GLOSSARY – I

GLOSSARY – I

by: Charles O’Dale

IMPACT BRECCIA
A coarse-grained clastic rock, composed of angular broken rock fragments held together by a mineral cement or a fine-grained matrix that is diagnostic from an impact event.

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 BRECCIA
A coarse-grained clastic rock, composed of angular broken rock fragments held together by a mineral cement or a fine-grained matrix of crystallized impact melt that is diagnostic from an impact event.

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 RATE (on earth)

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.

Classification of impactites from single impacts based on geological setting, texture, and degree of shock metamorphism as manifested by impact craters on Earth.

[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.

IRON
A heavy magnetic malleable and ductile chemically active mineral, the native metallic element Fe. Native iron is rare in terrestrial rocks but common in meteorites.

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]