IMPACT CRATER/STRUCTURE GLOSSARY
by: Charles O’Dale
ALLOCHTHONOUS
Of rocks whose primary constituents have not been formed in situ (formed elsewhere and clearly moved to their current location). Allochthonous impactites can be further subdivided into those within and around the final crater (proximal) and those some distance from the final crater (distal). The latter are always ejecta, including air – fall deposits.
Allochthonous – something unequivocally displaced (physically moved) from where it was to start with . Usually applies to things like nappes and thrust sheets that have been translated on major flat-lying faults during continental collisions and the like. However, could also apply at the scale of impact-generated movements too. Grieve may also be referring to thrust or nappe displacements in the bedrock that was impacted. but which pre-date the impact by a long time.
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. In the case of the Beaverhead Impact structure, the crater remnants are found in a place other than where it and its constituents were formed.
[see – PARAUTOCHTHONOUS, AUTOCHTHONOUS]
ASTEROID
Cosmic body in the asteroid belt between the orbits of Mars and Jupiter. The largest asteroid, Ceres, has a diameter of roughly 1,000 km. Asteroids from Earth-crossing orbits are potential impactors.
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 the Earth’s orbit and could strike its surface.
[see – METEOR, METEORITE, METEOROID.]
ASTROBLEME
“star wound”; crater formed by meteorite impact.
[see – CRATER CLASSIFICATION.]
[see – CRATER FORMATION]
AUTOCHTHONOUS
Originating where found (formed in place).
[see – ALLOCHTHONOUS, PARAUTOCHTHONOUS.]
BARRINGER (aka CANYON DIABLO) CRATER
Called also meteor crater, Arizona crater, Coon Butte, Canyon Diablo crater, Crater Mound.
1,200 m-diameter and 175 m-depth impact crater within the Colorado Plateau in Arizona, USA. Archetype of a meteorite crater. At the end of the 19th century, Meteor crater marks the beginning of the bitter controversy about the cosmic or endogenetic origin of terrestrial craters and ring structures.
BOLIDE
“Fire-ball”; meteor exploding in passing through the Earth’s atmosphere.
BRECCIA
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). Depending on the origin of the clasts, monomictic (monogenetic, monolithologic) and polymictic (polygenetic, polylithologic) breccias may be distinguished.
The word is a loan from Italian indicating both loose gravel and stone made by cemented gravel. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, tectonic breccia, igneous breccia, impact breccia and hydrothermal breccia.
BRECCIA (BUNTE)
(= multicolored breccia; local name) Impact ejecta deposit of the Ries impact structure (Germany). See Image.
At the Ries Crater in the image, there are two varieties of impact metamorphism. A grey well-consolidated suevite overlying a coarser bunte breccia. The suevite contains numerous pieces of black deformed glass and altered basement rocks held in a fine matrix of altered glass. Bunte breccia is largely a turbulent mixture of deformed sedimentary rocks. The suevite is formed from the basement below 700 m, and the bunte breccia from the overlying sedimentary rocks. (Dence 2005; “Half a Century of Searching for Impact Craters in Canada” – RASC presentation.)
BRECCIA DIKE
A dike in the common geological sense is a mostly tabular body of different materials (minerals, rocks, ores) cross-cutting the host rock. In impact structures, breccia dikes have played an important role in the understanding of the impact cratering process. It is generally suggested that for the most part the breccia dikes are formed in the excavation stage by injection of brecciated material into the walls and the floor of the expanding excavation cavity. Later formations of breccia dikes in the modification stage incorporating earlier formed ones may lead to generations of breccia dikes.
BRECCIA (IMPACT)
The rocks at an impact target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock, impact breccia in bodies tens to hundreds of meters in size, is the result.
Lithologies showing these unique diagnostic shock effects, formed at pressures ≥10 GPa, tend to be restricted to two locations:
- crater-fill materials (suevites, melt breccias, and fragmental impact breccias) deposited in the crater; and
- brecciated basement rocks, often containing shatter cones, near the center of the structure.
[see – MONOMICT, POLYMICT, SUEVITE.]
BRECCIA (MONOMICT)
Refers to the composition of clasts. Monomict breccia will contain clasts of identical lithology and origin. If the dislocation metamorphism is impact-related, the produced cataclasite may be termed a monomict impact breccia.
[see – POLYMICT BRECCIA.]
BRECCIA (POLYMICT)
Breccia composed of mixed clasts of different lithology and origin.
[see – MONOMICT BRECCIA]
BRECCIA (SUEVITE)
A polymict impact breccia composed of fragments more or less shocked, and melt clasts in a clastic matrix. 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.
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.
CENTRAL UPLIFT
Structural elevation (central peak) in complex impact structures. Originates from elastic rebound and transient-crater collapse in the modification stage of impact cratering.
Large buried impact structure in the Yucatan (Mexico) region that formed at the Cretaceous-Tertiary (K/T) boundary 65 Ma ago. The Chicxulub impact is generally assumed to be the main cause of the K/T mass extinction.
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.
COESITE (IMPACTITE )
High-pressure polymorph of quartz (SiO2). Found in impact rocks and in rocks subjected to extreme regional metamorphism.
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.
Coesite from the Wanapitei Impact structure, Dence 1974.
[see – SHOCK METAMORPHISM: PRESSURE-TEMPERATURE CONDITIONS.]
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.
COMPLEX IMPACT STRUCTURE/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 ~5 km diameter Gow structure with a central peak, is one of the smallest currently known complex impact structures on earth (Grieve 2006).
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
CONTACT AND COMPRESSION STAGE
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
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.
- multi ring: It is not known if there are examples of true multi-ring basins, equivalent to those observed on the moon, on Earth (Grieve 2006). The Sudbury Impact Structure may be a multi-ring impact crater.
While a single interior ring is required to define a basin, basins have been subdivided, with increasing diameter on other planetary bodies, into;
- central-peak basins, with both a peak and ring;
- peak ring basins, with only a ring; and multi-ring basins, with two or more interior rings (Wood and Head 1976).
[see – CRATER FORMATION]
CRATER FORMATION (three stages)
1. CONTACT AND COMPRESSION STAGE
First stage in the impact cratering process. On contact of the projectile (impactor) with the target, both become highly compressed leading to shock waves that move into both the target rocks and the projectile. The extreme temperatures in the shock fronts from the kinetic energy release are enough to completely vaporize the projectile and a comparable volume of the target rocks. The kinetic energy of an object of mass m traveling at a speed v is = mv2/2, provided v is much less than the speed of light.
2. 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.
3. MODIFICATION STAGE (ACOUSTIC FLUIDIZATION)
Third stage of impact cratering assigned to the modification of the transient crater after excavation and ejection. In this stage, small craters undergo only slight modifications. Collapse of large transient craters by elastic rebound and slumping of the crater walls leads to the formation of complex impact structures with central uplifts, inner rings and terraced walls.
Hypothesized (H.J. Melosh); fluidization of rock debris subjected to strong vibrations possibly enable the collapse of the transient crater in the modification stage of impact cratering.
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
CRATER SIZE
[see – CRATER FORMATION]
CRATER – size of METEOROID
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
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]
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-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.
Solid, liquid and vaporized material ejected from an impact crater during its formation.
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.
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.
An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge.
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.
EXCAVATION STAGE
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
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.
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.
GRAVITY ANOMALY Gravimetry;
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.
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.
IGNEOUS ROCKS
Group of rocks that have crystallized from a magma; e.g., granite, basalt. Also see sedimentary rocks and metamorphic rocks.
IMPACT CRATER
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.
Volcanic craters result from explosion or internal collapse.
impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain.
Impact cratering is one of the most common geological processes that have happened on planetary objects with solid surfaces (our home planet Earth included) and is unlike any other known natural geological process. Impact involves the transfer of massive amounts of energy to a relatively small area of the Earth’s surface, in an extremely short period of time (Kinetic energy).
The kinetic energy of an object of mass m traveling at a speed v is mv2/2, provided v is much less than the speed of light. The pressures and temperatures in the shock wave after impact are well above the magnitudes of pressures and temperatures occurring naturally on this planet.
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
[see – SHOCK METAMORPHISM: PRESSURE-TEMPERATURE CONDITIONS.]
A line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena (plural catenae).
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: PRESSURE-TEMPERATURE CONDITIONS.]
IMPACT STRUCTURE
Is 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 CLASSIFICATIONS]
[see – CRATER FORMATION]
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.
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 = mv2/2, provided v is much less than the speed of light.
[see – CRATER – size of METEOROID]
[see – METEORITE]
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.
KUIPER BELT
[see – OORT CLOUD.]
K/T boundary
[see – CRETACEOUS-TERTIARY/CRETACEOUS-PALEOGENE (K–Pg) BOUNDARY.]
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.
Geophysical method to measure variations of the Earth’s magnetic field related with rocks of different magnetic properties. Magnetic anomalies in and around impact structures may result from displacement of magnetized rocks in the impact cratering process, decomposition of existent rock magnetization (by shock, for example), and formation of new magnetic phases in rocks (e.g., by chemical alterations, by acquiring a thermal remnant magnetization). Magnetic geo-signatures are instrumental in identifying impact structures ie: Carswell.
MELT ROCK (IMPACT )
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. However, sometimes, so much melt is produced that it forms a pool in the central parts of an impact crater to form crater-fill deposits. This pool of melt then cools slowly over time and solidifies to form a new rock, which we term impact melt rock. An impact melt rock contains only a few fragments of target rock, maybe up to ~25% fragments in extreme cases. If the melt contains a lot of fragments of target rock, then we term this an “impact melt breccia”. Impact melt rocks can be found in crater-fill deposits and in ejecta deposits.
Shock pressures in excess of roughly 60 GPa (600 kbar) are required for total rock melts. 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.
Melt rocks in impact structures may also result from frictional melting in strong dynamic metamorphism
[see – PSEUDOTACHYLITE]
[see -BRECCIA SUEVITE]
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.
[see – IMPACTOR, METEORITE, METEOROID.]
METEOR CRATER
[see – BARRINGER CRATER.]
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 = mv2/2, provided v is much less than the speed of light.
[see – IMPACT CRATER, IMPACTOR.]
[see – METEOR, METEOROID.]
METEORITE CRATER
[see – IMPACT CRATER.]
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.]
MODIFICATION STAGE
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
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.
- LSM: Lake St Martin;
- M: Manicouagan;
- RWC: Red Wing Creek.
Multiple impacts are observed also on the Moon, Mars, Venus and on Jupiter’s satellites Ganymede and Callisto.
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
MULTI-RING IMPACT CRATER/STRUCTURE
The largest craters contain multiple concentric topographic rings, and are called multi-ringed basins, for example the lunar Orientale.
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
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).
[see – KUIPER BELT]
OVERTURNED STRATA (“overturned flap”)
Inverse stratigraphy at an impact crater rim related with the excavation process.
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
PDFs
[see – PLANAR DEFORMATION 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 – CRATON, AUTOCHTHONOUS, ALLOCHTHONOUS, PARAUTOCHTHONOUS.]
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 CLASSIFICATIONS]
[see- CRATER FORMATION]
PLANAR DEFORMATION FEATURES
Upon bolide impact, the passage of the resultant shock wave through the rock changes the structure of some of the enclosed minerals.
PRESSURE-TEMPERATURE CONDITIONS for SHOCK METAMORPHISM
[see – SHOCK METAMORPHISM]
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.)
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 (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 Haughton 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.
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)
SHOCK METAMORPHISM: PRESSURE-TEMPERATURE CONDITIONS
Unique deformation effects occurred as changes in minerals such as mineral deformations and melting under the extreme high pressure and temperature (e.g., the shock pressure may be than 60 GPa and post-shock temperature may be 2000◦C). Quartz begins to convert to coesite (another polymorph or atomic-structural form of silica) at about 20 kilobars (that pressure is reached in the solid Earth at about a depth of 70 km (43 miles). Solids begin to convert to glass at 400 kb, to melt at ~500 kb, and to vapor (gas) at a megabar. Thus, the range of shock metamorphism is from 0.02 to 1 megabar – these pressures are known to occur in the Earth only in its mantle and core but the rock types affected by shock metamorphism are dominantly those of the crust where the natural pressure gradient achieves values less than those affecting shocked rocks.
SHOCKED GNEISS
Gneiss (pronounced “nice”) is normally a dark dense rock, but at Haughton, 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: PRESSURE-TEMPERATURE CONDITIONS.]
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).
TOP: The shocked Coconino Sandstone is weakly shocked sandstone (<10 GPa) that lacks remnant porosity and contains abundant grain comminution and fracturing. Note the “rock flour” on the shocked sample.
BOTTOM: The unshocked Coconino Sandstone consists of a fine to medium-grained, moderately well-sorted, rounded quartz arenite with ~ 20 vol% porosity. Coconino sandstone layers are typically buff to white in color. It consists primarily of sand deposited by eolian processes (wind-deposited) approximately 260 million years ago.
[see – SHOCK METAMORPHISM: PRESSURE-TEMPERATURE CONDITIONS.]
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 CLASSIFICATIONS]
[see – CRATER FORMATION]
STISHOVITE (IMPACT)
Stishovite was first found in nature and named after Sergey M. Stishov, a renowned Russian high-pressure physicist who first synthesized this mineral. It is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide (SiO2). At normal temperature and pressure, stishovite is metastable, as it will eventually decay to quartz; however, this phase change is slow enough that it has never been observed.
Recently (2007), minute amounts of stishovite has been found within diamonds, and post-stishovite phases were identified within ultra-high pressure mantle rocks.
With a mass density of 4.287 g/cm3, stishovite is the heaviest polymorph of silica.
[see – SHOCK METAMORPHISM: PRESSURE-TEMPERATURE CONDITIONS.]
SUEVITE (IMPACT)
Defined as a polymict breccia with a particulate matrix, containing lithic and mineral clasts in all stages of shock metamorphism, including microscopic impact melt particles.
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.
The name tektite comes from the Greek word ‘tektos’, meaning ‘molten’. Tektites do not contain any water. They can be mistaken for obsidian or pitchstone (black volcanic glasses), but these will emit some water on strong heating. Their density is similar to, or a little lighter than, quartz beach sand.
TEKTITE STREWN FIELDS
References
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