18 Miles

by Christopher Dewdney

  But the Cold War was at its peak and the Russians took up the challenge. Four years later, in 1970, they started their own deep-drilling project. They might have lost the race to the moon, but they would be the first to get to the center of the Earth. They proved to be much more persistent, and they had the advantage of drilling on dry land, which made their equipment more stable. But after 10 years, they too abandoned their quest for magma. Still, they had bested the Americans. Their drills reached a respectable depth of 7.6 miles, on top of which they discovered two unexpected phenomena: at six miles down, the temperature was already much hotter than anyone anticipated, 180°C; even weirder, the rock was sopping wet.

  So water lurks within rock, rock dissolves into water, water evaporates into air, and air, in the form of carbon and nitrogen, gets trapped in rock. And all of it passes through Vulcan’s fiery forge deep beneath the Earth’s surface.

  Journey to the Center of the Earth

  I was nine when I saw Journey to the Center of the Earth in 1960. The film follows a group of four intrepid explorers — a famous geologist, his student, a beautiful philanthropist (who funds their exploration) and a tall Icelander with his pet duck — as they wend their way to the center of the Earth following a series of marks left by a previous explorer. Naturally, a rival team led by an evil scientist tries to sabotage them and they barely escape a dinosaur attack, but they eventually arrive at the center of the Earth where they encounter a strong magnetic field and all the metal objects they carry — rings, pocket watches, even their dental fillings — get sucked up into the air.

  The movie is based on Jules Verne’s novel of the same name, written in 1864. Though richly fanciful, and containing enough science to make it credible to the Victorians, the journey it describes is impossible, given what we now know about the Earth’s interior. Direct, manned exploration is completely out of the question, at least for now. But let’s take an imaginary ride through the layers of the Earth and see what’s down there.

  A supernova explosion might seem a strange place to start our journey but consider this: when a big star detonates, one with, say, 20 times the mass of our own sun, most of the energy of the explosion blows off the outer shell of the star. Meanwhile, the inner core implodes in a tremendous gravitational collapse, and all that remains of the original star is a remnant core so dense that electrons and protons are fused together into neutrons. That’s not easily done. This dark object, not quite a black hole, can have the mass of two of our suns yet be only seven miles in diameter. It is called a neutron star.

  Here’s where our thought experiment begins. A teaspoon of neutron star weighs 10 million tons, about 900 times the mass of the Great Pyramid of Giza. If we could enlist Superman to hold that teaspoon and tip it onto the surface of the Earth, it would freefall through the soil and rock and magma like spit through candy floss, reaching the center as fast as a rock falling through clouds. Since the center of the Earth is 3,960 miles beneath our feet, the freefall would take all of 45 minutes and look something like this:

  In the first few seconds of freefall, our spoonful of neutron star would cut through the Earth’s rocky mantle: the continental crust, the stuff we stand on, what mountains are made of. As Mohorovičić discovered, it’s about three to six miles thick under the seabed and about 22 miles thick under the continents. Beneath mountains, it bulges down an extra 15 to 35 miles.

  Temperatures rise steadily the deeper you go: 12 to 20 feet beneath the surface of the Earth, the temperature is a constant 11°C. Which explains why our ancestors lived in caves and why a house built into a hill is warmer than one built on flat land. It’s hotter still at the bottom of the world’s deepest gold mine in South Africa, the TauTona mine. There, 2.4 miles below the surface, the rock face temperature is 60°C. Even with industrial-scale air conditioners going full blast, the ambient temperature for the gold miners is 27°C.

  A few seconds later, our tiny dollop of neutron star would hit the upper mantle, just below the rocky crust at about 25 miles down, a depth equivalent to the mesosphere’s height above the Earth’s surface. This is the lava zone, where temperatures range from 500 to 900°C. Red hot. And it only gets hotter. Together, the upper half of the mantle and rocky crust make up the lithosphere, while the lower half of the upper mantle is called the athenosphere. The lithosphere “floats” on top of the athenosphere. It might take almost a minute for our dense spelunker to go through the upper mantle, which ends about 250 miles below the Earth’s surface. After that, it would plunge into the transition zone between the upper and lower mantle that starts at 250 miles below the surface and ends 400 miles beneath that, or roughly the same distance downwards as the thermosphere is above the surface of the Earth. This is the lowest depth that earthquakes have been detected.

  Interestingly enough, the transition zone between the lower and upper mantle is thought to contain up to three times as much water as all the world’s oceans combined. The magma is saturated like a sponge, a 1,900°C yellow hot sponge to be exact. Oceans in the fiery rock. These thicker layers are taking minutes rather than seconds for our heavy glob of neutron star to traverse, especially the next layer below the transition zone, the lower mantle. It starts at 400 miles and ends 1,800 miles further down.

  Below that is the D layer, a name derived from geophysicist Keith Bullen’s original term, D double-prime. He coined the tag during the 1960s at the height of geophysics’ exploration of the Earth’s core. The D layer, extending from 1,800 miles down to 1,900 miles, is thin but active. Here is where thermal fluctuations create hot spots, which propagate plumes of heat that rise through the mantle to the surface.

  Finally, at 1,900 miles deep, our stellar spelunker has reached the outer core. The temperature at the boundary of the mantle and the core is around 4,000°C. White hot. The tremendous pressures at this depth, almost three million times those of the surface, paradoxically turn the flowing magma at the bottom of the mantle into solid rock. By contrast, just below that, the outer core is liquid. It is also the source of Earth’s magnetism. Like a giant electric dynamo, the metals in the liquid outer core orbit the inner core and generate electromagnetic fields. It is these that give our planet its north and south magnetic poles, and the magnetic field that stretches into space. Solid planets without a liquid outer core, like Mars and our moon, have no magnetic fields. And it turns out that the magnetic field is crucial.

  A testament to the fluid nature of the outer core is how many times the Earth’s magnetic field has reversed over the past 100 million years — 200 times, or once every 500,000 years. As if we didn’t have enough to worry about, it’s possible we might be entering a reversal phase right now. Earth’s magnetic field has decreased by 15 percent over the past 200 years, and the process appears to be accelerating at a rate of 5 percent per decade. This isn’t good. Earth’s magnetic field creates the Van Allen belt, a field of charged particles captured from the solar wind that wraps around the planet just beyond the exosphere like a giant invisible donut with Earth in the middle. Each of the poles pokes out of the donut hole and the inner edge of the hole is clearly visible during auroral displays. The northern lights are the result of charged particles sliding down the edge of the hole in the donut. That’s why, from the vantage of the space station, auroral displays are often ring shaped.

  The Van Allen belt is more than the source of a beautiful display: it also plays a role in deflecting harmful cosmic rays from pelting down on us, a bit like the ozone layer. If the Van Allen belt magnetic field disappears, the effects on our DNA, on the entire planet’s DNA, might be damaging. Alarmingly, there is evidence that Earth’s magnetic field is already beginning to collapse in a region in the south Atlantic, between Africa and South America.

  Our neutron star explorer has finally arrived at the last layer, the inner core. This sphere at the heart of the Earth is an alloy of solid iron and nickel with its outer edge 3,200 miles beneath our feet. Its center is the Earth’s cente
r. Estimates of the temperature here range up to 6,927°C, or equivalent to the surface of the sun. And it has been that way for a long time. Despite the fact that the Earth sheds some 44 trillion joules of heat per second via mantle plumes, geophysicists believe that the inner core has cooled only some 400°C during the last four billion years.

  Yet it’s those mantle plumes that have the most direct relationship to the drifting continents and to the eventual release of naturally sequestered carbon dioxide. Even though carbon dioxide makes up less than 0.04 percent of the atmosphere, it has a disproportionate importance to life. Without it and the other gaseous buffers such as methane and water vapor, Earth’s average surface temperature would be -19°C instead of the current 14°C. Obviously, the balance of carbon dioxide in the atmosphere is critical, which is where the magma oceans of the deep mantle come in.

  Life processes have changed the composition of the Earth itself, right down to the lower mantle, about 435 miles below us. Carbon capture, or carbon sequestration as it is officially known, is in the news a lot these days. Many fossil-fuel power plants are using a technology that traps carbon dioxide before it is released into the atmosphere. They compress it into a liquid that is then pumped into geological formations deep beneath Earth. Nature has been doing the same thing, on a more massive scale, for billions of years. The world’s vast limestone deposits are carbon sequestration on a stupendous scale.

  Limestone is calcium carbonate, and it is almost totally composed of the compressed deposits of shallow tropical seas — fossil shells, fossil reefs, fossil algae and fossil oolites (spherical crystals of calcium carbonate that form in warm sea water). Perhaps one of the world’s best-known deposits of calcium carbonate are the White Cliffs of Dover. They represent millions of years of stable deposition in the oceans of the Cretaceous era, when tyrannosaurs walked the land and giant mosasaurs plied the oceans.

  A six-inch cube of chalk from the Dover cliffs sequesters more than 35 cubic feet of compressed carbon dioxide. And think about all the limestone that has been produced on our planet since the first stromatolites began to deposit calcium carbonates three billion years ago. To get a better idea of the sheer mass of limestone, let’s travel to the Bahama Banks, which started forming 150 million years ago, just after Africa detached from North America.

  The Bahama accretionary platform is one of the most seismically stable regions on the planet. Dinosaurs came and went while the Bahamas just kept depositing calcium carbonate in shallow tropical seas. Layer upon layer upon layer. So just how deep do these calcium carbonate deposits go? The Deep Sea Drilling Project drilled two cores in 1970. The first bore hole, near Andros Island, went to a depth of 15,600 feet, while the other bore hole, near Cay Sal Bank, reached a depth of 18,906 feet. Neither hit the bottom of the carbonate sediments, though they did get to early Cretaceous era limestone, 140 million years old. That’s just the Bahamas. An awful lot of oceanfloor sediments have already been subducted under the continental plates, and with them untold gigatonnes of sequestered carbon dioxide. It runs deeper than you can imagine. And volcanoes, it turns out, occasionally spew out clues to just how deep.

  Xenoliths, something that geologists describe as an intrusion or rock trapped inside another rock (usually magma), are carried up to the surface by volcanoes, and by analyzing them, geologists can get a good idea of what is going on in the mantle. Xenoliths come in many forms, but diamonds are the most famous. And some very special diamonds, while worthless for the diamond trade, are invaluable for geologists because they enclose intrusions — almost like xenoliths inside xenoliths — that contain pristine, unmelted samples of the deep mantle, 435 miles beneath the surface. Some contain water, carbon and even ocean sediments.

  These relics of ancient seafloors are three billion years old. Yet even more astounding than their age is how deeply they have penetrated the Earth. The deep mantle, once composed of the same rock as the rest of the planets in our solar system, has been altered in chemical composition by life itself. Only the planet’s core remains untouched. It has taken some time but life has created our atmosphere and has now reconstituted almost a quarter of the rocky substance of our planet. Earth is being transformed.

  Colliding Continents

  On a grand timescale, the magma beneath the continents acts like an ocean, with eddies, currents, upwellings and even whirlpools. In a sense, the continents are like clouds: just as the atmosphere both supports and propels clouds, so does the liquid magma interior of the Earth support and propel continents. They drift around on the surface of the magma-like rafts, steered by the currents and upwellings from the fiery depths. All the continents except one have a deeply submerged base that extends into the magma. Like the blocky underwater hull of a barge, the bases act to catch the flow of the magma and carry the continent along.

  Sometimes another player interferes with the continents as they bob along on the athenosphere. These are 375- to 500-mile-wide upwellings of magma, called mantle plumes, which rise from the D layer, 1,800 miles down, and climb through the mantle to the lithosphere, sometimes burning through it like a blowtorch. Mantle plumes are relatively stationary. A good example is the Hawaiian Island chain, part of the Pacific plate which is moving west-northwest at three to four inches a year. The Hawaiian mantle plume creates a hotspot that occasionally melts through the crust and erupts. The volcanic eruption forms an island that is then carried away by plate tectonics before the process repeats itself. An even larger mantle plume lurks beneath Yellowstone National Park and is the heat source for Old Faithful.

  One of the longest lasting mantle plumes currently sits under the island of Réunion in the South Pacific. A hundred million years ago, it ignited volcanoes that helped to break up the Gondwana supercontinent (which included today’s southern landmasses — Antarctica, Australia, South America, Africa and India). It also burned the bottom off the Indian plate after it had detached from Gondwana. Because the Indian plate was half as thick as most continental plates, it skidded over the athenosphere more quickly, covering the 1,900 miles between Gondwana and the Eurasian plate in less than 50 million years. The resulting impact was a tremendous bang-up. The Himalayas and the Tibetan plateau, the highest landmasses in the world, are still rising as a result.

  As the two continental plates continue to grind together and buckle in a massive, snail’s pace collision, their effect on climate has been equally massive. Indeed, the Himalayas and the high Tibetan plateau affect weather as far away as Australia in a seasonal phenomena called the Indian monsoon, the largest and wettest of the world’s monsoons. During the four months from June to late September, India receives 80 percent of its annual total rainfall and oftentimes monumental deluges.

  The driver of the whole cycle is the Himalayas mountain range and the high Tibetan plateau. When this region heats up in summer, it draws moist air from the Indian Ocean from as far away as Australia. The inflow is so strong that ocean currents are created beneath the monsoon winds that follow them. These currents are, in effect, underwater winds rather than true ocean currents. Every year, they dredge up plankton from the depths near the Maldives, and for a few weeks hundreds of manta rays congregate in an astonishing and graceful feeding frenzy as a result.

  The relationship between ocean currents and wind and climate patterns is quite intimate. The Coriolis effect not only deflects the winds but also ocean currents, causing the great gyres of the north Pacific and Atlantic oceans to rotate clockwise, while those in the southern hemisphere rotate counterclockwise. These gyres are vast currents that shunt warm and cold water thousands of miles, affecting huge swaths of climate. The Atlantic gulf stream supports palm trees on the west coast of Scotland while in the southern Pacific, the Humbolt current delivers cold water to the west coast of South America.

  El Niño

  It was Ecuadorian fishermen who first alerted meteorologists to El Niño, the warm Pacific current that would sometimes appear around Christmas. Normally the
coastal waters of Ecuador are cool, supplied by the nutrient-rich Humbolt current. Plankton populations explode in this cold water, and the anchovies that feed on them thrive. But when El Niño’s current brings warm water to the west coast of South America, the anchovy population collapses and affects the fishermen of Ecuador and Peru along with all the larger fish, sea birds and seals the swarming anchovy feed. The effects of El Niño are not confined to the eastern Pacific region. In the last few decades, meteorologists have begun to understand the complex relationship between wind patterns and ocean currents and have realized that the reach of El Niño is long indeed.

  Again, it all depends on the trade winds. Normally they blow at a steady rate from east to west across the Pacific, pushing warmer surface waters westward toward Australia and Indonesia. This forces the cold, deep water of the Humbolt current to well up off the coast of South America, as it replaces the eastern wake of displaced warm water. But sometimes the trade winds lessen in intensity, and the region of warmer water is not pushed west at its usual rate, with the result that it backtracks until it reaches South America. This is El Niño. In an El Niño year, as the ripple effect spreads, there may be flooding in Peru, while Brazil, India, Australia and Indonesia experience droughts. In North America, the winter will be warmer in the northeast and wetter in the south.

  La Niña is the mirror opposite of its brother: stronger than normal trade winds push the cooler water westward into the equatorial Pacific, cooling surface temperatures and changing weather patterns radically. It brings drought to Peru and flooding to Australia. Together, El Niño and La Niña are referred to as the Southern Oscillation. Weather patterns over half the Earth’s surface are influenced by this cycle, and it shows how ocean currents intermingle with the air above to form a supertroposphere, one that extends from the bottom of the deepest ocean trench up to the edge of the stratosphere.