February 4, 2012 12 Comments
One of the many things I adore about rock is the way it offers such a joyous sense of perspective. When I realize what a tiny blip in geological history I am, most of my troubles start to look pretty pathetic. After all, even the entire history of mankind will one day be reduced to a small brown stain in a few cliff faces. And yet, at the very same time, rocks make me feel wonderfully connected to everything that is and ever was, and I become acutely aware of the meaningful part I play in this huge and beautiful story. It’s quite paradoxical.
The other day I needed to go out for a walk to do some thinking about the brain, so I decided to stroll along the Lake Mary Fault, a few miles from my apartment. Lake Mary lies in a Graben – a block of the earth’s crust that has slumped downwards between two faults. The fault line itself is pretty dull to look at and yet, as my mother often used to reassure me, looks aren’t everything:
Actually, the other side of the lake – the Anderson Mesa fault – looks rather more dramatic, but the point is that Lake Mary is on a pretty big chunk of rock that’s slipped down between two cracks. For much of geological history, the land that is now Arizona was being squished together like a concertina by unbelievable tectonic forces. That’s why the rocks beneath my feet contain fossil sea shells when I’m actually 7,000 feet above sea level. Back in Permian times these rocks were forming on the sea bed, but since then the entire Colorado Plateau has been lifted up by at least a mile and a half, as the oceanic crust of what’s now the Pacific inexorably buried itself under the continent like a cat trying to hide under a rug. The majestic Rocky Mountains owe their existence to such squishing but, as it turned out, this was merely a petulant phase this part of the planet was going through. In more recent times, relatively speaking, the forces acting on Arizona have been in the opposite direction, pulling the state apart again like books slumping on a giant bookshelf after nature took away the bookends. The Lake Mary Graben is one of the smaller pieces of evidence for this stretching.
So anyway, I was merrily wandering along this fault-line and I suddenly remembered that local geologists are predicting a significant earthquake on this fault, sometime in the next decade or two. I whipped out my trusty iPhone and looked to see how much the fault is actually moving (there’s an app for everything these days). I admit I was kind of hoping the earthquake might happen that morning, since I was in the perfect place to watch it. The answer, though, much to my disappointment, was that the fault is currently slipping by no more than 0.2mm per year.
0.2mm? That’s not even as thick as my fingernail! Admittedly, if we are expecting a magnitude 6.9 earthquake soon then there must be a lot of pent-up energy waiting to be released, and in a photo below you can see some blistered rock along the Anderson Mesa fault that gives a hint of this tension. But 0.2mm is pathetic! It occurred to me that I’d have to wait five years just to see a single millimetre of movement.
Come to that, it means there’s only been a centimetre of slip in my entire lifetime. Less than the width of a fingertip.
Five hundred long years ago, back in 1512, when Martin Luther was receiving his doctorate and the ceiling of the Sistine Chapel was first being shown to the public, the lake was thus a mere ten centimetres higher than it is today. In order to get to a whole metre of slip – the distance from my outstretched fingertip to my nose, I’d have to go back, not five hundred but five thousand years, to when Stonehenge was being built and the Bronze Age was just getting underway. Multiplying by ten once more takes us back to a time long before the last glaciation, when the first people were just begininng to wander into North America from Siberia and a small asteroid the size of a parking lot was hurtling towards the planet, intent on creating Meteor Crater, which is not very far from Lake Mary. All of this happened just ten metres of fault movement ago; the height of a house.
Standing there on the shore of the lake, looking up to the top of Anderson Mesa, where I often go running, I suddenly felt rather small. Once upon a time, a very long time ago, the land I was currently standing on was all the way up THERE. Having huffed and puffed my way up that hill at this altitude I can tell you that it’s a great deal more than ten metres to the top, so we’re talking about many times greater than 50,000 years of movement.
Of course extrapolating from today’s 0.2mm per year any further than a metre is pretty silly. It’s like saying “if present trends continue, that tree will be ten miles tall by the end of the millennium.” Present trends rarely do actually continue, and that’s certainly true for earthquake faults. But just to keep the metaphor running, let’s multiply by ten yet another time. Now we have a hundred metres of hypothetical fault movement, which is about the distance the crust under Lake Mary has actually slumped since the earth around here first cracked. This takes us back half a million years into the past. At 0.2mm per year to create a hundred metres of movement this seems such a crazy long time ago, but half a million years is only yesterday in geological terms. In fact it used to be called the Recent Epoch. That’s like, last Tuesday!
Our increasingly absurd fault analogy will give us a kilometre of earth movement after five million years. That takes us to the beginning of the Pliocene. If we were to go back in a handy time machine, life wouldn’t look all that unusual. There would be Mastodons instead of elephants, but camels and armadillos looked pretty much the same then as they do today, and it wouldn’t be all that long before Australopithecines were wandering around Africa, tantalisingly leaving their bones behind to perplex future anthropologists. A kilometre of vertical movement along the fault isn’t actually possible at Lake Mary but it’s not at all unreasonable for faults in general. Many faults in Arizona have more than a kilometre of throw. Don’t forget that Lake Mary is itself two kilometres above the sea in which its bedrock originally formed and there was probably a good deal more rock above this point before it eroded.
Multiply by ten one more time and we reach back to fifty million years ago and a completely hypothetical ten kilometres of crustal movement – about the distance between here and the post office, traveling at a rate not much greater than the size of the period at the end of this sentence each year. Pause here for a moment and just think about that last sentence. Imagine someone setting out to post a letter and yet, a whole year later, their car has only moved by the width of a full stop. But after all these powers of ten we still haven’t even got back as far as the last possible moment in which to see dinosaurs (if you don’t count chickens).
It’s in the nature of powers of ten that they rise pretty rapidly, so including another power of ten actually skips most of the interesting stuff and lands us right in the middle of the Cambrian period, half a billion years ago. I was once lucky enough to stand on the famous Burgess Shale in British Columbia and hold in my hand some of the freaky alien animals that lived during this period but whose descendents never made it to our time. Of course, I couldn’t have stood near Lake Mary and done the same thing, since the rocks on which the lake now rests wouldn’t form for another 250,000,000 years.
One final power of ten and we get back to five billion years. Now we’re talking serious time. The earth didn’t even exist yet and nor did our sun. According to some scientists, we only have another five billion years left before the universe ends, so I guess I’d better stop with this fanciful analogy while I still have time. But my point is, a fault that slips one puny fifth of a millimetre per year really brought it home to me how astoundingly ancient this planet is. Some days I feel really old, but heck, my entire lifetime accounts for just a finger’s width of movement along the Lake Mary fault. And that’s considered “active.”
The general point I wanted to make is that standing on any old lump of rock, as long as you have some idea what you’re actually looking at, really puts life into perspective. In a way it makes me feel very, very, very small. The forces that shaped Lake Mary and raised the Permian sea bed over a mile into the air, and the fact that the fossil sea shells I picked up that morning last drew breath a quarter of a billion years before human beings were even thought of, puts me right in my place.
And yet I don’t feel at all bad about feeling small. Quite the opposite, in fact. Geology also makes me feel intimately connected with the earth and its great story. There’s an unbroken thread that connects me personally to every other living thing on earth today, and to everything that has ever lived on this planet. We are all related; all the same family. And I’m today’s representative of one fine strand of that beautiful unbroken thread. Rocks enable me to feel this. No man-made thing, no religion, could ever, ever do that.
A few days before my Lake Mary stroll, I’d been hiking on a mountainside among bright red rocks that formed in an ancient desert during the Triassic period. The Triassic actually contains a very boring collection of rocks, but for very interesting reasons. There aren’t many fossils in them, partly because all the continents on earth were joined into one giant supercontinent at the time, whose interior was a blistering hot desert, but more interestingly because the junction between the Permian and Triassic periods marks a truly massive extinction event, during which up to 96% of all marine species and 70% of terrestrial vertebrate species were wiped out, never to be seen again. There aren’t many fossils in the Triassic because life on earth had been almost competely destroyed and it took millions of years for it to crawl back from the brink.
But in these Technicolor red rocks that I was hiking on, I came across some thin white bands. These turned out, on closer examination, to be evaporites from the bed of an ancient, dried up lake. And in the chunks of friable white rock I found some thin, tendril-like grooves and some tiny black dots. With a hand-lens I could see that the dots were the same thing as the tendrils, just seen in cross section as they disappeared into the rock. They were black because they still contained their original carbon. I was looking at tiny plant roots, a quarter of a billion years old.
I sat down on this ancient lake bed and held these frail remains in my hand. The owners of these tiny roots had once clung desperately to life on a hostile desert lake shore and maybe even had to contend with early dinosaurs trampling all over them and crushing their leaves (they had no flowers, since flowering plants hadn’t yet evolved). Perhaps the one I was holding had only lived a year. The local earthquake faults might only have shifted by a fingernail’s thickness before this vegetation succumbed to the summer heat and left its delicate root fibres behind in the salty, preservative mud for all eternity. But nevertheless we had shared this beautiful planet, this little plant and me. We were both lucky enough to have had our moment in the sun. It had its moment two and a half million centuries ago, and here was I, two extinctions later, in the same warm Arizona sunlight, having mine.
I held my delicate fellow earthling in my hand and we communed.