The Bangui Magnetic AnomalyYou may not think about it, but right now you’re bathed in a magnet

The Bangui Magnetic AnomalyYou may not think about it, but right now you’re bathed in a magnetic field. Flowing currents of extremely hot iron in Earth’s outer core are generating a dipole magnetic field; basically the same pattern is produced in Earth’s core as would be produced if there were a giant bar magnet running across the core.This dipole field is the largest fraction of Earth’s magnetic field, but it’s not the only part of it. There are smaller currents within the core that create distinct anomalies; zones of additional strength or weakness in the magnetic field observed at the surface. The Earth’s magnetic field also interacts with charged particles released from the sun out in space; this interaction moves charged particles around the Earth and also generates a magnetic field that is ~1-3% the total field strength at the surface.Finally, rocks at the Earth’s surface are also cold enough to take in and store a magnetic field. When rocks form at the planet’s surface, they record the magnetic field direction surrounding them; generally that field is the one generated by the planet’s core. Because rocks form at different times and then move around the planet’s surface, the rocks of the planet’s crust forms a series of anomalies. Some of these anomalies are in the same direction as the current magnetic field and add to it, others are in the opposite direction and when added to the current magnetic field they weaken it. This effect is generally small, again about 1-2% the strength of the full planet’s magnetic field.A particularly unusual magnetic anomaly is found centered in the Central African Republic. That anomaly was termed the Bangui Magnetic Anomaly after that nation’s capitol in the early 1980s.The Bangui magnetic anomaly is one of the strongest crustal magnetic anomalies on Earth. At the surface its strength is about 1000 nT (nanoTeslas, the unit for magnetic anomaly strength), which in this area is just under 5% of the strength of the full planet’s magnetic field.On this plot, the Bangui anomaly is shown as a large body of strong magnetic intensity in central Africa, as dark reddish colors bracketed by two zones magnetized in the opposite direction in the weaker blue colors.Other magnetic anomalies of this strength are associated with large bodies of iron-rich magnetic minerals, but not this one. The Bangui Magnetic Anomaly is a large, elliptical zone of odd magnetic intensity in the middle of Africa with no obvious linkage to a known geologic unit. Something buried deep in the Earth’s crust in this area is extremely anomalous and it has never been sampled at the surface.There are two major ideas for what could cause this anomaly. It could be a deeply-buried igneous unit as igneous rocks like those of the mid-ocean ridges commonly contain significant amounts of magnetite (that’s why mid-ocean ridges do a good job of recording the changes in Earth’s magnetic field). A thick sequence of basaltic rocks at depth could explain this anomaly.Alternatively, it has also been proposed that this anomaly is the result of an ancient asteroid impact. Asteroid impacts heat and shock the target rocks, changing the chemistry in such a way that would make them more easily magnetized and also remagnetizing the entire province at once so that it has a single, strong direction. Other craters around the world show strong and uniform magnetic anomalies, but none of them rival the Bangui anomaly in intensity.Both of these hypotheses remain possible to this day and until deep crustal rocks are sampled or drilled in this area, the debate may remain unsettled.-JBBImage credit:https://en.wikipedia.org/wiki/File:Bangui_anomaly.JPGReferences:http://adsabs.harvard.edu/full/1999M%26PSA..34Q…9Ahttp://bit.ly/1YmE3sghttp://www.lpi.usra.edu/meetings/metsoc99/pdf/5087.pdfhttp://bit.ly/1Oic7Tu -- source link
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