Planet Earth

A Beginner's Guide

John R. Gribbin

• Publisher: Oneworld Publications
• Year: 2012
• eBook ISBN: 9781780740423
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

A highly entertaining and accessible introduction to our planet from the bestselling author of In Search of Schrödinger's Cat, The Scientists, and In Search of the Multiverse In this lively expedition into the origins, evolution, and workings of our planet, John Gribbin does what he does best: gathers 4.5 billion years of geological history and shares the best bits. Taking an astronomer's perspective, Gribbin follows Earth's development from its beginnings in cosmic gas and dust to the explosion of human life after the last ice age, combining stories of scientific discovery with gripping accounts of geological activity - earthquakes, volcanoes, and climate change. Along the journey we consider Lord Kelvin's time-scale for the life of the sun; the meteorologist who first championed the idea of continental drift; and an intriguing proposal that Earth has expanded substantially in recent millennia. Told in Gribbin's dynamic and beloved voice, this is the perfect introduction to geology and an essential guidebook for anyone wanting to better appreciate the wonders of our shared home.

 

1

A brief history of terrestrial time

Before we begin the story of our home planet, we should first consider another story of terrestrial time, the story of how pioneering geologists came to realize the true extent of the history of the Earth, and the nature of the forces that have shaped, and continue to shape, the world we live in. That history goes back scarcely more than two hundred years, to the time when scientists in Europe first began seriously to question the idea, derived from a literal interpretation of biblical stories, that the Earth is only some six thousand years old.

The birth of geology

The modern understanding of the Earth began with the work of the Scotsman James Hutton, who had qualified as a physician, but was wealthy enough never to have to work and devoted his life to science. He presented his ideas to the Royal Society of Edinburgh in 1785, and published them in a book, Theory of the Earth, in 1795. The Geological Society of London, the first organization devoted to the study of what are now called the Earth sciences, was founded just twelve years later, in 1807. By then, Hutton’s pioneering ideas had been popularized by his friend John Playfair in his book Illustrations of the Huttonian Theory of the Earth.

At the end of the eighteenth century, although it was still widely believed that the Earth had been created in 4004 BC, scientists like Hutton were well aware that our plant must be much older than this. The question was, how old? And how had the landscape been shaped into its present form?

The puzzle had been hinted at as far back as 1665, by Robert Hooke, in his book Micrographia. Hooke was one of the founding fathers of science, but doesn’t always get the credit he deserves because he was a contemporary of Isaac Newton, compared with whom even the cleverest people look ordinary. Hooke was one of the first people to realize that fossils aren’t just curiously shaped rocks that happen to resemble living things, but are the preserved remains of living things themselves. Describing ammonites in his book, he wrote that they are ‘the Shells of certain Shelfishes, which, either by some Deluge, Inundation, Earthquake, or some such other means, came to be thrown to that place, and there to be filled with some kind of mud or clay, or petrifying water, or some other substance, which in tract of time has been settled together and hardened’. In lectures at Gresham College, in London, he also said that ‘Parts [of the Earth] which have been sea are now land’, and that ‘mountains have been turned into plains, and plains into mountains, and the like’. He was exactly right on every count. But few people took any notice of these ideas at the time, because such dramatic changes clearly could not have happened in the span of a few thousand years.

Exactly at the same time, in the mid 1660s, the Dane Niels Steensen also recognized that fossils found far inland today, and even on high mountains, are the remains of creatures that once lived in the sea. He wrote under a Latinized version of his name, as Steno, and said that different rock layers (‘strata’) must have been laid down underwater at different times in the past, as a result of floods covering the land. So the idea of fossils as the remains of living creatures became linked with the idea of the biblical Flood, which some people saw as just the latest in a series of watery cataclysms that had inundated the Earth. This was quite different from Hooke’s idea that the land itself could be raised up or lowered, while sea levels stayed the same. But it still left the question of how long all this would have taken – and where all the water came from.

In the 1740s, Carl Linnaeus, remembered today for his invention of a classification system for plants and animals, came up with his own version of the Flood idea. He realized that even if it had really happened, the Flood described in the Bible had not lasted long enough for all these processes to have taken place. Instead, he thought that the Earth had originally been completely covered with water, which has been gradually receding ever since, leaving behind fossils of sea creatures as dry land emerged from beneath the waves. The whole point of this idea was that it would have taken much longer than the six thousand years of Earth history suggested by adding up chronologies in the Bible, but Linnaeus was careful not to get into trouble with the Church by actually saying as much in print. The most daring comment he allowed himself to make for public consumption appeared in 1753, when, in his book Museum Tessinianum, he wrote, ‘The infinite number of fossils of strange and unknown animals buried in the rock strata beneath the highest mountains, animals that no man of our age has beheld, are the only evidence of the inhabitants of our ancient earth at a period too remote for any historian to trace.’

But another scientist didn’t exercise such restraint. While Linnaeus was in Sweden thinking about the implications of fossils for the terrestrial timescale, his contemporary the Comte de Buffon was in France carrying out the first actual experiments to try to determine the age of the Earth. Buffon was a hardworking and successful scientist, even though he lived off inherited wealth and could have spent his days in idle luxury. He was so dedicated that he actually hired a servant whose job was to drag Buffon out of bed each morning at 5 a.m. and make sure he was awake and ready to start work. His great achievement was to produce a huge survey of science, the Histoire naturelle, in no less than forty-four volumes; his most inspired piece of work is described in the book.

Buffon thought that the Earth had been formed from a blob of molten material torn out of the sun by a passing comet, which wasn’t a completely crazy idea in the eighteenth century. So he tried to work out how long it would take for a ball of molten iron the size of the Earth to cool down to its present temperature. He did this by heating balls of iron of different sizes until they were red hot and on the point of melting, then timing how long it took them to cool down to the point where they could just be touched without burning the skin. The story goes that his assistants in this experiment were aristocratic ladies with delicate hands, protected by the finest silk gloves, who he used, in effect, as thermometers – there were, of course, no accurate thermometers that could do the job in his day.

Scaling the results up from his experiments to a ball the size of the Earth, Buffon calculated that the Earth must be at least seventy-five thousand years old, and dared to say as much in the Histoire. Even though this age is so much less than the billions of years suggested by modern science, this was a landmark event. Buffon had dared to publish an age for the Earth more than ten times greater than the age suggested by Bible scholars. The next person to make such an estimate pushed the date back even farther – but, unlike Buffon, lacked the courage to publicize his findings.

Joseph Fourier is remembered today as a mathematician; but he developed the mathematical techniques for which he became famous for a practical reason – to describe the way heat flows from a hot object to a colder one. The equations can describe, for example, the rate at which heat flows along an iron bar if one end is kept red hot in a furnace and the other end is at room temperature. Fourier used these equations to estimate the rate at which the Earth would have cooled down from a molten state, and, unlike Buffon, he also realized that once a crust of solid rock had formed at the surface of the Earth it would act like an insulating blanket and slow down the rate at which heat was being lost to space. Putting everything together, he came up with a formula which, when solved, gives the age of the Earth as 100 million years. Fourier was so shocked by this that he never published that number. He did publish the formula, in 1820, and any half-decent mathematician could have used it (as they surely must have) to work the number out. But Fourier just couldn’t bring himself to stir up controversy by suggesting that the Earth was nearly a hundred thousand times older than Bible scholars believed. By 1820, though, this kind of timescale was, if anything, too short for the requirements of geologists following in the footsteps of James Hutton.

The uniformitarians

The key thing that Hutton appreciated was that there is no need to invoke global catastrophes to explain how the Earth got to be the way it is today. He studied the way strata that must have originally been laid down one on top of the other are now seen to be bent and twisted into distorted patterns, and appreciated that instead of explaining these features as due to a global catastrophic upheaval, they could be caused by the same processes we see at work on Earth today, but operating over immensely long periods of time. These processes include volcanic eruptions and great earthquakes, which are catastrophic enough for anyone living in their vicinity. But, Hutton realized, such short-lived events are common on a geological timescale, and as normal in the life of the Earth as sneezing is in the life of a human being. The idea that gradual processes operating over immense periods of time can explain the origin of all the features of the Earth, from mountain ranges to ocean basins, became known as uniformitarianism, since the same uniform processes we see at work today can explain everything from how mountain ranges are uplifted from the sea floor to the way they are worn away by erosion, with the resulting sediments being laid down under the sea, eventually to become the raw material of new mountain ranges. This image of the changing Earth neatly explains why there are just three kinds of rock on Earth – igneous rock, which flowed from volcanoes in a molten state and set hard; sedimentary rock, laid down underwater from tiny pieces of older rock worn away by erosion; and metamorphic rock, such as granite, which is formed when either of the two basic kinds of rock becomes at least partially molten and reworked.

Hutton couldn’t calculate the age of the Earth, but he understood that all these processes need a very long time to operate. Indeed, he thought that there might not have been a ‘beginning’ at all, and that the Earth had always existed, and always would exist, with these processes going on forever. In a paper published in 1788, he wrote, ‘The result, therefore, of our present enquiry is, that we find no vestige of a beginning – no prospect of an end.’

The person who built on Hutton’s foundations and really made people take notice of the idea of uniformitarianism was another Scot, Charles Lyell, who published a great book, Principles of Geology, in three volumes, between 1830 and 1833. In the typical style of the day, he provided a subtitle which explained what the book was all about: ‘Being an Attempt to Explain the Former Changes of the Earth’s Surface by Reference to Causes Now in Operation’. Among the evidence gathered in his book, Lyell described how at Mount Etna, layers of igneous rock formed by lava flows are separated by layers of sedimentary rock. In one place, a bed containing fossilized oysters ‘no less than twenty feet in thickness, is there seen resting on a current of basaltic lava; upon the oyster bed again is superimposed a second mass of lava’. So the time interval between the lava flows was long enough for sediments twenty feet thick to be laid down. And the lava beds themselves weren’t laid down overnight. Lyell calculated that it would require ‘ninety flows of lava, each a mile in breadth at their termination, to raise the present foot of the volcano as much as the average height of one lava-current’.

Among the people on whom Lyell’s book made a deep impression was the young Charles Darwin, who set off on his famous voyage on the Beagle in 1831, taking the first volume of Principles of Geology with him – the other volumes caught up with him on his travels. Over the years that followed, as Darwin developed his ideas about the origin of species as a result of evolution by natural selection, he grasped that this, too, is a uniformitarian process requiring immense timescales. Thanks to Lyell (and to Hutton before him) Darwin knew that nature had indeed provided a sufficient timescale for natural selection to do its work. He said that Lyell had given him ‘the gift of time’, and much later commented, ‘I always feel as if my books came half out of Lyell’s brain ... the great merit of the Principles was that it altered the whole tone of one’s mind.’

Darwin’s book On the Origin of Species was published in 1859. Not everyone was an immediate convert to his ideas, but it is fair to say that by the last quarter of the nineteenth century many geologists and biologists were convinced that the Earth must be at least hundreds of millions of years old. The snag was, the physicists and astronomers were telling them that this was impossible, according to all the known laws of physics. Both sides of the argument were right. The conflict would only be resolved, and the dating story brought up to date, when previously unknown laws of physics were discovered.

Dating up to date

By the middle of the nineteenth century, physicists had realized that nothing lasts forever; everything wears out, eventually. This key principle became enshrined in a scientific law, the famous second law of thermodynamics. It meant that there must indeed have been a ‘vestige of a beginning’ at some remote time in the past, and that, one day, albeit in the far future, there would be an end to the kind of conditions that exist on Earth today. So in 1852, the British physicist William Thomson (who later became Lord Kelvin, the name by which he is better known) wrote:

Within a finite period of past time the earth must have been, and within a finite period of time to come the earth must again be unfit for the habitation of man as at present constituted, unless operations have been or are to be performed which are impossible under the laws to which the known operations going on at present in the material world are subject.

The Earth as we know it could not exist as a home for life without the sun, so Thomson tried to work out how long the sun could keep pouring out the vast quantities of heat and light that make the Earth habitable. Using an example with which his Victorian contemporaries would have been comfortable, Thomson pointed out that even if the sun were made entirely of coal, burning in a pure oxygen atmosphere, it would only last for a few thousand years before becoming a cinder. But he found that there is another source of energy that a star like the sun can draw on.

Thomson realized (as his German counterpart Hermann von Helmholtz also did, independently) that a ball of gas the size of the sun, 108 times bigger in diameter than the Earth, could be kept hot inside if it was shrinking slowly under its own weight. Such shrinking releases gravitational energy, which is converted into heat. The rate at which the sun must be shrinking to maintain its present heat output can easily be calculated, and it is only about fifty metres (150 feet) per year. Unfortunately, slow though that is, it means that the sun would fizzle out within about twenty million years, a number that is known as the Kelvin–Helmholtz timescale. That was still far too short a span for the biologists and geologists, who required timescales of many hundreds of millions of years to produce the changes they saw in the living and nonliving world (though today we know that this process is the way young stars get hot inside when they are born).

It turned out that both parts of Thomson’s statement are correct. ‘Within a finite period of past time’ the Earth was indeed unfit for life, but that ‘finite period’ is much, much longer than the Kelvin–Helmholtz timescale. Further, there are indeed laws of physics that were unknown in the middle of the nineteenth century. In particular, radioactivity.

X-rays were discovered by the German physicist Wilhelm Röntgen in 1895, and this led to the discovery of other forms of radiation and the study of materials that release such radiation – radioactive material. The person who took up these new ideas and used them both to point the way towards a ‘new’ energy source for the sun and to come up with a new timescale for the Earth was New Zealander Ernest Rutherford, who worked at various times in Cambridge, and Manchester, as well as at McGill University in Montreal. But there is no need for us to go through all the steps that led him to the discovery of an accurate terrestrial timescale. We can skip to the end of the story.

The first thing this kind of radiation provides is energy, produced by mechanisms that Thomson knew nothing about in the 1850s. The radiation is produced when the central parts of the atoms of some heavy elements (their nuclei) re-arrange themselves into states with lower energy. Lower energy states are always preferred in nature. As the nucleus adjusts itself in this way, the excess energy is carried off in the form of the radiation that we detect, and the atom may be converted into an atom of a different substance. For example, one form of uranium ‘decays’ in this way to make lead.

The energy released in these processes comes ultimately from the conversion of a tiny amount of matter into energy, in line with Albert Einstein’s famous equation E = mc². But Einstein only came up with that equation in 1905, and it took several decades for astronomers to discover that what keeps the sun and stars hot is not radioactive decay but a set of processes that fuse very light nuclei together (in particular, converting hydrogen into helium), which also releases energy. What mattered at the beginning of the twentieth century was that it was clear to people like Rutherford that there were laws of physics that were unknown to Thomson’s generation, and that processes going on inside atoms could provide energy for the sun for a very long time indeed. How long? That was where Rutherford’s most important contribution to the dating debate came in.

Rutherford discovered that if you start with any particular amount of a radioactive element, half of it will have decayed into another form after a certain time, called the half-life. During the next half-life, half of what is left (a quarter of the original) will decay, and so on. The half-life is different for various radioactive substances, and in each case it can be measured in the laboratory. It means that if you have a sample of rock that contains a mixture of a radioactive element and its so-called ‘daughter’ products (such as radioactive uranium and its daughter lead) you can measure the proportions of each substance present and use that to work out how old the rock is – how long it is that the radioactive decay has been going on. In 1905, using this technique, Rutherford and his colleague Bertram Boltwood measured the age of a sample of rock as 500 million years – twenty times the Kelvin-Helmholtz timescale. But even this turned out to be a relatively young piece of rock. Since 1905, the age of the oldest known rocks found on Earth has been pushed back, using this utterly reliable and accurate technique, to more than four billion years, neatly matching modern estimates of the age of the sun. The timescale mystery was solved.

In the following chapters, we do not go into great historical detail about how scientists who study the Earth – Earth scientists – have discovered what they know about our home planet. Instead, we concentrate on the most up-to-date version of the story they have uncovered, the story of the Earth as it is today. It makes sense to start this story by setting the Earth in its context as a planet, looking at how the Earth formed as part of the solar system – not least because that explains where the radioactive elements still decaying on Earth today, so important to an understanding of terrestrial time, actually came from.

2

Our place in space

The Earth is made of stardust. In the space between the stars, there are huge clouds of gas and dust, the raw material from which new stars and planets are formed. Most of the gas is primordial – hydrogen and helium left over from the Big Bang, in which the universe as we know it began. But the dust is different. It is material that has already been processed inside previous generations of stars and thrown back out into space to be recycled.

The story of the Earth begins with the collapse of one of those clouds of gas and dust to make the sun and its family of planets – and probably also several other stars and their planetary partners. In order to understand our place in space, and in particular where the radioactive elements that are so useful in measu...

Table of contents

• Cover
• A Beginner’s Guide
• Title page
• Copyright page
• Contents
• Foreword
• 1 A brief history of terrestrial time
• 2 Our place in space
• 3 Drifting continents and spreading seas
• 4 What goes up must come down
• 5 Inside and outside the Earth
• 6 The changing Earth
• 7 Fire on Earth
• 8 Our changing planet
• 9 Our changing climate
• 10 Life and Earth
• Appendix 1: The Earth in numbers
• Appendix 2: The timescales of planet Earth
• Further reading
• Acknowledgements
• Index