About 1% of the earth's magnetic field at conventional aeromagnetic survey altitudes can be ascribed to crustal geology. The 2007 WDMAM global compilation of survey data with its systematic subtraction of non-geological effects has given the first uniform global view of crustal anomalies at a scale useful to global geological studies. The magnetization of rocks is partly due to their magnetic susceptibility coupled to their situation in the present-day geomagnetic field and partly due to remanent magnetization acquired in an earlier field. The familiar ocean-floor 'stripes' testify to the preponderance of the latter at sea.
On land, the situation is more complex, not least on account of the polyphase metamorphism that many older rocks have undergone. We have used the WDMAM anomaly grid to calculate a global equivalent susceptibility contrast map (ESCM) in an attempt to reduce the more obvious effects of variations in the direction and strength of the present inducing field over the earth's surface. While the assumptions made have limited validity where remanent magnetization is present, the image allows more objective comparison of geology in different parts of the world since the appearance of the geology in the image should be independent of its present location on the earth. Equivalent susceptibility values less than zero often signal the presence of remanence. The predominant magnetic effects are due to the igneous and metamorphic Precambrian rocks.
Using Dapple (www.dapple.geosoft.com) we have compiled outline interpretation maps of the major features of global Precambrian geology and set them in the context of geodynamic paleomaps illustrating the movements of the Precambrian shields over the earth's surface during the Phanerozoic.
