One of the most striking changes in Earth Science in the last 20 years has been the way meteorite and associated impacts (or bolides and astroblemes, if you prefer) are viewed by Geologists. In the dark days of the 1990s they tended to be viewed as an annoying thing that people from NASA kept going on about. Now they are a standard part of mainstream Geology, not least because of the large number of geological features now linked with extra-terrestrial impacts.
The most dramatic form of evidence is a crater, but large or small, these only affect a small area of the earth. Excitingly, there are more subtle forms of evidence that are much more common and might be stumbled across by any geologist in the field.
You may have read about suevite, a deposit containing glass found close to impact craters. The glass was formed by shock melting at the impact site and is thrown out from the crater. Suevite is formed by material that falls close-by, but some of the glass is thrown high into the atmosphere and can land 100s of kilometres away. Big impacts can form a thin layer of bits of glass that cover most of the earth’s surface. The glass that goes a long way is most likely to have flown in the air as small droplets, known as spherules.
These layers are thin and not particularly dramatic, but they are the evidence of an impact event most likely to be preserved in the geological record. The latest copy of Geology has a paper (doi: 10.1130/G31526.1) that nicely illustrates this. It describes a spherule layer, 2.57 Ga in age from Western Australia. The description of the layer itself is nice enough; the glass is long since devitrified, plus there is evidence that crystals grew in flight (before they were quenched by landing in the ocean). The glass is of mafic composition, suggesting the impact hit oceanic crust.
From Australia to Outer Space
The main interest of the paper is that this is by no means the first layer to be found. Three other layers are known, both in this Western Australian basin and in one of similar age in South Africa. The age interval of the sediments containing these four spherule layers is 140 m.y., which suggests major impacts were quite often at this time. It is dangerous to extrapolate from so few data points, but it suggests an unusually high level of impacts at this time (Archean-Proterozoic boundary). Rates of impact frequency are generally thought of as a solar-system wide phenomena with the intensity reducing over time. This can be thought of as the solar system gradually getting tidier as old junk left over from the formation of the solar system is swept up by planets. Evidence that rates briefly increased suggests this picture is over-simplified.
Dating of features on other planets is generally done with reference to the number of craters cross-cutting them: the Earth is by far the easiest place to get absolute dates. The Geology paper’s suggestion that spherule layers on Earth can be used to infer variations in the rate of impacts is intriguing. If correct, it would mean that we can put constraints on solar-system wide processes by the detailed study of old sediments. Looking at dirty-looking outcrops in Australia can maybe help us understand other planets more.
Not just Australia
The paper shows a picture of an outcrop of the spherule layer. It is narrow and hardly jumps out at you. Yet it has been identified in three different places, which is impressive. The paper thanks a couple of mining companies for access to sites, and the area is the site of major iron ore mining, so I suspect there has been some very detailed logging of the sediments done. Mining in the area boomed due to Chinese demand. Look around you, you probably see something made in China and if it has any iron in it, then Western Australia seems nearer than it did before.
There are bound to be many more spherule layers around that have yet to be identified. Phanerozoic ones may be harder to spot due to bioturbation, but there is an example from the Triassic of England (nice description here). So, the next time you are looking at sediments, keep your eyes peeled for spherules; you are the first generation of geologists to understand how significant they really are.