Impact melts

Abstract

Almost forty terrestrial structures are known in which igneous rocks or glasses are associated with rocks showing shock deformation. In Quaternary craters, glasses containing Ni‐Fe particles are undoubtedly impact melted, At older, larger craters, igneous materials occur as (1) fresh, recrystallized or altered glass in mixed breccias; (2) subhorizontal layers tens to hundreds of meters thick, depending on crater size; and (3) dikelike intrusions into basement rocks beneath the crater floor. These igneous rocks are distinguished from normal volcanic rocks by their heterogeneity, abundant inclusions of shocked country rocks, and lack of phenocrysts. In general they agree closely in composition with adjacent country rocks, but they commonly are relatively enriched in K and Mg and depleted in Si and Na. These chemical differences are attributed to reaction with vapors and solutions under conditions of near‐surface crystallization with access to atmospheric oxygen. In some melts Ni and Fe are enriched, suggesting meteorite contamination. No contributions from deep magmatic sources are required to explain the chemistry of the melts. The theory of cratering by hypervelocity impact as applied to natural terrestrial events satisfactorily accounts for the form and distribution of the igneous rocks. The large volumes of impact melt in terrestrial craters >20 km across suggests (1) that the strength of target materials must be considered in extrapolating cratering theory to impacts of such dimensions; (2) the floors of large lunar craters. For example, Tycho, if of impact origin, should be underlain by several hundred meters of impact melt.

Lobate and flow-like features on asteroid Vesta

“…These features are common on Mercury, Venus, the Moon, and Mars (Dence, 1971;Gault, 1974;Howard and Wilshire, 1975;Carr et al, 1977;Melosh, 1989;Barlow, 2005). Based on previous spacecraft observations of the smaller asteroids, e.g., 951 Gaspra and 243 Ida by the Galileo spacecraft (Carr et al, 1994;Sullivan et al, 2002); 433 Eros by the NEAR spacecraft (Chapman et al, 2002;Sullivan et al, 2002); and 21 Lutetia by the Rosetta spacecraft (Massironi et al, 2012), impact craters and their ejecta deposits were expected to be commonplace on Vesta, with impact craters at all size scales and degrees of degradation, including rayed craters.…”

“…Lunar impact melt flows (Fig. 1) typically contain a variety of blocks transported by the partially-molten flow (Dence, 1971;Howard and Wilshire, 1975;Hawke and Head, 1977;Bray et al, 2010).…”

The 2002 rock/ice avalanche at Kolka/Karmadon, Russian Caucasus: assessment of extraordinary avalanche formation and mobility, and application of QuickBird satellite imagery

“…Such impacts are furthermore often accompanied by fluidization effects where the target material begins to flow even if it is rock or debris. Existence of water under the surface of the target greatly eases fluidization (Dence, 1971). A mechanism proposed for flow initiation is thereby acoustic fluidization of debris in relation with an acoustic field produced by shock waves (Melosh, 1987).…”

Chemical composition and origin of the shock metamorphic rocks of the Sääksjärvi area, Finland

“…Since no potassium-rich and calcium-poor rocks similar to the breccias have been found during the geological bedrock mapping of the area, it would be hard to assume that a petrological anomaly such as they are, could be situated as the target rock at the impact of a meteorite. However, it has been assumed of Canadian Craters (Dence 1971), that some igneous rocks, which were formed by melting at the meteorite impact, are relatively enriched in K and Mg and depleted in Si and Na; also Ni and Fe may be enriched by meteoritic contamination. Dence (1971) explains the differences between impact melt and country rock as a result of selective melting of the target rock.…”

“…However, it has been assumed of Canadian Craters (Dence 1971), that some igneous rocks, which were formed by melting at the meteorite impact, are relatively enriched in K and Mg and depleted in Si and Na; also Ni and Fe may be enriched by meteoritic contamination. Dence (1971) explains the differences between impact melt and country rock as a result of selective melting of the target rock.…”