Large Aerial Bursts: An Important Class of Terrestrial Accretionary Events

Abstract

Large aerial bursts similar to the 1908 Tunguska bolide but much larger in magnitude have surely been responsible for many catastrophic events in the history of the Earth. Because aerial bursts produce shallow (or even negligible) craters, their existence is difficult to document in the geological record. Even aerial bursts as small as Tunguska deposit enough energy to melt approximately 1mm of dry soil. Silica-rich glass formed in such melts has the potential to survive in the soil for many Ma, thus a potential indicator of large aerial bursts is glass that was formed as thick regions within silicate melt sheets. The layered tektites from Southeast Asia and the Libyan desert glass may have formed by a combination of sedimentation and downslope flow of silicate melt heated by radiation from large aerial bursts. The alternative, formation of layered tektites as crater ejecta, cannot account for observations such as uniformly high 10Be contents, the orientation of the magnetic remanence field, and the absence of splash-form (e.g., teardrop or dumbbell) tektites in regions where layered tektites are common. The largest asteroids or comets make craters no matter what their strength. Recent reviews suggest that, for events in the energy range up to 10(19)-10(20) J (about two orders of magnitude larger than the Meteor Crater impact), aerial bursts are more likely than cratering events, and the layered tektites of Southeast Asia imply the existence of aerial bursts one to two orders of magnitude larger still.

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

“…5), suggesting an origin by one of those causes. Lechatelierite is found in material from Meteor Crater (16), Haughton Crater, the Australasian tektite field (17), Dakhleh Oasis (18), and the Libyan Desert Glass Field (17), having been produced from whole-rock melting of quartzite, sandstones, quartz-rich igneous and metamorphic rocks, and/or loess-like materials. The consensus is that melting begins above 1,700°C and proceeds to temperatures >2;200°C, the boiling point of quartz, within a time span of a few seconds depending on the magnitude of the event (26,27).…”

“…Lechatelierite cannot be produced volcanically, but can form during lightning strikes as distinctive melt products called fulgurites that typically have unique tubular morphologies (15). It is also common in cratering events, such as Meteor Crater, AZ (16), and Haughton Crater, Canada § , as well as in probable high-temperature aerial bursts that produced melt rocks, such as Australasian tektites (17), Libyan Desert Glass (LDG) (17), Dakhleh Glass (18), and potential, but unconfirmed, melt glass from Tunguska, Siberia (19). Similar lechatelierite-rich material formed in the Trinity nuclear detonation, in which surface materials were drawn up and melted within the plume (20).…”

Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

“…Al-Si-rich spherules have been produced under laboratory conditions from the combustion of charcoal at ∼1,600-2,000°C (35). Also, numerous spherules and melt-glass have been produced in atomic explosions (36,37), including the Trinity detonation in New Mexico in 1945, where the airburst produced spherules similar to those from the Tunguska cosmic airburst in 1908 (8,38). More specifically, the Trinity explosion was a surface burst because the aerial fireball intersected the ground.…”

Brownish inclusions and dark streaks in Libyan Desert Glass: Evidence for high-temperature melting of the target rock

“…Especially the absence of an impact crater unambiguously related to LDG has caused alternative models to the classical impact hypothesis to be explored. Recently, it was proposed that Libyan Desert Glass supposedly formed by radiative melting from lowaltitude airbursts explosions (e.g., Wasson 2003;Svetsov and Wasson 2007;Boslough and Crawford 2008;Shuvalov and Trubetskaya 2008). These authors suggested that the hot jet of a vaporized projectile comes into contact with the Earth's surface where it expands radially at temperatures high enough to melt silicate minerals (e.g., Boslough and Crawford 2008).…”