Geomagnetic surveys

The geomagnetic signature of impact structures may be much more complex than the gravity signature. There are three basic processes to produce magnetic anomalies in and around impact structures:

-- displacement of magnetized rocks in the impact cratering process
-- decomposition of existent rock magnetization (by shock, for example)
-- formation of new magnetic phases in rocks (e.g., by chemical alterations, by acquiring a thermal remnant magnetization). For more details see, e.g., Pohl, J. (1994): The effect of shock on magnetic properties of rocks and minerals. European Science Foundation Network: Impact cratering and evolution of Planet Earth. Third International Workshop on Shock wave behaviour of solids in nature and experiments, Limoges, France. Abstract book, p. 51.

A distinct magnetic negative anomaly is related with the Ries impact structure in Germany (see image below). The causative body is a several 100 m thick suevite breccia layer within the crater. Predominantly, the suevite has an inverse thermal remnant magnetization which was acquired upon cooling of the highly shocked breccia material below the Curie temperature (Pohl, J. (1965). Die Magnetisierung der Suevite des Rieses. - N. Jb. Min., Mh., H. 9-11, 268-276.)

A peculiar observation has been made in the Ries crater by measuring the magnetic properties of the suevite impact breccia and the overlying post-impact lake sediments. As mentioned above, the suevite has an inverse remnant magnetization, but the sediments deposited immediately after the impact show a normal magnetization. Thus it may be speculated (Pohl, J., 1978: Evidence for the coincidence of a geomagnetic reversal with the Ries impact event. - Meteoritics, 13, 600.) whether the impact could have caused a polarity reversal of the Earth's magnetic field.

Continuation, under construction ...