I’ve spent most of the last five years studying rocks formed in the last 10-20 million years. By normal human standards, that’s quite old. If each of those 20 million years was compressed into a single second, we’re still looking at about 8 months’ worth of geological history: by contrast, the average human lifespan would be about a minute, the whole of recorded history would take up the last hour and a half, and anatomically modern humans were almost literally born yesterday (to be precise, just over two days ago).
But now that I’m in South Africa, suddenly 20 million years doesn’t seem very old at all. Johannesburg owes its existence and wealth to the gold bearing rocks of the Witwatersrand Group (some background), which are almost 3 billion – 3 thousand million – years old. That’s 150 times older; going back to our compressed 1 year=1 second timescale, the Witwatersrand is on the verge of celebrating its first centenary and receiving a congratulatory telegram from the Queen, although it still has to defer seniority to the 140 year-old planet which hosts it. From 8 months to 100 years – it seems that one of the guys here was actually being pretty accurate when he sneeringly referred to my New Zealand mudstones as ‘baby rocks’.
So why am I here? Pretty as they can be, geologists are not so much interested in rocks for their own sake as they are in what they can tell us about the Earth’s distant past: what was the climate like? How fast were the tectonic plates moving relative to each other? What, if any, sorts of life were thriving? However, just as one’s grandparents can sometimes become a little forgetful in their old age, a rock’s memory of the time that it was deposited can also fail. As they age, they get faulted and folded by rifts and orogenies, heated by nearby volcanic intrusions, or buried deeply beneath upstart younger rocks; in the process, new minerals grow, or the properties of the original ones get altered, and the record of their younger days becomes increasingly hard to decipher.
However, many ancient South African rock sequences are, if not in pristine condition, remarkably sprightly for their age. There’s been some deformation, and there’s certainly been some igneous activity, but not enough that the memories of their youth have been completely obscured. That makes South Africa one of the few places in the world where we can study the early geological history of the Earth in detail: for example, if I can retrieve an early paleomagnetic signal from these rocks, we might get some valuable data about how plate tectonics worked back in the Archean (or even if there was plate tectonics as we understand it) – and if we’re very lucky, we might even be able to get some idea of what the Earth’s magnetic field was like. It’s potentially very exciting; but it’s also a bit of risk, because there’s no guarantee that we’ll actually be able to find and measure those early signals. I’m hoping that somewhere out there, there’s a few outcrops which still remember the days of their youth.