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Glossary of geologic terms

The Explorer's Guide to Impact Craters


Aggregate -- A mass or body of rock particles, mineral grains, or both. Any of several hard, inert materials, such as sand, gravel, slag, or crushed stone, used for mixing with a cemeting or bituminous material to form concrete, mortar, or plater; or used alone, as in railroad ballast or graded fill.

Algae -- Photosynthetic, almost exclusively aquatic plants of a large and diverse group, including seaweeds and their fresh-water allies. They range in size from simple unicelluar forms to giant kelps several meters long, and display extremely varied life cycles and physiological processes, with, for example, different complexes of photosynthetic pigments.

Alluvial Fan -- An outspread, gently sloping mass of alluvium deposited by a stream in an arid or semi-arid region where a stream issues from a narrow canyon onto a plain or valley floor.

Angular -- Having sharp angles or borders. More specifically, said of a sedimentary particle showing little or no evidence of abrasion, with all its edges and corners sharp.

Ash Fall -- A rain of airborne volcanic ash falling from an eruption cloud.

Asteroid -- One of the many small celestial bodies in orbit around the sun. Most asteroid orbits are between those of planets Mars and Jupiter.

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

Bedrock -- The solid rock that underlies gravel, soil, or other surficial material.

Bioherm -- A mound-like or circumscribed mass of rock built by sedentary organisms such as corals, stromatoporoids, or algae, and enclosed in rock of different lithological character.

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

Calcite -- A common rock-forming mineral, CaCO 3 . Commonly gray or white, it has perfect rhombohedral cleavage and reacts readily with cold dilute hydrochloric acid.

Caldera -- A large basin-shaped volcanic depression, more or less circular, the diameter of which is many times greater than that of the included ventor vents, irrespective of steepness of the walls or form of the floor.

Canyon -- A stream-cut chasm or gorge, the sides of which are composed of cliffs or a series of cliffs rising from its bed. Canyons are characteristic of arid or semiarid regions where downcutting by streams greatly exceeds weathering. Sometimes spelled cañon.

Carbonate -- A mineral compound characterized by a fundamental anionic structure of CO 3 -2 . Calcite and aragonite (CaCO 3 ) are examples of carbonates.

Cavity -- A solutional hollow, often found in a limestone cave or in cavernous lava.

Cement -- Chemically precipitated mineral material that occurs in the spaces of a sedimentary rock, thus binding the grains into a rigid mass. The most common cements are silica, carbonates, and iron oxides.

Clast -- An individual constituent, grain, or fragment of a detrital sediment or sedimentary rock, produced by the physical disintegration of a larger rock mass.

Clay -- 1) A detrital mineral particle of any composition having a diameter less than 1/256 millimeter (4 microns). This is approximately the upper limit of size of particle that can show colloidal properties. 2) An earthy, extremely fine-grained sediment or soft rock composed primarily of clay-size or colloidal particles, having high plasticity and a considerable content of clay minerals. Clays may be classified by use, origin, mineral composition, or color.

Coesite -- A monoclinic mineral, SiO 2 . It is a very dense polymorphic form of quartz that is stable at room temperatures only at pressures above 20,000 bars. Found in impact craters and associated structures.

Comet -- A celestial body, observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous coma up to 2.4 million kilometers (1.5 million miles) in diameter and an elongated curved vapor tail arising from the coma when sufficiently close to the sun. Comets are thought to consist chiefly of ammonia, methane, carbon dioxide, and water.

Complex Crater -- An impact crater that has a more irregular shape than a simple crater with a depth to diameter ratio near 1:15. This type of crater has an uplift that forms in the center due to the earth rebounding from the impact event.

Crust -- The outermost layer of the Eart; that part of the Earth above the Mohorovicic discontinuity, made up of sial , or of sial and sima . It represents less than 0.1% of the Earth's total volume.

Crystal -- A homogenous, solid body of a chemical element, compound, or isomorphous mixture, having a regularly repeating atomic arrangement that may be outwardly expressed by plane faces.

Deformation -- A general term for the processes of folding, faulting, shearing, compression, or extension of rocks as a result of various earth forces.

Datum -- A fixed or assumed point, line, or surface, in relation to which others are determined.

Delta -- The nearly flat alluvial tract of land at the mouth of a river, commonly forming a triangular or fan-shaped plain resembling the greek letter "delta" in plan view. It is crossed by many distributaries, and results from the accumulation of sediment supplied by the river.

Desert -- A region with a mean annual precipitation of 10 inches or less, and so devoid of vegetation as to be incapable of supporting any considerable population. Four kinds may be distinguished: (1) polar deserts, marked by perpetual snow cover and intense cold; (2) middle-latitude deserts, in the basinlike interiors of the continents, such as the Gobi, characterized by scant rainfall and high summer temperatures; (3) trade-wind deserts, notably the Sahara, with negligible precipitation and large daily temperature range; (4) coastal deserts, as in Peru, where there is a cold current on the western coast of a large land mass.

Dune -- A mound, ridge, or hill of wind-blown sand, either bare or covered with vegetation.

Ejecta -- Glass, rock fragments, and other material thrown out of an explosion or impact crater during formation.

Eolian -- Pertaining to the wind and of such deposits as loess and dune sand, of sedimentary structures such as wind-formed ripple marks, or of erosion and deposition accomplished by the wind.

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

Fault plane -- A fault surface that is more or less planar.

Fine-grained -- Said of a sedimentary rock, and of its texture, in which the particles have an average diameter less than 1/16 mm (62 microns, or silt size and smaller). The term is used in a relative sense, and various size limits have been utilized.

Formation -- 1. A body of rock strata that consists dominantly of a certain lithologic type or combination of types. It is the fundamental lithostratigraphic unit. Formations may be combined into groups or subdivided into members . 2. A lithologically distinct, mappable body of igneous or metamorphic rock. 3. An informal term applied by drillers to a sedimentary rock with certain drilling characteristics, e.g. "cherty formation". 4. A group of plant or animal associations that exist together because of closely similar life patterns, habits, and climate requirements. 5. A topographic feature differing conspicously from adjacent features, e.g. a striking erosional form on the land surface. 6. A speleothem .

Fossil -- Any remains, trace, or imprint of a plant or animal that has been preserved in the earth's crust since some past geologic or prehistoric time; loosely, any evidence of past life.

Fracture -- A crack, joint, or other break in rocks.

Glacier -- A large mass of ice formed on land by the compaction and recrystallization of snow, creeping downslope or outward due to the stress of its own weight, and surviving from year to year.

Glass -- A state of matter intermediate between the close-packed, highly ordered array of a crystal, and the poorly-packed, highly disordered array of a gas. Most glasses are supercooled liquids, i.e., metastable, but there is no break in the change in properties between the metastable and stable states. The distinction between glass and liquid is on the basis of viscosity.

Gneiss -- A foliated rock formed by regional metamorphism, in which bands or lenticles of granular minerals alternate with bands or lenticles of minerals with flaky or elongate prismatic habit.

Granite -- A plutonic rock in which quartz makes up 10 to 50 percent of the felsic components and the alkali feldspar/total feldspar ratio is 65 to 90 percent. Broadly applied, any holocrystalline quartz-bearing plutonic rock.

Gravel -- 1) An unconsolidated natural accumulation of rounded rock fragments, mostly of particles larger than sand (diameter greater than 2 millimeters), such as boulders, cobbles, pebbles, granules, or any combination of these; the unconsolidated equivalent of a conglomerate. 2) A popular term for detrital sediment along streams or beaches, composed cheifly of pebbles and sand.

Gypsum -- A widely distributed mineral consisting of hydrous calcium sulfate: CaSO 4- H 2 O. It is the commonest sulfate mineral, and is frequently associated with halite and anhydrite in evaporites, forming thick, extensive beds, especially in rocks of Permian and Triassic age.

Hydrothermal -- Of or pertaining to hot water, to the action of hot water, or to the products of this action, such as a mineral deposit precipitated from a hot aqueous solution; also said to be the solution itself. This term is generally used for any hot water, but has been restricted by some to water of magmatic origin.

Igneous-- Said of a rock or mineral that solidified from molten or partly molten material, i.e. from a magma; also, applied to processes related to the formation of such rocks. Igneous rocks constitute one of the three main classes into which rocks are divided, the others being metamorphic and sedimentary.

Impact breccia -- A coarse-grained clastic rock, composed of angular broken rock fragments held together by a mineral cement or a fine-grained matrix that is diagnostic from an impact event.

Impact melt breccia -- A coarse-grained clastic rock, composed of angular broken rock fragments held together by a mineral cement or a fine-grained matrix of crystallized impact melt that is diagnostic from an impact event.

Iron -- A heavy magnetic malleable and ductile chemically active mineral, the native metallic element Fe. Native iron is rare in terrestrial rocks but common in meteorites.

Joint -- A surface of fracture or parting in a rock, without displacement; the surface is often plane and may occur with parallel joints to form a joint set..

Limestone -- A sedimentary rock consisting chiefly of the mineral calcite (calcium carbonate, CaCO 3 ), with or without magnesium carbonate. Common impurities include chert and clay. Limestone is the most important and widely distributed of the carbonate rocks and is the consolidated equivalent of limy mud, calcareous sand, and/or shell fragments. It yields lime on calcination.

Massif -- A massive topographic and structural feature, especially in an orogenic belt, commonly formed of rocks more rigid than those of its surroundings.

Matrix -- The finer-grained material enclosing the larger grains in a sediment or sedimentary rock.

Mesa -- A flat-topped mountain or plateau bounded on at least one side by a steep cliff.

Metamorphic -- Pertaining to the process of metamorphism or to its results.

Metamorphism -- The mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions imposed at depth below the surface zones of weathering and cementation, which differ from the conditions under which the rocks originated.

Meteorite -- Any solid object from interplanetary space that has fallen to the Earth's surface without being vaporized by frictional heating during its passage through the atmosphere; a stony or metallic object large enough to reach the ground. Most meteorites are believed to be fragments of asteroids and to consist of primitive solid matter similar to that from which the Earth was originally formed.

Microscope -- An optical instrument that uses a lens or a combination of lenses to produce magnified images of small objects, especially of objects too small to be seen by the unaided eye.

Mineral -- A naturally occurring inorganic elementy or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties.

Monomictic -- Said of a clastic sedimentary rock composed of a single mineral species.

Obsidian -- A back or dark-colored volcanic glass, usually of rhyolitic composition, characterized by conchoidal fracture. It has been used for making arrowheads, jewelery, and art objects.

Pitchstone -- A volcanic glass with a waxy dull resinous luster. Its color and composition vary widely; it contains a higher percentage of water than obsidian. Crsytallites are detectable in thin section.

Polymictic -- Said of a clastic sedimentary rock composed of many rock types, e.g. a graywacke; also, said of the clasts of such a rock.

Polymorph -- A crystal form of a substance that displays polymorphism.

Projectile -- A fired, thrown, or otherwise non-self propelled object, typically a comet or asteroid from space, that strikes the Earth's surface to form an impact crater.

Proximal -- 1. Said of an ore deposit formed adjacent to a volcanic feature to which it is genetically related and from which its constituents have been derived. 2. Said of a sedimentary deposit consisting of coarse clastics, formed nearest the source area. 3. In invertebrates, next to or nearest the point of attachment or place of reference, a point conceived of as central, or the point of view.

Pumice -- A light-colored cellular glassy rock commonly having the composition of rhyolite. It is often sufficiently buoyant to float on water and is economically useful as a lightweight aggregate and as an abrasive.

Pyroclastic -- Pertaining to clastic rock material formed by volcanic explosion or aerial expulsion from a volcanic vent.

Quartz -- Crystalline silica, an important rock-forming mineral, SiO 2 . It is, next to feldspar, the most common mineral, occuring either in transparent hexagonal crystals or in crystalline or cryptocrystalline masses. Quartz is most common gangue mineral of ore deposits, forms the major proportion of most sands, and has a widespread distribution in igneous, metamorphic, and sedimentary rocks.

Ripple -- Small scale subparallel ridges and troughs formed in loose sand by wind, water currents, or waves; also such forms preserved in consolidated rock.

Sandstone -- A clastic sedimentary rock composed of grains of sand size set in a matrix of silt or clay and more or less firmly united by a cementing material (commonly silica, iron oxide, or calcium carbonate); the consolidated equivalent of sand. The sand particles usually consist of quartz, and the term "sandstone" when used without qualification, indicates a rock containing about 85-90% quartz.

Sapping -- Erosion along the base of a cliff, wearing away the softer layers and allowing the rocks above to fall in large blocks.

Sediment -- Solid material that has settled down from a state of suspension 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 such as sand, mud, or till.

Sedimentary -- Pertaining to or containing sediment, or formed by its deposition.

Selenite -- The clear, colorless variety of gypsum, occurring (esp. in clays) in distinct, transparent monoclinic crystals or in large crystalline masses that easily cleave into broad folia.

Silica -- Silicon dioxide, SiO 2 . It occurs as crystalline quartz, cryptocrystalline chalcedony, and amorphous opal; dominantly in sand, diatomite, and chert; and combined in silicates as an essential constituent of many minerals.

Simple Crater -- An impact crater with a smooth bowl shape and a small depth to diameter ratio, typically near 1:6.

Shale-- A fine-grained detrital sedimentary rock, formed by the compaction of clay, silt, or mud. It has a finely laminated structure, which gives it a fissility along which the rock splits readily, especially on weathered surfaces. Shale is well indurated, but not as hard as argillite or slate. It may be red, brown, black, or gray.

Shatter Cone -- A distinctively striated conical fragment of rock along which fracturing has occurred, ranging in length from less than a centimeter to several meters, generally found in nested or composite groups in the rocks of crypto-explosion structures, and generally believed to have been formed by shock waves generated by meteorite impact.

Slickenside -- A polished or striated rock surface that results from friction along a fault plane.

Strain -- Change in the shape or volume of a body as a result of stress; a change of the relative configuration of the particles of a substance.

Stishovite -- A tetragonal mineral, SiO 2 . It is a high-pressure, extremely dense polymorph of quartz, produced under static conditions at pressures above about 100 kilobars and found naturally associated with coesite and only in shock-metamorphosed quartz-bearing rocks. Its occurrence provides a criterion for meteorite impact.

Stress -- In a solid, the force per unit area, acting on any surface within it, and variously expressed as pounds or tons per square inch, or dynes or kilograms per square centimeter.

Suevite -- A grayish or yellowish breccia that is associated with meteorite impact craters and that contains both shock-metamorphosed rock fragments and glassy inclusions that occur typically as aerodynamically-shaped bombs. It closely resembles a tuff breccia or pumiceous tuff but is of non-volcanic orgin and can be distinguished by the presence of shock-metamorphic effects.

Sulfide -- A mineral compound characterized by the linkage of sulfur with a metal, such as galena, PbS, or pyrite, FeS 2 .

Tektite -- A rounded pitted jet-black to greenish or yellowish body of silicate glass of non-volcanic origin, usually walnut-sized, found in groups in several widely separated areas of the Earth's surface. Most tektites are high in silica (68-82%) and very low in water content (average 0.005%): their composition is unlike that of obsidian and more like that of shale. Tektites average a few grams in weight. They are believed to be of extraterrestrial origin or alternatively the product of large hypervelocity meteorite impacts on terrestrial rocks.

Tectonic -- Pertaining to the forces involved in, or the resulting structures of, tectonics.

Tectonics -- A branch of geology dealing with the broad architecture of the outer part of the Earth, that is, the major structural or deformational features and their relations, origin, and historical evolution. It is closely related to structural geology, but tectonics generally deals with larger features.

Tuff -- A general term for all consolidated pyroclastic rocks.

Uplift -- A structurally high area in the crust, produced by movements that raise the rocks, as in a broad dome or arch.

Volcanic -- Pertaining to the activities, structures, or rock types of a volcano.

Weathering -- The destructive processes by which rocks are changed on exposure to atmospheric agents at or near the earth's surface, with little or no transport of the loosened or altered material.


*  Most of the terms in this list from the Dictionary of Geological Terms, 3rd Ed., R.L. Bates and J.A. Jackson (Eds.), American Geological Institute, 1984 .

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