14 February, 2006

The Art of Palaeomagic

Some of you may be curious about what I’m referring to when I list ‘Palaeomagic’ under my Specialist Subjects on the left. Even if you’re not, I’m going to explain it to you anyway. Palaeomagic is the in-house nickname for the field of palaeomagnetism, or paleomagnetism (as a concession to our American friends – just another example of their dodgy spelling), a discipline with which I have become rather familiar during the course of my PhD.

Palaeomagnetism, literally ‘ancient magnetism’, is the study of the signals left in volcanic and sedimentary rocks by the Earth’s magnetic field; signals which can be preserved over millions of years of geological time. On a broad scale, the geomagnetic field closely approximates a dipole centred near the axis of rotation.

The shape of this field should look familiar to most people: it resembles that of a giant bar magnet. This does not mean that the geomagnetic field is caused by a giant bar magnet at the Earth’s core; in fact the ultimate source is thought to be convection of liquid iron in the outer core (this is a subject worthy of its own post, which I may supply in due course). A compass needle will align with the field lines in the figure above, which curve from the south to north pole (due to the way magnetic polarities were first defined, the current magnetic north pole is currently located near the geographic south pole). This curvature means that although a compass needle will always align north-south in the horizontal plane, which is why compasses are useful for navigation, in the vertical plane there is a variation in the inclination (or dip) of the magnetic vector with latitude, from being horizontal at the equator to vertical at the poles. Also, a compass needle will point downward in the northern hemisphere, and upward in the southern hemisphere. Measuring the direction of the magnetic vector gives us information about where we are on the Earth’s surface, which will become important later.

The “palaeo” in palaeomagnetism comes from the influence the geomagnetic field has on ferromagnetic minerals, which can record the direction of an applied magnetic field even after it has been removed, as rocks are forming. In igneous rocks, minerals such as magnetite crystallise directly out of the lava as it cools. Below a certain temperature (~600oC) magnetite becomes ferromagnetic, and the crystals become magnetized parallel to the direction of the geomagnetic field. In sedimentary rocks, ferrmagnetic minerals tend to align with the geomagnetic field as they settle, and preserve this orientation as they are compacted and lithified. In both types of rocks, this alignment can be preserved as a remanent magnetization over millions of years of geological time.

The essence of palaeomagnetism is the measurement of these remanent magnetisations. Why bother? Because the magnetisation directions measured, in rocks of different ages at the same location, change over time. This information is useful in several ways, including:

  • Reconstructing continental drift.Changes in inclination indicate a change in location relative to the northern geographic pole over time, which is evidence for movement of the continents. For example, in the UK, Devonian rocks have magnetisations with a very shallow inclination, indicating that they formed very close to the equator. This cheerfully coincides with the fact that most of the Devonian Rocks found in Britain are part of the Old Red Sandstone, which appears be formed mainly from sediments eroded from a dry, desert-like landmass which as the rain beating against the window as I write this reminds me, is hardly consistent with the weather at our present latitude.

  • Dating sequences using magnetostratigraphy. If you measure a continuous sequence of rocks, spanning a few tens of millions of years, you will make the surprising discovery that every few million years on average, the inclinations change polarity. Rocks in the northern hemisphere will not record a downward pointing magnetisation as they do today, but an upward pointing magnetisation; inclinations from rocks in the southern hemisphere are likewise reversed. This is evidence of polarity reversals, when the two magnetic poles switch geographic location: the north pole goes to the south pole and vice versa.

    Because geomagnetic reversals are global events, rocks of a similar age will record the same sequence of reversals, allowing the correlation and dating of rock sequences worldwide. This is the basis of the geomagnetic polarity timescale

  • Measuring crustal rotations. This was what I spent my PhD doing. If you measure the remanent magnetization direction of 5-20 million year old rocks from New Zealand, you will find that in many places, the declination (the orientation of the magnetic vector in the horizontal plane) has changed substantially, so that instead of pointing north, they point north-east or east. In other words, parts of New Zealand have rotated almost 90o clockwise in the last 20 million years. It turns out that such rotations are very common in areas where deformation is spread across several faults. Without palaeomagnetic measurements, measuring this type of deformation would be very difficult.

I hope to explore some of these applications in more detail in future posts. For now, one question remains to be answered: why “palaeomagic”? Well, it does almost seem like magic sometimes: you can’t see a remanent magnetisation, you can’t feel it or smell it, and it is often a property of only a tiny fraction of the mineral grains in your sample. Also, the post above sketches over the details of exactly how you measure a remanent magnetisation. When I started my PhD, a study of tectonic rotations in New Zealand, I viewed palaeomagnetism as a simple tool: stick rocks in, get direction out. It turns out its not that easy. Some rocks guard their magnetic secrets jealously, behind a wall of present day overprints and remagnetisations; it requires a lot of work and head-scratching to coax any useful information out. To illustrate, I’ll note that on submission three and a half chapters of my PhD thesis were devoted to unravelling the remanent magnetisation of New Zealand sediments. Only then could I actually start talking about the tectonic story, which was what I was actually interested in. Without knowing the palaeomagnetic story, however, there was no coherent tectonic story; it was palaeomagic which transformed confusion into understanding.

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