E-F-G-H

IMPACT CRATER/STRUCTURE GLOSSARY

by: Charles O’Dale

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

 

FRACTURED ROCK

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces.

 

GEOMORPHOLOGY

The scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth’s surface.

 

GEOLOGIC DATING

[see – DATING- GEOLOGIC]

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.

HYDROTHERMAL 

The origin and emergence of life under impact bombardment (Cockell 2006)

Craters formed by asteroids and comets offer a number of possibilities as sites for prebiotic chemistry, and they invite a literal application of Darwin’s ‘warm little pond’. Some of these attributes, such as prolonged circulation of heated water, are found in deep-ocean hydrothermal vent systems, previously proposed as sites for prebiotic chemistry. However, impact craters host important characteristics in a single location, which include the formation of diverse metal sulphides, clays and zeolites as secondary hydrothermal minerals (which can act as templates or catalysts for prebiotic syntheses), fracturing of rock during impact (creating a large surface area for reactions), the delivery of iron in the case of the impact of iron-containing meteorites (which might itself act as a substrate for prebiotic reactions), diverse impact energies resulting in different rates of hydrothermal cooling and thus organic syntheses, and the indiscriminate nature of impacts into every available lithology—generating large numbers of ‘experiments’ in the origin of life.

Darwin’s warm little pond—the impact crater as a prebiotic reactor. Some of the diversity of characteristics of impact structures that make them favourable sites for prebiotic reactions are shown.

Theories on the pathways of prebiotic evolution and the formation of the first complex self-replicating macromolecules have to take into account several common requirements, including: (i) a source of energy to drive molecular and macromolecular synthesis, (ii) a mechanism for the localized concentration of reactants to favour the required chemical reactions, (iii) suitable catalysis and (iv) a suitable geochemical environment for these reactions and their products to be sustained for sufficiently long periods to lead to the origin of life.

 

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.