Impact melts


  • Michael R. Dence


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.