C-D

IMPACT CRATER/STRUCTURE GLOSSARY

by: Charles O’Dale

The petrographic and geochemical study of actual rocks from the potential impact structure will bring final confirmation of the presence of an impact structure. In case of a structure that is not exposed on the surface, drill-core samples are essential. Good materials for the recognition of an impact origin are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or a contamination from the extraterrestrial projectile.

 

CAI

Calcium-aluminum-rich inclusions (CAIs) are found in chondritic meteorites. CAIs are primitive objects that formed in the solar nebula before the planets formed. CAIs are light-colored objects rich in refractory elements (that condense at a high temperature). Besides calcium and aluminum, this includes magnesium, titanium, and rare earth elements. CAIs range in size from about a millimeter to a centimeter. Meteoriticists have identified several distinct varieties of CAIs, but all share a high temperature origin. Some might be condensates from the solar nebula. Other CAIs might be evaporation residues. FCAI – rare type of CAI. F stands for fractionation, UN stands for unidentified nuclear isotope properties. FUN CAIs are characterized by 26Al/27Al ratios much lower than the canonical value of ~5×10-5(at the time of our Solar System’s formation); they also can have large isotopic anomalies in many elements.

Refractory inclusion: Inclusions, enriched in the rare earths and the other elements mentioned in the definition of refractory. These inclusions are often referred to as Ca-, Al-rich inclusions, or “CAIs.” Most refractory inclusions contain the minerals spinel and melilite and/or hibonite.

 

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.

The ~5 km diameter Gow structure with a central peak, is one of the smallest currently known complex impact structures on earth (Grieve 2006).

 

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.

Horseshoe Island, the central peak of the Mistastin impact crater.

 

 

CHICXULUB IMPACT STRUCTURE

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.

Chicxulub crater image of its gravitational field (NASA) .

 

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.

Impact breccia from the Ile Rouleau structure illustrating country rock clasts (fragments of geological loose material).

 

COESITE (IMPACTITE )

High-pressure polymorph of quartz (SiO2). Found in impact rocks and in rocks subjected to extreme regional metamorphism.

The presence of coesite in unmetamorphosed rocks may be evidence of a meteorite impact event or of an atomic bomb explosion. In metamorphic rocks, coesite commonly is one of the best mineral indicators of metamorphism at very high pressures.
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.

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.

[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.

Comet Hyakutake, taken by Peter Ceravolo March 17, 1996 with film. Later processed by Debra Ceravolo.

 

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.

This false-color image shows a green ring depression that surrounds a central peak. The ring depression contains the Manicouagan Reservoir. A fracture halo, which extends out to -150 km from the center, was first noted on Skylab photography. This halo is best developed in the west and south.

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]

 

CRATER CLASSIFICATIONS

(A) HiRISE image of an unnamed simple crater on Mars (38.7° N, 316.1° E) displaying an elevated crater rim and steeply dipping upper cavity walls. The mid and lower parts of the wall are covered by talus deposits. (B) Kaguya/SELENE image (S0000001616_1906) of the complex impact crater Aristarchus on the Moon, showing a central peak, a fl at crater fl oor with isolated hummocks and an extensive slump-terrace zone. Note the different scale bars in the two images. (Collins et al 2012)

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.
Reflection seismic cross-section of Chicxulub along Chicx-A and -A1 (Bell et al. Forthcoming). The post-impact Tertiary sediments are clearly identifiable as high-frequency reflections from 0 to ~1 sec two-way travel time (TWTT). A topographic peak ring, with draped sediments, is identifiable on the floor of Chicxulub and separates the central basin from a surrounding annular trough. (GRIEVE et al 2003)
  • 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.

The modification stage of impact cratering is mostly over “by the time the dust settles”.
Crater modification by gravity – A mechanical analysis of slumping

[see – CRATER CLASSIFICATIONS]

[see –  CRATER FORMATION]

 

CRATER SIZE

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]

[see –  CRATER FORMATION]

 

CRATER – size of METEOROID

The Earth is immersed in a swarm of Near Earth Asteroids (NEAs) capable of colliding with our planet, a fact that has become widely recognized within the past decade. The first comprehensive modern analysis of the impact hazard resulted from a NASA study requested by the United States Congress. This Spaceguard Survey Report (Morrison 1992) provided 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]

[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.

The form of collapse is governed almost completely by the dimensionless parameter (pgh)/c, when <5.5 = stable, 5.5.to 10 = slope failure, ~>20 = failure (where p is the density of the excavated rock, g is the acceleration of gravity,  c is the yield strength of the substance), [from Melosh 1977 – The Role of Slumping in Crater Modification, Melosh, H. J. 1977]
[see – CRATER CLASSIFICATIONS]

[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.

The Sudbury Structure sandwiched between the Superior Geologic or Structural Province and the Southern Geologic or Structural Province Huronian Supergroup cratons, deformed by the Penokean orogeny.

 

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.

Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K–T) boundary – at the Royal Tyrrell Museum Drumheller Alberta.

K/T Boundary: concentrations of the rare platinum group elements (PGEs; Ru, Rh, Pb, Os, Ir, and Pt) and other siderophile elements (e.g., Co, Ni) are enriched by up to 4 orders of magnitude in the thin clay layer marking the K-T boundary compared to those of normal terrestrial crustal rocks. 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 Chicxulub impact structure.

 

DATING – RADIOMETRIC

The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:

Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values
Uranium-238 Lead-206 4.5 billion years
Hafnium-182 Tungsten-182 9 Million years
Uranium-235 Lead-207 704 million years
Thorium-232 Lead-208 14.0 billion years
Rubidium-87 Strontium-87 48.8 billion years
Potassium-40 Argon-40 1.25 billion years
Samarium-147 Neodymium-143 106 billion years

Impact melts and glasses (or minerals that have recrystallized from the melt; e.g., Krogh et al., 1993; Izett et al., 1994) have another important use, as they often are the most suitable material for the dating of an impact structure. The methods most commonly used for dating of impact melt rocks or glasses include the K-Ar, 40Ar-39Ar, fission track, Rb-Sr, Sm-Nd, or U- Th-Pb isotope methods. Isochron dating is useful in the determination of the age of igneous rocks, which have their initial origin in the cooling of liquid magma from volcanos. It is also useful to determine the time of metamorphism, shock events and other events depending of the behaviour of the particular isotopic systems under such events.

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.