When you hear a word like ‘collided’ you might think of a sudden crash, as when a truck collides with a car. That isn’t the way it was – and is. The movement of the continents happens agonizingly slowly. Somebody once said it happens about as fast as fingernails grow. If you sit and stare at your fingernails, you don’t see them growing. But if you wait a few weeks, you can see that they have grown, and you need to cut them. In the same way, you can’t see South America in the act of moving away from Africa. But if you wait 50 million years, you notice that the two continents have moved a long way apart.
‘The speed with which fingernails grow’ is the average speed at which the continents move. But fingernails grow at a pretty constant speed, whereas the continents move in jerks: there’s a jerk, then a pause of a hundred years or so while the pressure to move again builds up, then another jerk, and so on.
Perhaps now you are beginning to guess what earthquakes really are? That’s right: an earthquake is what we feel when one of those jerks happens.
I’m telling you this as a known fact, but how do we know it? And when did we first discover it? That’s a fascinating story, which I now need to tell.
Various people in the past have noticed the jigsawy kind of fit between South America and Africa, but they didn’t know what to make of it. About 100 years ago, a German scientist called Alfred Wegener made a bold suggestion. It was so bold that most people thought he was a bit mad. Wegener suggested that the continents drifted about like gigantic ships. Africa and South America and the other great southern land masses had, in Wegener’s view, once been joined together. Then they tore apart from each other and cruised off through the sea in their separate directions. That was what Wegener thought, and people laughed at him for it. But it now turns out that he was right – well, almost right, and certainly much more right than the people who laughed at him.
The modern theory of plate tectonics, which is supported by a huge amount of evidence, isn’t quite the same as Wegener’s idea. Wegener was definitely right that Africa and South America, India, Madagascar, Antarctica and Australia had once all been joined up and later split apart. But the way it happened, according to the theory of plate tectonics, is a bit different from the way Wegener saw it. He thought of the continents as ploughing through the sea, floating, not on water but on the soft, molten or semi-molten layers of the Earth’s crust. The modern theory of plate tectonics sees the whole crust of the Earth, including the bottom of the sea, as a complete set of interlocking plates. (That’s ‘plates’ as in ‘armour plates’, not the kind of plates you eat off.) So it isn’t just the continents that move: it’s the plates that they sit on, and there is no bit of the Earth’s surface that isn’t part of a plate.
Most of the area of most of the plates lies under the sea. The land masses we know as the continents are the high ground of the plates, sticking up above the water. Africa is just the top of the much larger African plate, which stretches halfway across the South Atlantic. South America is the top of the South American plate, which stretches across the other half of the South Atlantic. Other plates are the Indian and Australian plates; the Eurasian plate, which consists of Europe and all of Asia except India; the Arabian plate, which is rather small and slots in between the Eurasian plate and the African plate; and the North American plate, which includes Greenland as well as North America and reaches halfway across the bottom of the North Atlantic ocean. And there are some plates that have hardly any dry land on them at all, for example the vast Pacific plate.
The divide between the South American plate and the African plate runs right down the middle of the South Atlantic, miles from either continent. Remember that the plates include the bottom of the sea, and that means hard rock. So how could South America and Africa have nestled together 150 million years ago? Wegener would have had no problem here, because he thought the continents themselves drifted about. But if South America and Africa once snuggled together, how does plate tectonics explain all the undersea hard rock that nowadays separates them? Have the undersea parts of the rocky plates somehow managed to grow?
Sea-floor spreading
Yes. The answer lies in something called ‘sea-floor spreading’. You know those moving walkways that you see at large airports to help people with luggage cover the long distances between, say, the entrance to the terminal and the departure lounge? Instead of having to walk all the way, they step on a moving belt and are carried along to some point where they have to start walking again. The moving walkway at an airport is only just wide enough for two people to stand side by side. But now imagine a moving walkway that is thousands of miles wide, stretching most of the way from the Arctic to the Antarctic. And imagine that, instead of moving at walking pace, it moves at the speed with which fingernails grow. Yes, you’ve guessed it. South America, and the whole South American plate, is being carried away from Africa and the African plate, on something like a moving walkway that lies deep under the sea bed and stretches from the far north to the far south of the Atlantic Ocean, moving very slowly.
What about Africa? Why isn’t the African plate moving in the same direction, and why doesn’t it keep up with the South American plate?
The answer is that Africa is on a different moving walkway, one that is travelling in the opposite direction. The African moving walkway goes from west to east, while the South American moving walkway goes from east to west. So what is going on in the middle? Next time you are at a big airport, stop just before you step on the moving walkway and watch it. It wells up out of a slit in the floor, and moves away from you. It is a belt, going round and round, travelling forwards above the floor and coming back towards you under the floor. Now imagine another belt, welling out of the same slit but going in exactly the opposite direction. If you put one foot on one belt and the other foot on the other belt you’d be forced to do the splits.
The equivalent of the slit in the floor at the bottom of the Atlantic Ocean runs all along the deep sea floor from the far south to the far north. It is called the mid-Atlantic ridge. The two ‘belts’ well up through the mid-Atlantic ridge and head off in opposite directions, one carrying South America steadily westwards, the other carrying Africa away to the east. And, like the belts at the airport, the great belts that move the tectonic plates roll around and come back deep within the Earth.
Next time you are at an airport, get on the moving walkway and let it carry you, while you imagine you are Africa (or South America if you prefer). When you get to the other end of the walkway and step off, watch the belt dive underground, ready to make its way back to where you’ve just come from.
The moving belts at an airport are driven by electric motors. What drives the moving belts that carry the great plates of the Earth with their cargo of continents? Deep beneath the Earth’s surface there are what are called convection currents. What’s a convection current? Maybe you have an electric convector heater in your house. Here’s how it works to heat a room. It heats air. Hot air rises because it is less dense than cold air (that’s how hot-air balloons work). The hot air rises until it hits the ceiling, where it can’t rise any more and is forced sideways by the fresh hot air pushing up from beneath. As it travels sideways, the air cools down, whereupon it sinks. When it hits the floor, it again moves sideways, creeping along the floor until it gets caught up in the heater and rises again. That explanation is a bit too simple, but the basic idea is all that matters here: under ideal conditions a convector heater can get the air moving round and round – circulating. This kind of circulation is called a ‘convection current’.
The same thing happens in water. In fact, it can happen in any liquid or any gas. But how can there be convection currents under the Earth’s surface? It isn’t liquid down there, is it? Well, yes, it is – sort of. Not liquid like water, but sort of half liquid like thick honey or treacle. That’s because it is so hot that everything is melting. The heat comes from deep down. The centre of the Earth is very hot indeed, and it goes on being hot until muc
h closer to the surface. Occasionally the heat bursts out through the surface at a place we call a volcano.
Driven by heat
The plates are made of hard rock, and, as we’ve seen, most of them is under the sea. Each plate is several miles thick. This thick layer of armour plating is called the lithosphere, which literally means ‘sphere of rock’. Under the sphere of rock is an even thicker layer, if you can believe it, which isn’t actually called the sphere of treacle but probably should be (it’s actually the upper mantle). The hard rocky plates of the sphere of rock could be said to ‘float’ on the sphere of treacle. Deep heat beneath and within the sphere of treacle causes agonizingly slow, grinding convection currents in the treacle, and it is those convection currents that carry the great rocky plates floating above.
Convection currents follow pretty complicated paths. Just think about all the different ocean currents, and even the winds, which are sort of high-speed convection currents. So it’s no wonder that the various plates on the Earth’s surface are carried in all sorts of directions, rather than round and round as if they were all on a simple merry-go-round. No wonder the plates bump into each other or tear rendingly away from each other, dive one under the other or grate sideways against each other. And no wonder we feel these titanic forces – grinding, wrenching, roaring, scraping forces – as earthquakes. Terrible as earthquakes can be, the wonder is that they aren’t even more terrible.
Sometimes a moving plate slides underneath a neighbouring plate. This is called ‘subduction’. Part of the African plate, for example, is being subducted under the Eurasian plate. This is one reason why there are earthquakes in Italy, and it is one reason why Mount Vesuvius erupted in ancient Roman times and destroyed the towns of Pompeii and Herculaneum (because volcanoes tend to sprout along the edges of the plates). The Himalayan mountains, including Mount Everest, were forced up to their great height as the Indian plate was steadily subducted under the Eurasian plate.
We began with the San Andreas Fault, so let’s end there. The San Andreas Fault is a long, rather straight ‘slippage’ line between the Pacific plate and the North American plate. Both plates are moving north-west, but the Pacific plate is moving faster. The city of Los Angeles lies on the Pacific plate, not the North American plate, and is steadily creeping up on San Francisco, most of which is on the North American plate. Earthquakes are constantly to be expected in this whole region, and experts are predicting that there will be a big one within the next ten years or so. Fortunately, California, unlike Haiti, is well equipped to deal with the terrible plight of earthquake victims.
One day, parts of Los Angeles might end up in San Francisco. But that is a long way off, and none of us will be around to see it.
11
WHY DO BAD
THINGS HAPPEN?
WHY DO BAD things happen? After a dreadful disaster such as an earthquake or a hurricane, you’ll hear people saying things like this:
‘It’s so unfair. What did those poor people ever do to deserve such a fate?’
If a really good person gets a painful disease and dies, while a really bad person remains in the best of health, once again we cry, ‘Unfair!’
Or we say, ‘Where’s the justice in that?’
It is hard to resist this feeling that, somehow, there ought to be a kind of natural justice. Good things should happen to good people. Bad things, if they must happen at all, should only happen to bad people. In Oscar Wilde’s delightful play The Importance of Being Earnest, an elderly governess called Miss Prism explains how, long ago, she wrote a novel. When she is asked whether it ended happily, she replies: ‘The good ended happily, and the bad unhappily. That is what fiction means.’ Real life is different. Bad things do happen, and they happen to good people as well as bad. Why? Why is real life not like Miss Prism’s fiction? Why do bad things happen?
Lots of peoples believe that their gods intended to create a perfect world but unfortunately something went wrong – and there are almost as many ideas about what that something was. The Dogon tribe of West Africa believe that at the beginning of the world there was a cosmic egg from which two twins hatched. All would have been well if the twins had hatched at the same time. Unfortunately, one of them hatched too soon, and spoiled the gods’ plan of perfection. That, according to the Dogon, is why bad things happen.
There are lots of legends about how death came into the world. All over Africa, different tribes believe that the chameleon was given the news of everlasting life and told to carry it to humans. Unfortunately the chameleon walked so slowly (they do, I know: as a child in Africa I had a pet chameleon called Hookariah) that the news of death, carried by a nippier lizard (or other faster animal in other versions of the legend), arrived first. In one West African legend, the news of life was brought by a slow toad, unfortunately overtaken by a fast dog bringing the news of death. I must say I’m a bit puzzled why the order in which news arrives should matter so much. Bad news is still bad, whenever it arrives.
Disease is a special kind of bad thing, and it has spawned plenty of myths of its own. One reason is that for a long time diseases were rather mysterious. Our ancestors faced other dangers – from lions and crocodiles, from enemy tribes, from the threat of starvation – but you could see them coming, and understand them. Smallpox, on the other hand, or the Black Death, or malaria, must have seemed to pounce from nowhere, without warning, and it wasn’t obvious how to guard against these assaults. It was a terrifying mystery. Where did diseases come from? What did we do to deserve this painful death, this agonizing toothache or these hideous spots? No wonder people resorted to superstition when desperately trying to understand disease, and even more desperately trying to protect themselves from it. In many African tribes, until quite recently, anybody who got ill, or had a sick child, would automatically look around for an evil magician or witch to blame.
If my child has a high fever, it must be because an enemy paid a witch doctor to cast a spell on her. Or maybe it is because I couldn’t afford to sacrifice a goat when she was born. Or perhaps it is because a green caterpillar walked across the path in front of me and I forgot to spit out the evil spirit.
In ancient Greece, sick pilgrims would spend the night in a temple dedicated to Asclepius, the god of healing and medicine. They believed the god would either heal them himself or reveal the cure in a dream. Even today, a surprisingly large number of sick people travel to places like Lourdes, where they plunge into a sacred pool in the hope that the holy water will heal them (actually, one might suspect that they are more likely to catch something from all the other people who have bathed in the same water). About 200 million people have made the pilgrimage to Lourdes during the past 140 years, hoping for a cure. In many cases there is not much wrong with them, and thankfully they mostly get better – as they would have anyway, with or without the pilgrimage.
Hippocrates, the ancient Greek ‘father of medicine’ who gives his name to the oath of good conduct that all doctors are supposed to observe, thought that earthquakes were important causes of disease. In the middle ages, many people believed that diseases were caused by the movements of the planets against the backdrop of stars. That’s part of a system of beliefs called astrology, which, ridiculous as it may seem, still has quite a few followers to this day.
The most persistent myth about health and disease, lasting from the fifth century BC right up to the eighteenth century AD, was the myth of the four ‘humours’. When we say, ‘He’s in a good humour today,’ that’s where the word comes from, although people don’t believe in the idea behind it any more. The four humours were black bile, yellow bile, blood and phlegm. Good health was thought to depend on a good ‘balance’ between them, and you can still hear something a bit similar from quack ‘healers’ today who will wave their hands over you in order to ‘balance’ your ‘energies’ or your ‘chakras’.
The theory of the four humours certainly couldn’t help doctors to cure illnesses, but it might have done no great harm
except that it led to the practice of ‘bleeding’ patients. This involved opening a vein with a sharp instrument called a lancet, and drawing off quantities of blood into a special basin. This, of course, made the poor patient even sicker (it contributed to George Washington’s death) – but the doctors believed so strongly in the ancient myth of the humours that they did it again and again. What’s more, people didn’t only get bled when they were ill. Sometimes they asked the doctor to do it in advance of getting ill, in the hope that it would ward off sickness.
Once, when I was at school, our teacher asked us to think about why diseases happen. One boy put his hand up and suggested that it was because of ‘sin’! There are many people, even today, who think something like that is the cause of bad things generally. Some myths suggest that bad things happen in the world because our ancestors did something wicked long ago. I’ve already mentioned the Jewish myth of the founding ancestors Adam and Eve. You’ll remember that Adam and Eve did a simply terrible thing: they allowed themselves to be persuaded by the snake to eat the fruit of a forbidden tree. This mythical crime has reverberated down the ages and is still regarded by some people as responsible for all the bad things that happen in the world to this day.
Lots of myths talk about a conflict between good gods and bad gods (or devils). The bad gods are responsible for the bad things that happen in the world. Or there may be a single spirit of evil, called the Devil or something similar, who fights with the good god or gods. If only there wasn’t this tussle between devils and gods, or good gods and bad gods, bad things wouldn’t happen.