IMPACT CRATER/STRUCTURE GLOSSARY
by: Charles O’Dale
Solid, liquid and vaporized material ejected from an impact crater during its formation.
Impact ejecta found at distances greater than 5 crater radii from the rim of the source crater, as opposed to proximal ejecta, which are found closer than 5 crater radii from the crater rim, and which make up about 90% of all material thrown out of the crater during the impact event.
All ejecta that are found up to 5 crater radii from the rim of the impact crater; 90% of all ejecta are found within this region. Note that the limit of proximal ejecta scales with the crater size. Ejecta found at greater distances are called distal ejecta.
An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge.
Part of a theory explaining the forces that cause earthquakes. In impact cratering , elastic rebound describes the readjustment of the highly compressed floor of the transient cavity in the modification stage.
A common class of achondrite meteorites composed of pigeonite and plagioclase. These meteorites formed as basaltic flows on a parent body, probably asteroid 4 Vesta. (Data collected by NASA’s Dawn Mission, in orbit around Vesta from 2011-2012, strengthed the association between Vesta and eucrite meteorites.)
[see – CRATER CLASSIFICATIONS]
[see – CRATER FORMATION]
Extinction of many groups of organisms at a particular time by environmental catastrophe related with collapsing ecosystems. There are strong indications that some mass extinctions may be caused partly or completely by large asteroidal or cometary impacts.
[see – DATING- GEOLOGIC]
Gigapascal, 1 GPa = 1,000 MPa (Megapascal) = 109 Pascal, the SI unit of pressure. GPa is commonly used in the high-pressure range of shock deformation, 1 GPa = 10 kbar.
GRAVITY ANOMALY Gravimetry;
Geophysical method to measure variations of the gravity field related with subsurface density variations. Impact structures commonly show pronounced gravity negative anomalies due to the occurrence of low-density breccias, rock fracturing, and replacement of ejected material by post-impact young sediments. In very large impact structures, relative positive anomalies may be produced by the uplift (see; central uplift) of high-density material from the Earth’s lower crust and upper mantle.
A velocity approximately over 3,000 meters per second (6,700 mph, 11,000 km/h, 10,000 ft/s, or Mach 8.8). In particular, hypervelocity is velocity so high that the strength of materials upon impact is very small compared to inertial stresses.