Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session S2: Geophysics & Planetary Science II |
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Chair: Konstantinos Tsembelis, Cavendish Laboratory, University of Cambridge Room: Hyatt Regency Constellation C |
Thursday, August 4, 2005 9:30AM - 9:45AM |
S2.00001: Crustal Rock: Recorder of Oblique Impactor Meteoroid Trajectories Thomas J. Ahrens, H. Anita Ai Oblique impact experiments in which 2g lead bullets strike samples of San Marcos granite and Bedford limestone at 1.2 km/s induce zones of increased crack density (termed shocked damage) which result in local decreases in bulk and shear moduli that results in maximum decreases of 30-40\% in compressional and shear wave velocity (Budianski and O'Connell). Initial computer simulation of oblique impacts of meteorites (Pierazzo and Melosh) demonstrate the congruence of peak shock stress trajectory with the pre-impact meteoroid trajectory. We measure (Ai and Ahrens) via multi-beam ($\sim$ 300) tomographic inversion, the sub-impact surface distribution of damage from the decreases in compressional wave velocity in the 20 $\times$ 20 $\times$ 15 cm rock target. The damage profiles for oblique impacts are markedly asymmetric (in plane of pre-impact meteoroid pre-impact trajectory) beneath the nearly round excavated craters. Thus, meteorite trajectory information can be recorded in planetary surfaces. Asymmetric sub-surface seismic velocity profiles beneath the Manson (Iowa) and Ries (Germany) impact craters demonstrate that pre-impact meteoroid trajectories records remain accessible for at least ${\sim} 10 ^ 8$ years. [Preview Abstract] |
Thursday, August 4, 2005 9:45AM - 10:00AM |
S2.00002: A Comparison of Oceanic and Continental Asteroid Impacts Paul S. De Carli, Adrian P. Jones, G. David Price Ever since the recognition that Lunar craters are impact craters rather than volcanos, it has been possible to seriously consider the role of comet or asteroid impacts as possible drivers of geologic events on the Earth. For example, the Chicxulub impact event is widely accepted as a cause of the mass extinction at the end of the Cretaceous. Currently, the subject of impact-induced volcanism is fiercely debated. In the absence of compelling field evidence, the debate is centered on the results of computational modeling of large impacts and on the estimated frequency of such large impacts. Both sides of the debate agree that a sufficiently large impact would excavate to such a depth that decompression melting of the Earth's mantle would be triggered. The anti-impact volcanism faction argues that the impact event would have to be so large that it is unlikely to have occurred during the past 3.5 billion years. The pro-impact volcanism faction argues that an event comparable to or slightly larger the the Chicxulub impact would be sufficient, provided that the impact were oceanic. Here we present detailed calculations to address the systematic differences between oceanic and continental impacts. In partiular, we incorporate the hysteretic phase transition behavior of granite in our material model. [Preview Abstract] |
Thursday, August 4, 2005 10:00AM - 10:30AM |
S2.00003: Ballography: A Billion Nanosecond History of the Bee Bluff Impact Crater of South Texas Invited Speaker: Study of an area of disturbed geology south of Uvalde, Texas in Zavala County has resulted in evidence for a meteorite impact site.* The local sedimentary geology, with an approximate two-to-three m thick hard cap of sandstone and siltstone, a two cm layer of hydrous iron-oxide, and a deep, soft-calcareous silt, dominates both residual site characteristics and impactite samples. A crater of about 1.6 km in diameter and about 10 m in depth is evident from 1942 aerial photographs. Loose silt debris flow and deposit of the prompt venting of the silt is evident from a wake deposit behind local uplifted rocks and throughout the region. A suite of samples obtained in the area shows evidence for high-pressure shock modification. Impact modification of the hydrous iron-oxide provides expression for complex shock-processes in these materials. Especially important to the scientific documentation is discovery of a 300 kg rock `The Uvalde Crater Rosetta Stone' that contains numerous features of the shock modification in a configuration that can be directly related to the target configuration and the impactite collection. Calculations show that the impact of an approximate 60 m diameter iron meteorite at a velocity of 14.2 km/sec with an impact pressure of 350 GPa produces a crater diameter of about 2 km. In the present thin-target configuration the meteorite quickly penetrates the hard rock with release waves from the bottom resulting in a prompt bottom-up rather than the usual top-down pressure release and melt. Such upward high pressure-temperature flow results in prompt melting and radial flow along the iron meteorite interface as evident in residual impactite instability patterns and the presence of Uvalde suevite. * R. A. Graham and W. F. Wilson, Lunar Planet. Sci., 2005, 1086.pdf [Preview Abstract] |
Thursday, August 4, 2005 10:30AM - 10:45AM |
S2.00004: The Stability of MgSiO$_{3}$ Perovskite at Lower Mantle Conditions Li Zhang, Zizheng Gong, Xiufang Chen, Liwei Dong, Yingwei Fei, Fuqian Jing The stability of MgSiO$_{3}$-perovskite at lower mantle conditions has been a subject of extensive investigation and debate. Shock recovery experiments with the initial sample of (Mg$_{0.92}$, Fe$_{0.08})$SiO$_{3}$ enstatite and MgO+SiO$_{2}$ were conducted up to 120GPa. The analysis of XRD results indicate that there is no possibility for the chemical decomposition from (Mg$_{0.92}$, Fe$_{0.08})$SiO$_{3}$ to oxides SiO$_{2}$ and (Mg$_{0.92}$, Fe$_{0.08})$O under experimental shock pressure. The Gibbs energy and molar volume of all phases in the reaction MgSiO$_{3}$(Pv) = MgO(Pe) + SiO$_{2}$(St) are calculated using both the latest experimental thermodynamic parameters and the first principle molecular dynamic (MD) simulations under lower mantle conditions (1000-3500K and 30-150GPa)., and the results show that perovskite is thermodynamically stable relative to the stishovite and periclase assemblage at lower mantle conditions. [Preview Abstract] |
Thursday, August 4, 2005 10:45AM - 11:00AM |
S2.00005: Sound Velocity Of (Mg$_{0.92}$, Fe$_{0.08}$)Sio$_{3}$ Perovskite up to 140 Shock Pressure and Its Geophysical Implications Zizheng Gong, Lin He, Li Zhang, Liwei Deng, Yingwei Fei, Fuqian Jing New experimental data of compressional sound velocity for polycrystalline enstitate (Mg$_{0.92}$, Fe$_{0.08})$SiO$_{3 }$initial specimens were measured up to 140 GPa shock pressure using the optical analyzer techniques. The results confirm the discontinuity and negative jump of compressional sound velocity between 70-85GPa. This pressure range is comparable with that of the HS to LS transition of iron in (Mg$_{0.92}$, Fe$_{0.08})$SiO$_{3}$ Perovskite. Moreover, the estimated phase boundary dT/dP, 66$\sim $92 K/GPa, is consistent with that of spin transition estimation. The corresponding depth in the lower mantle of 70-85GPa just is 1600-1800km, so, the radial anomaly of seismic wave velocity in this region of the lower mantle is possible a result of solid to solid structure transition of (Mg$_{0.92}$, Fe$_{0.08})$SiO$_{3}$ Perovskite. [Preview Abstract] |
Thursday, August 4, 2005 11:00AM - 11:15AM |
S2.00006: The Dynamic Behaviour of Micro-Concrete K. Tsembelis, W.G. Proud A series of plate impact experiments has been performed to assess the dynamic behaviour of micro-concrete (70{\%} crushed dolerite and 30{\%} cement paste by weight) in both longitudinal and lateral directions. Information was obtained for the Hugoniot curve and dynamic shear stress properties. Hugoniot results are compared with published data on cement paste, mortar and concrete from the UK, Germany and the US. Furthermore, the shear strength of micro-concrete is compared to the cement paste where very small differences are observed. Therefore, the shear strength appears to be independent of the aggregates and only depends on the matrix material. [Preview Abstract] |
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