E-F-G-H

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.

EJECTA

Solid, liquid and vaporized material ejected from an impact crater during its formation.

Impact ejecta on the rim of the Barringer Crater. The large rocks on the top of the rim were ejected upon the impact explosion from within the crater.

 

EJECTA (DISTAL)

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.

Distal ejecta, at Thunder Bay Ontario, from the Sudbury impact event. The Sudbury impact occurred 650–875 km to the east of this site at 1850 ± 1 Ma.

 

EJECTA (PROXIMAL)

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.

Proximal ejecta from the Whitecourt Impact Crater.

 

EJECTA BLANKET

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.

Cross sections of the Whitecourt Crater ejecta blanket along 038° and 110° with a reference figure showing the location of the sections. (Kofman et al – Meteoritics & Planetary Science 2010)

 

ELASTIC REBOUND

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.

Charlevoix impact structure Digital Elevation Model with Earthquake Epicentres.

 

EUCRITE

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.)

 

EXCAVATION STAGE

[see – CRATER CLASSIFICATIONS]

[see –  CRATER FORMATION]

 

EXTINCTION vs IMPACT

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.

Odale extinction.jpg

 

GPa

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.

 

HYPERVELOCITY

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.