This is the World Magnetic Anomaly Map produced by the US National Geophysical Data Center (NGDC) from its extensive holdings of marine, airborne and satellite magnetic field measurements. These magnetic anomalies are caused by the magnetization of the Earth's crustal rocks. Blue means that the magnetic field is weaker than normal, red indicates a stronger field. The original map has a 3 arc minute resolution. Further information, related products, and acknowledgments of data providers can be found at http://geomag.org/models/wdmam.html

March 12, 2009
Note that a significantly improved version (EMAG2) of this map is now available.

Stefan
Nice job! My congratulations and 5 stars
Welcome to GEC!

Stefan:

This is just an outstanding dataset. It's just amazing to be able to see the trace of continental motion embedded in the seafloor. As I understand it, the magnetic polarity is a 'fossilized' remnant of the earths magnetization at the time the sea-floor was created. .

I'm trying to understand the patterns on the continents. For instance in California the magnetization seems to align with the topography and geology. The Central Valley is positive and the northern Coast Ranges are negative. Is this pattern also due to fossil magnetism or is there another effect going on here?

Yes, you are right about the oceanic magnetization. New oceanic crust is formed along the Mid-Ocean-Ridges and spreads away from the ridge axis. When the basaltic material cools to below about 500 Centigrade, the magnetic minerals "freeze-in" the direction of the Earth's magnetic field. Since the magnetic field reverses every couple of 100,000 years, one finds striped magnetization patterns parallel to the Mid-Ocean Ridges with alternating polarity.

Over the continents it is quite a different story. Here the dominant factors for magnetic anomalies are crustal composition and the thickness of the magnetized layer. Usually, young crust is relatively thin and has low content in magnetic minerals (e.g. western Europe). The old cratons, on the other hand, are thick and have high magnetic mineral content. A prominent anomaly can be seen, for example, at the boarder between young western Europe and the old Baltic Shield. Temperature also plays a role: When the crust is very hot locally and the temperature is higher than the Curie Temperature of the magnetic minerals, the crust is demagnetized, creating a contrast with the surrounding magnetized crust.

Magnetic anomalies are also caused by the contrast between oceanic and continental crust. Furthermore, subduction zones show up as strong anomalies. See for example the Summatra Trench which was the source of the large Indonesian Tsunami. Whether the subduction-zone related anomalies are just due to the "doubling" of the crustal thickness, or due to the specific mineralogy is still not well understood.

Stefan

Stefan:

Thanks for the tutorial - One more question

Quote:

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Temperature also plays a role: When the crust is very hot locally and the temperature is higher than the Curie Temperature of the magnetic minerals, the crust is demagnetized, creating a contrast with the surrounding magnetized crust.

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Is this the source of the blue magnetization pattern in central Nevada which is kind of a blob and the Snake River plain which seems to show the track of the Yellowstone hot spot?