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.
Russian term for IMPACT MELT ROCK.
Area and rocks exposed to the impacting projectile, sometimes called country rock.
Natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as aerodynamically shaped droplets during terrestrial impact events. The process of tektite formation is disputed, but many researchers believe that they are formed in the early
contact and compression stage of impact cratering. They range in color from black or dark brown to gray or green. Tektites have been found in “strewn fields” on the Earth’s surface.
Tektites are small, glassy pebble-like objects that form during meteorite impact. They represent droplets of molten target rock that are ejected up into the Earth’s atmosphere, which then fall back to the surface up to several hundred kilometers from where their source impact crater. They often acquire aerodynamic shapes as they fly through the atmosphere.
The name tektite comes from the Greek word ‘tektos’, meaning ‘molten’. Tektites do not contain any water. They can be mistaken for obsidian or pitchstone (black volcanic glasses), but these will emit some water on strong heating. Their density is similar to, or a little lighter than, quartz beach sand.
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TEKTITE STREWN FIELDS
Tektites often occur in so-called strewn fields, areas over which tektites with similar chemical and physical properties are found.
Several Australasian tektites (from Thailand), showing the variety in shapes and forms. Tektites are distal impact ejecta, which formed by total melting of continental crustal target rocks (source crater still unknown, although a large crater in Western Cambodia, Lake Tonle Sap, has been proposed)..
Chicxulub Impact structure spherules (microtektites) are abundant components of the K-T boundary that encircles the Earth. They are less than 0.5mm in diameter and consist mostly of Ni-bearing magnesioferrite spinel crystals (at the Canadian Museum of Nature in Ottawa).
Darwin glass is a natural glass found south of Queenstown in West Coast, Tasmania. It takes its name from Mount Darwin in the West Coast Range, where it was first reported, and later gave its name to Darwin Crater, a probable impact crater, and the inferred source of the glass.
Ivory Coast (linked to the Bosumtwi crater in Ghana, West Africa)
The origin of Libyan Desert glass is uncertain. Meteoritic origins have long been considered possible, and recent research links the glass to impact features, such as zircon-breakdown, vaporized quartz and meteoritic metals, and to an impact crater. Some geologists associate the glass with radiative melting from meteoric large aerial bursts, making it analogous to trinitite created from sand exposed to the thermal radiation of a nuclear explosion. Libyan Desert glass has been dated as having formed about 26 million years ago.
Moldavite is an unusual type of tektite with a beautiful translucent green clarity. The moldavites are tektites derived from the Ries impact structure, German. Moldavite is a special term coming from German and means ‘Vltava River Stone’.
The age of the 85-kilometer-diameter Chesapeake Bay impact structure (35 million years old) and the composition of some of its breccia clasts are consistent with the structure being the source of the North American tektites.