Glossary of Geologic Terms
| Got a question about some other geologic term? Send e-mail to Dick Gibson. |
amphibole A group of ferromagnesian minerals, usually dark in color, and common as constituents of many rocks. Hornblende is a mineral of this group, sometimes used as a catch-all term for amphibole.
anticline Figure An anticline is a fold in rocks that brings rocks up from depth, forming an arch-like structure. The rocks in the center of an anticline are the older rocks.
basalt Figure Basalt is a rock that is very fine-grained (often the grains are not visible) and contains dark ferromagnesian minerals (such as pyroxene) and calcic plagioclase (a feldspar). It is usually extrusive (solidifying from magma) or occasionally intrusive in narrow dikes.
cactolith
calcite Calcite is a mineral, calcium carbonate, or CaCO3. It is the primary constituent of the rock limestone, and it is also formed by many organisms (as in mollusk shells, for example). Calcite is relatively soft, and it will dissolve in acid. For this reason many limestone gravestones in northeastern US are becoming illegible because acid rain is eroding their carvings. See also Amethyst Galleries.
dike Figure A dike is an intrusive igneous body that cuts across the grain of the rock it intrudes. Compare sill.
extrusive Extrusive refers to igneous rocks that are forced out onto the Earth's surface. Volcanic ash and lava flows solidify to form extrusive rocks. More or less synonymous with volcanic.
fault Figure Faults are surfaces of breakage in rocks, along which some movement has taken place. The movement may be measured in millimeters or miles. Fault "planes" are often not actually flat surfaces, but may be curved or undulatory.
feldspar Feldspars are aluminum-rich silicate minerals that are very common in rocks like granite. 
Most feldspars are white or pink, and form rectangular crystals with excellent cleavage (or preferred breaking surfaces). The name of one of the most common feldspars, orthoclase, means "straight-breaking" in reference to this cleavage. As a group, feldspars are the most common minerals in the Earth's crust.
geologic ages and periods Go to the University of California Museum of Paleontology Web Time Machine - a really great starting point for lots of geology! With details of stratigraphy and fossils for most of the geologic periods, and nice links to more information.
gneiss A term for a common category of metamorphic rocks in which heat and pressure have rearranged the minerals into planar compositional bands. Commonly, though by no means always, the minerals are dominantly quartz and feldspar, making a quartzo-feldspathic gneiss. The planar compositional bands define the rock's foliation.
granite A common igneous rock that crystallizes slowly, within the earth, so that mineral crystals have time to grow to relatively large size. Granites are usually low in iron-magnesium minerals and contain abundant quartz and feldspar.
igneous rocks Rocks derived from molten material such as magma. The rocks may be fine-grained, if they cooled quickly on the Earth's surface (basalt, etc.) or coarse-grained if they cooled slowly in the interior of the Earth. "Igneous" is from the Latin word for fire.
intrusive Said of rocks that are forced into pre-existing rocks. Dikes and sills are intrusive igneous rocks, injected in the molten state into previously solidified rocks.
limestone A sedimentary rock that is predominantly calcium carbonate (the mineral calcite). Such rocks may result from the activity of calcite-precipitating organisms, but most limestone is probably the result of inorganic chemical precipitation from sea water. Chalk is a form of limestone.
metamorphic rocks "Metamorphic" means "changed form," and this broad category of rocks indicates that the minerals and chemicals present have been significantly rearranged, usually through the action of intense heat and/or pressure. The original rocks might have been sedimentary or igneous, or even older metamorphic rocks.
mica Micas are a group of silicate minerals that have a platy or sheet-like structure. This makes thin flakes of mica very easy to peel off. Muscovite and biotite are two of the commonest micas.
mineral A mineral is a naturally occurring compound with a specific chemical composition and regular crystalline structure.
phenocryst A large crystal of some mineral, usually much larger in size than the grains in the main mass of the rock in which it is embedded. Usually applied to crystals in igneous rocks that have had a complicated cooling history.
ptygmatic Convolute, and often giving the appearance of folds (and sometimes called ptygmatic folds). Usually applied to thin bands of intrusive igneous material.
pyroxene A group of rock-forming ferro-magnesian minerals, dark, and superficially similar to amphiboles. Pyroxenes crystallize at higher temperatures than amphiboles, and commonly have a square or rectangular cross-section while amphiboles show angular and prismatic (even needle-like) cross-sections. Augite is a common pyroxene.
quartz Silicon dioxide, one of the commonest minerals in the Earth's crust. Quartz is a common constituent of a great many rocks, including granite, sandstone, some volcanic rocks, and the metamorphic rocks that are derived from them. As well-formed, vitreous crystals, it displays hexagonal prisms topped with points shaped like hexagonal pyramids (actually two rhombohedrons). See also Amethyst Galleries.
rock A rock is an aggregate of minerals, sometimes together with non-crystalline (i.e., non-mineral) natural materials.
schist A metamorphic rock characterized by abundant micaceous mineral development. The micas form on planar surfaces and separate the other minerals in the rock into bands, and this banding is called the rock's foliation.
sedimentary rocks Layered rocks that are formed by the consolidation of loose particles (sediments) of older rocks. They may result from river, wind, or glacial transport, as well as through slow accumulation on the sea floor. Sedimentary rocks also include chemical precipitates such as limestone and rock salt.
seismic Refers to earth vibrations, such as those created by earthquakes, usually related to movement on faults. Also applies to man-made acoustic (sound) waves used to analyze the structure of the subsurface, for resource exploration and environmental studies. Figure
sill An intrusion of igneous rock into older rock, but one in which the intrusion is parallel to or concordant with pre-existing layering such as sedimentary bedding or metamorphic foliation. Compare dike.
stratigraphy The study of sequences of rock and their history. Commonly applied to sedimentary rocks, but igneous and metamorphic sequences are studied as well. The concept of stratigraphy includes some of the most important tenets in geology: that sedimentary strata were originally deposited horizontally, that older beds underlie younger beds, and that older rocks are cross-cut by younger ones.
syncline Figure A syncline is a fold in rocks in which the layers are bowed downward, resulting in a bowl-shaped structure. The younger rocks are toward the center of a syncline.
trap In petroleum geology, a trap is a place in the subsurface to which oil or natural gas migrates, and from which it cannot migrate any further. A trap requires an impermeable seal above the porous reservoir rock that contains the oil or gas. A trap may be an anticline, a change in rock type along one particular unit of rock, a fault-bounded zone, and other structures.
trilobite Figure Trilobites were a huge class of marine arthropods, whose modern relatives include horseshoe crabs, insects, and spiders. They can be found in rocks ranging in age from Cambrian (550 million years ago) to Permian time (240 million years ago). They all became extinct in the greatest decimation of life ever to occur on earth, at the end of the Permian Period.
twinning The intimate intergrowth of two (or more) crystals of the same mineral in a regular manner that is mathematically and crystallographically predictable. See also Amethyst Galleries.
volcanics Extrusive igneous rocks that are ejected by volcanoes. The word is often used to include "pyroclastics", which are volcano-derived materials that are reworked in a sedimentary manner by wind or water action. There is no sharp boundary between sedimentary and volcanic igneous rocks. Figure
