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Scotland’s Earth-shattering secret: how to find a meteorite impact crater

Samuel Lickiss, Production Editor in the Geological Society’s Publishing House, on two papers published today in the Journal of the Geological Society with different ideas about the potential location of a 1.2 billion year old meteorite crater in north west Scotland…

Download the papers free from our Lyell Collection:

We hear the term ‘earth-shattering’ a lot in the media. Everything is ‘earth-shattering’, be it the 2016 Brexit referendum and the subsequent ‘meteoric rise’ of the Brexit Party (which I’ve always thought was a weird idiom: meteorites fall) or Bianca receiving some ‘earth-shattering’ news about Ricky on Eastenders.

But you know what’s actually earth-shattering (and meteoric)?

Meteorites.

Specifically, one that collided with northwest Scotland 1.2 billion years ago.

Back then in the pre-Cambrian, Scotland hovered a little north of the equator where it experienced plentiful sunshine and a dry climate (i.e. nothing like contemporary Scotland). NASA’s Planetary Defence Team (cool job) didn’t exist to save our skins from incoming hunks of rock, nor did Bruce Willis and his scrappy crew of drillers and geologists. If something was going to collide with us, there wasn’t a whole lot the bacterial slime which covered the seabed could do about it.

The Stac Fada Member deposits

Stac Fada Member

Stac Fada Member

The Stac Fada Member deposits near the village of Stoer are strange to a geologist’s eyes: greenish-grey rocks flecked with fragments of glass squeezed and folded in between blocky layers of the Torridonian sandstone that characterises the area. How did they get there?

For years, the best explanation was volcanism, which fits with the presence of glassy minerals in the rocks, but even then, the intricacies of the rocks defied easy interpretation. That changed in 2008 when Kenneth Amor, then a PhD student at the University of Oxford, stopped at Stoer to look at the Stac Fada Member rocks while demonstrating a first-year undergraduate field trip.

‘I was immediately struck by the textual similarity of the green devitrified glass and a piece of the 15-million-year-old suevite from the Ries impact crater in Germany,’ he says. Suevites are a type of impactite: rocks formed from the massive temperatures and pressures associated with impact events, usually meteorites or, more commonly in modern times, nuclear weapons testing.

The small town of Nördlingen in Bavaria, southern Germany, sits within the Nördlinger Ries impact crater 24 km in diameter on graphite-rich bedrock. The force of the impact of a meteorite estimated to be 1 km in diameter was so powerful that it forced the carbon atoms to rearrange themselves from the characteristic hexagonal structure of graphite to the tetrahedral structure of diamonds, something that ordinarily only happens deep within the earth’s crust: the buildings of Nördlingen are built from local stone that glistens with billions of tiny diamonds.

There are no diamonds in Stoer, but meteorite impacts leave behind other tell-tale signs, the most characteristic being shocked quartz.

Photomicrograph of a shocked quartz grain (0.13 mm across) from the Chesapeake Bay impact crater, showing shock lamellae

‘I brought back a block of the Stac Fada to Oxford and asked the thin section lab to make up a couple of thin sections, with the idea of searching for shocked quartz. I didn’t expect to find any because the rock has been examined by countless geologists over the last hundred years.’ Impatience got the better of Ken, because while the lab was doing the painstaking work of grinding down rocks to sheets little thicker than that of a piece of paper for microscope observations, he remembered they had slides already prepared in the undergraduate teaching collection.

‘I spent the afternoon scanning those and that was when I found my first grain of shocked quartz.’ Decades of geology students and their teachers had looked at these same slides, not noticing the tell-tale signs of shocked quartz striped with lines called shock lamellae.

After presenting his discovery to his supervisor, Stephen Hesselbo, they arranged a series of subsequent field trips to the area for more detailed examination, confirming that the Stac Fada rocks, previously thought to be of volcanic origin, were in fact debris ejected from a mighty meteorite impact: likely Europe’s largest.

The next question: where is the impact crater? Unlike Nördlingen, which is nestled in its roughly circular bowl-like impact crater, the geology around Stoer is rather more complicated (fiendishly so). The Ries impact event occurred 14.5 million years ago in the Miocene, yesterday in geological terms, whereas the rocks of northwest Scotland have had a little over a billion years to mess themselves up.

This question is the subject of two papers published in the Journal of the Geological Society this week.

Download the papers from the Lyell Collection free:

An alternative theory

Science, like the geology of Stoer, is rarely neat and uncontentious, and geologists presented with the same evidence can come to rather different conclusions. Over a billion years, much of that evidence has been destroyed by forces like erosion, folding and faulting, and the skill lies in being able to figure out what would have been there.

Challenging Ken Amor’s interpretation that the site of impact lies 20 km northwest off the coast in The Minch, roughly halfway between the mainland and the Isle of Lewis, is Dr Mike Simms, who first visited Stoer in 2011 while on holiday.

‘I just wanted to pay homage to the site, collect a couple of pieces, and then on get on with the rest of the holiday!’ While at Stoer, he found himself particularly drawn to the 3 billion-year-old chunks of Lewisian gneiss, the bedrock, embedded in the sandstone immediately below the Stac Fada Member deposits. ‘They had all the hallmarks of spallation ejecta blocks launched in the first moments of the impact, and they seemed to have been overlooked.’ In other words, they’d been forced from the ground upon impact then redeposited in with the younger sedimentary rocks.

After this flying visit, Mike found himself gripped by his observations, returning to Scotland again three months later to do a more detailed study, particularly looking for anything that could give some indication of the direction of movement of the sediment. Mike published his original findings in PGA in 2015 following detailed field studies around Stoer and the wider area, concluding an eastern impact crater. This latest research re-evaluates his original conclusions. Now he believes the impact crater is much bigger than originally thought, and therefore further east.

Mike interpreted that the folded sandstone structures wrapped around the strange Stac Fada rocks like half a Swiss roll was the result of super-heated steam from the impact exploding through the sandstone with enormous force and opening up cavities for the Stac Fada sediments to be blasted into. From this, he realised he could try to figure out where the sediment had come from: east to west.

When meteorites slam into the crust, they shatter and fragment the rocks in a large area affected by the shockwave around the impact zone. These rocks then have a lower density than the surrounding country rock, which results in small, but measurable, changes in the surface gravitational pull: less dense equals lower gravitational pull. Knowing this, Mike returned home to Northern Ireland and continued his investigation, ‘I pulled out the British Geological Survey’s gravity map of the UK and, lo and behold, there was a huge negative anomaly just where I might have predicted the crater to be!’

Regional geology of northern Scotland showing the outcrop of the Stoer Group and its relationship to the residual gravity field (contoured at 2 mGal intervals; from Rollin 2009) for the Lairg Gravity Low. Arrows indicate directional azimuths within the Stac Fada Member (from Simms 2015). From Michael J. Simms and Kord Ernstson, Journal of the Geological Society,

This gravitational anomaly, an area of lower gravitational pull than the surrounding region, centred on the village of Lairg in eastern Scotland. His detailed study on the Lairg Impact Structure is published in JGS. He acknowledges that his interpretations are controversial; indeed, his proposed location for the impact site is ‘diametrically opposed’ to Ken Amor’s study, also published in JGS. Should further investigation prove Mike’s theory it will have significant repercussions for our current understanding of the geological structure below Scotland.

‘When considering a crater location, I did look at a gravity map,’ says Ken, ‘[but] as Mike points out, there are similar gravity anomalies in the Minch Basin, Lairg and the Moray Firth.’ Ken emphasises that the Moine Thrust, the dominant tectonic feature of northwest Scotland produced when the Caledonian orogeny elevated the Scottish Highlands above the surrounding landscape, removed much of the country rock from the Lairg area where it subsequently infilled with lower-density sediments. This, according to a model first put forward by a team led by Graham Leslie in 2010, plus the intrusion of the Grudie granite, accounts for the gravity low near Lairg.

Ken’s team used a wide variety of field observations to conclude that the origin of the Stac Fada sediments, those that came from the meteorite impact, came from the west as a debris flow – not with ballistic force as Mike thinks. Analysis of the Torridonian sandstone shows a palaeocurrent heading from the west to the east that cut into the Lewisian gneiss through canyons and valleys that have been buried and since unearthed by erosion. This agrees with measurements taken from the region’s structural geology, geochemical and petrographical analysis, and palaeomagnetic data from the clasts. Ken’s team furthermore observed that the metamorphic clasts show little evidence of extreme shock metamorphism as you might expect from Mike Simms’ ballistic model, instead showing lower-grade metamorphism that could have already been present prior to impact, and the rounded clasts are consistent with a debris flow.

(a) Basal breccia of Stac Fada Member at Enard Bay, randomly oriented angular and rounded gneiss blocks up to 0.5 m across resting on Stoer Group sandstone and surrounded by a fine-grained matrix. The hammer is 38 cm in length. (b) Interlocking ‘pillow’ in the upper part of the basal breccia. The left of the ‘pillow’ is draped over an earlier deposited one. To the right and beneath the ‘pillow’ is mixed breccia and melt-rich impact rock. The hammer is 38 cm in length. (c) Fining-upward graded sandstone beds in post-impact sediments, assumed to have been deposited in a standing body of water. These lay a few metres above the undulating airfall bed at the top of the impactite. (d) Incised square-cut channel in the overlying graded bedded sandstone of the Meall Dearg Formation at Enard Bay about 2 m above the airfall bed of the impactite. The gneiss clasts are matrix-supported. The channel has an orientation of 6°N. The coin is 21.4 mm in diameter. From Kenneth Amor et al, Journal of the Geological Society,

All this, Ken concludes, put the impact site out to sea with the Stac Fada rocks coming from a partially molten flow resulting from the intense heat from the impact. The impact site itself, back a billion years ago, would have been a stark Mars-like landscape that’s since been buried by sediment and water.

Clearly the debate about the precise location of Scotland’s impact crater will continue to go on. For Dr Ken Amor, the next step is a thorough geophysical survey in the Minch Basin where he thinks the crater lies, buried deep beneath the ground. For Dr Mike Simms, drilling deep into the Lairg anomaly to acquire samples of the underlying rocks is needed along with more detailed geophysical surveys.

At any rate, both papers demonstrate that the UK, geologically the most intensely studied nation on the planet (arguably), still holds plenty of fascinating secrets waiting to be discovered. This one, at least, is truly earth-shattering.

References:

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