GLOSSARY – I
by: Charles O’Dale
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 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]
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)
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.
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 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.
[see – SHOCK METAMORPHISM – Impactite]
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.
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.
[see – IRON METEORITE]
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]