IMPACT CRATER GLOSSARY

IMPACT CRATER/STRUCTURE GLOSSARY

by: Charles O’Dale

  1. IMPACT CRATER GLOSSARY
  2. REFERENCE

1. IMPACT CRATER GLOSSARY

ACCRETIONARY LAPILLI
Pellets that form by the accretion of fine ash around condensing water droplets or solid particles, particularly in steam-rich volcanic eruptive columns. Similar processes are related with the turbulent explosion plume raising above the expanding excavation cavity in an impact cratering event. So, accretionary lapilli that commonly exhibit a concentric internal structure, have been found also in impact deposits.

ACOUSTIC FLUIDIZATION
Hypothesized (H.J. Melosh) fluidization of rock debris subjected to strong vibrations possibly enabling the collapse of the transient crater in themodification stage of impact cratering.

ALLOCHTHONOUS
Of rocks whose primary constituents have not been formed in situ (formed elsewhere and clearly moved to their current location).

[see – AUTOCHTHONOUS, PARAUTOCHTHONOUS]
[more data – IMPACT CRATER EJECTA]

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 Earth crossing orbits. The average impact velocities of asteroids hitting the earth is ~18Km/s (Kring)
[see – METEOR, METEORITE, METEOROID.]

AUTOCHTHONOUS 
Originating where found (formed in place).

[see – ALLOCHTHONOUS, PARAUTOCHTHONOUS]
[more data – IMPACT CRATER EJECTA]

BALDWIN’S CURVE

Baldwin’s curve relating crater depth to diameter, showing how well the Pingualuit, Holleford, Brent and Deep Bay meteorite craters all fit the curve (after C. S. Beals, M. J. S. Innes, and J. A. Rottenberg, The search for fossil meteorite craters–I, Current Science 29: 206).

BALLEN STRUCTURE
Microscopic shock-deformation feature in quartz. Oval quartz with rims of tiny vugs filled with amorphous material.

BALLISTIC EROSION AND SEDIMENTATION
(Oberbeck 1975) Emplacement of ballistically transported impact ejecta, and ejecta-surface interaction.

BIOTITE
See mica.

BOLIDE
Exploding fireball.
[see – METEORITE]

BRECCIA – from Italian indicating both loose gravel and stone made by cemented gravel
A clastic sedimentary rock composed of angular clasts in a consolidated matrix. Breccias can be produced in several geologic processes: tectonic breccia, volcanic breccia (eruption breccia, vent breccia), sedimentary breccia (e.g., rock fall breccia), collapse breccia (e.g., in karst areas). Breccias may be distinguished depending on the origin of the clasts, monomictic (monogenetic, monolithologic) and polymictic (polygenetic, polylithologic).

[see – BRECCIA]
[see – SHOCK METAMORPHISM]

CAI
Calcium-aluminum-rich inclusions (CAIs) are found in chondritic meteorites. Rubinite was identified as tiny crystals in calcium-aluminum-rich inclusions (CAIs), and is among the first solids formed in the solar nebula. As the inner regions of the protoplanetary disk cooled below 1650°C (3,000° F),  those elements condensed out of the hot vapor to form delicate mineral crystals.

A close-up of an Allende meteorite fragment shows the white calcium-aluminum-rich inclusions and the darker chondrules. Scientists believe the former are the first rocks in the solar system, and the latter helped form the planets. Chip Clark/National Meteorite Collection/Smithsonian Institution

[see – METEORITE]

CALCITE
calcium carbonate mineral, CaCO3. Major constituent of carbonate sedimentary rocks, e.g., limestone.

CATACLASITE
Rubble breccia formed by shearing and granulation in dislocation metamorphism. Also seemonomict(ic) breccia.

CLAST
A fragment of geological loose material, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.
[see – SHOCK METAMORPHISM– Breccia]

COESITE
A high-pressure polymorph of quartz (SiO2). High pressure destructs the crystal lattice characteristic of quartz and compresses the silicon and oxygen atoms into an amorphous system. The result is high-density glass. Once the pressure has surpassed a certain threshold, the amorphization process becomes irreversible and the material can no longer return to a crystalline configuration.

COESITE (IMPACTITE ) 
A high-pressure polymorph of quartz (SiO2)  that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz.

High pressure destructs the crystal lattice characteristic of quartz and compresses the silicon and oxygen atoms into an amorphous system. The result is high-density glass. Once the pressure has surpassed a certain threshold, the amorphization process becomes irreversible and the material can no longer return to a crystalline configuration.

Coesite has two morphologies: fine grade needle-like crystals or as greenish aggregates (a.k.a. “granular coesite”).

In 1960, coesite was found by Edward C. T. Chao, in collaboration with Eugene Shoemaker, to naturally occur in the Barringer Crater. This was evidence that the crater must have been formed by an impact.

Geologist Eugene Shoemaker (1928-1997) published the landmark paper conclusively demonstrating an impact origin for the Barringer Meteorite Crater. Photo: USGS

Coesite from the Wanapitei Impact structure, Dence 1974.

Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied at the Chesapeake Bay impact crater were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.

[see – SHOCK METAMORPHISM – coesite]

COMET
Cosmic body in a parabolic or highly elliptical orbit around the sun. Composed of meteoric dust and frozen C, O, H -compounds. Near the Sun, the icy material vaporizes and streams off the comet, forming a glowing tail. Comets are potential projectiles in impact cratering.

COMMINUTION
The reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of the Earth’s crust.
[see SHOCK METAMORPHISM – Shocked target rock]

COMPLEX IMPACT STRUCTURE/CRATER (CENTRAL PEAK CRATER)
An impact structure exhibiting a central uplift and/or inner rings that are formed by elastic rebound and slumping of the walls of the transient crater in the modification stage. The transition from simple to complex craters depends on the gravity of the impacted planetary body. On Earth, complex craters have diameters of roughly more than 4 km. 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.
[see – CRATER CLASSIFICATION – Complex crater]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

CONTACT AND COMPRESSION STAGE
A process  in which a large object strikes an even larger one at hypervelocity, which locally releases a huge amount of energy producing an impact crater.
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION – Contact & Compression]
[see – CRATER IDENTIFICATION]

CRATER  (ASTROBLEME)
An approximately circular depression in the surface of a solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Interplanetary collisions of planetary bodies represent a fundamental process that affected all planets and moons of the solar system since its formation. They occur on an extremely wide scale of projectile and target sizes, and with a large range of impact velocities. Hypervelocity collisions result in the propagation of shock waves in the colliding bodies and as a consequence in “shock metamorphism” of the impacted regions.


On this planet, impact craters are divided into basic morphologic subdivisions:

  • simple: The transition size between simple to complex craters is 2km in sediments and 4km in crystalline rocks (Dence 1972).
  • complex: The transition size between complex to ringed basin craters is 10 to 50 km (Osinski, G. 2008).
  • peak ring: With increasing diameter, impact structures become proportionately shallower and develop more complicated rims and floors, including the appearance of central peaks and interior rings.

[see – CRATER CLASSIFICATION]

CRATER DATING
Using a variety of methods to determine the age of geological materials. Relative dating methods are used to describe a sequence of events. These methods use the principles of stratigraphy to place events recorded in rocks from oldest to youngest. Absolute dating methods determine how much time has passed since rocks formed by measuring the radioactive decay of isotopes or the effects of radiation on the crystal structure of minerals. Paleomagnetism measures the ancient orientation of the Earth’s magnetic field to help determine the age of rocks.
[see – DATING]

CRATER FORMATION
Impact crater formation is divided into three basic subdivisions:

  • Contact & Compression:  a large object strikes an even larger one at hypervelocity, which locally releases a huge amount of energy producing an impact crater.
  • Excavation: The crater excavation stage overlaps somewhat with the compression stage and involves two processes:
    • upward ejection (spalling) of large near-surface fragments and smaller ejecta (ejecta curtain);
    • subsurface flow of target material to form the transient crater.
  • Modification: The initial transient crater is unstable and the modification stage commences. Small craters of <4 km (on Earth) are relatively stable after the excavation stage. For larger craters, the impact structure is gravitationally unstable and its modification stage will include uplift of the crater floor and collapse of the unstable steep walls (slumping). These movements will be completed in a few minutes and could result in a complex or multi-ring crater. Minor faulting, mass movement and/or hydrothermal activity in the larger craters could last indefinitely.

[see –  CRATER FORMATION]

CRATER IDENTIFICATION

  • GEOMORPHOLOGY – One of the first indicators of a possible impact site is “circular geology”.
  • SHATTER CONES – Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events.
  • FRACTURED ROCK – While travelling toward impact sites I documented fractured rocks increasing in magnitude as we neared the crater site.
  • GRAVITY ANOMALIES – Gravity contours illustrate anomalies caused by fractured country rock under an impact site.
  • MAGNETIC ANOMALIES – Magnetic studies  document the magnetic disturbances within impact structures.
  • SHOCK METAMORPHOSIM – The extreme pressures and temperatures at hypervelocity impacts have caused shock metamorphic effects on target rocks.

CRATER SIZE vs PLANET/MOON
The depth to diameter ratio of craters smaller than a certain size is a constant, as predicted by the Maxwell Z-model. Below a break point (10 km for the Moon), the ratio follows a power law, decreasing as size increases [Hiesinger, 2006, Sharpton, 1994]. Source: [Hiesinger, 2006].
[see – CRATER CLASSIFICATIONS]

CRATER SIZE vs SIZE OF METEOROID
A quantitative estimate of the impact hazard as a function of impactor size (or energy) and advocated a strategy to deal with such a threat (Morrison, 2007).
[see – CRATER CLASSIFICATIONS]

CRATER TRANSIENT
The crater that exists at the end of the excavation stage of impact cratering. The transient crater undergoes only slight modification in the case of a small, bowl-shaped crater. Large transient craters exhibit a gravity-dependent instability which leads to its collapse by elastic rebound and slumping of the walls and, to a large extent, to filling up of the cavity. Consequently, these complex impact structures/craters show a much smaller depth-to-diameter ratio compared with simple, bowl-shaped craters.
[see –  CRATER FORMATION]

CRATON
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.
[see – SUDBURY IMPACT STRUCTURE]

CRETACEOUS-TERTIARY/CRETACEOUS-PALEOGENE (K–Pg) BOUNDARY
A major stratigraphic boudary 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.
[see –  EXTINCTIONS]

CRYPTOVOLCANIC STRUCTURE
Term used especially in the twenties and thirties and assigned to terrestrial circular structures that showed heavy destructions of rocks evidently produced by a tremendous underground explosion. Because of the absence of any volcanic activity in many of these structures (e.g., Steinheim, Serpent Mound, Decaturville, Wells creek, Kentland), a muffled or hidden volcanism was suggested (especially by the American geologist W. H. Bucher). Later, these structures proved to be impact structures.

DATING

Name of Method Age range of Application Material Dated Methodology
Radiocarbon 1 – 70,000 years Organic material such as bones, wood, charcoal, shells Radioactive decay of 14C in organic matter after removal from bioshpere
K-Ar dating 1,000 – billion of years Potassium-bearing minerals and glasses Radioactive decay of 40K in rocks and minerals
Uranium-Lead 10,000 – billion of years Uranium-bearing minerals Radioactive decay of uranium to lead via two separate decay chains
Uranium series 1,000 – 500,000 years Uranium-bearing minerals, corals, shells, teeth, CaCO3 Radioactive decay of 234U to 230Th
Fission track 1,000 – billion of years Uranium-bearing minerals and glasses Measurement of damage tracks in glass and minerals from the radioactive decay of 238U
Luminescence (optically or thermally stimulated) 1,000 – 1,000,000 years Quartz, feldspar, stone tools, pottery Burial or heating age based on the accumulation of radiation-induced damage to electron sitting in mineral lattices
Cosmogenic Nuclides 1,000 – 5,000,000 years Typically quartz or olivine from volcanic or sedimentary rocks Radioactive decay of cosmic-ray generated nuclides in surficial environments
Magnetostratigraphy 20,000 – billion of years Sedimentary and volcanic rocks Measurement of ancient polarity of the earth’s magnetic field recorded in a stratigraphic succession
Tephrochronology 100 – billions of years Volcanic ejecta Uses chemistry and age of volcanic deposits to establish links between distant stratigraphic successions

DATING GLOSSARY(from “The Nature Education“)

absolute dating: Determining the number of years that have elapsed since an event occurred or the specific time when that event occurred
atomic mass: The mass of an isotope of an electron, based on the number of protons and neutrons
atomic nucleus: The assemblage of protons and neutrons at the core of an atom, containing almost all of the mass of the atom and its positive charge
daughter isotope: The isotope that forms as a result of radioactive decay
electrons: Negatively charged subatomic particles with very little mass; found outside the atomic nucleus
electron spin resonance: Method of measuring the change in the magnetic field, or spin, of atoms; the change in the spin of atoms is caused by the movement and accumulation of electrons from their normal position to positions in imperfections on the crystal structure of a mineral as a result of radiation.
elements: Chemical substances that cannot be split into a simpler substances
fault: A fracture in a rock along which movement occurs
geomagnetic polarity time scale: A record of the multiple episodes of reversals of the Earth’s magnetic polarity that can be used to help determine the age of rocks
half-life: The amount of time it takes for half of the parent isotopes to radioactively decay to daughter isotopes
index fossil: A fossil that can be used to determine the age of the strata in which it is found and to help correlate between rock units
isotopes: Varieties of the same element that have the same number of protons, but different numbers of neutrons
magnetic field: A region where lines of force move electrically charged particles, such as around a magnet, through a wire conducting an electric current, or the magnetic lines of force surrounding the earth
magnetism: The force causing materials, particularly those made of iron and other certain metals, to attract or repel each other; a property of materials that responds to the presence of a magnetic field
normal polarity: Interval of time when the earth’s magnetic field is oriented so that the magnetic north pole is approximately in the same position as the geographic north pole
neutrons: A subatomic particle found in the atomic nucleus with a neutral charge and a mass approximately equal to a proton
optical stimulating luminescence: Dating method that uses light to measure the amount of radioactivity accumulated by crystals in sand grains or bones since the time they were buried
paleomagnetism: Remanent magnetization in ancient rocks that records the orientation of the earth’s magnetic field and can be used to determine the location of the magnetic poles and the latitude of the rocks at the time the rocks were formed
parent isotope: The atomic nucleus that undergoes radioactive decay
polarity (magnetic polarity): The direction of the earth’s magnetic field, which can be normal polarity or reversed polarity
potassium-argon (K-Ar) method: Radiometric dating technique that uses the decay of 39K and 40Ar in potassium-bearing minerals to determine the absolute age
principle of cross-cutting relationships: Any geologic feature that cross-cuts across strata must have formed after the rocks they cut through were deposited.
principle of faunal succession: Fossil species succeed each other in a definitive, recognizable order and once a species goes extinct, it disappears and cannot reappear in younger rocks.
principle of original horizontality: Layers of strata are deposited horizontally, or nearly horizontally, and parallel or nearly parallel to the earth’s surface.
principle of superposition: In an undeformed sequence, the oldest rocks are at the bottom and the youngest rocks are at the top.
protons: Positively charged subatomic particles found in the nucleus of an atom
radioactivity (radioactive): An unstable isotope spontaneously emits radiation from its atomic nucleus
radioactive decay: The process by which unstable isotopes transform to stable isotopes of the same or different elements by a change in the number of protons and neutrons in the atomic nucleus
radiocarbon dating: Radiometric dating technique that uses the decay of 14C in organic material, such as wood or bones, to determine the absolute age of the material
radiometric dating: Determination of the absolute age of rocks and minerals using certain radioactive isotopes
relative dating: Rocks and structures are placed into chronological order, establishing the age of one thing as older or younger than another
reversals (magnetic reversals): Changes in the earth’s magnetic field from normal polarity to reversed polarity or vice versa
reversed polarity: Interval of time when the earth’s magnetic field is oriented so that magnetic north pole is approximately in the same positions as the geographic south pole
strata (singular: stratum): Distinct layers of sediment that accumulated at the earth’s surface.
stratigraphy: The study of strata and their relationships
thermoluminescence: Dating method that uses heat to measure the amount of radioactivity accumulated by a rock or stone tool since it was last heated

DEFORMATION TWINNING
Lattice gliding due to stress subjected to a crystal. Twinned crystals may show deformation lamellae. Twinning in crystals may be caused by shock deformation.

DIAMOND
The crystalline form of carbon (C) is the hardest naturally occurring material. May be formed in meteorite impact when carbon, e.g. in graphite-bearing rocks, is subjected to extraordinarily high temperatures and pressures. Is observed in meteorites and impactites (e.g., Younger Dryas event, Ries, Popigai impact structures).

Asteroid Impact Diamonds: Diamonds have been found in and around the craters of many asteroid impact sites. Earth has been repeatedly hit by asteroids throughout its history. These asteroids hit with such force that pressures and temperatures high enough to form diamonds are produced. If the target rock contains carbon, the conditions needed to form diamonds might occur within the impact area.
Extraterrestrial Diamonds: Diamonds have been discovered in some meteorites. These diamonds are thought to have formed in space in response to asteroid impacts or other severe events.

DIAPLECTIC CRYSTAL
Partly isotropic crystal originating from patchy shock damage of the crystal lattice.

DIAPLECTIC GLASS
Glass formed by shock damage of a mineral and not by melting. According to current knowledge, diaplectic glass cannot be formed in endogenetic processes.

EJECTA
Solid, liquid and vaporized material ejected from an impact crater during its formation.

  • Distal  – Impact ejecta found at distances greater than 5 crater radii from the rim of the source crater;
  • Proximal – Impact ejecta found up to 5 crater radii from the rim of the impact crater;
  • Blanket – A generally symmetrical apron of ejecta that surrounds an impact crater; and
  • Curtain – Impact-excavated clasts ejected ballistically on parabolic trajectories combine into a cone-shaped curtain, the ejecta curtain, expanding outward from the crater rim and turning into the ejecta blanket around the crater.

[see – EJECTA]

ELASTIC REBOUND
Part of a theory explaining the forces that cause earthquakes. In impact cratering , elastic rebound describes the readjustment of the highly compressed floor of the transient cavity in the modification stage.

EUCRITE
A common class of achondrite meteorites composed of pigeonite and plagioclase. These meteorites formed as basaltic flows on a parent body, probably asteroid 4 Vesta. (Data collected by NASA’s Dawn Mission, in orbit around Vesta from 2011-2012, strengthed the association between Vesta and eucrite meteorites.)

EXCAVATION STAGE
The crater excavation stage (Melosh, 1980) overlaps somewhat with the compression stage and involves two processes:

  • upward ejection (spalling) of large near-surface fragments and smaller ejecta (ejecta curtain);
  • subsurface flow of target material to form the transient crater.

[see – CRATER CLASSIFICATION – Complex crater]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

EXTINCTION vs IMPACT
Extinction of many groups of organisms at a particular time by environmental catastrophe related with collapsing ecosystems. There are strong indications that some mass extinctions may be caused partly or completely by large asteroidal or cometary impacts.
[see –  EXTINCTIONS]

FRACTURED ROCK
A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. One of the impact shock effects I noticed in my crater explorations was the fractured rocks around the perimeter of the structure where there should have been solid rock outcrops. Though this phenomenon IS NOT firm evidence of an impact but is a feature of impacting and would indicate that further investigation of the structure is warranted.
[see – FRACTURED ROCK]

FULLERENES
Carbon molecules of the form C60, C76, C84 … that may trap gases in their soccer ball-shaped structure. Extraterrestrial gases in fullerenes found in terrestrial rock deposits have been related with impact events, e.g., extraterrestrial helium trapped in fullerenes in the Sudbury impact structure, and extraterrestrial noble gases in fullerenes at the Permian-Triassic boundary.

GEOLOGY
Science of the study of the accessible parts of the Earth’s crust, their rocks, their structures,their fossils and their resources with the aim to get a picture of the development of the planet and the life on it. So, geology is partly a historical science and partly a natural science. Although “geo” means earth, the term geology is used also with the study of other planetary bodies, e.g., planetary geology, geology of the Moon.

GEOMORPHOLOGY
The scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth’s surface.


The following example illustrates the shortcoming of using geomorphology in crater identification:

Alsever Lake compared to Brent impact crater

Brent Crater – IMPACT
Alsever Lake – NON-IMPACT

Alsever Lake (image LEFT) is located at the southern boundary of Algonquin Park. It is similar in appearance to the Brent impact crater (image RIGHT) with its two distinct bodies of water forming a circular pattern. Alsever has a central “land mass” like Brent and what appears to be circular outline. This is best viewed on some topographical maps.


 

GEOLOGIC DATING
Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth’s surface to determine the relative ages of geological events preserved in the rock record. For example, in the rocks exposed in the walls of the Grand Canyon there are many horizontal layers, which are called strata. The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
[see – DATING– geologic]

GEOPHYSICS
Physics of the Earth related with all aspects of physical properties, structures, and processes of the Earth (and other planetary bodies). Geophysics of impact structures comprises gravity, magnetic, seismic, geoelectric, and other measurements. Several buried impact structures have been detected by geophysical studies

GPa
Gigapascal, 1 GPa = 1,000 MPa (Megapascal) = 109 Pascal, the SI unit of pressure. GPa is commonly used in the high-pressure range of shock deformation, 1 GPa = 10 kbar.

GRADY-KIPP FRAGMENTS
In an impact cratering model, the rock fragments resulting from tensile stress related with therarefaction wave.

GRANOPHYRE
Igneous rock with granitic composition. Also local term for an impact melt rock in the Vredefort impact structure, South Africa.

GRAVIMETRY – Gravity anomaly 
Geophysical method to measure variations of the gravity field related with subsurface density variations. Impact structures commonly show pronounced gravity negative anomalies due to the occurrence of low-density breccias, rock fracturing, and replacement of ejected material by post-impact young sediments. In very large impact structures, relative positive anomalies may be produced by the uplift (see; central uplift) of high-density material from the Earth’s lower crust and upper mantle.
[see – GRAVITY ANOMALY ]

GRIES (= gravel, grit),
German term especially used to describe a heavymonomictic grit brecciation in rocks of the Ries impact structure.

GRIT BRECCIATION
Cataclastic deformation by shearing and granulation of hard (competent) rocks typically found in impact structures. Also see monomictic movement breccias.

HIGH PRESSURE MINERALS:

Coesite is a form (polymorph) of silicon dioxide SiO2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C or 1,300 °F), are applied to quartz.

HUGONIOT EQUATIONS
Also termed Hugoniot-Rankine equations, describing the behavior of material subjected toshock waves.

HYDROTHERMAL
The origin and emergence of life under impact bombardment (Cockell 2006)
Craters formed by asteroids and comets offer a number of possibilities as sites for prebiotic chemistry, and they invite a literal application of Darwin’s ‘warm little pond’. Some of these attributes, such as prolonged circulation of heated water, are found in deep-ocean hydrothermal vent systems, previously proposed as sites for prebiotic chemistry. However, impact craters host important characteristics in a single location, which include the formation of diverse metal sulphides, clays and zeolites as secondary hydrothermal minerals (which can act as templates or catalysts for prebiotic syntheses), fracturing of rock during impact (creating a large surface area for reactions), the delivery of iron in the case of the impact of iron-containing meteorites (which might itself act as a substrate for prebiotic reactions), diverse impact energies resulting in different rates of hydrothermal cooling and thus organic syntheses, and the indiscriminate nature of impacts into every available lithology—generating large numbers of ‘experiments’ in the origin of life.
[see – CRATER CLASSIFICATION– hydrothermal]

HYPERVELOCITY
A velocity approximately over 3,000 meters per second (6,700 mph, 11,000 km/h, 10,000 ft/s, or Mach 8.8). In particular, hypervelocity is velocity so high that the strength of materials upon impact is very small compared to inertial stresses.

IMPACT CRATER CHAIN
A line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena (plural catenae).

IMPACT MELT ROCK
Impact melt rocks are basically volcanic rocks, such as basalt lava, and they attest to the extreme conditions generated by the impact event. Pressures and temperatures in the target rocks surrounding the point where the asteroid or comet hits are so high that large volumes or rock can be instantaneously melted. Pieces of this melt can cool rapidly to form glass and be incorporated in suevites. Melt rocks in impact structures may also result from frictional melting in strong dynamic metamorphism, pseudotachylite.
[see SHOCK METAMORPHISM – suevite, pseudotachylite]

IMPACT METAMORPHISM
In the broader sense: changes of minerals and rocks acquired in the impact cratering process including shock metamorphism, pseudotachylite and shattercone formation. In the narrow sense: metamorphism of minerals and rocks caused by shock from meteorite impact.
[see – SHOCK METAMORPHISM]

IMPACT SPALLATION
Sum of effects related with rarefaction waves in the impact cratering process. Near the free surface of the target where the rarefaction tensile stress is maximum, thin spall plates of rock are thought to be expelled at very high speed. Peculiar spallation effects may occur in conglomerates by multiple shock-wave reflections within spherically shaped clasts.

IMPACT STRUCTURE
Closely related to the terms impact crater and meteorite impact crater, and is used in cases in which erosion or burial has destroyed or masked the original topographic impact feature with which one normally associates the term crater.
[see – CRATER CLASSIFICATION – Complex crater]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

IMPACTITE
Impactite is the term used for all rocks produced or affected by a hypervelocity impact event (a.k.a. instant rocks). Impactites range from completely reconstituted lithologies, such as impact melt rocks, to fractured target rocks. They generally, but not always, contain evidence of shock metamorphism.
[see – SHOCK METAMORPHISM – Impactite]

IMPACTOR
The cosmic projectile, meteoroid, asteroid, comet, or other celestial object which causes an impact event. The kinetic energy of an object of mass m traveling at a speed v is = (½)mv2, provided v is much less than the speed of light.


[see – CRATER CLASSIFICATION – size of METEOROID]
[see – METEORITE]

INTERFERENCE ZONE
In impact cratering models, the near-surface of the target where compressional shock wave and tensional rarefaction wave interfere to reduce maximum pressure.

ISOCHRON – DATING
Is a common technique of radiometric dating and is applied to date certain events, such as crystallization, metamorphism, shock events, and differentiation of precursor melts, in the history of rocks. The initial amount of the daughter product can be determined using isochron dating.
[see – DATING– isochron]

JETTING
Ejection process postulated for the very earlycontact and compression stage of impact cratering. Many researchers believe that tektites originate from jetting of material extremely compressed in the impactor/target contact zone.

Kbar
Kilobar, 1 kbar (1 kb) = 1,000 bar; unit of pressure, frequently replaced by the SI unit Pascal, Pa, and Gigapascal, GPa (1,000 kbar = 1 Mbar = 100 GPa). The hydrostatic pressure in the center of the Earth amounts to about 3,000 kbar (300 GPa). Shock pressures in the contact and compression stage of impact cratering may exceed this value.
[see –  CRATER FORMATION]

KINETIC ENERGY
KE, of a massive falling meteorite is given by the following equation: KE = (½)MV2

KINK BAND
Characteristic pressure deformation in minerals, most common in mica but also observed inquartz and other minerals. Kink bands are formed by pressure-induced gliding in the crystal combined with external rotation of the lattice.

KREEP
An acronym built from the letters K (the atomic symbol for potassium), REE (rare-earth elements) and P (for phosphorus), is a geochemical component of some lunar impact breccia and basaltic rocks.

KUIPER BELT

The Kuiper belt or, sometimes called the Edgeworth–Kuiper belt, is a circumstellar disc in the Solar System beyond the planets, extending from the orbit of Neptune to approximately 50 AU from the Sun.

[see – OORT CLOUD.]

K/T boundary
Cretacious/Tertiary boundary (the C abbreviation is already assigned to the Cambrian system), at present practically synonymous with marking the giant mass extinction 65 Ma ago. The extinction of the dinosaurs at that time is only a subordinate part of this remarkable event.
[see – CRETACEOUS-TERTIARY/CRETACEOUS-PALEOGENE (K–Pg) BOUNDARY.]
[see –  EXTINCTIONS]

LECHATELIERITE

Fused silica glass SiO2. Lechatelierite has three different origins: Meteorite impacts, volcanism and lightning strikes (fulgurites). See Libyan desert glass.

LITHIC (impact) breccia
Polymictic impact breccia that contains shocked and unshocked clasts in a clastic matrix but lacks cogenetic melt particles.

LITHOLOGY
Of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition.

MAGNESIOFERRITE, MgFe2O
A rare spinel mineral crystallizes as black metallic octahedral crystals and has been documented in the recrystallized impact breccias of the Steen River Impact Structure (SRIS).  It is named after its chemical composition of magnesium and ferric iron. The density is 4.6 – 4.7 (average = 4.65), and the diaphaniety is opaque.

MAGNETIC ANOMALIES
In geophysics, a magnetic anomaly is a local variation in the Earth’s magnetic field resulting from variations in the chemistry or magnetism of the rocks. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study. Mapping of variation over an area is valuable in detecting structures obscured by overlying material.

MAJORITE, Mg3(MgSi)(SiO4)3 
A type of garnet mineral found in the upper mantle of the Earth. It is distinguished from other garnets in having Si in octahedral as well as tetrahedral coordination. Majorite was first described in 1970 from the Coorara Meteorite of Western Australia and has been reported from various other meteorites in which majorite is thought to result from an extraterrestrial high pressure shock event. Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.

MASKELYNITE
A clear, glassy pseudomorph of plagioclase produced by a relatively low pressure (250-300 kilobars) and low temperature (350°C) shock wave. It is found in the rocks of the central peaks of Clearwater West and Manicouagan craters, Quebec, Canada. Heated maskelynite reverts to crystalline plagioclase, indicating only slight structural disordering unlike fused plagioclase glass.
NATURAL TERRESTRIAL MASKELYNITE, Dence, et al THE AMERICAN MINERALOGIST 1967

METAMORPHIC ROCK
Rock that was formed by the recrystallization of a pre-existing rock in response to a change of mainly temperature and pressure (metamorphism). Metamorphic rocks are, e.g., marble (metamorphic limestone), gneiss, schist.
[see – SHOCK METAMORPHISM]

METEOR
Incoming meteoroids enter the earth’s atmosphere at 11 km/sec to 72 km/sec. Ram pressure between the air and the object create a very high temperature plasma at the front of the meteor. This plasma becomes visible at between about 120 km and 75 km above the earth. Energy goes into melting and vaporizing stone and metal. Energy is shed as material ablates.  In a couple of seconds most meteors are have been consumed. The left-over debris is called meteoric dust or just meteor dust.

METEORITE
If a meteoroid’s size, composition, speed and entry angle allow it to survive  the “meteor” phase of entry, it will slow to about 4 km/sec and enter “dark flight” at 20 km to 15 km above earth. Light emission from incandescence and ion recombination ceases. The meteor will arch into a more vertical trajectory, slow to terminal velocity of  about 0.1 km/sec and fall as a meteorite.
If the meteoroid is of sufficient size to keep it’s hyper-velocity  >12 km/sec through the atmosphere becoming an impactor, it will impact the ground and explode.  The kinetic energy of an object of mass m traveling at a speed v is = (½)mv2, provided v is much less than the speed of light.

METEORITE: Organic
Direct evidence of complex prebiotic chemistry from a water-rich world in the outer solar system is provided by the 4.5-billion-year-old halite crystals hosted in the Zag and Monahans (1998) meteorites. This study offers the first comprehensive organic analysis of the soluble and insoluble organic compounds found in the millimeter-sized halite crystals containing brine inclusions and sheds light on the nature and activity of aqueous fluids on a primitive parent body.
Organic matter in extraterrestrial water-bearing salt crystals
Chan et al

METEORITE CRATER
[see – CRATER  (ASTROBLEME)]

METEOROID (smaller than ASTEROIDS)
A meteoroid is a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom.
[see – ASTEROID, METEOR, METEORITE.]

MICAS
Group of sheet-silicate minerals; e.g., muscovite, biotite. Kink bands in mica may be produced by shock.

MODIFICATION STAGE
The initial transient crater is unstable and the modification stage commences. Small craters of <4 km (on Earth) are relatively stable after the excavation stage. For larger craters, the impact structure is gravitationally unstable and its modification stage will include uplift of the crater floor and collapse of the unstable steep walls (slumping). These movements will be completed in a few minutes and could result in a complex or multi-ring crater. Minor faulting, mass movement and/or hydrothermal activity in the larger craters could last indefinitely.
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION – Contact & Compression]
[see – CRATER IDENTIFICATION]

MOSAICISM
Shock feature in mineral crystals. Under crossed polarizers of the polarization microscope, mosaicism shows as a checkered, mosaic-like extinction pattern resulting from shock-induced disorder in the crystal lattice.

MULTIPLE IMPACT
Synchronous impact of two (paired impact) or more impactors. A Late Triassic multiple impact has been proposed to have produced a chain of five large impact structures on the European and the American continents.

Compilation of selected terrestrial meteorite impacts during the Triassic and the postulated Late Triassic multiple impact theory, modified after Spray et al.(1998). Lucas et al.(2012)suggested an age of ∼220 Ma for the Carnian/Norian boundary, which has an age of ∼227Ma in the current International Stratigraphic Chart (Cohen et al., 2013). Impact age data from Koeberl et al.(1996), Ramezani et al.(2005), Schmieder and Buchner (2008), Schmieder et al.(2010).

Multiple impacts are observed also on the Moon, Mars, Venus and on Jupiter’s satellites Ganymede and Callisto.
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION – Contact & Compression]
[see – CRATER IDENTIFICATION]

MULTI-RING IMPACT CRATER/STRUCTURE

The largest craters contain multiple concentric topographic rings, and are called multi-ringed basins, (ie: Sudbury), for example the lunar Orientale.
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION – Contact & Compression]
[see – CRATER IDENTIFICATION]

NAPPE
A sheet of rock that has moved sideways over neighboring strata as a result of an overthrust or folding.

OORT CLOUD
Cloud of comets hypothesized to be the source of the long-period comets. Periodical disturbance of the Oort cloud has been proposed to be related with a disputed periodical increase of cometary impacts on the Earth (Shiva theory).

Visual representation of the Oort Cloud, which is littered with ice and rocks left over from the formation of the Solar System. (Image: NASA) The Oort Cloud is an extended shell of icy objects that exist in the outermost reaches of the solar system. It is named after astronomer Jan Oort, who first theorised its existence. The Oort Cloud is roughly spherical, and is thought to be the origin of most of the long-period comets that have been observed.

[see – KUIPER BELT]

OVERTURNED STRATA (“overturned flap”)
Inverse stratigraphy at an impact crater rim related with the excavation process.
[see – Barringer Impact Crater]
[see –  CRATER FORMATION – Excavation Process]

OXYGENATION EVENT
2.4 billion years ago, the irreversible increase in the oxygen content of Earth’s near-surface atmosphere. Not only did it affect biological survival on our planet, but it also resulted in an extraordinary increase in mineral diversification.

PDFs
Upon bolide impact, the passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.
Decorated PDFs – Planar deformation features (PDFs) decorated by fluid and/or mineral inclusions as the result of annealing.
[see – SHOCK METAMORPHISM– Planar Deformaion Features]

PARAUTOCHTHONOUS
Ground which has been disturbed by impact, thrust or nappe displacement, but where the displacement is small enough that the rocks are still in contact with their source (moved but appear to be in place).

[see – ALLOCHTHONOUS, AUTOCHTHONOUS]
[see – IMPACT CRATER EJECTA]

PEAK RING IMPACT CRATER/STRUCTURE
Peak ring craters develop within the rim of larger complex craters. The ring structure forms as the central peak collapses and creates a peak ring before all motion stops (Melosh 1989).
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

PETROGRAPHY
Systematic description of rock texture and composition.

PGE
The platinum group elements (PGE) comprise platinum (Pt), palladium (Pd), iridium (Ir), osmium(Os), rhodium (Rh) and ruthenium (Ru).

PLANETESIMALS
Bodies ranging in size from meters up to hundreds of kilometers in diameter that formed during the process that formed the planets by accretion. Most planetesimals accreted to form the planets. A rocky and/or icy body, a few to several tens of kilometers in size, that was produced in the solar nebula.

PLANAR DEFORMATION FEATURES – PDF
Upon bolide impact, the passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.
[see – SHOCK METAMORPHISM– Planar Deformaion Features]

PLANAR FEATURES – PF
Cleavage. Fractures in minerals following crystallographic orientations. In quartz crystals, PFs (cleavage) is practically unknown and originates in rare instances from tectonicdeformation in very strong regional metamorphism. On the other hand, PFs in quartz are an indicator of shock metamorphism.

PRESSURE-TEMPERATURE CONDITIONS for SHOCK METAMORPHISM
[see – SHOCK METAMORPHISM]
[see – SHOCK METAMORPHISM] – PRESSURE-TEMPERATURE CONDITIONS]

PSEUDOTACHYLITE (friction melt)
Pseudotachylite is formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. It may contain unshocked and shocked mineral and lithic clasts in a fine-grained aphanatic [aphanatic = very fine-grained], crystalline texture matrix. (A tachylite is a black volcanic glass formed by the chilling of basaltic magmas.)

Sudbury pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of SB are found up to 100 km north of the SIC . The pseudotachylite here is injected into the pink gneiss country rock (the toe of my boot is for scale).

[see – SHOCK METAMORPHISM – pseudotachylite]

QUARTZ
Rock-forming silicate mineral SiO2. Quartz is a sensitive indicator of shock metamorphism.

RADIOMETRIC DATING
Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed
[see – DATING– radiometric]

RAREFACTION WAVE
Tensional wave originating from the reflection of a compressional wave at a free surface or at a boundary of material of reduced impedance (impedance is the product of density and sound velocity). In impact cratering, rarefaction (tensional) waves play an important role. They are formed when the compressional shock wavesare reflected back downwards from the free surface of the impacted target. Combined with the rock mass flow behind the shock front, they determine the excavation flow field. Because the tensile strength of rocks is always much lower than the compressive strength, rarefaction waves are in general more destructive than the compressional shock waves (see Grady-Kipp fragments). On a smaller scale, rarefaction may lead to characteristic spallation effects in shocked rocks.

REE
The Rare Earth Elements (REE) are the 15 lanthanide series elements, plus yttrium. Scandium is found in most rare earth element deposits and is sometimes classified as a rare earth element.

The Rare Earth Elements are the 15 lanthanide series elements, plus yttrium. Scandium is found in most rare earth element deposits and is sometimes classified as a rare earth element. Image by Geology.com.

Heavy and light rare earth elements: The rare earth elements are often subdivided into “Heavy Rare Earths” and “Light Rare Earths.” Lanthanum, cerium, praseodymium, neodymium, promethium, and samarium are the “light rare earths.” Yttrium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium are the “heavy rare earths.” Although yttrium is lighter than the light rare earth elements, it is included in the heavy rare earth group because of its chemical and physical associations with heavy rare earths in natural deposits.

REIDITE (METAMORPHIC ZIRCON), ZrSiO
A high-pressure polymorph of zirconium orthosilicate that forms >30 GPa and is an important accessory mineral in studies of shock metamorphism as its formation conditions have been experimentally constrained. Naturally occurring reidite is rare. Reidite has been found in crater impacts: the Chesapeake Bay Crater in Virginia, Rock Elm Crater in Wisconsin. Laboratory and theoretical results show how zircon transforms into reidite and then back into zircon.
Structure and stability of ZrSiO4 under hydrostatic pressure, Marqués, et al APS PHYSICS  2006
[see – SHOCK METAMORPHISM]

RINGWOODITE, Mg2SiO4
Formed at high temperatures and pressures of the Earth’s mantle between 525 and 660 km (326 and 410 mi) depth. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).

RUBINITE, Ca3Ti3+2Si3O12

Rubinite was identified as tiny crystals in calcium-aluminum-rich inclusions (CAIs), and is among the first solids formed in the solar nebula. Researchers say the mineral either condensed from solar nebula gas or it crystallized from an 16O-rich Ca, Al, and Ti-rich melt under highly-reduced conditions about 4.6 billion years ago. Post-crystallization oxygen-isotope exchange occured either while still in the solar nebula and/or on the meteorite parent asteroid.

Type material is deposited in the meteorite collection of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA, section VCM3 (Vigarano), and the Division of Earth and Planetary Materials Sc

SEDIMENTARY ROCK
Rock that has formed through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice (glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils.

SELENITE, CaSO4.H2O (hydrothermal)
The colorless and transparent variety of gypsum (calcium sulfate: CaSO4.H2O) that shows a pearl like luster and has been described as having a moon-like glow. The word selenite comes from the greek word for Moon and means moon rock. Gypsum is one of the more common minerals in formed sedimentary environments, such as tropical seas.
The heat source at the Haughton Impact Crater were the pale gray impact melt breccias which were originally at temperatures of >1000°C. As groundwater and rainwater came into contact with these hot rocks, these fluids were heated and circulated through the crater. Some of the target rocks at Haughton contained sedimentary gypsum, which was dissolved by these hot hydrothermal fluids. These fluids then migrated through the crater and re-deposited gypsum or selenite within cavities in the impact melt breccias as they cooled.

At Haughton, selenite was formed by hydrothermal activity associated with the impact event. The only hydrothermal systems active today are associated with volcanic regions (e.g., Yellowstone National Park), but it turns out that impact craters can also provide the two most important components of a hydrothermal system: heat and water.

SHATTER CONE
Formed at >8 GPa, shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism. The occurrence of shatter cones is the only accepted meso- to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. The apices of the cones tend to point towards the shock source. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometres in diameter. ( Baratoux, Reimold 2016)

 “Model for shatter cone surface modification: (a) offset shock front, generated due to host rock density variations, causes tearing in the out-of-sequence zone between leading and trailing fronts. The resulting fault transient evolves to a passive fracture as the trailing front passes through; (b) post-shock decompression leads to opening of the fracture” (Gibson, Spray 1998).
[see – SHOCK METAMORPHISM – shattercone]

SHOCK COMPRESSION – Contact and Compression Stage
Most of the structural and phase changes in minerals and rocks are uniquely characteristic of the high pressures (diagnostic shock effects are known for the range from 8 to >50 GPa) and extreme strain rates (up to 108 /s) (for comparison: a bat hitting a baseball generates a strain rate of ~102/s) associated with impact. The products of static compression, as well as those of volcanic or tectonic processes, differ from those of shock metamorphism, because of peak pressures and strain rates that are lower by many orders of magnitude.
[see –  CRATER FORMATION – Contact & Compression]

SHOCK FRONT
Front of a shock pulse from hypervelocity impact propagating into the target rocks (and into the impactor). In the shock front, rock ambient pressures and temperatures are rapidly raised to shock pressures and temperatures which may attain to several 100 Gigapascal (GPa) and several 10,000 Kelvin (K) near the impact point.

SHOCK METAMORPHISM:  PRESSURE-TEMPERATURE CONDITIONS

Conditions of endogenic metamorphism and shock metamorphism in the pressure-temperature fields. This comparison diagram exhibits the onset pressures of various irreversible structural changes in the rocks due to shock metamorphism and the relationship between pressure and post-shock temperature for shock metamorphism of granitic rocks (modified after Koeberl 1997). For the formation of total rock melts, shock pressures in excess of roughly 60 GPa (600 kbar) are required.

[see – SHOCK METAMORPHISM]

SHOCK PRESSURES

Shock pressures and their effects (after French, 1998: 33).

SHOCKED GNEISS
Gneiss (pronounced “nice”) is normally a dark dense rock, but at Haughton Impact Crater, the gneiss resembles pumice stone – it is ash-white, porous and very lightweight. In fact, some of these fragments float in water! The reason why this gneiss is so light is due to the air spaces or bubbles, which formed as the gneiss was compressed by the shock wave, and then released. Certain minerals in the rock were also vaporized, leaving behind a porous ghost of the gneiss it originally was.
[see – SHOCK METAMORPHISM]

SHOCKED TARGET ROCK
Is an informal term describing a rock created or modified by the impact of a meteorite. The term encompasses shock-metamorphosed target rocks, melts, breccias, suevites and mixtures, as well as sedimentary rocks with significant impact-derived components (shocked mineral grains, tektites, anomalous geochemical signatures, etc).
[see – SHOCK METAMORPHISM]

SHOCK WAVE
In a material, a shock wave is a deformation, a non-elastic wave that moves at a velocity exceeding the sound velocity of that material. The sound or seismic velocity is defined by the propagation of elastic waves. Shock waves are produced in hypervelocity impact and are the cause of shock metamorphism in rocks and minerals.

SIDEROPHILE ELEMENTS
Literally, “iron-loving” elements, such as iridium, osmium, platinum, and palladium, that, in chemically segregated asteroids and planets, are found in the metal-rich interiors. Consequently, these elements are extremely rare on Earth’s surface.

SIMPLE IMPACT CRATER/STRUCTURE
A bowl-shaped crater having undergone only slight modifications of its transient crater.
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

SLICKENSIDE
Slickenside is a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. Slickensides are naturally polished rock surfaces that occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved. Slickensides are characterized by a diagnostic unidirectional step-like pattern that actually allows investigation of the sense of movement on fractures (Passchier and Trouw 1996).

STISHOVITESiO2 (IMPACT)
Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide with a mass density of 4.287 g/cm3. Until recently, the only known occurrences of stishovite in nature formed at the very high shock pressures (>100 kbar or 10 GPa) and temperatures (> 1200 °C) present during hypervelocity meteorite impact into quartz-bearing rock. It is very rare on the Earth’s surface, however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle.
Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied at the Chesapeake Bay impact crater were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.
[see – SHOCK METAMORPHISM – PRESSURE-TEMPERATURE CONDITIONS]

SUEVITE (IMPACT)
Defined as an impact-derived, polymict breccia containing a mixture of shocked and unshocked, lithic and melt fragments and generally considered to possess clastic matrices [von Engelhardt, W. and Graup, G. 1984. Geologische Rundschau 73:2:447–481.].
[see – SHOCK METAMORPHISM – PRESSURE-TEMPERATURE CONDITIONS]

TAGAMITE
Russian term for IMPACT MELT ROCK.

TARGET ROCKS
Area and rocks exposed to the impacting projectile, sometimes called country rock.

TEKTITE
Natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as aerodynamically shaped droplets during terrestrial impact events. The process of tektite formation is disputed, but many researchers believe that they are formed in the early contact and compression stage of impact cratering. They range in color from black or dark brown to gray or green. Tektites have been found in  “strewn fields” on the Earth’s surface.
[see – CRATER EJECTA – tektite]

TEPHRA/PYROCLASTIC TERMINOLOGY

PARTICLE NAME PARTICLE SIZE
Blocks/Bombs >64 mm
Lapilli <64 mm
Volcanic Ash <2 mm
Volcanic Dust <0.063 mm

“Tephra” and “pyroclastics” are general terms used in reference to particles of igneous rock material of various sizes that have been ejected from volcanoes. They are classified by size. The terms “ash” and “dust” communicate a specific size of tephra or pyroclastic particles. These are summarized in the table above.

WADSLEYITE, β-Mg2SiO4 
A high-pressure phase of polymorphous Mg2SiO4. An orthorhombic mineral. It was first found in nature in the Peace River meteorite from Alberta, Canada.

ZIRCON
Zircon, zirconium orthosilicate (ZrSiO4), is found in most igneous rocks and some metamorphic rocks as small crystals or grains. Zircon transforms into reidite when the latter is subjected to very high pressures.
Reidite is a rare mineral,  a dense form (polymorph) of the fairly tough gemstone zircon.  Reidite has been found in crater impacts: the Chesapeake Bay Crater in Virginia, Rock Elm Crater in Wisconsin.
[see – SHOCK METAMORPHISM – reidite]


2. REFERENCE

Baratoux D., Reimold W.; The current state of knowledge about shatter cones – Meteoritics & Planetary Science, August 2016.

Gareth S. Collins, Gordon R. Osinski, Jay Melosh; The Impact-Cratering Process, Elements · February 2012

Greeley, R. 2011, Planetary Geomorphology, Cambridge.

Richard A. F. GRIEVE* and Ann M. THERRIAULT; Observations at terrestrial impact structures: Their utility in constraining crater formation, Meteoritics & Planetary Science 2003

Richard A. F. GRIEVE and Ann M. THERRIAULT; Observations at terrestrial impact structures: Their utility in constraining crater formation, Meteoritics & Planetary Science 39, Nr 2, 199–216 (2004)

Grieve, R.A.F. 2006, Impact Structures in Canada, Geological Association of Canada.

Morrison, D. 2007 The Impact Hazard: Advanced NEO Surveys and Societal Responses, Comet/Asteroid Impacts and Human Society 2007, pp 163-173

Wood, C.A. 2003, The Modern Moon, Sky Publishing