It seems fitting that my first real post should be about the Earth’s magnetic field very early in the planet’s history. This also ties in nicely with a paper that I’m first author on that just gone into press. The paper has the rather long-winded title: “Palaeomagnetism of Archaean rocks of the Onverwacht Group, Barberton Greenstone Belt (southern Africa): Evidence for a stable and potentially reversing geomagnetic field at ca. 3.5 Ga”. The number is the important bit – it tells you that we’re trying to look as far back as possible into the planet’s history (Ga translates as “billions of years” – the age of the Earth is 4.5 Ga) and see what the Earth’s magnetic field was doing. There are older rocks on the surface of the planet (in Greenland and Canada, these date back to 3.8 or even 4.0 Ga) but these have been pretty well-toasted and therefore have lost any info they might have held about the magnetic field at that time.

The oldest rocks which haven’t been too heated or mashed up are in the Pilbara Craton in northwestern Australia and the Barberton Greenstone Belt in eastern South Africa. It’s the latter that we’re talking about here (see picture). Why should we be interested in the Earth’s magnetic field so long ago? At some point, I will post a general piece on why it is important to study the magnetic field in general, so for the moment I’ll just say two very brief things. The first is that it might help us piece together what was going on in the deep interior of the Earth (the core and around) at that time which we really don’t have much of an idea about. The second is that it can potentially tell us about how much protection the atmosphere and earliest life on the planet (evidence for which can be found in these rocks) had from the barrage of particles hurled towards from the sun in the form of the solar wind.  

Some recently published results from rocks in the Barberton area which were used to measure the strength of the magnetic field at 3.5 Ga received some fairly spectacular media attention (front page of BBC news!). Measuring the strength of the ancient magnetic field (it’s “palaeointensity”) is particularly difficult – in our study we were just interested in measuring its direction. Our new data, when taken as a whole, support the reliability of the magnetic record in rocks from this area generally but do cast some doubt on the direction associated with the famous intensity result. They also add to our body of knowledge about the Earth’s magnetic field at this early time.

So, back to the title of this post, what do we know about the early Earth’s magnetic field? Well, the first thing to note is that it is now looking increasingly likely that it did actually exist. It is certainly not a given that the Earth has generated a global magnetic field through it’s entire history and to know that it probably did back then implies that conditions in the core of the planet were suitable to do so. The consistency of the directions that we and others have measured from rocks differing in age by tens of millions of years imply that the field was probably more or less stable like today’s. That is, the magnetic poles had an average position that was fairly fixed (very likely close to the geographic poles). This consistency also suggests that, if the landmasses on which these rocks formed were moving as result of early plate tectonics, then they were doing so at a rate not all that different from today’s (i.e. not much more than 10-20 cm/year). One of the most exciting results to come out of our new study is the suggestion (and I go to great lengths to stress the tentative nature of this evidence) that these rocks record a geomagnetic reversal. Reversals are a phenomenon whereby the north and south magnetic poles suddenly (ok, that’s a geologist’s suddenly, it takes thousands of years) swap positions with one another. They are a defining characteristic of the Earth’s magnetic field at least back to 2.8 Ga but this is the first evidence of their occurrence as far back as 3.5 Ga. Finally, there’s the strength of the field as measured by the Rochester group (and modified by this study), a little weaker than today’s field but well-inside the range measured during the last few hundreds of millions of years.

Does all this sound a bit boring? (Actually, don’t answer that). Well, it certainly would have been exciting to have found the ancient field behaving very differently to that of more recently. Nonetheless, the fact that it appears to be so strikingly similar is interesting in its own way. Our understanding of the dynamo processes generating magnetic fields in planets, stars, asteroids, etc says that the way the field behaves is rather sensitive to the conditions at the location where they are produced. Only a fairly narrow set of conditions can produce a field like the Earth’s that is dominated by a dipole (a single pair of N and S poles rather than something more complicated), aligned with the rotation axis (i.e. the magnetic poles are close to the geographic poles) and which reverses polarity now and again after random amounts of time. The Earth was rather different 3.5 billion years ago. Although it’s difficult to know by how much, it was certainly hotter and rotating faster than today. It was also very likely missing the solid inner core which it has today, its core back then being entirely liquid iron alloy. That, despite these potentially enormous differences, the field generated in the core was so similar strikes me as very interesting.

Before finishing this post, I should stress that I am certainly not saying that the field was identical back then. We only have the roughest of rough ideas so far. Research by myself and others has suggested that the field around 2.5 billion years ago (when we have better records) was rather more stable than today’s – the poles seemed to be wandering less and flipping less frequently. This could also be true for the older time I’m talking about here, there just aren’t the results to say that yet.

To finish on a personal note, this paper was not the easiest I’ve ever written and has been a long time coming (it feels like 3.5 billion years). I’m proud but also bl**dy relieved to finally see it in print. I’ve roped Laura Roberts Artal, a new PhD student here at Liverpool into working on Barberton stuff from hereon in. Hopefully that means the next publications will come easier (for me at least :-)).