A short history of nearly everything

studies had led the scientists to predict, and pretty confidently, that they would encounter sedimentary rock to a depth of 4,700 meters, followed by granite for the next 2,300 meters and basalt from there on down. In the event, the sedimentary layer was 50 percent deeper than expected and the basaltic layer was never found at all. Moreover, the world down there was far warmer than anyone had expected, with a temperature at 10,000 meters of 180 degrees centigrade, nearly twice the forecasted level. Most surprising of all was that the rock at that depth was saturated with water-something that had not been thought possible.

Because we can’t see into the Earth, we have to use other techniques, which mostly involve reading waves as they travel through the interior. We also know a little bit about the mantle from what are known as kimberlite pipes, where diamonds are formed. What happens is that deep in the Earth there is an explosion that fires, in effect, a cannonball of magma to the surface at supersonic speeds. It is a totally random event. A kimberlite pipe could explode in your backyard as you read this. Because they come up from such depths-up to 120 miles down-kimberlite pipes bring up all kinds of things not normally found on or near the surface: a rock called peridotite, crystals of olivine, and-just occasionally, in about one pipe in a hundred-diamonds. Lots of carbon comes up with kimberlite ejecta, but most is vaporized or turns to graphite. Only occasionally does a hunk of it shoot up at just the right speed and cool down with the necessary swiftness to become a diamond. It was such a pipe that made Johannesburg the most productive diamond mining city in the world, but there may be others even bigger that we don’t know about. Geologists know that somewhere in the vicinity of northeastern Indiana there is evidence of a pipe or group of pipes that may be truly colossal. Diamonds up to twenty carats or more have been found at scattered sites throughout the region. But no one has ever found the source. As John McPhee notes, it may be buried under glacially deposited soil, like the Manson crater in Iowa, or under the Great Lakes.

So how much do we know about what’s inside the Earth? Very little. Scientists are generally agreed that the world beneath us is composed of four layers-rocky outer crust, a mantle of hot, viscous rock, a liquid outer core, and a solid inner core.[28] We know that the surface is dominated by silicates, which are relatively light and not heavy enough to account for the planet’s overall density. Therefore there must be heavier stuff inside. We know that to generate our magnetic field somewhere in the interior there must be a concentrated belt of metallic elements in a liquid state. That much is universally agreed upon. Almost everything beyond that-how the layers interact, what causes them to behave in the way they do, what they will do at any time in the future-is a matter of at least some uncertainty, and generally quite a lot of uncertainty.

Even the one part of it we can see, the crust, is a matter of some fairly strident debate. Nearly all geology texts tell you that continental crust is three to six miles thick under the oceans, about twenty-five miles thick under the continents, and forty to sixty miles thick under big mountain chains, but there are many puzzling variabilities within these generalizations. The crust beneath the Sierra Nevada Mountains, for instance, is only about nineteen to twenty-five miles thick, and no one knows why. By all the laws of geophysics the Sierra Nevadas should be sinking, as if into quicksand. (Some people think they may be.)

How and when the Earth got its crust are questions that divide geologists into two broad camps-those who think it happened abruptly early in the Earth’s history and those who think it happened gradually and rather later. Strength of feeling runs deep on such matters. Richard Armstrong of Yale proposed an early-burst theory in the 1960s, then spent the rest of his career fighting those who did not agree with him. He died of cancer in 1991, but shortly before his death he “lashed out at his critics in a polemic in an Australian earth science journal that charged them with perpetuating myths,” according to a report in Earth magazine in 1998. “He died a bitter man,” reported a colleague.

The crust and part of the outer mantle together are called the lithosphere (from the Greek lithos, meaning “stone”), which in turn floats on top of a layer of softer rock called the asthenosphere (from Greek words meaning “without strength”), but such terms are never entirely satisfactory. To say that the lithosphere floats on top of the asthenosphere suggests a degree of easy buoyancy that isn’t quite right. Similarly it is misleading to think of the rocks as flowing in anything like the way we think of materials flowing on the surface. The rocks are viscous, but only in the same way that glass is. It may not look it, but all the glass on Earth is flowing downward under the relentless drag of gravity. Remove a pane of really old glass from the window of a European cathedral and it will be noticeably thicker at the bottom than at the top. That is the sort of “flow” we are talking about. The hour hand on a clock moves about ten thousand times faster than the “flowing” rocks of the mantle.

The movements occur not just laterally as the Earth’s plates move across the surface, but up and down as well, as rocks rise and fall under the churning process known as convection. Convection as a process was first deduced by the eccentric Count von Rumford at the end of the eighteenth century. Sixty years later an English vicar named Osmond Fisher presciently suggested that the Earth’s interior might well be fluid enough for the contents to move about, but that idea took a very long time to gain support.

In about 1970, when geophysicists realized just how much turmoil was going on down there, it came as a considerable shock. As Shawna Vogel put it in the book Naked Earth: The New Geophysics: “It was as if scientists had spent decades figuring out the layers of the Earth’s atmosphere-troposphere, stratosphere, and so forth-and then had suddenly found out about wind.”

How deep the convection process goes has been a matter of controversy ever since. Some say it begins four hundred miles down, others two thousand miles below us. The problem, as Donald Trefil has observed, is that “there are two sets of data, from two different disciplines, that cannot be reconciled.” Geochemists say that certain elements on Earth’s surface cannot have come from the upper mantle, but must have come from deeper within the Earth. Therefore the materials in the upper and lower mantle must at least occasionally mix. Seismologists insist that there is no evidence to support such a thesis.

So all that can be said is that at some slightly indeterminate point as we head toward the center of Earth we leave the asthenosphere and plunge into pure mantle. Considering that it accounts for 82 percent of the Earth’s volume and 65 percent of its mass, the mantle doesn’t attract a great deal of attention, largely because the things that interest Earth scientists and general readers alike happen either deeper down (as with magnetism) or nearer the surface (as with earthquakes). We know that to a depth of about a hundred miles the mantle consists predominantly of a type of rock known as peridotite, but what fills the space beyond is uncertain. According to a Nature report, it seems not to be peridotite. More than this we do not know.

Beneath the mantle are the two cores-a solid inner core and a liquid outer one. Needless to say, our understanding of the nature of these cores is indirect, but scientists can make some reasonable assumptions. They know that the pressures at the center of the Earth are sufficiently high-something over three million times those found at the surface-to turn any rock there solid. They also know from Earth’s history (among other clues) that the inner core is very good at retaining its heat. Although it is little more than a guess, it is thought that in over four billion years the temperature at the core has fallen by no more than 200°F. No one knows exactly how hot the Earth’s core is, but estimates range from something over 7,000°F to 13,000°F-about as hot as the surface of the Sun.

The outer core is in many ways even less well understood, though everyone is in agreement that it is fluid and that it is the seat of magnetism. The theory was put forward by E. C. Bullard of Cambridge University in 1949 that this fluid part of the Earth’s core revolves in a way that makes it, in effect, an electrical motor, creating the Earth’s magnetic field. The assumption is that the convecting fluids in the Earth act somehow like the currents in wires. Exactly what happens isn’t known, but it is felt pretty certain that it is connected with the core spinning and with its being liquid. Bodies that don’t have a liquid core-the Moon and Mars, for instance-don’t have magnetism.

We know that Earth’s magnetic field changes in power from time to time: during the age of the dinosaurs, it was up to three times as strong as now. We also know that it reverses itself every 500,000 years or so on average, though that average hides a huge degree of unpredictability. The last reversal was about 750,000 years ago. Sometimes it stays put for millions of years-37 million years appears to be the longest stretch-and at other times it has reversed after as little as 20,000 years. Altogether in the last 100 million years it has reversed itself about two hundred times, and we don’t have any real idea why. It has been called “the greatest unanswered question in the geological sciences.”

We may be going through a reversal now. The Earth’s magnetic field has diminished by perhaps as much as 6 percent in the last century alone. Any diminution in magnetism is likely to be bad news, because magnetism,