Magnetic Signatures of Terrestrial Meteorite Impact Craters: A Summary

Abstract

This chapter summarizes the magnetic characteristics of meteorite impact craters. Magnetic mineralogies of both impact melts and target rocks are described, as are the paleomagnetic signals they retain and the magnetic field anomalies they produce. Particular emphasis is given to five craters studied under the umbrella of the Deutsche Forschungsgemeinschaft’s Schwerpunktprogramm, Planetary Magnetism: Manicouagan (Canada), Mistastin (Canada), Ries (Germany), Rochechouart (France), and Vredefort (South Africa), with a synthesis from other craters worldwide. A special problem addressed here is whether shock waves generated during impact influence the geodynamo. We conclude that the seismic energy released during the formation of craters up to 90 km in diameter is insufficient to disturb the dynamo process in a way that would provoke observable changes in field direction or intensity at the Earth’s surface. We show that shock can permanently modify magnetic properties of the target rocks; however, it is difficult to assess the relative influence between thermal and pressure effects on their remanent magnetizations. Distinguishing between shock and thermal overprinting and then unraveling these signals from the original remanence remain important problems that bear on the interpretation of magnetic anomalies in impact craters as well as our understanding of heat production from collision. Paleomagnetic directions from impact melts and suevites are well clustered at each crater, which suggests that building of the structures was completed before the ferrimagnetic minerals cooled through their Curie temperatures.