Terrestrial Impact Craters


allochthonous — Material that is formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer to the fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.

asteroid — Any of the numerous small rocky bodies in orbit around the Sun. Most asteroids reside in the "main belt" between Mars and Jupiter, but some have orbits that cross Earth's orbit and could strike its surface.

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

cataclastic — A texture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing.

central peak — The exposed core of uplifted rocks in complex meteorite impact craters. The central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism.

comet — One of the primitive icy bodies originating in the outer reaches of the solar system that are in elliptical orbits around the Sun. Near the Sun, the icy material vaporizes and streams off the comet, forming a glowing tail.

cratons — The relatively stable portions of continents composed of shield areas and platform sediments. Typically, cratons are bounded by tectonically active regions characterized by uplift, faulting, and volcanic activity.

Cretaceous — A geological term denoting the interval of Earth history beginning around 145 million years ago and ending 65 million years ago with the formation of the Chicxulub impact structure.

Cretaceous-Tertiary (K-T) boundary — A major stratigraphic boundary on Earth, marking the end of the Mesozoic Era, best known as the age of the dinosaurs. The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. It has been dated to 65 million years ago, coeval with the age of the 200-kilometer-diameter Chicxulub impact structure in Mexico.

crystalline — Rock types made up of crystals or crystal fragments, such as metamorphic rocks that recrystallized in high-temperature or pressure environments, or igneous rocks that formed from cooling of a melt.

diaplectic glass — A natural glass formed by shock tranformation from any of several minerals without melting. It is found only in association with meteorite impact craters and crater ejecta.

distal ejecta — Impact ejecta found at distances greater than 5 crater radii from the rim of the source crater, as opposed to proximal ejecta, which are found closer than 5 crater radii from the crater rim, and which make up about 90% of all material thrown out of the crater during the impact event.

ejecta — Material such as glass and fragmented rock thrown out of an impact crater during its formation.

high-pressure mineral phases — Mineral forms that are stable only at the extremely high pressures typical of Earth's deep interior, but not its surface. Such pressures are generated instantaneously during meteorite impact. For example, stishovite is a high-pressure polymorph of quartz, a common crustal mineral, and diamond is the high-pressure modification of graphite.

hummocky — Uneven, "lumpy" terrain.

impact melt rock — Rocks melted during impact, including small particles dispersed in various impact deposits and ejecta, and larger pools and sheets of melt that coalesce in low areas within the crater. Impact melts are extremely uniform in their composition, but highly variable in texture. They are composed predominantly of the target rocks, but may contain a small, but measurable, amount of the impactor.

meteorite — An extraterrestrial rock that has fallen to Earth. Most meteorites are pieces of asteroids and are of stony, stony-iron, or iron composition.

meteoroid — A small solid body moving through interplanetary space; after falling to Earth it is called a meteorite.

Paleozoic — A geological term denoting the time in Earth history between about 570 and 245 million years ago.

peak ring — A central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial craters above about 50 kilometers in diameter.

planar deformation features — Microscopic features in grains of (for example) quartz or feldspar consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.

Precambrian — A geological term denoting the time in Earth history prior to 570 million years ago.

proximal ejecta — All ejecta that are found up to 5 crater radii from the rim of the impact crater; 90% of all ejecta are found within this region. Note that the limit of proximal ejecta scales with the crater size. Ejecta found at greater distances are called distal ejecta.

shatter cone — Striated conical fracture surfaces produced by meteorite impact into fine-grained brittle rocks, such as limestone.

shield — Any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth's surface.

shock metamorphism — The production of irreversible chemical or physical changes in rocks by a shock wave generated by impact, or detonation of high-explosive or nuclear devices.

siderophile elements — Literally, "iron-loving" elements, such as iridium, osmium, platinum, and palladium, which are relatively common in undifferentiated meteorites, and, in chemically segregated asteroids and planets, are found in the metal-rich interiors. Consequently, these elements are extremely rare on Earth's surface.

stishovite — A dense, high-pressure phase of quartz that has so far been identified in nature only in shock-metamorphosed quartz-bearing rocks from meteorite impact craters.

suevite — A breccia composed of angular fragments of different rock types as well as glass inclusions. Glass can make up more than half of the volume of a suevite. The minerals in the rock fragments within suevites (also called suevitic breccias) commonly display shock-metamorphic effects. Suevite was named after a rock found at Ries crater in southern Germany.

synthetic aperture radar — Synthetic aperture radar, or SAR, uses a relatively small antenna to produce a broad beam, and makes use of the Doppler shift of the radar signal moving across the target to synthesize (with extensive computer processing) the azimuth resolution of a very narrow beam.

target rocks — The surface rocks that an asteroid or comet impactor smashes into in a meteorite impact event.

tektites — Natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as droplets during terrestrial impact events. They range in color from black or dark brown to gray or green and most are spherical in shape. Tektites have been found in four geographically extended deposits or "strewn fields" on Earth's surface: North America, Central Europe, Ivory Coast, and Australasia. In contrast to most impact glasses, which are found inside or within the immediate vicinity of impact structures, tektites are distal impact ejecta. Source craters are known for three of the four tektite strewn fields: The 11-kilometer-diameter Bosumtwi Crater (1.07 million years old) is linked to the Ivory Coast tektites, the 24-kilometer-diameter Ries Crater (15 million years old) is the source of the Central European tektites, and the 85-kilometer-diameter Chesapeake Bay impact structure (35 million years old) is the source crater of the North American tektites.

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