Planetary science: Tunguska at 100
The most dramatic cosmic impact in recent history has gathered up almost as many weird explanations as it knocked down trees, writes Duncan Steel.
Comet/asteroid impacts and human society : an interdisciplinary approach
Anthropology, Archaeology, Geology.- The Geologic Record of Destructive Impact Events on Earth.- The Archaeology and Anthropology of Quaternary Period Cosmic Impact.- The Sky on the Ground: Celestial Objects and Events in Archaeology and Popular Culture.- Umm al Binni Structure, Southern Iraq, as a Postulated Late Holocene Meteorite Impact Crater.- Tree-Rings Indicate Global Environmental Downturns that could have been Caused by Comet Debris.- The GGE Threat: Facing and Coping with Global Geophysical Events.- Astronomy and Physical Implications.- The Asteroid Impact Hazard and Interdisciplinary Issues.- The Impact Hazard: Advanced NEO Surveys and Societal Responses.- Understanding the Near-Earth Object Population: the 2004 Perspective.- Physical Properties of NEOs and Risks of an Impact: Current Knowledge and Future Challenges.- Evaluating the Risk of Impacts and the Efficiency of Risk Reduction.- Physical Effects of Comet and Asteroid Impacts: Beyond the Crater Rim.- Frequent Ozone Depletion Resulting from Impacts of Asteroids and Comets.- Tsunami as a Destructive Aftermath of Oceanic Impacts.- The Physical and Social Effects of the Kaali Meteorite Impact - a Review.- The Climatic Effects of Asteroid and Comet Impacts: Consequences for an Increasingly Interconnected Society.- Nature of the Tunguska Impactor Based on Peat Material from the Explosion Area.- The Tunguska Event.- Tunguska (1908) and Its Relevance for Comet/Asteroid Impact Statistics.- Atmospheric Megacryometeor Events versus Small Meteorite Impacts: Scientific and Human Perspective of a Potential Natural Hazard.- Socio-Economic and Policy Implications.- Social Science and Near-Earth Objects: an Inventory of Issues.- Perception of Risk from Asteroid Impact.- Hazard Risk Assessment of a Near Earth Object.- Social Perspectives on Comet/Asteroid Impact (CAI) Hazards: Technocratic Authority and the Geography of Social Vulnerability.- May Land Impacts Induce a Catastrophic Collapse of Civil Societies?.- The Societal Implications of a Comet/Asteroid Impact on Earth: a Perspective from International Development Studies.- Disaster Planning for Cosmic Impacts: Progress and Weaknesses.- Insurance Coverage of Meteorite, Asteroid and Comet Impacts - Issues and Options.- The Economic Consequences of Disasters due to Asteroid and Comet Impacts, Small and Large.- Communicating Impact Risk to the Public.- Impact Risk Communication Management (1998-2004): Has It Improved?.- Towards Rational International Policies on the NEO Hazard.- A Road Map for Creating a NEO Research Program in Developing Countries.
Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks
Abstract Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. Al,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500°C in albite and possibly to ∼ 200°C in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ∼ 75–150°C depending on pressure, low albite being stable up to about 620–650°C and high albite above about 725°C at low pressure. Symmetry is broken at ∼ 980°C mainly by a cooperative shearing of the whole framework and not by Al,Si ordering alone; there is a thermal crossover near 700°C shearing being dominant above (high albite) and ordering dominant below (intermediate albite). In potassium feldspar symmetry is broken by Al,Si ordering at a temperature of about 500°C The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ∼ 1075 to ∼ 550°C and to a lesser extent in microcline from 450 to 200°C. Ordering rates in sanidine down to 500°C and ordering rates in microcline between 450 and 200°C are almost as fast as in albite. Ordering in sanidine at 500°C and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of Al,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite. The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Ab80-75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships. These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.