Date of Award
Doctor of Philosophy (PhD)
The formation of an impact crater ejecta blanket can be viewed as a form of organized chaos. Material that is ejected from a crater is heavily brecciated, but falls back to the surface along ballistic trajectories, generally preserving an inverted sense of the original stratigraphy. As the ejecta re-impacts the area surrounding the crater it forms a thick blanket of ejected material and reworked target surface that gradually thins away from the crater rim. Within the crater, crater modification processes, such as wall terrace formation and impact melt drainage, transform the crater in expectable ways.
The approach adopted in this research is to use what is known about impact cratering and ejecta emplacement processes to geologically map craters on the Moon using remote-sensing data, determine the timing of individual impacts on the Moon, and investigate terrestrial impact melt glass. Research has been divided into three parts: 1) a detailed geomorphologic and geologic map of the lunar crater Aristarchus; 2) detailed crater size-frequency distribution measurements on the ejecta blankets of the lunar craters Aristarchus and Tycho; and 3) characterization of zircon decomposition in impact melt glass from the Mistastin Lake impact structure, Labrador, Canada.
Mapping the geomorphology and geology of Aristarchus has shown that there are differences in the distribution of morphologic and compositional units related to pre-existing topography. I use the basic principles of inverted stratigraphy and remote-sensing data to investigate the geology of the subsurface material excavated by the crater and determine that Aristarchus likely excavated a buried pluton, or hypabyssal intrusive body, related to the large, possibly bi-modal, Cobra Head volcanic complex on the southern Aristarchus Plateau. Measuring crater size-frequency distributions on the ejecta blankets of Aristarchus and Tycho were done to determine the timing of these impacts; however, my measurements revealed that there is a significant difference in crater density, irrespective of crater diameter, between impact melt and ejecta blanket units. I show that the difference in crater density between these units can most likely be explained by a mechanism of self-secondary cratering, where late-arriving fragments of ejecta crater the surface of the ejecta blanket after it forms, but prior to the arrival of impact melt flows. These measurements call into question the long-held notion that ejecta blankets represent completely resurfaced units through ballistic sedimentation, free of impact craters immediately after formation, and these measurements suggest that cratering flux over the last billion years of the Solar System may be considerably lower. Lastly, I use field observations and a number of state-of-the-art laboratory analyses of a sample of impact melt glass from the Mistastin Lake impact structure to study the decomposition of zircon grains and the provenance of the impact melt. From my measurements, I show that zircon grains from a mangerite target rock were entrained in a superheated melt of very low viscosity and quenched, preserving high temperature mineral phases, and revealing how zircon grains undergo decomposition in a natural sample.
Chair and Committee
Bradley L Jolliff
Ramesh Agarwal, Robert Dymek, Randy Korotev, Harald Hiesinger, William McKinnon
Zanetti, Michael Raymond, "Investigating the Complexity of Impact Crater Ejecta" (2015). Arts & Sciences Electronic Theses and Dissertations. 694.