October 3, 2012 | 2
Rocks rock. You might deduce from the title of my blog, and the poem that it references, that I’m attracted to the powerful manifestations of time and persistence that are found in stone as a substance. It’s not only that rock is materially durable and resistant, and, luckily for my research on Neanderthal tools, outlasts human lifespans by many times. The real magic of geology lies in the incredible capacity of rocks to be at once unimaginably ancient, and yet to preserve in them a few moments in time: billion year old ripples worked into a sandy lake bed, or the impressions of fat raindrops hammering into soft muds almost as long ago.
I’ve recently returned from one of the places where you can see examples of this for yourself, in the very beautiful setting of Assynt, part of the north west Scottish Highlands. Although almost everywhere on Earth has something geologically interesting to offer, this part of the world is something special. Here some of the most ancient rocks in Europe are found, with stories to tell of huge wastes of time as sandgrains accumulated in empty landscapes and water washed across lakebeds, interspersed by brief, energetic (and sometimes explosive) dramas. Here you can see both deep sediment accumulation, and the lacunas in the record where tens of millions of years worth of solid rock have vanished: ground away to nothing but an “unconformity”, a temporal void between two contacting entities of stone.
The very oldest rocks in this region are the Lewisian gneisses, a strange looking crystalline stone that seems to express its tortured history in its appearance. Sitting on this rock, you’re touching over 3 billion years of time- more than two thirds the age of the Earth-, and a journey of thousands of miles from the deep crust to the surface, worn down to hills and canyons, covered by kilometers of later deposits and finally revealed again through the rough caresses of ice sheets. What was once probably a mix of even more ancient sedimentary stone and igneous rocks formed at depth were altered through many stages of metamorphism over the course of at least 2 billion years of mountain building to become granitic gneisses. First stretched and pulled into bands of varied compositions, then folded and injected by many other minerals, today they have a somewhat crazed, zebra-stripe appearance. Rocks such as these are likely to lie at the base of most of the Earth’s continental crust.
I have a lump of Torridonian sandstone I collected many years ago from an outcrop at the beautiful white sand beach of Clachtoll, the location of idyllic childhood holidays when it was still just a few dilapidated caravans amongst the dunes. Although I knew nothing of the age or formation of the rocks, I was fascinated by the strange knobbly Lewisian country inland, and the angled, stepped Torridonian slabs that made fantastic fishing spots on the coast. I still love visiting Scotland, but now my experience of its breathtaking landscapes are enriched by understanding their deep history. This year we climbed up Stac Pollaidh, a mountain rising over 600m formed of layers upon layers of almost horizontal layers of Torridonian rock. Much dawdling enroute occured as I stared, fascinated at the thin beds of worn pebbles visible every so often through the sandstone sequence.
Ringing the mountain like necklaces, these pebble beds represent brief episodes of more violent deposition, probably flood events after extreme weather. In fact, the material making up the Torridonian, all those sandgrains and pebbles, are likely to be weathered debris from mountains formed of Lewisian gneiss lying to the west (at least, the direction that is now west!). The size of those source mountains, and the gigantic spans of time taken to wear them down, wash them eastwards, piling up into the sandstones and pebble beds is made clear by estimates of the original depth of the Torridonian: some 6-8 kilometers, almost the height of Mt Everest. Much of that has eroded away, yet in other places, on the peaks of Torridonian mountains like Beinn Eighe to the south, it is capped in turn by yet more rock, much younger marine quartitzes from around 500 millions years ago in the Cambrian.
After the heights of Stac Pollaidh, at coast level (and therefore further back in time) I was on the trail of more geology thanks to an excellent book, the Highland Geology Trail. This features geological excursions you can follow yourself. As luck would have it, the stunning beach near us at Achnahaird was not only a wonderful place to watch birds, but the start of a section of the trail we could follow. As we crossed moors and walked around the jagged line of th coast, we could trace outcrops of the Lewisian gneiss, shifting to deep beds of finely layered Torridonian… and something else surprising.
I promised you Mars in the title of this post. And here’s where all this geology, already fascinating in its own right, starts to rock even harder. The Torridonian is a pretty good analogue for some Martian sediments, certainly better than almost any environments we can see on Earth today (the Antarctic Dry Valleys and the Atacama Desert being exceptions). It formed long before there was any complex life: no animals or plants broke up these desolate landscapes, although there were mats of microbial life called stromatolites living in shallow seas, and even in crusts at lake margins on land. The form of the Earth during the Torridonian was shaped by the twin tyrannies of water and wind, creating deserts, rivers, flood breccias and even, recorded in the rocks, the imprints of raindrops.
But something else is also preserved, something of a very different character to the eons of time as sand grains accumulated, rivers ran, and rain fell. For many years some deposits at the Bay of Stoer had been interpreted as remnants of an isolated volcanic event. These rocks, the Stac Fada formation, were thought to result from a mudflow following volcanic activity: they had no bedding (were one large mixed mass), seemed to have displaced the Torridonian sandstones below them, and included greenish fragments of rock similar to volcanic glasses. However Stac Fada didn’t quite sit right with many geologists, as it was a loner: there was no other volcanic activity at the time in the region, no source to explain its origin, despite its very extensive size: over 50km across. Then in 2008, the penny dropped: following detailed analysis by a team of geologists, it was realised that this huge formation did have a violent, sudden origin, but not a volcanic one. Evidence from ‘shocked’ quartz grains, platinum group elements and an overabundance of a Chromium isotopes in the rock showed that instead, over a billion years ago, a large meteorite had hit this region, and the Stac Fada rocks are actually its ejecta blanket, the material thrown up and out by the impact. In between all the slowly accumulated, sandy typical Torridonian rocks, there is a record of a massive, sudden drama: searing heat, flying partially melted rock, falling debris over a huge area.
Perhaps the most incredible part of the impact formation however is a layer of what are termed “accretionary lapilli”. Looking like small brown berries, these hard little balls formed because some of the rocks hit by the meteorite were volatile element and water-bearing. In the maelstrom of the impact, steam clouds were formed mixed with pulverised rock. As the clouds condensed, they did so around dust, creating layered spheres, like tiny stony onions. These fell to the surface, and can now be seen in the tilted exposed layers, some sticking up proud, others partially eroded allowing you to see their internal structure.
Finding these geological gems was the ultimate goal of my romp across the moors and coast at Achnahaird. After several hours enjoying the rocky highlights along the way, and a picturesque tiny bay with a ruined bothy, we reached the right small promontory where the accretionary lapilli were supposed to be found. My guidebook suggested “careful searching”, yet after a good 20 minutes scouring the impressive terraced rocks, I still hadn’t found anything remotely berry-like. Disappointed, I trudged up to where my husband was sitting (watching a group of seals in the bay), sat down to announce defeat, and noticed the rocks there looked awfully bobbly… Of course, here they were, right in the last place I look (the same is universally true of archaeological digs by the way). I returned to the lovely shore-side cottage we were staying in, contented to have found and touched such an amazing material record of one of the most apocalyptic events to have occurred in the history of British Isles.
A few days later, when reading the latest NASA press release I saw something familiar. The fabulous Opportunity Rover, still gamely rolling across Mars checking out new geology years after its ‘best before’ date, had made a new discovery. In the “Kirkwood” section of an outcrop it had been studying at Endeavour Crater, Oppy had looked closely at the rocks and found intriguing tiny rocky spherules. NASA has been at pains to point out that these are very different from the previous discovery of “blueberry”-like formations made by Opportunity right at its landing site more than 8 years ago. However although the title of the press release is “Puzzling Little Martian Spheres”, I’m certain that NASA has many planetary geologists who will have some pretty clear ideas. I’m certainly not a geological expert, but to me these new tiny balls do look awfully like the accretionary lapilli I saw up in Assynt.
Impact craters on Earth are difficult to study due our planet’s extensive surface re-modeling thanks to erosion and sedimentation. Two of the best understood major impacts are the Ries crater, Germany, around 15 million years old, and the massive Chicxulub crater, Mexico, created by an asteroid which impacted around 65 million years ago and which has been suggested as a factor in the extinction of most dinosaur species. Both these craters have accretionary lapilli deposits. The physical processes involved in creating impact ejecta blankets are therefore an active area of research, but it’s more than possible that conditions on Mars when Endeavour Crater was created were similar to Earth during the Torridonian. Opportunity has found evidence at the crater rim for minerals that formed in wetter conditions than at present (phytosillicates and gypsum), which might have created a large steam cloud during impact, causing accretionary lapilli to form. It’s interesting that the size of the Kirkwood spherules is slightly smaller on average than those from Achnahaird: c. 3 mm rather than 5 mm. However, Mars has had a thinner atmosphere for a long time than Earth, and also has lower gravity, so it’s possible that accretionary lapilli might form differently there. As a final consideration, the Assynt crater substantially remodeled the local Precambrian landscape, with evidence that a massive post-impact crater lake formed. Perhaps in Mars’ deep past, there were also massive surface lakes formed by asteroid and meteorite impacts, which may have formed very particular and potentially habitable conditions.
It’s precisely by studying the geology that we have on Earth that we can create a structure of knowledge within which we can situate truly alien worlds like Mars. Planetary geology is another of my ‘alternate universe’ careers: there’s so much uncharted territory just in our own solar system, so many oddly familiar and totally, unimaginably bizarre landscapes and worlds waiting to be explored and understood. It’s a nice coincidence that in the same week, the 2012 100 Year Starship Symposium was being held in Houston. This initiative is aimed at nothing less than true interstellar human exploration, by creating a non-governmental organization that will promote long-term private investment in moving us beyond the confines of our local stellar environment. I find it inspirational that finally we’re making major efforts at becoming more than a provincial, self-regarding species.
Although I, and probably all of you reading this, probably won’t be around to see the launch of the first interstellar human mission, there’s still a lot of exploration in the meantime to look forward to in our own solar system. We will see the first people standing on Mars (including a very lucky geologist), and missions to the fascinating solid-surfaced moons such as Enceladus, Titan, Europa and Triton and the dwarf planets like Pluto. The geology of Earth’s remote past, accumulated over timescales ridiculously far removed from a human lifetime, waited billions of years for our species to mature enough to begin to explore it. It’s only over the past century and a half that we’ve really begun to comprehend how ancient our home world is, and yet how much younger and smaller it is than the rest of the universe. We have so many new horizons in front of us, but the Earth will remain our touchstone, both scientifically and emotionally. Those tiny spheres in a rocky corner of Assynt connect us to the craters on Mars, Mercury and other as yet unnamed planets around distant suns. To quote the great geologist James Hutton, one of the first to perceive the great age of Scotland’s rocks, the eons of time locked up in the Lewisian and the Torridonian formations have not flowed in vain.
Amor, K., Hesselbo, S.P., Porcelli, D., Thackrey, K. and Parnell, J. 2008. A Precambrian proximal ejecta blanket from Scotland. Geology 36 (4): 306-4
Prave, A.R. 2002. Life on land in the Proterozoic: Evidence from the Torridonian rocks of northwest Scotland. Geology 30 (9): 811-4
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