Terry Pratchett - The Science of Discworld I

+++Yes? +++

'Is there any way we can get onto the world?'

+++ Nothing Physical May Enter The Project +++

'I want someone down there to observe things from the surface.'

+++ That Is Possible. Virtually Possible +++

'Virtually?'

+++ But You Will Need A Volunteer. Someone To Fool +++

'This is Unseen University,' said the Archchancellor 'That should present no problem.'

WE DON'T KNOW IF THE EARTH IS A TYPICAL PLANET. We don't know how common 'aqueous' planets with oceans and continents and atmospheres are. In our solar system, Earth is the only one. And we'd better be careful about phrases like 'earthlike planet', because for about half of Earth's history it has not been the familiar blue-green planet that we see in satellite photos, with its oxygen atmosphere, white clouds, and everything else that we are used to. In order to get an earthlike planet, in today's sense, you have to start with an unearthlike planet and wait a few billion years. And what you get is quite different from what, only a few decades ago, we thought the Earth was like.

We thought it was a very stable place, that if you could go back to the time when the oceans and continents first separated out, they'd have been in the same places they are now. And we thought that the interior of the Earth was pretty simple. We were wrong.

We know a lot about the surface of the Earth, but we still know much less about what's inside it. We can study the surface by going there, which is usually fairly easy, unless we want to look at the top of Everest. We can also penetrate the ocean depths using vehicles that can protect frail humans against the huge pressures of the deep seas, and we can dig holes down into the ground and send people down those too. We can get further information about the top few miles of the Earth's crust by drilling, but that's just a thin skin, comparatively speaking. We have to infer what it's like deeper down from indirect observations, of which the most important are shock-waves emitted by earthquakes, laboratory experiments, and theory. The surface of our planet generally seems fairly placid, apart from weather and the sometimes severe effects of the seasons, but there are plenty of volcanoes and earthquakes to remind us that not so far below our feet it's a lot less hospitable. Volcanoes form where the molten rocks inside the Earth well up to the surface, often accompanied by massive clouds of gas or ash, all of it emerging under high pressure. In 1980 Mount St Helens in Washington State, USA blew up like a pressure-cooker whose lid had been tied down, and about half of a large mountain simply disappeared. Earthquakes happen when the Earth's crustal rocks slide past each other along deep cracks. Later we'll see what drives these two things, but they need to be put into perspective: despite occasional disasters, the surface of the Earth has been sufficiently hospitable for life to have evolved and survived for several billion years.

The Earth is nearly spherical, having a diameter of 7,928 miles (12,756 km) at the equator but only 7,902 miles (12,714 km) from pole to pole. The slight broadening at the equator is the result of centrifugal forces from the Earth's spin, and originally set in when the planet was molten. The Earth is the densest planet in the solar system, with an average density 5.5 times that of water. When the Earth condensed from the primal dustcloud the chemical elements and compounds that formed it separated into layers: the denser materials sank to the centre of the Earth and the lighter ones floated to the top, much as a layer of light oil floats on denser water.

In 1952 the American geophysicist Francis Birch set out a description of the general structure of our planet which has been modified in only minor ways since. The inside of the Earth is hot, but the pressure there is also very high: the most extreme condi­tions occur at the centre where the temperature is about 6,000°C and the pressure is 3 million times atmospheric pressure. Heat tends to make rocks and metals melt, but pressure tends to solidify them, so it is the combination of these two conflicting factors that determines whether the materials are liquid or solid. The centre of the Earth is a rather lumpy spherical core, mainly made of iron, with a radius of roughly 2,220 miles (3,500 km). The innermost regions of the core, out to a radius of 600 miles (1000 km), are solid, but a thick outer layer is molten. The very top layers of the Earth form a thin skin, the crust, which is only a few miles thick. Between crust and core lies the mantle, which is solid, formed from a variety of silicate rocks. The mantle also divides into an inner layer and an outer layer, with the division occurring at a radius of about 3,600 miles (5,800 km). Above this 'transition zone' the main rocks are olivine, pyroxine, and garnet; below it their crystal structures become more tightly packed, forming such minerals as perovskite. The outermost parts of the mantle, and the deeper parts of the crust where the two join, are again molten.

The crust is between 3 and 12 miles (5 and 20 km) thick, and there's a lot going on there. Those parts of the crust that form the continental land masses are mainly made of granite. Beneath the oceans, the crustal layer is predominately basalt, and this basalt layer continues underneath the continental granite. So the conti­nents are broad, thin sheets of granite stuck on top of a basalt skin. From the Earth's surface the most evident features of the granite layers are mountains. The highest ones look big to us, but they rise no more than 5 miles (9 km) above sea level, a mere seventh of a per cent of the Earth's radius. The deepest part of the ocean, the Mariana Trench in the northwest Pacific, plunges 7 miles (11 km) beneath the waves. The overall deviation from an ideal sphere (strictly, spheroid, because of the flattening of the poles) is about one-third of a per cent, about as irregular as the shallow indenta­tions you find on a basketball, which add to its grip. Our home planet, give or take a bit of squashing, is remarkably round and sur­prisingly smooth. Gravity made it that way, and it keeps it that way, except that some small but interesting movements in the mantle and the crust add a few wrinkles.

How do we know all this? Mainly because of earthquakes. When an earthquake hits, the whole Earth rings like a bell hit by a ham­mer. Shockwaves, vibrations emitted by the earthquake, travel through the Earth. They are deflected by transition zones between different kinds of material, such as that between core and mantle, or lower and upper mantle. They bounce off the Earth's crust and head back down again. There are several kinds of wave, and they travel with different speeds. So the short sharp shock of an earth­quake gives rise to a very complex pattern of waves. When the waves hit the surface they can be detected and recorded, and recordings made in different places can be compared. Working backwards from these recorded signals, it is possible to deduce a certain amount about the underground geography of our planet.

One consequence of the Earth's internal structure is a magnetic field. A compass needle points roughly north. The standard 'lie-to-children' is that the Earth is a giant magnet. Let's unpack the next layer of explanation.

The Earth's magnetic field has long been something of a puzzle since magnets are seldom made out of rock, but once you realize that the Earth has a whopping great lump of iron inside it, every­thing makes much more sense. The iron doesn't form a 'permanent' magnet, like the ones you inexplicably buy to stick plastic pigs and teddy bears on the fridge door; it's more like a dynamo. In fact it's called the geomagnetic dynamo. The iron in the core is, as we've said, mostly molten, except for a slightly lumpy solid bit in the mid­dle. The liquid part is still heating up, the old explanation of this was that radioactive elements are denser than most of the rest of the Earth, and therefore sank to the middle where they became trapped, and their radioactive energy is showing up as heat. The current theory is quite different: the molten part of the core is heat­ing up because the solid part is cooling down. The liquid iron that is in contact with the solid core is itself slowly solidifying, and when it does so it loses heat. That heat has to go somewhere, and it can't just waft away unnoticed as warm air because everything is thou­sands of miles underground. So it goes into the molten part of the core and heats it up.