A brief history of Earth – part 1

A couple of weeks ago, I got into a spot of bother for suggesting that only geologists have the required sense of perspective to appreciate how contingent Life on Earth actually is (i.e. that things could quite easily have not turned out so good)… This is, therefore, the first part of a much-delayed review of the Australian Broadcasting Corporation’s Australia: The Time Traveller’s Guide, as shown in the UK a few weeks ago on the Eden Channel; and still viewable in its entirety on a very flash part of the ABC’s website (which takes a while to load over all but the fastest fibre-optic connections). The latter is reachable from this synopsis (i.e. if you follow the link to the [ABC’s] ‘Official Website’). If this programme was not part-financed by the Australian Government’s Department of Foreign Affairs and Trade (i.e. to promote tourism) it should have been… Presented by Professor Richard Smith, apparently Australia’s equivalent to the University of Plymouth’s Iain Stewart here in the UK, the programme is simultaneously a marvelous testimony to the very extensive geological record Australia contains – and a wonderful evocation of its amazing landscape. It certainly succeeded in making me wonder, having worked in the north-west of Western Australia (WA) for 3 years in the 1980’s, why I ever left. There is so much that could be written about this brilliant series of programmes that I cannot promise to ever finish it: Instead, I propose to just see how far I get and, however far that is, it should be borne in mind that the over-arching purpose is to illustrate just how lucky we are to be here to marvel at our own existence. Episode 1 of the series covers the first 90% of Earth History (up to the first flourishing of complex life in the Cambrian Period) and Richard Smith starts his journey driving across what he reminds viewers is the “oldest, flattest, driest continent on Earth” (i.e. Australia is extreme in all 3 senses – Antarctica is larger and in places drier but not uniformly so and, ignoring the ice, neither is it flat). People are familiar with the size of Australia (i.e. Perth to Sydney being comparable with Los Angeles to New York) but many struggle with the vastness of geological time. To illustrate this point, Smith sets his Time Travelling Sat Nav to the beginning of Earth History and sets off at an imaginary speed of 60 million years per hour. At this speed, progress would be as follows: — After 4 hours, you would have almost travelled back to the Permian/Triassic extinction (251 Million years [Ma] before present [BP]); — After 8 hours, you would have almost travelled back as far as the Cambrian Period (542-488 Ma BP); — After 2 and a half days, you would have travelled back to the emergence of simple forms of life; and — Only after 3 days, would you finally arrive at the beginning of Earth history (some 4540 Ma BP). Amidst all the current talk of global warming, it was striking that Richard Smith should point out that it is generally accepted that, having coalesced from the leftovers of an earlier supernova – and survived an early collision between two proto-planets (resulting in the formation of our Moon) – the Earth has spent the rest of its history trying to cool down. However, the fact that the Earth’s core is still warmer than the surface of the Sun is no cause for complacency regarding the reality that humanity is currently warming the planet up again. Life on Earth is adapted to the way things are on the surface of the Earth; not the surface of the Sun. Convection within the Earth’s interior is of course the reason for plate tectonics and – thanks to the Earth being bigger – it has not cooled down as much as Mars, where plate tectonics has ceased. However, fortunately, it has not been as active as Venus, where the build-up of greenhouse gases (GHGs) in the atmosphere has pushed surface temperatures and pressures there to levels approximately 90 times those here on Earth: Thus our Goldilocks planet has retained conditions capable of preserving water in liquid form at its surface. However, the lesson we should take from observing both Mars and Venus is that this could quite easily not be the case. Having set the scene, Richard Smith’s first stopping point on his time traveller’s guide to (Australian) geology is the Jack Hills in WA, where crystals of zircon – a rare rock-forming mineral (with a very high melting point) – have been geochronologically dated (using the fact that radioactive decay of elements within the crystals began when they first formed and has not been interrupted since) to approximately 4400 Ma BP. The rocks of the Jack Hills may be a little younger (although still amongst the oldest on the planet) but these zircon crystals are the oldest known minerals on Earth. Next, Smith visited Mileura Station in the Pilbara (NW of WA) looking for evidence of the earliest life forms on a planet that for a very long time had no oxygen in its atmosphere. In many ways, it is astonishing that any can be found because, as is evidenced by the surface of our Moon, for much of the Earth’s early history it was being bombarded by meteorites. Therefore, the first pre-requisite for the emergence of life was almost certainly the end of this bombardment, which is believed to have slowed significantly about 3800 Ma BP. To illustrate this point, Smith also visited the comparitively recent (300,000 year-old) Wolfe Creek Crater (which at nearly 900 metres in diameter was formed by a meteorite just 10 or 20 metres across). Sure enough, evidence of early bacteriological life has been found in rocks dated to 3500 Ma BP (at the hottest place in WA – Marble Bar); and the bacteria, Stromatolites, can of course still be found living in Shark Bay – where evaporation maintains the seawater at twice its normal level of salinity. Over a considerable period of time these early photosynthesising bacteria oxygenated the seawater (although it still contained large amounts of other gases we consider toxic like hydrogen sulphide). These stromatolites therefore shared the planet with other extremophiles (i.e. organisms that can live in extreme conditions that most life cannot; now found only at the bottom of the ocean where there is no light and/or in volcanic vents where acidity can be as high as a pH of 1). However, by 2500 Ma BP, the oxygen released by the Stromatolites allowed the most abundant element on Earth to combine with oxygen for the first time to form iron oxide. This gave rise to one of the Pilbara’s most widespread rock types – Banded Iron Formation (BIF) – alternating layers of grey iron oxide (deposited in oxygenated sea water) and white silicon oxide (deposited in oxygen-deficient seawater). Although generally thought of as being red-brown in colour, along with much of the remainder of Australia, this (e.g. see my photos here) is a consequence of a much more recent process of subaerial oxidation of all iron-rich rocks exposed to an oxygen-rich atmosphere. I am not even half way through the story of Episode 1 but – I think this is enough for one day…

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