rock produced in a region of sporadic heavy rainfall by differential weathering or erosion of horizontal strata, facilitated by joints and by layers of varying hardness, and occurring in varied and often eccentric or grotesque forms.

intrusion:

The process of emplacement of magma in pre-existing rock; magmatic activity.

laccolith:

A concordant igneous intrusion that has domed the overlying rocks and has a known or assumed flat floor and a postulated dikelike feeder beneath its thickest point. It is roughly circular in plan, less than five miles in diameter, and from a few feet to several hundred feet in thickness.

lode:

A mineral deposit consisting of a zone of veins, veinlets, or disseminations; also, a mineral deposit in solid rock as opposed to a placer deposit.

Mesozoic:

An era of geologic time, from the end of the Paleozoic to the beginning of the Cenozoic, or from about 225 to about 65 million years ago; also the rocks formed during that era. It includes the Triassic, Jurassic, and Cretaceous periods.

Miocene:

An epoch of the early Tertiary Period, after the Oligocene and before the Pliocene; also, the corresponding worldwide series of rocks. It is considered to be a period when the Tertiary is designated as an era.

Mississippian:

A period of the Paleozoic era, thought to have covered the span of time between 345 and 320 million years ago; also, the corresponding system of rocks.

monocline:

A local steepening in an otherwise uniform gentle dip.

Ordovician:

The second earliest period of the Paleozoic era, thought to have covered the span of time between 500 and 440 million years ago; also, the corresponding system of rocks.

Paleozoic:

An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, or from about 570 to about 225 million years ago.

pegmatite:

An exceptionally coarse-grained igneous rock, with interlocking crystals, usually found as irregular dikes, lenses, or veins, esp. at the margins of batholiths.

Permian:

The last period of the Paleozoic era, thought ot have covered the span of time between 280 and 225 million years ago; also, the corresponding system of rocks.

placer:

A surficial mineral deposit formed by mechanical concentration of mineral particles from weathered debris. The common types are beach placers and alluvial placers. The mineral concentrated is usually a heavy mineral such as gold, casiterite, or rutile.

plate tectonics:

A theory of global tectonics in which the lithosphere is divided into a number of plates whose pattern of horizontal movement is that of torsionally rigid bodies that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries.

Pleistocene:

An epoch of the Quaternary period, after the Pliocene of the Tertiary and before the Holocene; also, the corresponding worldwide series of rocks. It began two to three million years ago and lasted until the start of the Holocene some 8,000 years ago.

Pliocene:

An epoch of the Tertiary period, after the Miocene and before the Pleistocene; also, the corresponding worldwide series of rocks. It is considered to be a period when the Tertiary is designated as an era.

phreatomagmatic:

An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.

plug:

An upheaved, consolidated mass filling a conduit.

Precambrian:

All geologic time, and its corresponding rocks, before the beginning of the Paleozoic; it is equivalent to about 90% of geologic time.

pyroclastic:

Pertaining to clastic rock material formed by volcanic explosion or aerial expulsion from a volcanic vent; also, pertaining to rock texture of explosive origin.

radiometric dating:

Calculating an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g. carbon-14, or potassium-40/argon-40. The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes.

rhyolite:

A group of extrusive igneous rocks, typically porphyritic and commonly exhibiting flow texture, with phenocrysts of quartz and alkali feldspar in a glassy to cryptocrystaline groundmass; also, any rock in that group; the extrusive equivalent of granite.

roll-front ore body:

A roll ore body bounded on the concave side by oxidized altered rock typically containing hematite or limonite, and on the convex side by relatively reduced altered rock typically containing pyrite and organic matter.

scoria:

A vesicular cindery crust on the surface of lava flows, the cellular nature of which is due to the escape of volcanic gasses before solidification; it is heavier, darker, and more crystalline than pumice.

shield volcano:

A broad, gently sloping volcanic cone of flat domical shape, usually several tens or hundreds of square miles in extent, built chiefly of overlapping interfingering basaltic lava flows. Typical examples are the volcanoes of Mauna Loa and Kilauea on the island of Hawaii.

sill:

A tabular igneous intrusion that parallels the planar structure of the surrounding rock.

speleothem:

A mineral deposit formed in a cave by the action of water.

stratification:

The arrangement of sedimentary rocks in strata; bedding. It may be indicated by differences in texture, cementation, color, or composition.

stock:

An igneous intrusion that is less than 40 square miles (100 km²) in surface exposure, is usually but not always discordant, and resembles a batholith except in size.

syncline:

A fold of which the core contains the stratigraphically younger rocks; it is generally concave upward.

Tertiary:

The first period of the Cenozoic era, thought to have covered the span of time between 65 million and 2 million years ago; also, the corresponding system of rocks.

Triassic:

The first period of the Mesozoic era, thought to have covered the span of time between 225 and 190 million years ago; also, the corresponding system of rocks.

uraninite:

An isometric metallic mineral, essentially UO₂. It is strongly radioactive and is the chief ore of uranium. Uraninite often contains thorium, radium, the cerium and yttrium metals, and lead; when heated, it yields helium. It occurs in veins with the minerals of lead, tin and copper, and in sandstone deposits, and is a primary constituent of granites and pegmatites.

Definitions courtesy of [Bates and Jackson, 1984] .