GLOSSARY – C

GLOSSARY – C

by: Charles O’Dale

CAI
Calcium-aluminum-rich inclusions (CAIs) are found in chondritic meteorites. The very largest CAIs are up to 2–3 cm in size, type (CV3) meteorites, but most CAIs are < 1 mm in maximum size.

Three types of CAI:

  • A CAI – dominated by melilite, (Ca, Na)₂[SiO₇] (>75%) with spinel,  MgAl₂O₄ (5 to 20%) and minimal clinopyroxene;
  • B CAI – crystallized from partly molten droplets (less primitive than A CAI);
  • C CAI – coarse grained rich in anorthite, CaAl₂Si₂O₈ and contain little melilite.

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. The primary mineralogy of CAIs is remarkably similar to the phases predicted to condense out of a hot solar vapor during cooling

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

CAIs range in shape from irregular, highly porous aggregates of tiny crystals, to strings of crystals that stretch out across several mm of meteorite matrix with expanses of matrix intervening, to nearly spherical, densely crystalline objects. These diverse morphologies reflect diverse and complex histories, including deformation due to impact processes.

CALCIUM·RICH MINERALS in the inclusion in the Allende meteorite appear in this scanning electron micrograph. The fact that there are distinct well·formed crystals projecting into the cavity suggests that the minerals were formed by condensation from a vapor. The width of the field of view is about eight micrometers. (L.Grossman 1975 SCIENTIFIC AMERICAN)

The most precise ages for CAIs are Pb-Pb measurements from the Efremovka CV3 chondrite, at 4.5672±0.0006 Ga (Amelin, Y., Krot, A. N., Hutcheon, I. D., & Ulyanov, A. A. 2002, Science, 297,2).

[see – METEORITE]

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

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

CARBONACEOUS CHONDRITE
Carbonaceous chondrites are the most primitive meteorites in a chemical way  that contain water-bearing minerals and carbon compounds including a variety of organic molecules such as amino acids. For example, the CI group of carbonaceous chondrites are closest in composition to the photosphere (visible surface) of the Sun.

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

CENTRAL UPLIFT
Structurally uplifted central volume, which can be manifested as a central peak (commonly with an irregular circular shape in plain view) in complex impact craters of intermediate size formed by the dynamic collapse of the transient crater cavity.

CHONDRITE
An abundant class of stony meteorites with chemical compositions similar to that of the Sun and characterized by the presence of chondrules (see definition below). Chondrites come from asteroids that did not melt when formed and are designated as H, L, LL, E, or C depending on chemical compositions. The H, L, and LL types are called ordinary chondrites. The L chondrites are composed of silicate minerals (mostly olivine and pyroxene, but feldspar as well), metallic nickel-iron, and iron sulfide (called troilite). Most L chondrites are severely shocked-damaged, probably by a large impact on the asteroid in which they formed. The E type are called enstatite chondrites, a rare type that formed under very reducing conditions and are composed primarily of a magnesium silicate called enstatite. They are subdivided into the low-iron (EL) chemical group and the high-iron (HL) group.

CHONDRULE
Roughly spherical objects found in a type of meteorite called chondrites. Most chondrules are 0.5 to 2 millimeters in size and are composed of olivine and pyroxene, with smaller amounts of glass and iron-nickel metal. Two main chondrule types have been identified;

  • Type I contain only small amounts of oxidized iron (FeO); olivine crystals in them contain only about 2 mole percent of the iron-rich-olivine fayalite (Fe2SiO4) end member.
  • Type II chondrules contain much more FeO; olivine crystals in them typically contain 10-30 mole percent fayalite.

The shapes of the mineral grains in them indicate that chondrules were once molten droplets floating freely in space.

CHONDRULE (shocked)
Six stages of shock (S 1 to S6) are defined, based on shock effects in olivine and plagioclase as recognized by thin section microscopy. The characteristic shock effects of each shock stage are: S 1 (unshocked)-sharp optical extinction of olivine; S2 (very weakly shocked)-undulatory extinction of olivine; S3 (weakly shocked)-planar fractures in olivine; S4 (moderately shocked)-mosaicism in olivine; S5 (strongly shocked)-isotropization of plagioclase (maskelynite) and planar deformation features in olivine; and S6 (very strongly shocked)-recrystallization of olivine, sometimes combined with phase transformations (ringwoodite and/or phases produced by dissociation reactions). S6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked.

Olivine (a) with sharp extinction in the unshocked H3(Sl) chondrite Dhajala; (b) with weak undulatory extinction and irregular fractures in the very weakly shocked H4(S2) chondrite Avanhandava; and (c) with undulatory extinction, irregular fractures, and rare planar fractures (one set of three parallel fractures in grain marked by arrow) in the weakly shocked H4(S3) chondrite Farmville. Transmitted light, crossed polars. Widths of images: (a) and (b) 710 µm: (c) 650 µm.

Shock metamorphism of ordinary chondrites
D. Stoffler, K. Keil, E. Scott 1991

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.

Experimental results and phase boundary of the coesite-stishovite transition in SiO2. Solid squares and circles denote the stability conditions of coesite and stishovite respectively. The dashed line shows the phase boundary determined in this study. Open squares and circles denote coesite and stishovite reported by previous experimental study (Zhang et al., 1996).

[see – SHOCK METAMORPHISM – coesite]

{see – STISHOVITE]

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.