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Overestimation of threat from 100 Mt-class airbursts? High-pressure evidence from zircon in Libyan Desert Glass

Aaron J. Cavosie and Christian Koeberl
Overestimation of threat from 100 Mt-class airbursts? High-pressure evidence from zircon in Libyan Desert Glass
Geology (Boulder) (May 2019) 47 (7): 609-612


Atmospheric airbursts over Russia at Chelyabinsk in 2013 and Tunguska in 1908 provide dramatic examples of hazards posed by near-Earth objects (NEOs). These two events produced 0.5 and 5 Mt of energy, respectively, which dramatically affected surface environments and, in the case of Chelyabinsk, injured humans. Enigmatic natural glasses have been cited as geologic evidence of the threat posed by large airbursts. Libyan Desert Glass (LDG) is a natural glass found in western Egypt that formed approximately 29 m.y. ago, however its origin is disputed; the two main formation hypotheses include melting by meteorite impact or melting by a large, 100 Mt-class airburst. High-temperature fusion occurs during both processes, however airbursts do not produce shocked minerals; airbursts generate overpressures at the level of thousands of pascals in the atmosphere, whereas crater-forming impacts generate shockwaves at the level of billions of pascals on the ground. Here we report the presence in LDG of granular zircon grains that are comprised of neoblasts that preserve systematic crystallographic orientation relations that uniquely form during reversion from reidite, a 30 GPa high-pressure ZrSiO (sub 4) polymorph, back to zircon. Evidence of former reidite provides the first unequivocal substantiation that LDG was generated during an event that produced high-pressure shock waves; these results thus preclude an origin of LDG by airburst alone. Other glasses of disputed origin that contain zircon with evidence of former reidite, such as Australasian tektites, similarly were also likely made during crater-forming events. Public-policy discussions and planning to mitigate hazards from airbursts caused by NEOs are clearly warranted, but should be cautious about considering LDG or other glasses with evidence of high-pressure shock deformation as products of an airburst. At present, there are no confirmed examples of products from a 100 Mt-class airburst in the geologic record.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 47
Serial Issue: 7
Title: Overestimation of threat from 100 Mt-class airbursts? High-pressure evidence from zircon in Libyan Desert Glass
Affiliation: Curtin University, School of Earth and Planetary Science, Perth, West. Aust., Australia
Pages: 609-612
Published: 20190502
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 29
Accession Number: 2019-046833
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2019218
Illustration Description: illus. incl. sketch map
N25°00'00" - N26°00'00", E25°00'00" - E26°00'00"
Secondary Affiliation: Natural History Museum, AUT, Austria
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201924
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