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Jan Zalasiewicz, Department of Geology.

Points of View: Down Below, Something Stirs…

Jan Zalasiewicz, Department of Geology, comments on how plate tectonics has entered public demonology

“Tectonic plates are moving beneath the Prime Minister…” Now here’s a phrase that hit the headlines a while back, and showed John Prescott, his Deputy, to have an unexpected gift for geological metaphor. Indeed, it’s another example of a geological term infiltrating the common language. It’s wonderfully evocative, a few brief words that encapsulate the feeling of a landscape, hitherto sunlit, stable and benign… suddenly, treacherously, falling away from beneath an emperor’s feet: a landscape devastated, and no longer his dominion.

What do you associate with the movement of tectonic plates? A thousand documentaries about killer earths and savage planets provide an instant, almost subliminal answer. Cities in ruins, or entire coastlines swept into the sea; forests thrown to the ground, and fleeing populations. If only, if only the earth wasn’t cracked into a lethal, unpredictable crazy pavement, then we’d all be able to live prosperously forever, and sleep safely in our beds.

Plate tectonics, that perfectly-proportioned offspring of continental drift, has certainly made the transition from being the most far-reaching and successful hypothesis of the earth sciences, to entering public demonology as the begetter of earthquakes and tsunami, hyper-eruptions and landslides.

Let’s try, though, to apply a different spin to it. The most astonishing thing about plate tectonics, something which I still can’t fit into a perpetually astonished cranium, is just how astoundingly benign and non-catastrophic a process it is, all things considered. For the catastrophes, if you just think on what actually happens to the planet’s crust, should, you imagine, be far worse. Give a dog a bad name… To remix our metaphors, it’s as if a homely old golden retriever was ambling down the road, only to be denounced as a Rottweiler on the loose. It’s bountiful Mother Earth, painted as Lady Macbeth.

Consider, for instance, that awesome, ever-opening, planetary-sized scar, the mid-Atlantic Ridge, where it rises above sea level to form Iceland. What do Icelandic people do? Do they flee the conflagrations, the all-consuming magma? No, they settle. They live, prosperously (if, as a visitor, you want to see how prosperously, just try buying a beer). They farm. They fish. Sure, now and again, farms or sections of roads or occasionally even towns are buried under lava or ash. But people move away, annoyed at the loss of real estate, and then move back and start building again. Given that they’re living exactly where the Atlantic is cracking apart, they seem mostly to live a pretty quiet life.

It’s true that the collision zones of plate tectonics can provide some more serious violence. Right now, for instance, a thousand kilometre-long, 100 kilometre-thick section of Pacific ocean floor is being forced down beneath South America. Things getting in the way suffer badly. Entire ocean islands are simply sliced into pieces, scrunched and mangled. This is the territory of big earthquakes, and of the more viciously explosive volcanoes.

Still, considering the sheer scale of the ongoing crustal rearrangement, the landscape, if not perfectly benign, is still generally habitable. When the devil does come out of the bottle, what kills people mostly stems from poverty. An earthquake that can kill tens of thousands in, say, Bam in Iran, will kill just hundreds in San Francisco or Tokyo, simply because of better building design. And big fierce volcanoes are hard to overlook and, these days, easy to monitor. Really, you shouldn’t live near them. Trouble is, in an overcrowded world, many people don’t get the choice, and volcanic soils are fertile, so today’s full belly outvotes tomorrow’s disaster.

There’s also a flip side to plate tectonics. They give us a nice dry home. The continents, the accumulated scrapings, as it were, of the whole messy process, float eternally, like so many Flying Dutchmen, high on the backs of the ever-moving plates. So what? – you might say. If there wasn’t plate tectonics, then there would be no continents, but we humans would still have evolved, wouldn’t we?. We would just be web-footed merpeople, gliding elegantly through a deep calm global ocean, instead of being clumsy landlubber humans.

Fat chance. Without the land, there would be no rivers, and without the rivers, no large-scale cycling of nutrients into and through the oceans. With such a scenario, it would be hard to imagine gill-bearing humanoids appearing. Evolution might, perhaps, have got as far as some bad-tempered bacteria fighting over a few nutrient scraps.

There’s another reason for thanking our lucky stars for those shifting plates. For our planet generates an awful lot of internal heat, because of the radioactivity inside it. That heat has to escape somehow, and conduction isn’t fast enough, because the earth is simply too big. So the almost-but-not-quite solid interior of the earth is slowly convecting, like jam being simmered, and this motion drives the overlying plates, which in turn provides a wonderfully regular means of magma to reach the surface to release its heat.

Let’s imagine an earth without such a smoothly-running heat exchange device. In fact, we don’t have to imagine. Venus is a rocky planet, and about Earth-sized. It doesn’t have plate tectonics. It seems to have evolved a different, and much scarier, form of heat release. From the amount of the meteorite impact craters on it, its land surface seems to be about half a billion years old throughout. Strange.

Half a billion years ago something seems to have happened. That something has been called ‘resurfacing’, when Venus created for itself a brand new landscape. The scenario involves half a billion years plus of pent-up heat suddenly coming to the surface in a planet-wide maelstrom of lava. The whole planet, in this interpretation, literally turned itself inside out. This would have been bad news for any recently-evolved Venusian life-form, no matter how many tentacles it possessed.

What makes the difference between these cosmic bodies, between planetary Jekyll and planetary Hyde? A good deal of the answer might be plain old water. The earth has lots of it. It’s a great lubricant, that can help even 100 kilometre-thick slices of crust slide past each other. The water on Venus boiled away a long time ago, and those thick swirling clouds aren’t steam but hot vitriol. On a dry planet, plate tectonics, seemingly, can’t work; there’s just too much friction. And so steady heat-release seems to have been replaced by intermittent Armageddon.

One can make a good case that we really are living on the best of all possible worlds. The good Doctor Pangloss was right all along, spot on the button. It’s amazing how it all works. The earth really is a dream machine.

That’s not to say that we should just take it for granted. There is a tendency – even, dammit, among the research funding councils – to imagine the earth as having inner workings which are somehow both perfectly constant, and fundamentally detached from its living skin.

But even a dream machine has occasional hiccups. The tiny irregularities in the earth’s heat regulation machine, the creaks and belches, the small interruptions of rhythm, may have more consequence than we generally imagine. When it comes to the great extinction events that have interrupted the onward march of Life, for instance, it’s the meteorites that tend to grab the biggest headlines. But at least a couple of the big extinctions coincided with enormous magmatic outpourings, if not (thankfully) on a Venusian scale.

These outpourings are rare events. More pertinently, though, it’s recently become clear that smaller globs of magma, injected into carbon-rich muds, may have released huge amounts of methane and carbon dioxide into the atmosphere, to cause sudden temperature hikes of five degrees centigrade or more.

Sounds familiar? Such gas outbursts may be the nearest analogue for humanity’s current carbon spree. The good news is that, in those ancient events, the climate stabilized geologically soon. The bad news for us is that ‘geologically soon’ meant hundreds of thousands of years.

Food for thought, indeed. We’ve got a lot to learn about how the internal plumbing of the planet works and how it interacts with its living passengers, and such questions are quickly becoming of more than just academic importance. Research councils please note.

Meanwhile, let’s pray that those plates beneath our feet keep moving. And perhaps John Prescott might have sought a more precise metaphor to describe the predicaments of his Prime Minister. Say, by describing him as placed, in an airtight metal canister, on the surface of planet Venus, waiting for the next resurfacing event. Some of his parliamentary colleagues – though one would naturally disapprove of any hint of uncharitable spirit – might quite enjoy that notion.

Jan Zalasiewicz, Department of Geology, University of Leicester

• This article first appeared in ES2k, the Irish earth sciences magazine.

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