I-J-K-L

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.

 

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.

Whitecourt Impact Crater –This image is derived by Light Detection And Ranging (LiDAR) technology. (Department of Earth and Atmospheric Sciences, University of Alberta)

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.]

[see – DATING – IMPACT CRATER]

IMPACT CRATER CHAIN

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

Kakiattukallak Lake + crater chain(?), Quebec, Canada. Note the circled lake on the bottom of the oval depicting the possible crater chain. courtesy LandSat

 

IMPACT KINETIC ENERGY

The kinetic energy, KE, of a massive falling meteorite is given by the following equation: KE = (½)MV2

 

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:  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.

Ground zero of the Carswell impact structure is imaged under the wing of GOZooM. The point of the meteorite impact is illustrated by the small superimposed circle. There is no surface expression of the original 39 Km diameter impact crater, hence the lable “structure”.

[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.

Typical extraterrestrial impactor entering Earth’s atmosphere. Image from Igor Zh/Shutterstock.

[see – CRATER – size of METEOROID]

[see – METEORITE]

 

ISOCHRON – DATING

[see – DATING – ISOCHRON]

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

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

[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.