It may not be obvious, but beneath our feet is a lot of hot, squishy rock.
If you could drive straight down, into a tunnel bored through the crust of the planet, you'd hit this molten mess in about an hour. It's called the asthenosphere -- a sluggish sea, several hundred miles thick, on which floats the Earth's cool epidermis -- the so-called tectonic plates. In case you've missed the last half-century of geology, our world is girded with these plates -- large slabs of rock often thousands of miles in size. If you're resident in the U.S., Canada, or Mexico, you live on the imaginatively named North American Plate, one of the many dozen that enamel the Earth, and make up its continents and ocean floor.
To the eventual discomfiture of map makers, these plates move -- shunting their way in a slow-speed, largely uncoordinated dance like pieces of styrofoam caught in a backwater. Where the plates run into one another, the inevitable jostling provokes both volcanoes and earthquakes.
The disaster that recently befell Japan occurred at the boundary of the Pacific and North American plates (yes, the latter actually reaches all the way across the northern Pacific to Japan). It's tragedy of the sort that's happened before, and it will happen again, despite the sardonic suggestion of California bumper stickers reading "Stop Unauthorized Tectonic Activity."
It's all unauthorized, but it's also unstoppable -- thanks to the asthenosphere's heat. The rock here is a toasty 3,000 fahrenheit. If you could crank your oven to that temperature, it would glow appealingly before dissolving onto the floor of your kitchen (and soon after that, your basement). Solid rock, which we seldom imagine in a molten state, loses its rigidity at the temperatures and pressures of the asthenosphere. Geology textbooks frequently describe it as having the consistency (if not the taste) of taffy.
It's the churning energy of this sticky hot rock that moves the plates, and occasionally destroys the precarious constructions of our civilization. But where did this heat come from, and when is it going to cool down? After all, if the asthenosphere could just chill a bit, we'd save many lives, not to mention a lot of money reinforcing the bridges in San Francisco.
The heat of the asthenosphere derives from several sources, but the main one is the collision energy of all the rocks -- big and small -- that slammed into our planet as it was born. Since that took place more than 4 billion years ago, you might expect that by now the heat would have been radiated away into space, leaving Earth as cool as jazz. But in fact, Earth's crusty outer surface is a pretty good insulator, and our planet retains its heat like a quality coffee pot. If you actually measure the energy coming up through the ground -- the amount of Earth's internal heat that's leaking away -- it amounts to about 0.08 watts per square meter. That's not much, even when you consider that our planet has a lot of square meters. The total, about 40 trillion watts, is roughly equal to all the energy consumed by our civilization -- small beer, in cosmic terms. (The sunlight energy falling on the planet is 3 thousand times greater by the way, which explains why nobody's growing crops with the heat from Earth's interior.)
So our world is still cooling off from the hellish era of its birth, billions of years ago. But it's cooling slowly. Mind you, some new heat is added by the radioactive decay of materials deep within the planet, but as the aeons grind on, the radioactivity becomes less important. The bottom line, according to Australian physicist Frank Stacey, is that the asthenosphere cools by about 120 degrees fahrenheit every billion years. So eventually, plate tectonics -- and killer earthquakes -- will go away.
Losing the quakes would be good news. But in the interest of full disclosure, there's also bad news. Plate tectonics is not all havoc and destruction. The slow movement of continents and ocean floors recycles carbon dioxide dissolved in the oceans back into the atmosphere. Without this slow speed carbon cycle, Earth's temperatures would cool dozens of degrees below your comfort zone. The big freeze.
In addition, we'd all be in Kansas forever, at least as far as topography's concerned. Mountains aren't eternal: even the most imposing massifs are smoothed away by weathering in a few hundred million years or less. Plate tectonics makes new ones, and without it, our future would be flat.
Indeed, planets that don't currently sport plate tectonics, such as Venus and Mars, are scarcely habitable. Tectonics might be a requirement of any world that aspires to a rich diversity of life.
Nonetheless, continents on Earth are destined to slow their crawl. Eventually, you'll have to sell the snowboard and the crampons. But when?
Don't hold your breath. There's enough heat in the asthenosphere to keep shuffling Earth's crusty armor around for another 10 billion years or so, keeping the planet warm and lifting new mountain ranges onto our fruited plains. So long before tectonic activity comes to a halt, the Sun, gasping for fuel, will swell up, boil away our oceans, and bake the planet hard.
In that apocalyptic, far-off future, earthquakes will be no more than a forgotten outrage of our planet's youth; a pernicious side effect of the environment that -- despite its terrors -- produced all that ever lived on Earth.
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