Magnetic anomaly map finished

[A guest post by palentologist and geologist Chris Nedin]

It's taken the best part of 50 years but it's finally here! 50 years after the International Geophysical Year (1957-8) that took a global geophysical view of the globe, one of the outcomes of that global geophysical view has just been published – the Digital Magnetic Anomaly Map (Korhonen et.al. 2007) (the BBC has the story and an annotated copy of the map here).

The map shows magnetic intensity readings of crustal rocks from around the world, combined on a single map for the first time.

Magnetic intensity is what's left after the magnetic field of the earth (which varies from 35,000 nanoTesla at the equator to 70,000 nanoTesla at the poles) has been removed. Rocks containing magnetic-friendly minerals, or metallic ore bodies, produce a strong magnetic intensity (+8,000 nT), rocks which do not contain such minerals, such a sandstone and other sediments, produce a weak magnetic intensity (-3500 nT). Magnetic intensity can therefore pinpoint areas with high magnetic anomalies for mineral or metallic ore exploration, or conversely, areas with low magnetic anomalies, can represent sedimentary basins for oil exploration.

It's a bit blobby because the resolution is very course – each data point represents a 5k square. This has been done to protect confidentiality as many countries and companies which have provided the data want to protect the information on their resources that a higher resolution may provide. This issue of confidentiality was one of the reasons that it has taken so long to gain approval to use the data and publish the map. Other reasons are political issues, and, well, it takes time to map the globe.

The course resolution still allows regional structures to be seen, so helps in a global study of the Earth, as well as pinpointing areas of interest for further study, and, through palaeo-reconstructions of the continents, allows us to study the evolution of regional structures and trends.

One of these regional structures happens to be an iconic image of 20th century geology – the magnetic stripes across the north Atlantic Ocean.

After the Second World War, while evidence began to mount in support of the idea that continents moved, the theory of continental drift suffered from one fatal flaw – the mechanism of continents ploughing over the lower mafic crust relied on the suspension of the laws of physics – the force required to push continental crust through the lower crust required enough energy to comfortably melt the crust. (the idea of an expanding Earth suffers from a similarly law-suspending, fatal flaw.)

At the same time, a new era of undersea mapping was underway, using sonar and magnetic equipment no longer required to hunt submarines.

In the late 50's, using the new maps of the ocean basins that included for the first time the extensive mid ocean ridges network, Harry Hess formulated a new theory about continental drift. His "History of the Ocean Basins" (finally published in 1962) theorised sea floor spreading, whereby oceanic plates were continually being formed at mid ocean ridges to be moved by underlying convection currents and subsequently destroyed at subduction zones.

Apart from mapping the mid ocean ridges that were central (ha!) to Hess's theory, studies showed strange magnetic anomalies in the north Atlantic. These anomalies appeared as stripes of high and low magnetic intensity running like a bar code across the ocean floor, paralleling the mid ocean ridges. There was no known mechanism to produce strong magnetic and weak magnetic compositions in such linear patters, so the anomalies remained, well . . . anomalous.

It was Fred Vine and Drummond Matthews who realise that the stripes did not represent compositional anomalies, but polarity anomalies.

Magnetic minerals above a certain temperature (the Curie point) will align their magnetic field to the Earths' magnetic field, so that the north pole of the mineral's field point to the Earth's North Pole. Once the rock cools past the Curie point, the magnetic field is frozen in place and subsequent changes to the earth's magnetic field will not alter the orientation of the mineral's magnetic field. V&M argued, in their 1963 paper "Magnetic anomalies over Oceanic Ridges", that the low intensity stripes represented oceanic crust that had formed at the mid ocean ridge and cooled past it's Curie point during times of polar reversal and so it's magnetic field had frozen with north pointing to the Earth's South Pole. Modern magnetometers, calibrated in the Earth's current magnetic field, would read a reversed field as a weak magnetic intensity. Thus the explanation did not require exotic compositional variation, but simple magnetic polarity reversal. Their idea showed that the oceanic crust was indeed formed at the mid ocean ridge, and moved away to be subducted at the subduction zones

This was not accepted immediately, as polar reversal was still a controversial theory, but during the 1960's as more and more evidence for reversals came it, both polar reversal and V&M's theory were accepted.

Actually it was Lawrence Morley, a Canadian, who first came up with the idea that the stripes represented magnetic reversals, but he couldn't get published and Vine and Matthews did. [Man I'd love to know the publication that bounced LM's paper! Talk about the one that got away! A pity though, because it robbed LM (and Canada) of the international recognition he (and it) deserved.]

Also during the 1960's, more supporting evidence came in, as Tuzo Wilson (another Canadian) formulated an explanation for the presence of transform faults associated with ridges, and in 1968, a number of cores cut into the Atlantic ocean floor showed that the sediments immediately above the ocean plate get older the further away you went from the ridge.

It turned out however, that while the evidence supported Hess's theory of sea floor spreading, his underlying (ha!) mechanism for moving the plates - convection cells in the mantle that welled up at mid ocean ridges and sank down at subduction zones - was wrong.

Work by Dan MacKenzie and Keith O'nions in the 1980's showed that the forces associated with ridge push at mid ocean ridges and slab pull at subduction zones, were sufficient to generate, move, and eventually sink oceanic plates without recourse to mantle convection cells.

The modern theory of plate tectonic is therefore made up of brilliant intuition coupled with hard research, but the false colour image of magnetic stripes across the north Atlantic remains one of the easiest accessible evidences for plate tectonics, and one of the most recognisable images in Geology.

Korhonen, J.V., Fairhead, J.D., Hamoudi, M., Hemant, K., Lesur, V., Mandea, M., Maus, S., Purucker, M., Ravat, D., Sazonova, T.,and Thebault, E., 2007, Magnetic Anomaly Map of the World (and associated DVD), Scale: 1:50,000,000, 1st edition, Commission for the Geological Map of the World, Paris, France.