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Enigmatic Meteorite Impact Signature: Field Evidence and Ground Penetrating Radar (GPR) Measurements Suggest Megascopic Impact Spallation Features.

  • December 2019
  • Conference: AGU Fall Meeting 2019
Authors:
Kord Ernstson at University of Wuerzburg
Kord Ernstson
Jens Poßekel at Geophysik Poßekel
Jens Poßekel
  • Geophysik Poßekel
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Abstract

Spallation is a well-known process in technical fracture mechanics. It describes strong tensile pulses, reflected from incident compression pulses at free surfaces, which can lead to decisive material damage due to the usually significantly lower tensile strength. In meteoric impacts with shock propagations, they play an important role in all dimensions, from mega to micro (Ernstson 2014). Early considerations led to the model of so-called spall plates (Melosh 1989), which describes near-surface platy rock bodies ejected at high speed during the formation of impact structures, a consequence of the spallation superposition of the propagating shock front with the rarefaction wave reflected at the free Earth surface. We describe known terrain observations (Ries impact event) of structurally very conspicuous impact layer deformations in Upper Jurassic limestones (Chao et al. 1978, Rutte 2003) , which so far cannot be explained geologically. GPR measurements show an extrapolation into the depth (8 m) and suggest that, based on the model of spall plates, shock spallation during the propagation of impact shock waves is the only reasonable explanation so far.-Very unusual GPR reflections are associated with a suggested impact airburst in Lower Bavaria. A 500 m x 50 m area of surficially melted granites shows narrow, circular curved, isolated reflection bands of high energy at apex depths down to a few meters. Conventional GPR interpretations and geologic considerations do not explain the observation. It is suggested that the GPR reflection traces an arcuate thin weak zone in the hard granite, caused by a rarefaction front of strong tensile stress propagating into the subsurface in response to an incident shock front. A specification of such a process in more detail is still pending, but shows how impressively GPR measurements can contribute to the clarification of near-surface impact processes (Poßekel & Ernstson 2019, Ernstson & Poßekel 2017). Plain Language Summary Spallation is a well-known process in technical fracture mechanics that describes the particularly destructive effect of dynamic tensile stresses as a result of reflected dynamic compressive stresses. In meteorite impacts, it is primarily reflected shock waves (rarefaction waves) that leave their mark in dimensions from mega to micro. We describe the investigation of quite unusual and so far geologically not understood layer deformations with geophysical ground penetrating radar (GPR) measurements and consider impact shock spallation as causal force.
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EP53F-2239 - Enigmatic Meteorite Impact
Signature: Field Evidence and Ground
Penetrating Radar (GPR) Measurements
Suggest Megascopic Impact Spallation
Features.
Abstract
Spallation is a well-known process in technical fracture mechanics. It describes strong tensile pulses,
reflected from incident compression pulses at free surfaces, which can lead to decisive material damage
due to the usually significantly lower tensile strength. In meteoric impacts with shock propagations, they
play an important role in all dimensions, from mega to micro (Ernstson 2014). Early considerations led to
the model of so-called spall plates (Melosh 1989), which describes near-surface platy rock bodies ejected
at high speed during the formation of impact structures, a consequence of the spallation superposition of
the propagating shock front with the rarefaction wave reflected at the free Earth surface. We describe
known terrain observations (Ries impact event) of structurally very conspicuous impact layer deformations
in Upper Jurassic limestones (Chao et al. 1978, Rutte 2003) , which so far cannot be explained
geologically. GPR measurements show an extrapolation into the depth (8 m) and suggest that, based on
the model of spall plates, shock spallation during the propagation of impact shock waves is the only
reasonable explanation so far. - Very unusual GPR reflections are associated with a suggested impact
airburst in Lower Bavaria. A 500 m x 50 m area of surficially melted granites shows narrow, circular
curved, isolated reflection bands of high energy at apex depths down to a few meters. Conventional GPR
interpretations and geologic considerations do not explain the observation. It is suggested that the GPR
reflection traces an arcuate thin weak zone in the hard granite, caused by a rarefaction front of strong
tensile stress propagating into the subsurface in response to an incident shock front. A specification of
such a process in more detail is still pending, but shows how impressively GPR measurements can
contribute to the clarification of near-surface impact processes (Poßekel & Ernstson 2019, Ernstson &
Poßekel 2017).
Plain Language Summary
Spallation is a well-known process in technical fracture mechanics that describes the particularly
destructive effect of dynamic tensile stresses as a result of reflected dynamic compressive stresses. In
meteorite impacts, it is primarily reflected shock waves (rarefaction waves) that leave their mark in
dimensions from mega to micro. We describe the investigation of quite unusual and so far geologically not
understood layer deformations with geophysical ground penetrating radar (GPR) measurements and
consider impact shock spallation as causal force.
Authors
Kord Ernstson
Friday, 13 December 2019
13:40 - 18:00
Moscone South - Poster Hall
... In both airbursts and crater-forming events, the fracturing of quartz grains may also occur from tensile forces and spallation [26,35,37,[74][75][76]. This shock occurs when a This grain is fractured, but the fractures are not oriented as in shock fractures. ...
... Several studies [34,36,37,75,86] have reported evidence that shock fractures are produced in cosmic airbursts when a high-pressure, high-temperature fireball intersects the surface, similar to the nuclear airbursts described here. These cosmic airbursts may produce shallow craters rather than classic hard-impact craters. ...
Microstructures in shocked quartz: linking nuclear airbursts and meteorite impacts
Article
Full-text available
  • Jan 2023
Many studies of hypervelocity impact craters have described the characteristics of quartz grains shock-metamorphosed at high pressures of >10 GPa. In contrast, few studies have investigated shock metamorphism at lower shock pressures. In this study, we test the hypothesis that low-pressure shock metamorphism occurs in near-surface nuclear airbursts and that this process shares essential characteristics with crater-forming impact events. To investigate low-grade shock microstructures, we compared quartz grains from Meteor Crater, a 1.2-km-wide impact crater, to those from near-surface nuclear airbursts at the Alamogordo Bombing Range, New Mexico in 1945 and Kazakhstan in 1949/1953. This investigation utilized a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Meteor Crater and the nuclear test sites all exhibit quartz grains with closely spaced, sub-micron-wide fractures that appear to have formed at low shock pressures. Significantly, these micro-fractures are closely associated with Dauphiné twins and are filled with amorphous silica (glass), widely considered a classic indicator of shock metamorphism. Thus, this study confirms that glass-filled shock fractures in quartz form during near-surface nuclear airbursts, as well as crater-forming impact events, and by extension, it suggests that they may form in any near-surface cosmic airbursts in which the shockwave is coupled to Earth’s surface, as has been proposed. The robust characterization of such events is crucial because of their potential catastrophic effects on the Earth’s environmental and biotic systems.
View
... Shock fracturing by tension. In both airbursts and crater-forming events, the fracturing of quartz grains may also occur from tensile forces and spallation [26,35,37,[74][75][76]. This shock occurs when a compressive shockwave enters a material, such as a quartz grain, and then reflects off the opposite grain boundary, producing a rarefaction wave that fractures the grain in the opposite direction. ...
... Future studies. Several studies [34,36,37,75,83] have reported evidence that shock fractures are produced in cosmic airbursts when a high-pressure, high-temperature fireball intersects the surface, similar to the nuclear airbursts described here. These cosmic airbursts may produce shallow craters rather than classic hard-impact craters. ...
Microstructures in Shocked Quartz: Linking Nuclear Airbursts and Meteorite Impacts
Preprint
Full-text available
  • Aug 2023
Many studies of hypervelocity impact craters have described the characteristics of quartz grains shock-metamorphosed at high pressures of > 10 GPa. In contrast, few studies have investigated shock metamorphism at lower shock pressures. In this study, we test the hypothesis that low-pressure shock metamorphism occurs in near-surface nuclear airbursts and that this process shares essential characteristics with crater-forming impact events. To investigate low-grade shock microstructures, we compared quartz grains from Meteor Crater, a 1.2-km-wide impact crater, to those from near-surface nuclear airbursts at the Alamogordo Bombing Range, New Mexico in 1945 and Kazakhstan in 1949/1953. This investigation utilized a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Meteor Crater and the nuclear test sites all exhibit quartz grains with closely-spaced, sub-micron-wide fractures that appear to have formed at low shock pressures. Significantly, these micro-fractures are closely associated with Dauphiné twins and are filled with amorphous silica (glass), widely considered a classic indicator of shock metamorphism. Thus, this study confirms that glass-filled shock fractures in quartz form during near-surface nuclear airbursts, as well as crater-forming impact events, and by extension, it suggests that they may form in any near-surface cosmic airbursts in which the shockwave is coupled to Earth’s surface. The robust characterization of such events is crucial because of their potential catastrophic effects on the Earth’s environmental and biotic systems.
View
... Shock fracturing by tension. In both airbursts and crater-forming events, the fracturing of quartz grains may also occur from tensile forces or, more specifically, due to spallation [26,35,37,[74][75][76]. This shock process occurs when a compressive shockwave enters a material, such as a quartz grain, and then is reflected off the grain boundary, producing a rarefaction wave. ...
... Future studies. Several studies [34,36,37,75,78] have reported evidence that shock fractures are produced in cosmic airbursts when a high-pressure, high-temperature fireball intersects the surface, similar to the nuclear airbursts described here. These cosmic airbursts may produce shallow craters rather than classic hard-impact craters. ...
Microstructures in Shocked Quartz: Linking Nuclear Airbursts and Meteorite Impacts
Preprint
Full-text available
  • May 2023
Many studies of hypervelocity impact craters have described the characteristics of quartz grains shock-metamorphosed at high pressures of >10 GPa, but in contrast, few studies have investigated shock metamorphism at lower shock pressures. In this study, we test the hypothesis that low-pressure shock metamorphism occurs in near-surface nuclear airbursts and that this process shares important characteristics with impact-cratering events. To investigate low-grade shock microstructures, we compared quartz grains from Meteor Crater, a 1.2-km-wide impact crater, to those from near-surface nuclear airbursts at the Alamogordo Bombing Range, New Mexico in 1945 and Kazakhstan in 1949/1953. This investigation utilized a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Meteor Crater and the nuclear test sites all exhibit metamorphosed quartz grains with closely-spaced, sub-micron-wide fractures that appear to have formed at low shock pressures. Importantly, these micro-fractures are closely associated with Dauphiné twins and are filled with amorphous silica (glass), widely considered to be a classic indicator of shock metamorphism. Thus, this study confirms that glass-filled shock fractures in quartz form during near-surface nuclear airbursts, as well as crater-forming impact events, and by extension, it suggests they also may form in near-surface cosmic airbursts.
View
... Based on current analyses, Abu Hureyra is the earliest example of a human settlement catastrophically affected by a cosmic impact event. There also are younger impacts proposed to have negatively affected contemporary human populations, including the Holocene Native American culture by Tankersley et al. [41]; the Chiemgau impact event in Bavaria during the Bronze Age/Iron Age (Ernstson et al. [42,43] Rappengluck et al. [44][45][46][47][48][49][50]); at Bronze Age Tall el-Hammam, Jordan (Bunch et al. [51]); in Northern Syria in the Bronze Age (Courty et al. [52][53][54]); and in western Kouvola, Finland in the Holocene (Ahokas [55]). Sample ES15 came from just outside a pit house, E305 from within a pit house, E326 from a pit, E301 from a work area, and E313 from a work area outside a pit house entrance. ...
Abu Hureyra, Syria, Part 2: Additional evidence supporting the catastrophic destruction of this prehistoric village by a cosmic airburst ~12,800 years ago
Article
Full-text available
  • Jan 2023
At Abu Hureyra, a well-studied archeological site in Syria, the onset boundary of the Younger Dryas climatic episode ~12,800 years ago has previously been proposed to contain evidence supporting a near-surface cosmic airburst impact that generated temperatures >2000°C. Here, we present a wide range of potential impact-related proxies representing the catastrophic effects of this cosmic impact that destroyed the village. These proxies include nanodiamonds (cubic diamonds, n -diamonds, i -carbon, and lonsdaleite-like crystals); silica-rich and iron-rich micro-spherules; and melted chromite, quartz, and zircon grains. Another proxy, meltglass, at a concentration of 1.6 wt% of bulk sediment, appears to have formed from terrestrial sediments and was found to partially coat toolmaking debitage, bones, and clay building plaster, suggesting that village life was adversely affected. Abundant meltglass fragments examined display remarkably detailed imprints of plant structures, including those of reeds. The nanodiamonds are proposed to have formed under anoxic conditions from the incineration of plant materials during high-temperature, impact-related fires, while geochemical evidence indicates that the micro-spherules formed from the melting of terrestrial sediments. Broad archeological and geochemical evidence supports the hypothesis that Abu Hureyra is the oldest known archeological site catastrophically destroyed by cosmic impact, thus revealing the potential dangers of such events.
View
... Based on current analyses, this would be the earliest example of a human settlement catastrophically affected by a cosmic impact event. There also are younger impacts proposed to have negatively affected contemporary human populations, including the Holocene Native American culture by Tankersley et al. [20]; the Chiemgau impact event in Bavaria during the Bronze Age/Iron Age (Ernstson et al. [21,22] Rappenglück et al. [23][24][25][26][27][28][29]); at Bronze Age Tall el-Hammam, Jordan (Bunch et al. [30]); in Northern Syria in the Bronze Age (Courty et al. [31][32][33]); and in western Kouvola, Finland in the Holocene (Ahokas [34]). ...
Abu Hureyra, Syria, Part 3: Comet airbursts triggered major climate change 12,800 years ago that initiated the transition to agriculture
Article
Full-text available
  • Jan 2023
This study investigates the hypothesis that Earth collided with fragments of a disintegrating comet, triggering Younger Dryas climate change 12,800 years ago. This collision created environmental conditions at Abu Hureyra, Syria, that favored the earliest known continuous cultivation of domestic-type grains and legumes, along with animal management, adding to the pre-existing practice of hunting-and-gathering. The proposed airburst coincided with a significant decline in local populations and led to architectural reorganizations of the village. These events immediately followed the deposition of the Younger Dryas Boundary layer that contains peak concentrations of high-temperature meltglass, nanodiamonds, platinum, and iridium. These proxies provide evidence of a nearby low-altitude airburst by a comet-like fragment of a former Centaur, one of many <300-km-wide bodies in unstable orbits between the giant planets. This large body is proposed to have undergone cascading disintegrations, thus producing the Taurid Complex containing Comet Encke and ~90 asteroids with diameters of ~1.5 to 5 km. Here, we present substantial new quantitative evidence and interpretations supporting the hypothesis that comet fragments triggered near-global shifts in climate ~12,800 years ago, and one airburst destroyed the Abu Hureyra village. This evidence implies a causative link between extraterrestrial airbursts, environmental change, and transformative shifts in human societies.
View
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