This handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures emphasizes terrestrial impact structures, field geology, and particularly the recognition and petrographic study of shock-metamorphic effects in terrestrial rocks. Individual chapters include: 1) Landscapes with Craters: Meteorite Impacts, Earth, and the Solar System; 2) Target Earth: Present, Past and Future; 3) Formation of Impact Craters; 4) Shock-Metamorphic Effects in Rocks and Minerals; 5) Shock-Metamorphosed Rocks (Impactities) in Impact Structures; 6) Impact Melts; 7) How to Find Impact Structures; and 8) What Next? Current Problems and Future Investigations.

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Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

The detailed morphology of impact craters is now believed to be mainly caused by the collapse of a geometrically simple, bowl-shaped “transient crater.” The transient crater forms immediately after the impact. In small craters, those less than approximately 15 km diameter on the Moon, the steepest part of the rim collapses into the crater bowl to produce a lens of broken rock in an otherwise unmodified transient crater. Such craters are called “simple” and have a depthto-diameter ratio near 1:5. Large craters collapse more spectacularly, giving rise to central peaks, wall terraces, and internal rings in still larger craters. These are called “complex” craters. The transition between simple and complex craters depends on 1/g, suggesting that the collapse occurs when a strength threshold is exceeded. The apparent strength, however, is very low: only a few bars, and with little or no internal friction. This behavior requires a mechanism for temporary strength degradation in the rocks surrounding the impact site. Several models for this process, including acoustic fluidization and shock weakening, have been considered by recent investigations. Acoustic fluidization, in particular, appears to produce results in good agreement with observations, although better understanding is still needed.

http://www.planetary.brown.edu/planetary/geo2810/papers/MeloshIvanov99.pdf

Impact crater collapse

The Chicxulub crater ejecta stratigraphy is reviewed, in the context of the stratigraphy of underlying and overlying rock sequences. The ejecta sequence is regionally grouped in (a) thick polymict and monomict breccia sequences inside the crater and within 300 km from the rim of the crater known from drill holes in and close to the breater, and exposures near the border of Yucatan and Belize; (b) Gulf of Mexico region, <2500 m from the crater, with up to 9 m thick, complex, tsunami-wave influenced, tektite-bearing sequences in shallow marine (<500 m deep) environments and tektite bearing, decimeter thick gravity-flow deposits in deep water sites; (c) an intermediate region between 2500 and 4000 km from the crater where centimeter thick, tektite-bearing layers occur, and (d ) a global distal region with a millimeter thin ejecta layer. The distal ejecta layer is characterized by sub-millimeter sized microkrystites, often rich in Ni-rich spinels and (altered) clinopyroxene. Wherever present, the ejecta layers mark exactly the sudden mass-mortality horizon of the K/T boundary. What exactly caused the mass mortality is still uncertain, but it appears the main event leading to the K/T mass extinctions.

The global stratigraphy of the cretaceous-tertiary boundary impact ejecta

Acknowledgements Foreword Introduction I BACKGROUND Long-term astrophysical processes Evolution theory and the future of humanity Millenial tendencies in responses to apocalyptic threats Cognitive biases potentially affecting judgement of global risks Observation selection effects and global catastrophic risks Systems-based risk analysis Catastrophes and insurance Public policy towards catastrophe II RISKS FROM NATURE Super-volcanism and other geophysical processes of catastrophic import Hazards from comets and asteroids Influence of Supernovae, gamma-ray bursts, solar flares, and cosmic rays on the terrestrial environment III RISKS FROM UNINTENDED CONSEQUENCES Climate change and global risk Plagues and pandemics: past, present, and future Artificial Intelligence as a positive and negative factor in global risk Big troubles, imagined and real IV RISKS FROM HOSTILE ACTS Catastrophe, social collapse, and and human extinction The continuing threat of nuclear war Catastrophic nuclear terrorism: a preventable peril Biotechnology and biosecurity Nanotechnology as global catastrophic risk The totalitarian threat Author's biographies Index

Global Catastrophic Risks

The Yucatan Peninsula karst aquifer is one of the most extensive and spectacular karst aquifer systems on the planet. This transboundary aquifer system extends over an area of approximately 165,000  km2 in Mexico, Guatemala and Belize. The Triassic to Holocene Yucatan limestone platform is located in the vicinity of the North American/Caribbean plate boundary and has been reshaped by a series of tectonic events over its long geologic history. At the end of the Cretaceous period, the Yucatan Peninsula was hit by a large asteroid, which formed the Chicxulub impact crater. The Yucatan Peninsula karst aquifer hosts large amounts of groundwater resources which maintain highly diverse groundwater-dependent ecosystems. Large parts of the aquifer are affected by seawater intrusion. Anthropogenic pollution of the aquifer has been increasing over the past few decades, owing to relentless economic development and population growth on the Peninsula. This review summarizes the state of knowledge on the Yucatan Peninsula karst aquifer and outlines the main challenges for hydrologic research and practical groundwater-resources management on the Peninsula.

Review: The Yucatán Peninsula karst aquifer, Mexico

The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.

Dave Snyder

Papers

Size and morphology of the Chicxulub impact crater