GLOSSARY – A

GLOSSARY – A

by: Charles O’Dale

ACCRETIONARY LAPILLI
Pellets that form by the accretion of fine ash around condensing water droplets or solid particles, particularly in steam-rich volcanic eruptive columns. Similar processes are related with the turbulent explosion plume raising above the expanding excavation cavity in an impact cratering event. So, accretionary lapilli that commonly exhibit a concentric internal structure, have been found also in impact deposits.

ACOUSTIC FLUIDIZATION
Hypothesized (H.J. Melosh) fluidization of rock debris subjected to strong vibrations possibly enabling the collapse of the transient crater in themodification stage of impact cratering.

ALLOCHTHONOUS
Of rocks whose primary constituents have not been formed in situ (formed elsewhere and clearly moved to their current location).

Something unequivocally displaced (physically moved) from where it was to start with. Usually applies to things like nappes and thrust sheets that have been translated on major flat-lying faults during continental collisions and the like. However, could also apply at the scale of impact-generated movements too. Grieve may also be referring to thrust or nappe displacements in the bedrock that was impacted. but which pre-date the impact by a long time.

Material that is formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer to the fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.
[see – AUTOCHTHONOUS, PARAUTOCHTHONOUS]
[more data – IMPACT CRATER EJECTA]

ASTEROID
Any of the numerous small rocky bodies in orbit around the Sun. Most asteroids reside in the “main belt” between Mars and Jupiter, but some have Earth crossing orbits. The average impact velocities of asteroids hitting the earth is ~18Km/s (Kring)
[see – METEOR, METEORITE, METEOROID.]

AUTOCHTHONOUS 
Originating where found (formed in place).

[see – ALLOCHTHONOUS, PARAUTOCHTHONOUS]
[more data – IMPACT CRATER EJECTA]