The Canon (39 page)

The surgery won't stop because Earth is a giant heat engine, and hot things are forever struggling to cool themselves down. This is the most telling way to think about our planet, said David Bercovici, a professor of geophysics at Yale University: as a great hot ball trying to shed thermal energy into space. After all, the second law of thermodynamics demands the transfer. Heat must travel from a spot of relative warmth to one of relative cold. The core of Earth is almost 6000 degrees Celsius. The space through which Earth is hurtling is about—270 Celsius. Ergo, the core keeps shrugging off its heat, Get out of here already, out into the frigid sinkhole of space, where you belong. Ah, but it's not always easy to play by the rules. Not only do the subterranean supplies of uranium and thorium keep pumping heat into the mix, but as thermal currents travel from the core through the thicket of inner earth, they
must pass through thousands of miles of densely packed rock and metal and putty and pudding, and up through the thin, brittle, insulating crust, and all the while never knowing how the substrate they're seeking to penetrate will react—will it shrink, collapse, crack, balloon outward? It's a real challenge. Yet this is what our world is about: there is heat inside, and it wants to get out.

"It's the same as with your cup of coffee," said Bercovici. "Everything is trying to get into equilibrium with the great, cold, vacuous space. And in the process of cooling off and becoming cold and undrinkable, it does all kinds of cool things."

What sort of cool things might heat transfer bring? Let's slit the world open and look from within.

As I'll discuss more fully in the next chapter, on astronomy, Earth condensed about 4.5 billion years ago from the ring of rock and dirt that remained after the formation of the sun, itself the result of a large gaseous cloud heeding the gravitational call to compaction. Earth and the other planets formed quickly by celestial standards, accreting their mass and assuming their spheroid shape (the expected geometry when every part of an object's surface is pulled evenly toward the center by gravity) in as little as 10 million to 35 million years. Those early days were hard days, lawless days. The interplanetary skies were littered with comets, asteroids, and other extrastellar trash, and orbital paths were still in violent dispute. Some 50 million years after the birth of the solar system, Earth collided with a planet approximately half its size, to spectacular, double-barreled effect. A portion of that doomed planet's mass was absorbed into our own, for a net weight gain of 10 percent. At the same time, a chunk of the original Earth was knocked free in the crash, a prize piece that became our incidental, parthenogenic daughter and sole satellite, the moon.

The newly enlarged edition of Earth then began to assume its current configuration. Denser materials like iron and nickel were tugged at most strongly by the planet's gravitational field and gradually migrated toward the center. Lighter elements, including oxygen and silicon, felt less of a pull and formed intermediate and outer layers. And this, in rough cut, is the Earth we have today: an orb composed of a ridiculously dense metal pit surrounded by comparatively lighter, fluffier layers and topped off with a crispy outer crust. But this is no finished dessert. This is not the end of the meal. Great pressure and radioactivity keep the fires burning; and when the chef is disgruntled, everyone else feels the heat.

For those of us who live at or around ground level—and that includes all known life forms, for even the deep-ocean dwellers puddling around hydrothermal vents subsist well within the confines of the crust—it's hard to appreciate the power of pressure. Humans put up with atmospheric pressures; but though Earth's atmosphere is thick, as these things go, extending upward for 50 miles and more, and though we at sea level navigate through its bottommost, heaviest stratum, still it is a relatively flimsy load: only 14.7 pounds of air bear down on any square inch of us. But inside Earth, things get weighty fast. Each layer is a solid, or some equivalent of a solid, and every successive layer has to hold up under the weight of all the solid layers above it. Penetrate eighteen miles down, and you're talking about an average pressure of 150,000 pounds per square inch. Go two hundred miles in, and it's up to 1.5 million pounds per square inch. By the time you reach the innermost core, you encounter crushing loads of 50 million pounds per square inch, or some 3.5 million times the pressure of air.

Earth's core, its compacted ball of iron, nickel, and other chunky elements, is really a ball within a ball, an inner zone the size of the moon—about 2,600 kilometers across—surrounded by an outer core as wide as Mars. The inner core is where we have our 10,000-degree-Fahrenheit Hadean heat. That would be more than hot enough to melt iron under most circumstances, including on the similarly searing face of the sun. But great pressure makes hay of ordinary chemical change and packs iron atoms into such tight rows that they can't get up and flow. As a result, the inner core is a solid, something akin to a huge crystal ball of iron.

In the outer core, pressures are a bit more relaxed, and so, too, are the resident ingredients. The outer core, like the inner, consists primarily of iron, but here it slips around as a liquid. That fluidity has one particularly welcome spinoff effect, which helps make Earth hospitable to life. As the molten metal of the outer core glides around the solid iron of the inner, the motions generate Earth's magnetic fields, which could well be called magnetic shields. Extending outward into space for thousands of miles, the magnetic fields help to deflect much of the solar wind, the crackling cataract of high-energy particles that streams nonstop from the surface of the sun, and that would, if left unchecked, strip away at our atmosphere as surely as turpentine does paint. Terrestrial magnetism then colludes with the cosseted atmosphere to defend the planet's surface against the sun's most dangerous light. Together air and magnetic fields scatter most solar X-rays, cosmic rays, and gamma rays before the radiation can reach us and tatter our cells and genes.

Magnetic fields also infuse the world with a sense of place, an inherent cartography of north and south, and many creatures are thought to navigate by tapping into terrestrial magnetism: pigeons, sparrows, bobolinks, humpback whales, salmon, spiny lobsters, loggerhead turtles, monarch butterflies, newts, the Central Australian Bushwalking Club. Then there are those of us with absolutely no sense of direction, whose idea of using a compass amounts to handing it over to a park ranger in exchange for a helicopter lift.

The core, inner and outer together, accounts for only about a sixth of the volume of Earth, but a third of its mass. It is defined by its heaviness, the density of its components. The most formidable atoms of which Earth is formed, those with the greatest number of protons and neutrons, have been lured inward by gravity, and in their pigheaded procession toward the midpoint have pushed aside slimmer players that stood in their way. The concentration of iron, nickel, and their atomic ilk in the core, to the near exclusion of lighter elements, makes for an unmistakable boundary between core and noncore. When you move outward from the core and into the adjacent layer of Earth-meat, the mantle, the difference in density is as extreme as it is between that of the ground we stand on and the sky above it.

Most of Earth's girth is taken up by the mantle, a word that comes from the German term for "cloak," as the mantle cloaks the core. And though the cloak is much less dense than the core, do not mistake it for gossamer. It is solid, rock-solid, a vast and varied mosaic of metals and silicates—materials built mainly of chains of silicon and oxygen, which pretty much covers the whole terrain we call stone. One of the misconceptions that people have about the mantle is that it is molten, a big vat of melted rock sloshing around underground like the molten lava bubbling from the mouth of a Hawaiian volcano. In fact, while much of the mantle is close to its melting point, particularly in the regions closest to the core, very little is truly liquid. Instead, the mantle is more like Silly Putty, a toy that more than one geologist keeps on hand for demonstration purposes, and to make funny imprints from the newspaper when they're bored. Like Silly Putty, the mantle is solid but springy, almost squishy, and it can move, and it does all the time. "Think of glaciers," David Bercovici suggests. "They are solid ice, and they move. They go very slowly, but they go." The mantle, too, is a very slow goer. It flows like a great sheet of rubbery rock around Earth's central core, at a rate of up to ten centimeters a year, slower than the speed of growing hair.

Above the mantle is the planet's outermost layer, the real cloak of Earth and the place we know best—the crust. On the one hand, "crust"
seems like an unnecessarily lackluster and dismissive word for something that has fed and housed us so well. The entirety of life on Earth lives on or in the crust. The seven continents and 100,000-plus inhabited islands of the world are part of the crust. The oceans and the floors they lie on are part of the crust. The beds from which we extract oil, natural gas, and coal are part of the crust. In crust we trust, and it has ever been thus.

On the other hand, the crust is
very
thin. It is a measly, miserly submorsel of Earth, accounting for less than one-half of 1 percent of Earth's mass and 1 percent of its volume. If you were in prison and somebody threw you a crust of bread with the same proportion of the original loaf as the earth's crust is of Earth, its breadth would be half a millimeter, barely thicker than a couple of eyelashes. The planetary crust is of such insignificant width compared to the entire sphere that if you reduced Earth to the size of a basketball, the skin on it would be much smoother than that of a basketball, closer to that on a bowling ball. All the nervy peaks and plunging vales we take such pride in conquering would vanish, leveled by force of contrast with the bulk of the mantle and core.

One fair way to think about the crust is as a layer of ice on a lake. The ice floats because it is lighter and less dense than the water below it, and it is crispily crystallized because it has been chilled by the winter air above. So, too, is the crust composed of relatively light rocks buoyed atop the condensed superputty of the mantle. The crust is also the coolest part of Earth, and thus is brittle and prone to fracturing. And just as the ice on a lake is thicker in some parts than others, which is why it's a very stupid thing to try driving your Volkswagen Beetle across it no matter what your cousin Jeb tells you, so the width of the crust varies considerably, from a thin point of three miles for the ocean floor beneath Hawaii, to a thickness of about forty-three miles for the crust of the Himalayan plateau. In general, continental crust is about six or seven times thicker than oceanic crust; and though the benthic realms of the ocean floor have a spooky, primordial mystique about them, the place where you might expect to find a few surviving trilobites, the lost island of Atlantis, or at least the original cast and crew of
The Love Boat,
in fact much of the seabed is quite young, hundreds of millions to billions of years younger than the dry land on which we stand. Which brings us to the sublime theory of plate tectonics, a fundamental organizing principle of geology and one of the grand discoveries of the twentieth century. It also returns us to the image of the very hot object that wants to cool down—a cup of coffee, a bowl of porridge, a planet
with an iron smelter of a core. No matter: the patterns that arise when heat bubbles up from below will resemble one another through whatever substrate they flow.

The idea that landmasses migrate slowly around the planet is not a new one. As maps improved, scientists and others couldn't help but puzzle over the puzzle-piece appearance of the continents. Incisively fusing evidence from fossils, rock deposits, and glacial striation patterns around the world, the German geologist and meteorologist Alfred Wegener in 1912 published his visionary "continental drift" hypothesis. Wegener proposed that 200 million years ago all the continents were lumped together in a giant landmass he called Pangaea, meaning "All Earth," and that somehow Pangaea had broken into pieces and the shards had drifted apart. Incidentally fusing the name of a fictional detective with that of the character's creator, the English geologist Arthur Holmes soon afterward suggested a possible mechanism for Wegener's continental peregrinations. Holmes, who studied physics and geology at what is now the Imperial College in London, conjectured that ongoing radioactive decay in the earth could be helping to generate giant heat currents that would convect up to the surface like soup cooking on a stove. Not until after World War II, however, did scientists gather empirical evidence that a steady spreading of the sea floors, fueled by chthonic radioactivity, was driving continental drift; and not until the 1960s did geologists pull all the pieces together into a grand unified theory of how the earth churns. The theory of plate tectonics is a bona fide theory, too, a comfortably capacious conceptual framework that explains an array of disparate findings, that grows stronger and sturdier with the steady accretion of new data, and that can be used to formulate and test all sorts of novel, unobvious hypotheses about how Earth behaves. Scientists may not yet be able to predict earthquakes or volcanic eruptions with anything near the precision that they, we, and the insurance industry would like, but they can make actuarial predictions about where and over what time period the bigger quakes are bound to happen.

The term "tectonic shift" has filtered into popular usage and competes with "quantum leap" to suggest a really big, generally constructive, but possibly risky change, all of which are appropriate nuances: "tectonic" comes from "tekton," the Greek word for "builder." Plate tectonics is the theory of how the shifting plates of the earth build the great bulk of our surroundings. And construction sites can be dangerous places—why do you think the workers wear hardhats, carry metal lunch boxes, and know how to whistle? Earth's tectonic plates build,
wrench apart, gerrymander entire nations on a lark, but they are not what they seem. You can't look at the globe and know where the plates are. They aren't defined by the shapes of the continents or where land meets sea. In fact, tracing the borders of the earth's plates is a tricky, sometimes fractious task. By general consensus, there are seven to ten large or "major" plates, and twenty-five to thirty minor ones. The precise head count counts far less than how the plates move, where they are headed, and what happens when two collide.