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.

Within the moderately eroded Manicouagan structure a sheet of clast‐laden impact melt 230 m thick and 55 km in diameter forms an annular plateau surrounding an uplift of shocked anorthosite. The melt sheet is divided into three vertically gradational units based on decreasing clast abundance and coarsening of the melt above the base. A very fine‐grained lower unit, rich in millimeter‐ and centimeter‐sized inclusions, thickens radially outward but is overlapped and replaced toward the center by a coarser middle unit containing fewer, larger inclusions. The upper unit is medium grained, virtually clast free, and texturally the most homogeneous of the three melt units. Within the lower and middle units a variety of textures are present. Textural development is a function of the cooling rate determined by stratigraphic position and the degree of supercooling determined by initial clast content. The mineralogy of the melt rocks is similar in all units and consists of zoned plagioclase, sanidine, Ca‐poor pyroxene, augite, quartz (rare in the lower unit), magnetite‐ilmenite intergrowths, smectite, and apatite, Pseudomorphs after olivine, and pigeonite in various stages of inversion to hypersthene, are widespread only in the upper unit, while minor biotite and hornblende are confined to the lower and middle units. Replacement of olivine and much of the Ca‐poor pyroxene by smectite, and alteration of iron oxides occurred during late stage crystallization and subsolidus cooling. The melt rocks as a group are chemically homogeneous, with a bulk composition similar to that of latite. No statistically significant regional chemical variations were found as a function of vertical, lateral, or radial position in the melt sheet. A local mafic variant represented by two samples with poikilitic texture indicates that the melt is not completely chemically homogeneous. The poikilitic rocks texturally resemble some Apollo 17 impact melt rocks and are inferred to have had a similar origin and thermal history.

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The characteristics investigated to date of ten circular structures in the Canadian Shield have supported their origin by meteoritic impact.Seven, ranging in diameter from 3 to 20 km, have a simple crater form modified by erosion. The underlying rocks have properties where known, which include:

Here, we report a previously unrecognized impactite from the Steen River impact structure in Alberta, Canada, which was intersected by continuous diamond drill core into the allochthonous proximal deposits of this buried 25-km-diameter complex crater. A suite of high-temperature minerals defines the matrix, formed by grain growth in a solid state by static recrystallization of an originally clastic matrix, deposited at temperatures ≥800 °C. This rock type is predominantly a result of the recrystallization of target material driven by the acceleration of hot gasses from volatilized sedimentary cover mixed with variably shocked crystalline basement. Approximately one-third of terrestrial impact structures occur in mixed target rocks; therefore, this type of impactite may be more common than previously realized. Contact metamorphism between entrained sedimentary target rocks and the juxtaposed hot matrix resulted in carbonate decomposition to form a rare spinel-group mineral, magnesioferrite. In crater environments, magnesioferrite has been found in the distal Chicxulub (Mexico) ejecta and may prove a novel indicator mineral for impact into carbonate-bearing target rocks.