Ejecta Research by Others
Abstracts as offered in The Sedimentary Record of Meteorite Impacts,
EVANS, K.R., HORTON, J.W., Jr., THOMPSON, M.F., and WARME, J.E.,
2005, eds., SEPM Research Conference, Springfield, Missouri, May 21-23, 2005
THE MECHANICS OF METEORITE IMPACT EJECTION AND SEDIMENTATION
MELOSH, H. Jay
Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, Arizona 86721-0092, USA,
The formation of a meteorite impact crater is sudden and violent by geologic standards. The crater structure and associated impact ejecta deposits form in a geologic instant and, when extensive ejecta deposits can be found, provide an excellent correlation tool. The mechanics of hypervelocity impact crater formation, however, may be unfamiliar to geologists more accustomed to dealing with volcanic eruptions or submarine landslides as a source of extensive deposits. Impact crater excavation is more akin to a powerful explosion than to the low velocity impact of, say, pebbles into water or mud that we experience in everyday life.
The formation of an impact crater begins when a rapidly moving object, a comet or asteroid, strikes the surface of the Earth or another planet. The projectile transforms its kinetic energy into heat and motion in the target over a time interval measured by its diameter divided by its velocity. In the process the target rocks may be raised to temperatures of tens of thousands of degrees while they are compressed to pressures comparable to those at the Earth's core-greatly exceeding pressures reached in even the most violent volcanic eruption. The initial ejected material is ionized plasma containing vaporized projectile and target material that expands away from the impact site and pushes back the ambient atmosphere. This plasma eventually creates a pocket of superheated, low density, gas near the impact site through which the proximal ejecta are deposited almost as if in a vacuum. While these hot gases vent above the atmosphere, solid material from close to the surface is expelled at high speed. Some of this material may even escape the Earth, eventually to fall on the Moon or other planets such as Mars or Venus. Deeperseated material is melted or highly shocked and expelled at lesser speed. Such material is the source of tektites, microtektites or microkrystites now found in deep-sea cores. The slowest and most voluminous ejecta emerges last and is deposited in thick, chaotic masses resembling glacial till in the vicinity of the crater itself.
The entire duration of the crater formation process is typically less than a minute—a few times a scale factor defined by the square root of the crater diameter divided by the acceleration of gravity. Some ejecta may travel ballistically nearly around the Earth, a process taking only a few hours, with final depositional times controlled by its rate of sedimentation through the air or water columns.
IMPACT EJECTA SEDIMENTATION PROCESSES
MELOSH, H. Jay, Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, Arizona 86721-0092, USA
OSINSKI, Gordon, R. Canadian Space Agency, 6767 Route de l'Aeroport, Saint-Hubert, Quebec, J3Y 8Y9, Canada
One of the most characteristic, but poorly understood, aspects of hypervelocity impact events is the generation of ejecta deposits. Proximal ejecta deposits are found in the immediate vicinity of an impact crater (i.e., external to the original transient cavity and up to the outer limit of the continuous ejecta blanket); whereas distal ejecta deposits are found distant from the crater (>5 crater radii). It is generally accepted that proximal ejecta deposits on airless bodies, such as the Moon, are emplaced via ballistic sedimentation (Oberbeck V. R., 1975, Rev. Geophys. Space Phys., 13, 337-362). Studies of the continuous ejecta blanket (Bunte Breccia) at the Ries impact structure, Germany, strongly support the importance of ballistic sedimentation on Earth (Hörz F. et al. 1983, Rev. Geophys. Space Phys., 21, 1667-1725). However, a variety of other ejecta types are found at the Ries overlying the Bunte Breccia, including suevites and impact melt rocks. It was generally accepted that these impactites were deposited subaerially from an ejecta plume. However, recently, it has been shown that proximal suevites were emplaced as surface flow(s), either comparable to pyroclastic flows (e.g., Newsom H. E. et al. 1986, JGR, 91, 239-251), or as a ground-hugging impact melt-rich flows that were emplaced outwards from the crater center during the final stages of crater formation (Osinski G. R. et al. 2004, MAPS, 39, 1655-1683). This has also been suggested for the impact melt rocks at the Ries (Osinski G. R., 2004, Earth Planet. Sci. Lett., 226, 529-543). Furthermore, there is a clear temporal hiatus between the ballistically-emplaced Bunte Breccia and the overlying suevites/impact melt flow deposits, which requires a two-stage ejecta emplacement model. This has also been invoked for similar deposits at the Haughton impact structure, Canada (Osinski G. R. et al. in press, MAPS). In terms of distal ejecta deposits, it has been suggested that more distal impact ejecta falling into the atmosphere may clump together into density currents that flow to the ground much more rapidly than might be expected for single particles themselves (Melosh H. J., 2004, MAPS., 39, A67). This paper will explore various models for the generation and deposition of ejecta deposits using a combination of field and numerical modeling studies of terrestrial impact craters, and volcanic analogues.
PROCESSES AND PRODUCTS OF METEORITE IMPACTS INTO SEDIMENTARY ROCKS
OSINSKI, Gordon, R., Canadian Space Agency, 6767 Route de l'Aeroport, Saint-Hubert, Quebec, J3Y 8Y9, Canada
Sedimentary rocks are present in the target sequence of ~70% of the world's known impact structures. In contrast to igneous and metamorphic rocks, sedimentary rocks are typically rich in volatiles (e.g., H2O, CO2, SO2), highly porous, and are typically layered. It is well known that such characteristics exert a considerable influence on the processes and products of meteorite impacts. However, despite the significance of sedimentary rocks in the target sequence of terrestrial impact structures, the response of such lithologies to hypervelocity impact remains poorly understood. In particular, the relative importance and role of impact melting versus decomposition in carbonate- and evaporite-rich targets remains controversial, although a considerable amount of work has been published on this subject in the past few years. Thus, the aim of this paper is two-fold: (1) to provide an up-to-date assessment of our current understanding of impacts into sedimentary-rich targets (carbonates, evaporites, and terrigenous clastic rocks); and (2) to discuss how impact-modified and impact-melted sedimentary rocks may be distinguished from their unshocked counterparts. In summary: (1) It is apparent that impact melting in sedimentary targets is much more common than previously thought (e.g., impact melting of carbonates, evaporites, sandstones, and shales, has now been recognized at a number of impact sites); (2) There is no unequivocal evidence for the decomposition of carbonates or evaporites at any terrestrial impact site; (3) The results of this study suggest that previous assumptions about the response of sedimentary rocks during hypervelocity impact events are incorrect; (4) It is suggested that the apparent "anomaly" between the volume of impact melt generated in sedimentary versus crystalline targets in comparably sized impact structures may be due to a misinterpretation of micro-textures. Textural and chemical evidence for the impact melting of sedimentary rocks during hypervelocity impact is provided by: (1) liquid immiscible textures; (2) quench textures; (3) melt spherules and globules; (4) anomalous chemical composition of various "sedimentary" mineral phases (e.g., SiO2-rich calcite); (5) crystallization of "new" silicate minerals from sediment-derived melts (e.g., pyroxene, olivine).
THE IMPORTANCE OF BEING CRATERED: THE NEW ROLE OF METEORITE IMPACT AS A NORMAL GEOLOGICAL PROCESS
FRENCH, Bevan M. Department of Paleobiology, Smithsonian Institution, P.O. Box 37012, Washington, DC 20560-7012
The young field of meteorite impact geology has passed through several stages: disregard, controversy, media attention, and gradual acceptance by geologists. Impact geologists no longer have to justify the ideas that meteorite impacts do occur, that such impacts produce major geological and biological effects, or that the resulting impact structures can be unequivocally identified. The challenges today are different: to determine the full range of impact effects preserved in 17 the geological record of the Earth, to continue the merger of impact geology with mainstream geosciences, and to promote communication and education about meteorite impact to the larger geology community and the public.
A particularly exciting trend, especially during the last decade, has been the frequent and unexpected discovery of major impact effects in the geological record, often by traditionally trained geologists doing conventional geological investigations. Examples include: (1) discovery of the 85-km-diameter Chesapeake Bay structure from geophysical and hydrological studies; (2) recognition that the widespread Alamo Breccia (Nevada) reflects the impact-induced collapse of a shallow continental platform; (3) discovery of spherule layers, especially in Precambrian rocks, which may represent ejecta from ancient, distant, and possibly destroyed impact structures. In related developments, other geologists have begun to use well-constrained and well-studied impact structures as "laboratory experiments" to explore fundamental geological processes: (1) cooling and crystallization of large, instantaneously-generated bodies of igneous melt (at Sudbury, Canada); (2) post-impact hydrothermal activity (e.g., at Haughton, Canada), which has implications for the formation of ore bodies and for the origin of life in suitable warm and sheltered environments.
A personal view of some areas where future impact studies could make major contributions to conventional geological problems include: (1) comparative studies between low-level (<7 GPa) shock deformation of quartz (commonly expressed as multiple cleavage sets) with tectonic deformation of quartz at comparable stress levels; (2) the nature, origin, and significance of bulk organic carbon ("kerogen") in some impact structures (Gardnos, Norway and Sudbury, Canada) and the implications of this material for possible biogenic processes related to meteorite impact events.
The field of impact geology is now at an exciting watershed between its past emphasis on the geology and history of other planets and a new and growing focus on the record of impact events preserved in terrestrial rocks. In addition to continuing impact geology studies, it is essential to improve communications between impact geologists and others and to expand the teaching of impact geology in standard geological curricula (the graduates may unexpectedly run across an impact structure in their future work). As these trends continue, we can look for more exciting and unexpected insights into the history of the Earth to appear.