Jules Verne's vision of burrowing deep into our planet remains fiction, but geophysicists have found ways of exploring its high-pressure, superheated core
Click to follow
The Independent Culture
Just 20 miles away from where we are now is a place no human has explored. People have mapped the entire surface of our planet, dived deep into the ocean, even visited the Moon, but the ground not far beneath our feet is unknown territory. When Jules Verne published Journey to the Centre of the Earth in 1864, it could only have been a work of fiction. Though geologists had begun mapping the surface in great detail, they knew little about the inner workings of the Earth. Only five years earlier, Charles Darwin had published his theory on the origin of species and the controversies it raised were still echoing through science and religion. Many people still thought the Earth had only been created a few thousand years earlier, fossils had been left by the biblical flood and the Sun was powered by burning coal. Against this background, Verne's fictional account seems remarkably perceptive. Today, a journey to the centre of the Earth need not be a work of fiction. It may not be possible in person, but techniques for probing, simulating and modelling the Earth's interior are beginning to bear real fruit.

Our recent ability to observe the Earth as a whole, from space, has added to geologists' knowledge and enabled them to measure processes on vast scales, and comprehend them. We now know that the Earth is a highly active, dynamic planet. Earthquakes and volcanoes are just the surface expression of that.

Structurally the Earth is like an onion, a series of concentric shells or layers. There is a thin crust of hard, cold rock about five miles thick under the ocean, and typically 20 or 30 miles thick under continents. The bulk of the Earth underneath this comprises a rocky mantle which, though solid, is hot and can flow slowly in much the same way as solid ice flows in a glacier. Beneath that is a core of molten iron with a small, solid, inner core of iron the size of the Moon. In fact, Earth is only 99 per cent like a perfectly concentric onion. The one per cent variation from that perfect form represents the frontier of geophysics and the key to the dynamic processes inside the Earth.

The crust and top few miles of mantle make up the rocky lithosphere which is divided into a series of plates. Some are as big as continents - indeed they are continents; others are mere splinters. They are pushed and pulled about on the semi-plastic mantle beneath. The rocks of the mantle are heated by radioactive decay from within and by the core beneath, setting up a circulation that takes hundreds of millions of years. Like very thick porridge on the stove, hot plumes rise and colder material sinks. Where a plume comes up underneath the crust, volcanoes erupt and can open a rift valley, as in East Africa, and ultimately an ocean such as the Atlantic. Such forces can destroy lives and change the face of the planet, but the plates of ocean floor and continent are the scum on the surface compared to the vast bulk of the mantle.

Even today, nobody can visit the mantle. They can't even drill that deep. A plan to drill through the comparatively thin ocean crust in the 1960s, the Mohole Project, foundered. A recent German attempt to drill deep into the crust had to be abandoned 11km down after it became so hot that the hole kept closing up. The depth record is still held by the Russians with a 14km borehole into the Kola peninsular. That found some interesting minerals, but came nowhere near reaching the mantle.

The lava thrown up by volcanoes comes from the mantle, but it is not representative of what the mantle is made of. It may consist of the four per cent that melts down as the pressure drops when hot mantle rock convects towards the surface. Sometimes it contains lumps of what may be mantle rock - rich in a dark green mineral, olivine - but even that can't occur very deep. Its crystal structure couldn't stand up to the incredible pressure.

One mineral that does come to us from great depth is diamond. Its crystal structure can only form at high pressure, and diamonds sometimes carry within them microscopic bubbles of material that can reveal details of their million-year journey through the mantle. Crushing diamonds to read their messages is an expensive way to do geology, but it is sobering to think, when you look at a diamond ring, what a tale it might have to tell.

Geologists can look into the mantle in other ways, too, such as seismic tomography. Just as the body scanners of medical tomography use X-rays to reveal the 3D structure of the inside of a living human, so seismic tomography uses seismic waves from earthquakes to reveal the structure of the Earth. That's how geologists know abut our planet's molten core. Some waves won't penetrate liquid, others travel more slowly through it than they do through hard rock. In the mantle, the waves travel faster in cold, hard rock than in hot rock. Each time there is a major earth tremor, thousands of seismic monitoring stations around the world record the arrival times of the waves, giving another angle of view through the mantle.

Other tools are making it possible to journey to the centre of the Earth without leaving the laboratory. It may sound a simple matter to take a sample of rock and see what happens when you subject it to high temperatures and pressures. But the temperature, and particularly the pressure, of the interior of the Earth are so high that it becomes very difficult indeed. The pressure at the boundary between the core and mantle is about 1.3m times the pressure on the Earth's surface, and that at the centre of the Earth is 3.6m times atmospheric pressure.

The first experiments to simulate the pressures of the planet's interior, and find out how minerals behave under such conditions, were done using big hydraulic presses. But such presses cannot simulate more than a few hundred kilometres depth - and even for that, they push materials technology to the limit in the design of sample holders that will not themselves break or melt.

Professor Thomas Ahrens has used a very different approach at the California Institute of Technology. His basement laboratory there boasts the fastest gun in the west: a giant two-stage cannon that first uses explosives to compress hydrogen to pressures of about 1,000 atmospheres, then uses that to fire a 15g (1oz) bullet to speeds of five miles per second. That is about the speed of a spacecraft orbiting the Earth, and the gun was first built to test materials for the Apollo space capsules. When the projectile hits a mineral specimen, it creates a shock wave that briefly matches the pressures and temperatures of the core of the Earth - but only for less than a millionth of a second, so there is no chance to establish equilibrium. A host of microscopes and detectors must analyse the flash of light in the same way as astronomers analyse starlight - and without getting blown up in the process.

The most popular technique today seems far simpler. One material that will withstand the temperatures and pressures involved is diamond. Unfortunately, the researchers cannot afford fist-sized diamonds, so they have become adept at handling microscopic specimens, typically weighing only a few millionths of a gram. They are placed between the flattened points of two sparkling, gem quality, diamonds to form a "diamond anvil". The diamonds are so-called brilliant cuts, with the flattened faces at their points perfectly parallel to concentrate the pressure so that nothing more than a precision thumbscrew is needed to reach the pressure of the centre of the Earth in the sample. Conveniently, the colour of ruby changes with increasing pressure, so a tiny speck of ruby next to the sample acts as a pressure meter. Also conveniently, diamond is transparent, so a laser beam can be shone through one face of the diamond to heat the sample and a microscope can see in to record what happens. But that does call for rare, white, nitrogen-free diamonds. In spite of the cost, the anvils are compact and teams in Britain, Germany and the US are using them.

One of the principal results that has come both from the diamond anvil work and from the shock-wave gun has been to estimate the temperature at the centre of the Earth. The lower mantle can be no hotter than the melting point of its constituents, and the liquid outer core can be no colder than the melting point of iron at that pressure. Most specifically, the temperature at the boundary between the outer and inner core must be at precisely the melting point since both have the same composition. Original estimates differed quite widely between experimental groups, but now most of the scientists are approaching a consensus. The Earth's centre is between 5,500 and 6,500K, the inner core/outer core boundary 500 cooler and the top of the outer core 500 cooler still. The base of the mantle, above the churning, molten core, provides further insulation and the temperature falls off to 4,000K. These temperatures are hotter than many expected. Indeed, the centre of the Earth seems to be hotter than the surface of the Sun. That raises the question of how it becomes so hot in the first place - a question with implications for how the Earth formed.

The combined knowledge of the minerals gained from laboratory simulations and the overall structure of the Earth gained from the seismic tomograph "body scans" provides a new window on the planet's interior. The mantle extends to a depth of about 2,900km but there are various layers within it that reflect earthquake waves. One, at 420km, represents what is known as a phase change: below it, minerals with the same chemical compositions are forced into a denser crystal lattice structure by pressure. This depth marks the transformation of green olivine into a denser, brown silicate called spinel. There is another feature about 670km down. That marks the boundary between the upper and lower mantle. Once again, there is a phase change across it, with the spinel structure above and an even more tightly packed one called perovskite below. Perovskite is probably the most abundant mineral in the Earth. Its tight-packed structure makes it of special interest to scientists looking for very hard materials and new types of super-conductor for the electronics industry. But at surface pressures perovskite is unstable and scientists have to struggle to make it, even in milligram quantities. No further beneath our feet than the horizontal distance between London and Edinburgh, there must exist millions of tons of it.

The phase change at 670km represents a more substantial barrier, both to mantle circulation and to seismic waves, than the one at 420km. The path of the descending slab of old, cold ocean crust can be traced from the epicentres of deep earthquakes. They map its descent down to 670km where it seems to grind to a halt, at least temporarily. One of the biggest controversies in geology is whether, after absorbing sufficient heat to undergo a further phase change, it then continues through the lower mantle so that the entire mantle takes part in the circulation, or whether the mantle is like a great double boiler with separate circulation in the upper and lower mantle and little or no chemical mixing between them.

Evidence from simulations is now beginning to suggest a compromise. The phase change from spinel to perovskite takes up a lot of heat, so the descending slab cannot undergo the change and cross the boundary until it has had time to warm up - probably millions of years. So it tends to spread out into a sort of holding reservoir that forms a pronounced layer in the seismic images. Eventually, when it is dense enough, computer models predict that vast slabs can break through quite quickly, like a slow-motion avalanche through the lower mantle.

What was probably the most powerful earthquake since the one in Alaska in 1964 took place in June 1994 beneath Bolivia. Its vibrations could be felt through cold, hard continental rocks as far apart as Canada and West Africa. But it caused comparatively little damage because the epicentre was extremely deep - about 640km. For a long time, geologists did not believe earthquakes were possible so deep, because the rocks were too soft to crack. What they now suspect may happen is that a phase change suddenly runs through a whole layer of rock in a sort of anti-crack. Even so, the Bolivian quake was bigger and deeper than anyone had expected. But it proved a marvellous tool for seismologists.

The boundary between the base of the mantle and the Earth's core is not a simple one. Seismic profiles imply that, at least in some places, there is a layer there up to 200km thick of material of a slightly different density to the bulk of the mantle. This layer is known as the D" (pronounced D double prime). It appears to be a discontinuous layer. It is simply not present under some areas of the globe. Perhaps the densest material of the mantle has accumulated there, or a sort of solid scum has formed above the core. The latest evidence, based on observations from Canada of seismic waves from earthquakes in South America, suggests that, under the Caribbean at least, the D" layer is itself layered. Seismic waves seem to pass more quickly along it than through it. One explanation of that could be that the layer is made of very ancient floor that sank all the way through the mantle, a sort of graveyard for the floors of oceans long since vanished.

There are also indications of physical unevenness at the base of the mantle. Very precise measurements of the Earth's rotation rate, made from space, reveal irregularities of the order of a millionth of a second in a day. Some are thought to be due to atmospheric circulation blowing on mountain ranges, the mountains acting like sails in the wind. But there is also evidence that the liquid core of the Earth has a similar effect on what may be ridges and valleys in the solid base of the mantle, like a ship's keel.

The entire liquid iron outer core is slowly churning about like water in a washing machine, moving at several millimetres a second. It is in this region that the Earth's magnetic field originates. Though the overall effect of the field is rather as if there were a giant bar magnet there, it is far too hot for permanent magnetism. But the liquid metal conducts electricity and, as it churns about, acts like a great dynamo generating the magnetic field. It is not an even circulation and it is not a smooth or unchanging magnetic field. Even over short periods of a few decades, the magnetic poles at the surface can wander over several degrees. Mathematical simulations of the Earth's core suggest that there are many more rapid fluctuations in the dynamo, but that these are to some extent screened and smothered out of the presence of the solid inner core and the mantle. As a result, the magnetic envelope around the Earth, the magnetosphere, which deflects charged particles from the Sun, has remained in place through historic times. There is clear evidence from the alignment of magnetic particles in rocks, however, that the entire field has reversed many times in the past, typically at intervals of 100,000 years or so. On that basis, we are overdue for a reversal now and since none has been observed, we do not know if it is a sudden event with little effect beyond disorientating our compasses, or if the field fades away completely for days or years during the reversal, in which case harmful radiation could reach the ground, cause radio blackouts, power surges in cables and even disrupt the climate.

The inner core of the Earth, at 5,000K or more, is under such pressure that it is solid. It is almost entirely iron, perhaps alloyed with some nickel. As the Earth cools, the inner core slowly grows. It is freezing at a rate of 1,000 tons per second. That seems fast but, after 4.5 billion years, still only four per cent of the total core has solidified, so the Earth is likely to have a magnetic field for billions of years to come. It is this freezing process that keeps the outer core in motion. In addition to heat released from the decay of radioactivity elements, heat is released by the freezing process. Also, lighter impurities in the outer core are left behind as the iron freezes and these float upwards through the outer core, stirring it up and disrupting the otherwise even magnetic field. In spite of the screening effect of the mantle, such magnetic anomalies can sometimes be detected at the surface.

The deep earthquake under Bolivia in 1994 produced some clear signals from the inner core. In particular, it confirmed suspicions that the inner core is not a simple, even sphere. It seems the seismic waves travel 3-4 per cent quicker through the inner core when going north-south, compared with when they cross between opposite points of the equator. Professor Guy Masters of the University of California at San Diego says the effect is similar to what might be seen if the inner core were a single crystal of iron the size of the Moon. He emphasises that this does not mean it is a single crystal, but that is one explanation. Another is that it could be made of many crystals all aligned the same way, as iron compass needles might be in a magnetic field. Though we know very little about the core of the Earth, Professor Masters points out that it is so inaccessible and its conditions so extreme that it is pretty amazing we know anything at all about it.

! Martin Redfern is executive producer at the BBC World Service Science Unit. His book, 'Journey To The Centre of the Earth', is published by Broadcast Books at pounds 14.95.

This window into the centre of the Earth is created from seismic data collected by survey stations worldwide, the largest being in the US and France. Shock waves from earthquakes travel at different speeds through liquids and solids, and through liquid rock at different temperatures. Areas that are hotter than average appear red in the diagram; those that are co oler than average appear blue. By analysing such data, scientists can build up a picture of the Earth's structure. They can tell, for example, where the upper mantle ends (about 670km down) and the outer core begins. This technique is known as seismic to mography