Venerdì, 15 Settembre 2017 07:34

SEMINARIO Distal Impact Ejecta Layers: Definition, Description, and Importance

Il 21/09/2017 presso il Dipartimento di Scienze della Terra in Via S. Maria 53, alle ore 12,00 il prof. Billy P. Glass dell’Università del Delaware terrà il seminario dal titolo: “Distal Impact Ejecta Layers: Definition, Description, and Importance”

Abstract

During the impact of an extraterrestrial body travelling at high velocity (greater than several km/s) two shock waves are produced: one in the target rock and one in the impacting body (or projectile). As the shock waves travel through the rock it produces high pressures and high temperatures. The shock wave is immediately followed by a rarefaction wave that decompresses the target rock and sets it in motion up and away from the point of impact and in large impacts the projectile is completely destroyed (mostly vaporized). The high pressure and temperature and the rapid change in pressure produces changes in the rock target rock called shock-metamorphic effects.  Rock material (both solid and melted) that is ejected to form the crater is called ejecta. About 90 % of the ejecta lands close to the crater to form an ejecta blanket.  Ejecta thrown more than 5 crater radii from an impact crater are called distal impact ejecta.  The thickness of a distal ejecta layer decreases from the source crater, but the average degree of shock metamorphism increases with distance and at a distance >10-30 crater diameters the ejecta consists primarily of impact spherules formed by solidification of melt droplets. At distances greater than ~100 crater diameters the ejecta consist almost entirely of spherules.  Two types of impact spherules have been recognized based on their petrography: microkrystites and microtektites.  Microkrystites contain primary crystallites (i.e., they partly crystallized during cooling).  Microtektites do not contain primary crystallites, but often contain lechatelierite (silica glass formed by melting of quartz). For impacts producing craters 100 km in diameter or greater a plume of vaporized projectile and target rock is formed that is dense enough for condensation to form droplets, which often partly crystallize upon cooling to form microkrystites that sometimes include Ni-rich spinels. Distal impact ejecta records are important for several reasons.  They can provide: ground truth for computer models of the cratering process including conditions within the vapor plume, reginal or global chronostratigraphic layers that can be used for correlation, a more concise record of how impacts are related to Earth events (climatic change, extinctions, tectonics), a record of the oxygen content of the atmosphere (if there are spinels in the microkrystites), and a more complete record of the Earth’s cratering history.