The Canon (40 page)

What, then, are the tectonic plates? Contrary to common misunderstanding, they are not simply broken pieces of the earth's crust, although the crustal rock is usually fractured along the boundary line between one plate and the next. But the plates extend deeper than the crust, into the upper part of the mantle. Each is about fifty miles thick, although, like the crust itself, the plates vary in width and in density, too. The plates bearing the continents are relatively thick and light, while those scalloped with ocean basins are thin and dense. The plates are defined as much by their motions as anything else. They are the segments of the outer earth that glide around as reasonably cohesive units. The upper part of each mobile plate, the crusty part, is brittle and prone to cracking and crumpling. The nether portion, in the mantle, is hotter and more plastic, more likely to yield when pressed. All the plates are sliding over, or in some cases with, the more viscous lower mantle beneath, at an average clip of one to ten centimeters per year—about the rate at which your fingernails grow. That may be snail-paced by human standards, but truly nail-biting by geologic ones. In a million years, a plate migrates some 30 miles. Give a plate 100 million years, and it will have globetrotted 3,000 miles, nearly the distance between New York and London.

Animating the plates is Earth's ceaseless effort to dispel its suffocating heat. The planet has a few cooling techniques at its disposal. It radiates a small amount through conduction, in which fast-moving atoms and molecules pawn some of their excess energy off onto slower-moving atoms and molecules that abut them—the same process that quickly heats your metal spoon when you stick it in a hot cup of coffee. Earth expels another modest degree of thermal energy by straightforward mechanical venting—volcanic explosions, geysers, and related geo-belches. Mostly, however, Earth relies on the conveyor belt method of heat dispersal, convection. Convection has the benefit of cooling not only by pushing hot things outward, but by pulling cooler items closer. The convective currents that course through our world are complicated and difficult to track, rather like large-scale weather patterns in the atmosphere, but here in crude cut is what happens. Heat flows from the iron core and into the rock of the lower mantle. As that boundary rock heats up, it expands and becomes less dense, and, just as hot air rises, so the heated, expanded rock starts to rise through the cooler mantle rock above it. The higher it manages to climb, the less pressure bears down upon it, and the softer it can become; and the more buttery it grows, the better it flows, which further eases its crustward cruise. At some point, however, another little technicality of physics intervenes, the flip side of the principle that sent the rocky mass bubbling upward to begin with. In rising, the rock dispenses its heat into its environs, and as it cools it gradually reverts to its former state of density. Finally, the bleb of stone has no choice, it is too heavy relative to the matrix around it, and it begins to sink, as stones predictably, platitudinously do. Down, down, back toward the hotter core, where the rocky mass can pick up more heat and start its yearning journey once more. This, then, is the basic convective cycle at work inside Earth. Hot rocks expand, rise, cool, contract, and descend; let's take a deep breath, and try that again. Some of these convective currents may eddy around near the border of the core, others convulse in larger swags across vast swaths of the mantle. And a few manage to fight their way to the surface and gurgle into the oceans, right where the crust is thin and the Earth's seams ever so slatternly gap.

Among the many lines of research that culminated in the theory of plate tectonics, some of the most important came from studies of the sea floor in the 1950s. That enterprise supplied a braided array of surprises. For one thing, there were long ridges of undersea mountains, the most prominent running down the middle of the Atlantic and Indian oceans and rising 3,000 meters or more above the sea floor; and there were undersea trenches, plunging 2,000 meters or more below that floor. For another, the rocks on the sea floor were outrageously youthful, 180 million years old at most, compared to terrestrial rock samples that date back billions of years. The youngest of those spritely seabed rocks were found closest to the midocean mountain ridges, with the ages steadily mounting as you moved farther from the mountains and up to the rim of the midocean trenches. Finally, the sea floor proved to be remarkably tidy and light on sediment, considering how long it had been subject to a steady drizzle of debris from the land above—posthumous plant and animal parts, sand, pebbles, mud, bones, shells, Naugahyde barstools, and 3,000 copies of Grand Funk Railroad's
We're an American Band,
still in their unopened jewel cases. It was as if the ocean bottom were being continuously scrubbed, vacuumed, and mercifully auctioned off on eBay.

Plate tectonics helped solve the riddle of the callow deep. The convecting loops carry roasted rock upward toward the carapace. Some of that hot young rock penetrates to the surface by welling up through the ocean ridges, where it emerges in the semisolid rock format called magma. That magma pushes the ocean floor apart, bobbling the oceanic plates on either side and plowing cooler, less young rock outward. Eventually, the cold leading edge of the spreading sea floor runs into other fissures in the crust, the deep ocean trenches, where it is sucked back, or subducted, into the mantle. In the mantle's mulching maw, the rock is smashed, pulverized, refitted, and pasteurized, so that whatever part of it might manage to emerge through a crustal ridge, to see the sea floor by the seashore once more, will do so as brand-new rock. Poseidon's conveyor belt never stops rattling along, and in Earth's hoary history the ocean basins, our crustal low points, have been recycled dozens of times. Not so the continents. Because continental rock is relatively light, it floats above the subduction zone of the trenches, getting pushed and pulled and battered without being routinely sucked into the mantle. Continental landmasses have changed their contours and allegiances repeatedly, as we said, but many of their rocks have remained above the molten fray for a billion years or more.

Everywhere the upwelling of hot rock keeps the plates in constant motion, and the moving plates in turn recast the crustal playhouse on which life gamely mouths its lines. The spreading of the sea floor at the midocean ridges drives some plates and the cargo they bear away from one another: such tectonic divergence is pushing North America and Eurasia in opposite directions, and widening the Atlantic Ocean by some five centimeters per year. Other plates are ramming into each other, as awkwardly and irritably as two pedestrians colliding on the sidewalk: You go this way, no, I'll go that way, whoops, now we're both going the same way again, that won't work. Maybe I'll just try to duck under your legs and get this farce over with. When a thick continental plate rubs up against a thin oceanic plate, the thinner plate does indeed start diving under the higher plate, making one of those subduction zones that return old sea floor to the mantle and seriously unsettling whatever landforms lie above it in the process: raising a string of volcanoes and outfitting them with explosive magma chambers, for example, or crumpling coastlines into high-altitude peaks best suited for llamas, kings with lots of slaves, and tourists with lots of medical insurance. The Cascade Mountains of the Northwest—home to Mount Saint Helens—and the Andes Mountains of South America both exemplify what happens when oceanic and continental plates collide.

If the converging plates both carry continents, the landmasses will be smashed together in slow motion, the leading edges buckling upward as the continents are forced into uneasy, captious alliance. Midcontinent mountain chains often reveal where once separate landforms were compressed together by converging continental plates. The Himalayas, for example, began clawing their way upward about 45 million years ago, when the plate bearing the Indian subcontinent collided with the rest of Asia. In Europe, the Alps delineate where the Italian peninsula, riding on the African plate, slammed into what are today Germany and France at about the same time, a churlish merger that two world wars, a common currency, and the frequent consumption of each other's pastries have not entirely placated.

Tectonic encounters are not always head-on collisions. Sometimes a pair of plates traveling in opposite directions merely scrape past each other, or try to. If the glancing encounter turns out to be a tight squeeze, parts of the plates will stick together, especially at their crispy ragged upper crusts. The plates underneath may insist on continuing in their respective, contradictory directions, but the rocks along the tacky upper boundaries are trapped in place. They become stressed and strained and resort to all sorts of tricks—therapy, yoga, renaming themselves "Gibraltar." But the pressure keeps building, and finally the strained rock surfaces snap, lurching away from each other in a seismic spasm. "Seismic" comes from the Greek word for "shaking," and sudden slippages along Earth's fault lines—fissures in the crust where underlying plate motions force rock to scrape against rock—are what shake and break the ground in a quake, as the pent-up energy stored in the long-suffering rocks is freed to spread outward in waves.

The most famous of these parlous plate boundaries is the San Andreas Fault in California, where the Pacific plate is crawling northward relative to the North American plate, and their stony interfaces alternately grip and slip, usually in incremental jerks, occasionally by harrowing heaves of several meters at once. In the catastrophic San Francisco earthquake of 1906, peak displacement was almost twenty feet near Olema, California. The chronic grinding and sliding of plates tend to fracture boundary rocks in many directions and along multiple planes, down an estimated six miles deep in the case of the 1906 quake. As a result, major fault lines like the San Andreas are not single slashes in the crust but crisscrossing thickets of cracked rock slabs, which sometimes absorb the querulous motions of the plates and sometimes recoil from the effort and splinter some more. The difficulty of determining the relative resilience of any strand in a fault line's sticky thicket
explains the considerable challenge of predicting when the next earthquake will strike, and exactly how bad it will be.

The prodigious upwelling of the mantle has done more for Earth's crust than hammering it
ad infinitum
with magmatic rancor. In addition to making the world's seabeds, the convecting forces of inner earth supplied the water that lies on them. Earth, of course, is awash in water. There are 326 million trillion gallons of it, enough to cover three-quarters of the planet's surface with flowing oceans that average 2.5 miles deep. Liquid water is essential to life as we know it, and none of our sibling planets can claim anything like our aqueous bounty. The precise sequence of events that pinned the bright blue rippling ribbon on Earth's lapel is still open to debate, but most scientists concur that it was likely a mix of the astral and the retentive. Liquid water may be rare in the solar system (and, as far as we can tell, in the universe generally), but H
₂
O in its other states is not. Comets abound at the fringes of our solar system, and we can in clean conscience refer to them as "dirty snowballs." A comet is nothing but an orbiting chunk of ice and dust maybe ten miles across; and the dramatic tail that cries "comet" so clearly that we recognize its image in the thousand-year-old Bayeux tapestry is a humble puff of steam, the surface ice boiling off as the speckled projectile careens close to the sun. Early in the evolution of the solar system, it seems, wild swarms of comets were drawn in from the exurbs by the highly credible gravitational pull of Jupiter, the giant of the planetary litter. A sizable number of those comets either forgot to bring their portable GPS device, or found it wasn't working because it hadn't been invented yet, but in any case they ended up overshooting their target by a few hundred million miles and crashing into Earth instead. Earth was still young, and so hot to the touch that much of the cometary water rapidly vaporized back into space; but some seeped into the depths of the young Earth, where it was bound up into the rock, greatly amplifying whatever water stores our world had from the start. Beginning about 4 billion years ago, volcanic eruptions steadily loosened the subterranean water from its mineral crypt and spewed it back out as steam, up to a planetary surface grown clement enough for the water to rally its forces, seize the initiative, and initiate the seas. Earth's crust had cooled down, and the swirling molten iron at its core had begun generating the magnetic fields that help deflect the searing solar wind. So shielded, the vast clouds of volcanic reflux were not peeled off into space, but instead hovered above the ground, gathering and glowering portentously, as storm clouds do. They gathered until there was no room for more. The skies were supersaturated out to the
limits of gravity's grip, in a nimbus of high humidity, and the water vapor had no choice but to condense out and fall to Earth as rain. The rains fell in relentless torrents of Noachian proportions, only bigger and longer, and there were no giraffes and zebras huddled on a wood boat, wishing they were back at the circus. For tens of thousands, hundreds of thousands of years, the antediluvian deluge fell, filling the dimples in Earth's newly caramelized silicate skin, filling them to the brim. Yet while this downpour may sound like an excessive rainy season, even for Seattle, by the timetable of Earth's history the building of our naval resources barely lasted a sneeze. "The geological record of sedimentary rocks, formed in the presence of liquid water," the geologist Robert Kandel has written, "proves that the oceans have existed for three billion years, maybe even four," and at a volume pretty much like what we have today. In other words, no sooner had Earth's crust managed to cool enough for discernible depressions to form than the skies had deposited in them the maximum allowable in liquid assets, our storied galleons of 326 million trillion gallons, enough water to fill a string of bathtubs that would reach from here to the sun and back again 5 million times. Just be sure to bring your own towel, shampoo, and a breathable atmosphere, too.