Onwards and downwards
Volcanoes are proof that even the depths of the mantle matter to us. Now we are going even deeper, to the parts of the planet wrhose components – we think – never have a hope of seeing daylight. While mantle material makes its way to the Earths surface daily, the core plays only a supporting role. The discovery of the core as a specific part of the Earth is a mighty scientific achievement, depending on the painstaking analysis of earthquake records and a good deal of ingenuity. Despite its inaccessibility, we have nowr built up a reasonable understanding of its properties. One thing we have learnt is that just like the boundaries between the other layers of the Earth, there is no hermetic seal between the core and the overlying mantle. The core is metallic, but we know that the liquid metal that makes it up also seeps into the rocks of the mantle above.
We also know that, like the mantle, the core consists of two halves. The outer core starts about 2700km below your feet while the inner core begins some 5200km down and keeps going to the centre, about 6371km below you if you are sitting at sea level.
As we saw earlier, there is one big difference betwreen the two. The outer core is molten. It consists mainly of iron with a few percent of nickel and about 10 percent of something else, probably oxygen or sulphur. This composition is consistent with its seismic properties and also with the chemistry of metallic meteorites. The inner core, however, is solid. Just what makes it solid despite its immense temperature is simple. It is the pressure. Even at its peak temperature of about 6000°C, the material that makes it up, with roughly the same composition as the outer core, cannot stay liquid. At the centre of the Earth, the pressure is about 14 million times the atmospheric pressure we know' at sea level. The density of the inner core averages out at about 15 grams per cubic centimetre, compared to 2.7 in the crust.
It had been thought that the inner core was a relatively uniform structure. Recently, however, research has suggested that it may be only the centre of the inner core which is homogeneous, while its outer parts are more varied in composition.
Heat from the core travelling outwards through the Earth is the main source of the energy driving the movement of the mantle, although the rocks there also contain radioactive atoms of their own that decompose and generate heat.
Perhaps the most unexpected recent finding about the inner core – derived from exquisite observations of many decades of earthquakes – is that it does not share the same 24-hour rotation as the rest of the Earth, including you and me. Instead it is rotating just slightly faster, by a degree, or 20km, a year. The rotation became apparent because the whole of the inner core has a structure in which the iron crystals point north-south and earthquake waves move through them faster in this direction than east-west. That also makes it possible to spot the slight rotation of the inner core. These aligned crystals form a “fast track” for seismic waves. Over years, it is possible to observe that track moving relative to the Earths surface by timing earthquake waves travelling from the Antarctic to Alaska. By the standards of the solid Earth, that rate of movement is wrarp speed, thousands of times as fast as the pace at which continents move. And all this movement of solid and liquid metal has a very real effect: it helps to create the Earths magnetic field.
This is a useful feat in several ways. As we saw in Chapter 2, it saves the Earth from the worst of the celestial radiation that wrould otherwise reach the surface. If it did not, life here would have taken a very different course from the one it has.
In addition, the roughly north-south alignment of the field drags any small magnetized object into line with it. The compass, first developed in China, remains a vital bit of navigational kit even in the electronic era. Take a GPS receiver out into the hills if you must, but don't think of going without a map and a compass as well, and make sure you know how to use them.
The machine that makes the Earth's magnetic field is called the geodynamo. The principles underlying it are the same as for any other dynamo. There are three components to the system: movement, an electric current and a magnetic field. As soon as two are present, the third will appear. In a dynamo used to generate electricity, for example, a metal wire is moved in a magnetic field, and a current is created. That is the principle behind a power station, whether the movement is created by burning coal or capturing power from the wind. In the same way, if a magnetic field is applied to a wire with electricity in it, it moves in response. This effect is used to generate force in the magnetic brakes used on theme park rides all over the world.
In the case of the Earth, there is movement, in the form of all that convecting metal, and an electric current, present inherently in the electrons in all that moving iron. And the inevitable result is a magnetic field. The Earth's magnetic field is not very powerful. Magnetic fields are measured in units called Teslas, and the Earth's is rated at just 0.00005T. By contrast, magnetic imaging machines in hospitals have fields from 1.5T upwards, and some bizarre stars called magnetars have fields of about 100 billion Teslas. But because the Earths core is large, the field it creates is massive as well, extending far out into space.
Our understanding of the geodynamo is being revolutionized by very large computers. Some of the biggest supercomputers in the world are being used to model the Earth's interior, such as the NEC Earth Simulator in Japan, at one time the most powerful in existence. In addition, satellites are gathering more and better data about the size, shape and strength of the Earth's magnetic field, and the core itself is being modelled – not only inside computers, but also through physical models in laboratories in which convecting masses of liquid sodium take the place of the iron found in the Earth's core.
Although the working of the geodynamo is controversial, there seems to be little doubt that it depends on the Earth having a solid inner core as well as a liquid outer one, to provide a structured shell within which metal can convect. The convecting iron seems to move at a stately speed of about 10km per year. It also seems that the north-south alignment of the convection is driven by the Earths rotation, something we see on other planets too, including Jupiter.
Effects at the surface
Edmond Hailey (of comet fame) was the first to write a scientific account of the shape of the Earths magnetic field. He used compasses on British naval ships to map the varying direction of the field, both its north-south orientation and its “dip”, the angle a compass needle at a specific point takes to the horizontal. These are the tangible signs at the Earths surface ol the way molten metal is moving far below.
The outer core convects for the same reason a boiling pan of water does: it is cooler at the top than at the bottom. The heat is produced from a number of sources, including the energy given out by iron solidifying on the outside of the inner core, which is growing slowly as the Earth cools. Add these to the force of the Earths rotation and complex, turbulent flow ensues in three dimensions and is maintained. One result is that the field we observe at the Earths surface is at its strongest in North America, Siberia and Antarctica, reflecting the pattern of convection cells beneath.
As we saw in Chapter 3, data from magnetic fields trapped in rocks shows that despite the inertia of billions of tonnes of molten iron, the Earths magnetic field is not fixed. Instead, it can switch direction totally over a few thousand years. The last such reversal was 780,000 years ago and the average time between reversals is about 250,000 years. But this does not mean we are “due” a reversal, any more than a gambler can be due a six just because one has not come up lately. Indeed, a look at the chart shows that at one point, there were no reversals for 35 million years.
However, we also know from direct measurement that the Earths magnetism has become about 10 percent less strong since reliable measurements got going in the 1830s. It seems from past reversals that it takes a few thousand years for the compass needle to turn by 180°, following a period of low Earth magnetism in which it might not point very decisively in any direction.
The key to how this happens seems to lie in the detailed interaction between turbulent eddies in the outer core and the base of the Earths mantle. This can produce patches of trapped molten iron in which the magnetism is opposite to the main strength of the field. Such a patch of “reverse polarity” might eventually spread to occupy the region of one magnetic pole while a second reversed the polarity of the other.
Richard Muller at the University of California points out that a whole range of events at the core-mantle boundary could set such changes going. These could include slumping material from old, subducted bits of continent. But he also points out that a hefty meteorite or asteroid impact would send a big shock wave into the boundary. As a result, in addition to leaving a big crater and altering the Earths climate, such an impact could cause a magnetic reversal and a major volcanic episode.
More optimistically, he adds that a major impact would remove all the loose matter near the boundary and protect the Earth from further changes in the next few million years. This may be what happened in the period from about 120 to 85 million years ago. This 35-million-year period in which there were no magnetic field reversals was preceded by an episode of major volcanic activity.
It is also possible that the rotation of the inner core relative to the outer core affects its short-term behaviour. It may contribute to the timing of reversals by affecting the convection patterns of the
While there is discussion of such a reversal happening “soon”, this is soon geologically, not on any human timescale. You can expect to be using your compass on country walks for the rest of your lifetime, and to see the red end of the needle still pointing more or less to the north.