Geoscientist’s Toolkit: Paleomagnetic Coring

Recording rock core orientation for paleomagnetic analysis.  Image credit: Bill Mitchell.
Recording rock core orientation for paleomagnetic analysis. Image credit: Bill Mitchell.

I’ve touched on paleomagnetism a little bit before, both as a technique for tying rocks in to the geologic timescale, and as something which can be found by using a fluxgate magnetometer. It’s a pretty interesting set of techniques and uses some cool science tools, so I thought I’d explain a little bit more.

Magnetism from the Earth’s magnetic field can be retained by individual layers of rocks, at least under some circumstances. If you have a bunch of layers stacked on top of each other like pancakes, the different layers (beds) can have different magnetic directions.

Stack of banana-walnut pancakes.  Although probably low on magnetic minerals and too thin individually for magnetic coring, they do illustrate the concept of layering quite nicely.  Image credit: Jack and Jason's Pancakes (CC-BY-SA).
Stack of banana-walnut pancakes. Although probably low on magnetic minerals and too thin individually for magnetic coring, they do illustrate the concept of layering quite nicely. Image credit: Jack and Jason’s Pancakes (CC-BY-SA).

As you might expect, the equipment needed to make sensitive measurements of the magnetic field are not particularly portable (and may be a topic for another post). Samples need to be collected in the field and brought back to the lab, and the sample orientation must be marked and recorded in such a way that the measured magnetic field can be related back to the magnetic field in the rock itself.*

To do that, paleomagnetists (or paleomagicians) will drill a small (1″ diameter by a few inches long) annular hole into the rock, leaving a plug of rock in the center. That will become the sample. Before it can be removed from the hole, a mark is made on the top of the plug with a brass rod. The direction of the hole is determined with a compass (or a sun compass when conditions allow), as is the angle away from vertical of the core (the hade).

When the plug is freed from the rock, the down-hole direction is marked with arrows along the mark using a permanent marker. The samples (several from each bed) are then placed into sample bags, labelled appropriately, and carefully transported back to the lab.

Are you irresistibly attracted to such a magnetic field of study? This is probably the best place to go for more information, and is freely accessible online.[1]

***
[1] Tauxe, L., Banerjee, S.K., Butler, R.F. and van der Voo R, Essentials of Paleomagnetism, 3rd Web Edition, 2014. [accessed Aug. 27, 2015]

* The field magnetic field?

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