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The Ramgarh structure (rim-to-rim diameter ~2.4 km) in the Vindhyan Supergroup of sedimentary rocks (including sandstone, shale and minor limestone) of the Mesoproterozoic age in the west-central India, is India’s third confirmed asteroid impact crater. This eroded structure is roughly rectangular in shape and resembles to the Barringer Crater, USA. The presence of central peak and its current crater diameter/depth ratio of ~12 well corroborate the range (10–20) of terrestrial complex asteroid impact craters. The mm-sized, iron-rich (FeO ~50 wt.% in average), spherule-like particles, recovered from the alluvium inside the Ramgarh structure, have internal morphology similar to those of the accretionary lapilli described in known impact craters. The in-situ LA-ICP-MS analyses also suggested high Co–Ni (up to 13,000 and 2500 ppm, respectively)-rich areas locally within these spherules/lapilli. A few non-in-situ, mm-sized particles, recovered from the rim of the structure show the presence of coesite, one of the diagnostic indicators of shock metamorphism. A few fragments of iron-rich, Ca–Al–silicate glasses recovered from the soil inside the structure and outside of the western crater rim include the presence of dendritic magnetite with occasional inclusions of relict native iron. Our microprobe analyses confirm that these metallic irons contain high proportions of Co (~350–3000 ppm), Ni (~200–4000 ppm) and Cu (~2200–7000 ppm) and possibly could be the relict component of a Cu-rich iron meteorite impactor. The field observation and relative enrichment of compatible and incompatible trace elements in the spherule-like substance (recovered from the alluvium inside the Ramgarh structure) as compared to target rocks suggests that hydrothermal activity played an important role in the evolution of the crater.

The Lonar impact crater, India, is one of the few known terrestrial impact craters excavated in continental basaltic target rocks (Deccan Traps, ~65 Ma). The impactites reported from the crater to date mainly include centimeter- to decimeter-sized impact-melt bombs, and aerodynamically shaped millimeter- and submillimeter-sized impact spherules. They occur in situ within the ejecta around the crater rim and show schlieren structure. In contrast, non–in situ glassy objects, loosely strewn around the crater lake and in the ejecta around the crater rim do not show any schlieren structure. These non–in situ fragments appear to be similar to ancient bricks from the Daityasudan temple in the Lonar village. Synthesis of existing and new major and trace element data on the Lonar impact spherules show that (1) the target Lonar basalts incorporated into the spherules had undergone minimal preimpact alteration. Also, the paleosol layer as preserved between the top-most target basalt flow and the ejecta blanket, even after the impact, was not a source component for the Lonar impactites, (2) the Archean basement below the Deccan traps were unlikely to have contributed material to the impactite parental melts, and (3) the impactor asteroid components (Cr, Co, Ni) were concentrated only within the submillimeter-sized spherules. Two component mixing calculations using major oxides and Cr, Co, and Ni suggest that the Lonar impactor was a EH-type chondrite with the submillimeter-sized spherules containing ~6 wt% impactor components.

The Ramgarh structure of SE Rajasthan, central India, situated within an almost undeformed, flat-lying Vindhyan Supergroup of sedimentary rocks of Meso- to Neoproterozoic age, is a potential candidate of asteroid impact crater for last many decades. A fresh observation on remote sensing images (ASTER, Landsat and Google Earth Imageries) along with structural analyses in field show that this rectangular structure has a rim-to rim diameter of ~

The ~50 or 570 ka old Lonar crater, India, was excavated in the Deccan Trap flood basalt of Cretaceous age by the impact of a chondritic asteroid. The impact-spherules known from within the ejecta around this crater are of three types namely aerodynamically shaped sub-mm and mm size spherules, and a sub-mm sized variety of spherule, described as mantled lapilli, having a core consisting of ash-sized grains, shocked basalt and solidified melts surrounded by a rim of ash-sized materials. Although, information is now available on the bulk composition of the sub-mm sized spherules (Misra et al. in Meteorit Planet Sci 7:1001–1018, 2009), almost no idea exists on the latter two varieties. Here, we presented the microprobe data on major oxides and a few trace elements (e.g. Cr, Ni, Cu, Zn) of mm-sized impact spherules in unravelling their petrogenetic evolution. The mm-sized spherules are characterised by homogeneous glassy interior with vesicular margin in contrast to an overall smooth and glassy-texture of the sub-mm sized spherules. Undigested micro-xenocrysts of mainly plagioclase, magnetite and rare clinopyroxene of the target basalt are present only at the marginal parts of the mm-sized spherules. The minor relative enrichment of SiO2 (~3.5 wt% in average) and absence of schlieren structure in these spherules suggest relatively high viscosity of the parent melt droplets of these spherules in comparison to their sub-mm sized counterpart. Chemically homogeneous mm-sized spherule and impact-melt bomb share similar bulk chemical and trace element compositions and show no enrichment in impactor components. The general depletion of Na2O within all the Lonar impactites was resulted due to impact-induced volatilisation effect, and it indicates the solidification temperature of the Lonar impactites close to 1,100 °C. The systematic geochemical variation within the mm-sized spherules (Mg# ~0.38–0.43) could be attributed to various level of mixing between plagioclase-dominated impact melts and ultrafine pyroxene and/or titanomagnetite produced from the target basalt due to impact. Predominance of schlieren and impactor components (mainly Cr, Ni), and nearly absence of vesicles in the sub-mm sized spherules plausibly suggest that these quenched liquid droplets could have produced from the impactor-rich, hotter (~1,100 °C or more) central part of the plume, whereas the morpho-chemistry of the mm-sized spherules induces their formation from the relatively cool outer part of the same impact plume.

Geophysical techniques based on radioactivity measurements are not generally used for exploration of asteroid impact craters. Our studies on the field and laboratory measurements of radioactivity on samples from the Lonar crater, India, show that this technique could be an important method for mapping the distribution of ejecta around the deeply excavated impact craters particularly when these structures are formed on relatively old target rocks/palaeosol. The Lonar ejecta shows ~1.3 times higher γ-ray count rates in the field on average compared to the underlying palaeosol and ~1.9 times higher values over the target basalt while measured by a portable Geiger–Muller pulse counter. The absorbed γ-dose rate (D) of the Lonar samples, computed from 232Th, 238U, and 40K abundances in these samples, also show that the ejecta has distinct bulk dose rates (average ~8.42 nGy h−1) as compared to those of the palaeosol (~18.34 nGy h−1), target basalt (~11.97 nGy h−1), and the impact-melts and spherules (~14 nGy h−1). Therefore, radioactivity mapping of the terrestrial and planetary impact craters by direct methods has importance in mapping ejecta distributions around these structures.

The anisotropy of magnetic susceptibility (AMS) technique, combined with the analyses of satellite images and the geological structure, are examined on Lonar crater, a small, ∼1.8-km-diameter impact crater in India, to evaluate the direction and obliquity of asteroid impact. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image (15-m resolution) of the ca. 52 ka Lonar crater in the subhorizontal Deccan basalt (ca. 65 Ma) shows that this simple, bowl-shaped impact crater has a near-circular rim with a circularity of ∼0.95. Most of the highly reflecting, continuous-ejecta blanket around the crater rim can be enveloped with an ellipse whose major E-W axis is coincident with the diameter of the crater rim and minor N-S axis is relatively displaced toward the west by ∼200 m. The present ejecta distribution, which appears to be close to its pristine shape, extends to a distance of ∼700 m in all the directions from the crater rim except to the west where it extends to a distance of a little more than 1 km. The circular shape of the crater rim, the E-W bilateral symmetry of the enveloping ellipse on the ejecta, and the greater extension of the ejecta toward the west appear to be the result of an oblique impact from the east with an angle of incidence of 30°–45° when compared with experiments. The AMS data suggest that the target basalts occurring at ∼2 km west-southwest of the crater rim are highly shocked, as indicated by the random orientation of their K 3 susceptibility axes in comparison to the unshocked basalts at ∼2 km east-southeast of the crater; the unshocked basalts show a bimodal distribution of susceptibility axes typical of lava flows. Moderate to strong westward shifts of the K 3 axes are seen for the majority of the shocked basalts on the crater rim and west-southwest of the crater; the shocked basalts also indicate an oblique impact from the east when compared with modeling and experiments. A general lowering of degree of anisotropy of the Lonar shocked basalts (∼1.01) compared to the surrounding unshocked basalts (∼1.03) is found to be a characteristic feature of impact crater target rocks. Variation in attitudes of the basalt flows on the Lonar crater rim shows a bilaterally symmetrical distribution about an E-W axial plane, which includes quaquaversal dips of the flows all around the crater rim, except to the west where overturned dips of the basalt flows are seen. It appears that oblique impact and the symmetry in structural variations around the crater rim have a relationship for a small crater such as Lonar.

The only well-known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the Lonar crater, India. For the first time, evidence of the impactor that formed the crater has been identified within the impact spherules, which are ~0.3 to 1 mm in size and of different aerodynamic shapes including spheres, teardrops, cylinders, dumbbells and spindles. They were found in ejecta on the rim of the crater. The spherules have high magnetic susceptibility (from 0.31 to 0.02 SI-mass) and natural remanent magnetization (NRM) intensity. Both NRM and saturation isothermal remanent magnetization (SIRM) intensity are ~2 Am2/kg. Demagnetization response by the NRM suggests a complicated history of remanence acquisition. The spherules show schlieren structure described by chains of tiny dendritic and octahedral-shaped magnetite crystals indicating their quenching from liquid droplets. Microprobe analyses show that, relative to the target basalt compositions, the spherules have relatively high average Fe2O3 (by ~1.5 wt%), MgO (~1 wt%), Mn (~200 ppm), Cr (~200 ppm), Co (~50 ppm), Ni (~1000 ppm) and Zn (~70 ppm), and low Na2O (~1 wt%) and P2O5 (~0.2 wt%). Very high Ni contents, up to 14 times the average content of Lonar basalt, require the presence of a meteoritic component in these spherules. We interpret the high Ni, Cr, and Co abundances in these spherules to indicate that the impactor of the Lonar crater was a chondrite, which is present in abundances of 12 to 20 percent by weight in these impact spherules. Relatively high Zn yet low Na2O and P2O5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume.

During the Pliocene–Pleistocene epoch, covering last ∼5.2 Ma of Earth’s history, altogether 34 terrestrial meteoritic impact craters are known. Most of these craters (29) have diameter ≤10 km, among which 11 craters fall in 1,000 to 100 m range, and 7 are still smaller in dimension and of recent age. The age versus impact-frequency plot shows that the meteoritic impacts during this time period occurred in discrete intervals but have a periodicity that shows the best possible coincidence with the ∼425 Ky climatic cycles observed by Fourier analysis and FFT filtering of composite high resolution benthic foraminiferal δ18O record. This observation is also supported by Monte Carlo test with 71% success where meteoritic impact(s) shows coincidence with climatic cooling within our error limit. The newly observed climatic–meteoritic cycle may be same with the ∼400 Ky Milankovitch cycle or it is a different newly understood cycle relating both the climatic variation and meteoritic impact events.

The Lonar crater, India, is the only well-preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous-Tertiary age. A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions. Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes. Comparable geochemical patterns of immobile elements (e.g., REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites. We found only little indication of post-impact hydrothermal alteration in terms of volatile trace element changes. No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non-chondritic or otherwise iridium-poor impactor.