მთავარი The Earth: A Very Short Introduction (Very Short Introductions)

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Around 30 years ago, two things happened that were to revolutionize the understanding of our home planet. First, geologists realized that the continents themselves were drifting across the surface of the globe and that oceans were being created and destroyed. Secondly, pictures of the entire planet were returned from space. Suddenly, the Earth began to be viewed as a single entity; a dynamic, interacting whole, controlled by complex processes we scarcely understood. 
This Introduction explores emerging geological research and explains how new advances in the understanding of plate tectonics, seismology, and satellite imagery have enabled us to begin to see the Earth as it actually is: dynamic and ever changing.
წელი:
2003
ენა:
english
ISBN:
B000SIKZAO
ფაილი:
EPUB, 1,27 MB
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2020
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3.5 / 5.0
The Earth: A Very Short Introduction

‘This is a splendid introduction to the Earth. There is enough detail here to get you really into an understanding of how our dynamic planet “works”, based in its history and structure.... Writing with great clarity and obvious enthusiasm Martin Redfern has managed to capture the excitement of the new discoveries in the Earth Sciences. I found it hard to put down!’ Aubrey Manning, University of Edinburgh

‘An excellent introduction to modern Earth Sciences Martin Redfern’s authoritative and timely account will be invaluable to secondary school students, entry level undergraduates, as well as all those interested in modern geoscience.’ Richard Corfield, author of Architects of Eternity and The Silent Landscape





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VERY SHORT INTRODUCTIONS

VERY SHORT INTRODUCTIONS are for anyone wanting a stimulating and accessible way in to a new subject. They are written by experts, and have been published in more than 25 languages worldwide.

The series began in 1995, and now represents a wide variety of topics in history, philosophy, religion, science, and the humanities. The VSI Library now contains over 200 volumes—a Very Short Introduction to everything from ancient Egypt and Indian philosophy to conceptual art and cosmology—and will continue to grow to a library of around 300 titles.

VERY SHORT INTRODUCTIONS AVAILABLE NOW

For more information visit our web site

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Martin Redfern





THE EARTH

A Very Short Introduction





Great Clarendon Street, Oxford OX2 6DP

Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York

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Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries

Published in the United States by Oxford University Press Inc., New York

© Martin Redfern, 2003

The moral rights of the author have been asserted Database right Oxford University Press (maker)

First published as a Very Short Introduction 2003

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organizations. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above

You must not circulate this book in any other binding or cover and you must impose this same condition on any acquirer

British Library Cataloguing in Publication Data Data available

Library of Congress Cataloging in Publication Data Data available

ISBN 13: 978–0–19–280307–8

ISBN 10: 0–19–280307–7

3 5 7 9 10 8 6 4

Typeset by RefineCatch Ltd, Bungay, Suffolk Printed in Great Britain by TJ International Ltd., Padstow, Cornwall





Acknowledgements


The author would like to thank: Arlene Judith Klotzko for introductions without which this book would never have been written; Shelley Cox for initial enthusiasm; Emma Simmons for continued patience; David Mann for instant cartoons; Pauline Newman and Paul Davies for helpful comments; Marian and Edmund Redfern for nurturing my enthusiasm and reading the results; Robin Redfern for beavering away; the un-named readers who have kept me precise; and the countless geologists who have shared with me their time and enthusiasm.





Contents


Acknowledgements

List of illustrations

1 Dynamic planet

2 Deep time

3 Deep Earth

4 Under the sea

5 Drifting continents

6 Volcanoes

7 When the ground shakes

Epilogue

Further reading

Index





List of illustrations


1 Planet Earth, as seen from Apollo 17 Corbis

2 The Earth’s magnetic envelope

3 The carbon cycle

4 ‘Onion’ layers in a radial section of Earth

5 The main divisions of geological time

6 Circulation in the Earth’s mantle

7 Possible model for the generation of Earth’s magnetic field

8 The ocean drilling ship JOIDES Resolution

9 The global system of ocean ridges

10 Stripes of magnetization in ocean floor volcanic rocks

11 The principal components of a mid-ocean ridge

12 How ocean lithosphere subducts beneath a continent

13 The major tectonic plates

14 How the continents have changed over the last 200 million years

15 Tectonic map of Southeast Asia

16 Seismic reflection profile of layers within the Earth’s crust

17 Types of volcano

18 Eruption of Mount St Helens

Popperfoto/United Press

International (UK) Limited

19 Features of an erupting composite volcano

20 Distribution of major earthquakes in the past 30 years

21 Ground movement in the Izmit earthquake in Turkey

22 Kobe City, Japan, after an earthquake

Sipa Press/Rex Features

23 San Andreas fault system, California

Cartoons

Copyright David Mann

The publisher and the author apologize for any errors or omissions in the above list. If contacted they will be pleased to rectify these at the earliest opportunity.



1. Planet Earth, as seen from Apollo 17, December 1972.





Chapter 1

Dynamic planet


Once a photograph of the Earth, taken from the outside, is available, a new idea as powerful as any in history will be let loose.

Sir Fred Hoyle, 1948

How can you put a big round planet in a small flat book? It is not an easy fit, but there could be two broadly different ways of attempting it. One is the bottom-up approach of geology: essentially, looking at the rocks. For centuries, geologists have scurried around on the surface of our planet with their little hammers examining the different rock types and the mineral grains which make them up. With eye and microscope, electron probe and mass spectrometer, they have reduced the planet’s crust to its component parts. Then they have mapped out how the different rock types relate to one another and, through theory, observation, and experiment they have worked out how they might have got there. It has been a huge undertaking and one that has brought deep insights. Collectively, the efforts of all those geologists have built a giant edifice on which future earth scientists can stand. It’s as a result of this bottom-up approach that I can write this book. But it is not the approach I will use. This is not a guide to rocks and minerals and geological map-making. It is a portrait of a planet.

The new view on our old planet is the top-down approach of what has come to be known as Earth systems science. It looks at the Earth as a whole and not just frozen in time in the moment we call now. Taken over the deep time of geology we begin to see our planet as a dynamic system, a series of processes and cycles. We can begin to understand what makes it tick.





The view from above


The prediction above was made by astronomer Sir Fred Hoyle in 1948, a decade before the dawn of space flight. When unmanned rockets took the first pictures of the Earth from outside, and when the first generation of astronauts saw for themselves our world in its entirety, the prediction came true. It’s not that those first views told us much we didn’t already know about the Earth, but they gave us an icon. And to many of the astronauts who witnessed the view first-hand, it gave an emotional experience of the beauty and seeming fragility of our world that has lived with them ever since. It is perhaps no coincidence that Earth sciences were undergoing their own revolution at the same time. The concept of plate tectonics was at last gaining acceptance, 50 years after Alfred Wegener originally suggested it. Exploration of the ocean floor revealed that it was spreading out from a system of mid-ocean ridges. It had to be going somewhere, forcing continents apart or into one another. The unimaginable masses of continent-sized plates of rock were on the move in an elaborate and ancient waltz.

It was around the same time, and with the same icon of that small blue jewel we call Earth floating in the blackness of space, that a global environmental movement began to form, a mixture of those with a sentimental attachment to endangered species and rainforests and scientists taking on board a new view of complex, interacting ecological systems. Today, most university departments and research groups use the term ‘Earth sciences’ rather than geology, recognizing a broadening of the discipline beyond the study of rocks. The term ‘Earth systems’ is becoming widespread, recognizing the inter-related, dynamic nature of processes that include not only the solid, rocky Earth but its oceans, the fragile veil of its atmosphere, and the thin film of life on its surface as well. It is as if our world were an onion; a series of concentric spheres, from magnetosphere and atmosphere, through biosphere and hydrosphere, to the layers of the solid earth. Not all are spherical and some are much less substantial than others, but each manages to persist in a delicate equilibrium. Each component of such a system is seen not as something fixed and unchanging but more like a fountain; maintaining its overall structure perhaps, but constantly changing as material and energy pass through it.





If rocks could talk


Rocks and stones are not the most forthcoming of storytellers. They have a tendency to sit there gathering moss, only rolling when pushed. But geologists have ways of making them talk. They can hit them and slice them; squeeze them, squash them, strain and stress them until they crack – sometimes quite literally. If you know how to look at them, rocks can tell you their history. There is the recent history of the rock on the surface: how it has been weathered and eroded; the tell-tale scars of wind, water, and ice. There can be deeper scars that record periods of heat and pressure and deformation when the rock was buried. Where these changes are extreme the rocks are known as metamorphic. Then there are clues to the origin of the rocks. Some show signs of having once been molten and pushed up from deep within the Earth to erupt out of volcanoes or to intrude into pre-existing rocks. These are the igneous rocks. The size of mineral grains within them can reveal how quickly they were cooled. A large mass of granite cools slowly so that crystals in it are large. Volcanic basalt solidifies rapidly and so is fine-grained. Rocks can be made of the ground-down remains of previous rocks. Here, the size of the fragments tends to reflect the energy of the environment that laid them down: from fine shale and mudstone deposited in still water, through sandstones to coarse conglomerates washed down by raging torrents. Others, such as chalk and limestone, are chemical deposits accumulated as living systems took carbon dioxide from the atmosphere and precipitated it in sea water, turning, as it were, the sky into stone.

Even individual mineral grains have their story to tell. Mineralogists can strip them apart atom by atom in mass spectrometers so sensitive that they can reveal different ratios of isotopes (different atomic forms of the same elements) even among trace constituents. Sometimes these can help date the grains so that we know if they came out of still more ancient rock. They can also reveal the stages of growth of a crystal, for example of diamond, as it passes through the Earth’s mantle. In the case of isotopes of carbon and oxygen in minerals derived from marine organisms, it’s even possible to estimate the temperature of the sea and the global climate when they formed.





Other worlds


The trouble with the Earth is that it is the only one we’ve got. We can only see it as it is today, and we can’t tell if it is here simply due to some happy accident. That’s why Earth scientists are taking a renewed interest in astronomy. Powerful new telescopes sensitive to infrared and sub-millimetre wavelength radiation can stare deep into star-forming regions to see what may have happened when our own solar system was born. Around some of the young stars they have revealed dusty haloes known as proto-planetary discs, perhaps new solar systems in formation. But the search for fully formed Earth-like planets is more difficult. To see directly such a planet in orbit around a distant star would be like trying to spot a small moth close to a powerful searchlight. But indirect methods have led to the discovery of planets in recent years, mainly by detecting tiny wobbles in the motions of the parent stars due to gravitational effects. The clearest effects and therefore the first discovered seem to be due to planets far bigger than Jupiter orbiting far closer to their stars than the Earth is to the Sun. So they could hardly be termed Earth-like. But evidence is beginning to accumulate for solar systems more like our own, with multiple planets. Though small and hospitable planets like Earth will be hard to detect.

To see such planets directly would take telescopes in space that we can scarcely dream of. There are ambitious plans underway in both the USA and Europe for a network of linked infrared telescopes. Each would have to be far bigger than the Hubble Space Telescope, and four or five of them would have to fly in close formation to combine their signals to resolve the planet. They would have to be as far out as Jupiter to get beyond the dusty infrared glow of our own planetary system. But then, they might be able to detect vital signs in distant planetary atmospheres and, in particular, they might detect ozone. That would imply Earth-like conditions of climate and chemistry plus the existence of free oxygen, something which, as far as we know, can only be maintained by life.





Signs of life


In February 1990, on its way out of the solar system after encounters with Jupiter and Saturn, the Voyager I probe beamed back the first image of our entire solar system as it might appear to visitors from another star. The picture is dominated by a single bright star, our Sun, seen from 6 billion kilometres away, 40 times the distance from which we are used to seeing it. The planets are scarcely visible. The Earth itself is smaller than one picture element in Voyager’s camera, its faint light caught in what looks like a sunbeam. This is our whole world, seemingly just a speck of dust. But to any alien visitor with the right instruments, that tiny blue world would immediately attract attention. Unlike the giant stormy gas bags of the outer planets, cold, dry Mars, or the acid steam-bath of Venus, the Earth has everything just right. Water exists in all three phases – liquid, ice, and steam. The atmospheric composition is not that of a dead world that has reached equilibrium but one that is active and must be constantly renewed. There is oxygen, ozone, and traces of hydrocarbons; things that would not exist together for long if they were not constantly renewed by living processes. This alone would attract the attention of our alien visitors, even if they could not detect the constant babble of our communications, radio and television.





Magnetic bubble


Geophysics goes way above our heads. I don’t mean by that that it is incomprehensible but that the physical influence of our planet extends far above its solid surface, way out into what we regard as empty space. But it is not empty. We live in a series of bubbles nested like Russian dolls one within another. The Earth’s sphere of influence lies within the greater bubble dominated by our Sun. That in turn lies within overlapping bubbles blown by the expanding debris of exploding stars or supernovae, long, long ago. They are all within our Milky Way galaxy, which is in turn a member of a super-cluster of galaxies within the known universe, which itself may be a bubble in a quantum foam of worlds.

The Earth’s atmosphere and magnetic field shield us, for the most part, from the radiation hazards from space. Without this protection, life on the Earth’s surface would be threatened by solar ultraviolet and X-rays as well as cosmic rays, high-energy particles from violent events throughout the galaxy. There is also a permanent gale of particles, mostly hydrogen nuclei or protons, blowing outwards from the Sun. This solar wind speeds past the Earth at typical velocities of around 400 kilometres per second, and goes three times faster during a solar storm. It extends for billions of kilometres out into space, beyond all the planets and maybe beyond the orbits of comets, which reach out many thousands of times further from the Sun than does the Earth. The solar wind is very tenuous but it is sufficient to blow out the tails of comets as they come closer in to the heart of the solar system, so the tails always point away from the Sun. It also features in imaginative proposals for propelling spacecraft with vast gossamer-thin solar sails.



2. Diagram of the Earth’s magnetic envelope, the magnetosphere, swept back into a comet-like structure by the solar wind. Arrows show the directions of electrical currents.

The Earth is sheltered from the solar wind by its magnetic field, the magnetosphere. Because the solar wind is electrically charged it represents an electrical current, which cannot cross magnetic field lines. Instead, it compresses the Earth’s magnetosphere on the sunward side, like the bow wave of a ship at sea, and stretches it out into a long tail down-wind which reaches almost as far as the orbit of the Moon. Charged particles caught within the magnetosphere build up in belts between the field lines where they are forced to spiral, generating radiation. These radiation belts were first spotted in 1958 when James van Allen flew the first Geiger counter in space on board the American Explorer 1 satellite. They are areas to be avoided by spacecraft hoping for a long life and would be lethal to unprotected astronauts.

Where the Earth’s magnetic field lines dive down towards the poles, solar wind particles can enter the atmosphere, sending atoms ricocheting downwards to produce spectacular auroral displays. At the top of the atmosphere, the hydrogen ions of the solar wind itself produce a pink haze. Lower down, oxygen ions produce a ruby-red glow, while nitrogen ions in the stratosphere cause violet blue and red auroras. Occasionally, magnetic field lines in the solar wind are forced close to those of the Earth, causing them to reconnect, often with spectacular releases of energy which extend the auroral displays.





The fragile veil


There’s no clearly defined height that marks the top of the atmosphere; 260 kilometres above the ground, in low Earth orbit where the space shuttle flies, you’re above almost all the air and the pressure is a billion times less than it is on the ground. But there are still about a billion atoms in a cubic centimetre up there, and they are hot and electrically charged and hence can have a corrosive effect on space vehicles. At times of maximum solar activity, the atmosphere expands slightly, exerting more frictional drag on low spacecraft, which have to be boosted up to stay in orbit. The upper atmosphere, above 80 kilometres, is sometimes known as the thermosphere because it is so hot, even though it is so rarefied that you would not burn your skin on it.

This region of the atmosphere also absorbs dangerous X-rays and some of the ultraviolet radiation from the Sun. As a result, many atoms become ‘ionized’, that is they lose an electron. For this reason, the thermosphere is also called the ionosphere. Because the ionosphere is electrically conducting, it will reflect certain frequencies of radio waves, making it possible for short-wave radio transmissions to be heard around the world, well over the horizon from the transmitter.

Even a mere 20 kilometres up, below the thermosphere, the mesosphere, and most of the stratosphere, we are still above 90% of the air in the atmosphere. It is at around this height that we encounter the tenuous ozone layer, molecules containing three oxygen atoms. Ozone forms when ordinary oxygen molecules of two atoms are split by solar radiation and some recombine in threes. Ozone is a highly effective sunscreen for the planet. If all the ozone in the Earth’s atmosphere were concentrated at ground level, it would form a layer only about three millimetres thick. But it still filters out virtually all of the most dangerous short-wave UV C radiation from the Sun and most of the medium-wavelength UV B rays as well. Thus it protects life from sunburn and skin cancer. The ozone layer has been severely depleted by chemicals such as CFCs (chlorofluorocarbons) released by human activity, leading to a generalized thinning of the layer and more specific holes over polar regions in the still, cold air of spring. International agreements have slowed the release of CFCs and the ozone layer should recover, but the chemicals are long-lived and it will be some time yet before it does.





Circles and cycles


It is in the lowest 15 kilometres of the atmosphere, the troposphere, that most of the action takes place. This is where weather happens. It’s where clouds form and disperse and where winds blow, transferring heat and moisture around the planet. In a dynamic planet, everything seems to go round in circles, flows of energy. And here, close to the surface, these cycles are driven by solar power. There are the obvious cycles of day and night as the Earth spins on its axis and the ground alternately heats and cools, and the annual cycle of the seasons as the Earth orbits the Sun, presenting first more of one hemisphere then more of the other to the sunshine. But there are longer cycles too, such as the wobble of the Earth’s axis over tens of thousands of years.

Just as the Earth orbits the Sun, so the Moon orbits the Earth. It takes about 28 days to complete an orbit, giving us our months. As the Earth spins on its axis, the Moon’s gravity pulls a bulge in the oceans around the planet, creating tides. This also acts as a brake on the rotation of the Earth, slowing down the days. Daily growth bands in fossil corals 400 million years old suggest that their days were several hours shorter than our own.

The Moon helps to stabilize the orbit of the Earth and hence the climate. But there are far longer cycles at work too. The Earth’s orbit around the Sun is not a perfect circle, but an ellipse, with the Sun at one focus. Hence the distance of the Earth from the Sun varies during its orbit. In addition, the degree of variation itself changes over a 95,800-year period. Also the Earth’s rotation axis slowly wobbles or precesses like a spinning top off balance. Over a period of 21,700 years the planet’s axis traces out a complete cone. At present, the Earth is nearest to the Sun during the northern hemisphere winter. The inclination of the Earth’s spin axis with that of its orbit around the Sun (the obliquity) also changes on a 41,000-year period. These so-called Milankovich cycles add up over tens or hundreds of thousands of years to affect climate. They have been blamed for such phenomena as the ice ages that have affected the Earth over the last three and a quarter million years. But the reality is probably even more complex, with their effects amplified or reduced by factors such as ocean circulation, cloud cover, atmospheric composition, volcanic aerosols, the weathering of rocks, biological productivity, and so on.





Solar cycles


Cycles of change are not restricted to the Earth. The Sun can change too. Over its 5-billion-year history, the Sun has been getting progressively warmer. Surface temperatures on Earth have remained more constant, however, as levels of greenhouse gases have fallen over the same time. This was largely due to the effects of life, as plants and algae consumed carbon dioxide that acted like a blanket to keep the young Earth warm. There have been other solar variations as well. There is a regular solar cycle of 11 years which sees a rise and fall in sunspot activity, in turn reflecting the cycle of solar magnetic activity which produces storms and the solar wind. Other Sun-like stars seem to spend about a third of their time free of sunspots, a state called a Maunder minimum. That happened to our Sun between 1645 and 1715 AD. Solar power only dropped by about 0.5%, but this was enough to plunge northern Europe into what has become known as the Little Ice Age, with a series of particularly severe winters. The River Thames in London froze over, and markets and frost fairs were held on it.





Hot air


The Sun distributes its warmth unevenly, warming up equatorial regions the most. As the air warms it tries to expand, increasing atmospheric pressure. To try to restore equilibrium, winds begin to blow and the air circulates. Whilst all this goes on, the Earth continues to rotate, giving the air angular momentum. That is greatest at the equator and results in the so-called Coriolis effect. The atmosphere is not firmly coupled to the solid planet, so, as winds blow away from the equator, they have a momentum that is independent of the rotating surface beneath. This means that, relative to the surface, the winds curve to the right in the northern hemisphere and left in the south. This leads to rotating systems of high and low air pressure, the weather systems that bring us rain or sunshine.

Land masses and mountain ranges influence the circulation of heat and moisture too. Until the Himalayan mountain range began to rise, there was no Indian monsoon, for example. And most importantly, the oceans play a huge part in storing heat and transporting it around the globe. The top 2 metres of the ocean have the same heat capacity as the entire atmosphere. At the same time, heat circulates in ocean currents. But currents on the surface are only half the picture. A good example is the Gulf Stream in the North Atlantic. That carries warm water north and east from the Gulf of Mexico and is one of the reasons why the climate of northwest Europe is much milder in winter than that of northeast America. As the warm water heads north, some evaporates into the clouds, which always seem to fall on British holidaymakers. The remaining surface waters in the ocean cool and become progressively more salty. As a result, they also become denser and eventually sink down to flow back south in the deep Atlantic, completing the conveyor belt of the ocean circulation.





Sudden freeze


About 11,000 years ago, the Earth was emerging from the last Ice Age. Ice was melting, sea levels were rising, and the climate was getting generally warmer. Then, suddenly, in the space of a few years, it turned cold again. The change was particularly marked in Ireland, where pollen in sediment cores shows that the vegetation suddenly reverted from temperate woodland back to tundra dominated by a little plant called the Dryas. Wally Broecker of the Lamont Doherty Geological Observatory has worked out what may have happened. As the ice sheet over North America receded, a vast lake of fresh meltwater, far bigger than the present Great Lakes, built up in central Canada. At first, it drained over a great ridge of rock into the Mississippi. As the ice receded, it suddenly opened a far lower passage down the St Lawrence River to the east. The vast lake of cold fresh water drained into the North Atlantic almost instantaneously. So much water was involved that it caused an immediate sea level rise of 30 metres. It diluted the salty surface water of the North Atlantic and put a virtual stop to the conveyor belt of ocean circulation. Thus there was no warming current into the North Atlantic and Arctic conditions returned. A thousand years later, the ocean circulation resumed as quickly as it had vanished and a temperate climate returned.

The North Atlantic deep water, together with cold bottom water from the Antarctic, finds its way at depth as far as the Indian and Pacific oceans. The deep current continues into the North Pacific, slowly accumulating nutrients as it goes, before it rises again to the surface.





Global greenhouse


Some of the gases in the Earth’s atmosphere act rather like the glass of a greenhouse, letting sunlight in to warm the ground but then preventing the resulting infrared heat radiation from escaping. Were it not for the greenhouse effect, average global temperatures would be around 15 degrees Celsius lower than they are, making life almost impossible. The principal greenhouse gas is carbon dioxide but others, including methane, play an important role. So does water vapour, an effect that is sometimes forgotten. Over hundreds of millions of years, an approximate balance has been struck with plants removing carbon dioxide from the atmosphere through photosynthesis and animals returning it by respiration. Vast quantities of carbon have been buried in sediments such as limestone, chalk, and coal. Volcanic eruptions have released carbon from inside the Earth.

In recent years, concern has grown over what should be termed the enhanced greenhouse effect, the very significant rise in greenhouse gas levels in the atmosphere resulting from human activity. The burning of fossil fuels such as coal and oil are prime culprits, but so are agricultural practices which produce methane, and deforestation which releases carbon dioxide from timber and soils as well as reducing the plant cover to absorb it again. Climate models suggest that these activities could result in a global temperature rise of several degrees over the next century, accompanied by greater extremes in weather and a possible sea level rise.





Climate change


The steady annual rise in carbon dioxide levels has been carefully recorded from an isolated mountain peak in Hawaii since 1958. More than 130 years of meticulous weather readings around the world confirm an average global warming of about half a degree, with the effect particularly pronounced over the last 30 years. But natural climate records go back much, much further. Tree rings record periods of drought and severe frosts, as well as the frequency of wild fires, over their lifespan. Extrapolating overlapping sequences in preserved timber can reveal climate conditions back to 50,000 years ago. Coral growth rings reveal sea surface temperatures over a similar span. Pollen grains in sediments record shifts in vegetation patterns over 7 million years. Landscapes reveal past glaciation and changes in sea level over billions of years. But some of the best records come from cores drilled from ice and from ocean sediments. Ice cores reveal not only rates of snow accumulation and trapped volcanic dust, but bubbles in the ice represent samples of the ancient atmosphere trapped in the snow. Isotopes of hydrogen, carbon, and oxygen can also indicate global temperature at the time. The ice record from Antarctica and Greenland now goes back over 400,000 years. Marine sediments all around the planet have been sampled by the ocean drilling programme and can carry records up to 180 million years old. Isotope ratios in microfossils trapped in the sediments can reveal temperature, salinity, atmospheric carbon dioxide levels, ocean circulation, and the extent of polar ice caps. All these different records reveal that climate change is a fact of life and that long periods in the past have been considerably warmer than the climate we experience today.





Web of life


The most insubstantial of the Earth’s layers has had perhaps the most profound effect on the planet: life. Without life, the Earth might be a runaway greenhouse world like Venus, or possibly a cold desert like Mars. There would certainly not be the temperate climate and oxygen-rich atmosphere in which we flourish. We’ve already heard how the first algae kept pace with the warming Sun by eating the carbon dioxide blanket that insulated the young Earth. The independent scientist James Lovelock suggests that such feedback mechanisms have managed the terrestrial climate for more than 2 billion years. He uses the term Gaia, after the ancient Greek earth goddess, as a name for this system. He does not pretend that there is anything conscious or deliberate about this control; Gaia does not have divine powers. But life, principally in the form of bacteria and algae, does play a key role in the homeostatic process that keeps the planet habitable. A simple computer model called ‘Daisyworld’ shows how two or more competing species can set up a negative feedback system that controls the environment within habitable limits. Lovelock suspects that the global system on Earth will adapt as human activity enhances the greenhouse effect, even if the adaptations are not favourable to human life.





The carbon cycle


Carbon is forever moving around. Each year, roughly 128 billion tonnes is released as carbon dioxide into the atmosphere by processes on land, and nearly as much is immediately absorbed again by plants and by the weathering of silicate rocks. At sea, the figures are comparable, though slightly more goes in than comes out. The system would be more or less in balance were it not for volcanic emissions and the 5 billion tonnes released each year by burning fossil fuels. The total amount of carbon held in the atmosphere is quite small – just 740 million tonnes, only slightly more than that held in plants and animals on land and slightly less than that held by living things in the ocean. By comparison, the amount of carbon stored in solution in the oceans is vast at 34 billion tonnes, and the amount stored in sediments is 2,000 times greater still. So the physical processes of solution and precipitation may be even more important in the carbon cycle than the biological ones. But life seems to hold some key cards. Carbon incorporated by phytoplankton would be released back to sea water and hence the atmosphere very quickly were it not for the physical properties of copepod faecal pellets. These tiny planktonic animals excrete their waste in small, dense pellets which can slowly sink into the deep ocean, removing them, at least temporarily, from the cycle.



3. The carbon cycle. This simplified diagram shows estimates of the amount of carbon (in billions of tonnes) stored in the atmosphere, oceans, and land. Figures by the arrows show the annual fluxes between stores, figures in brackets the annual net rise. Though small compared with most other fluxes, the input from burning fossil fuels seems enough to upset the balance.





Almost an onion


The interior of the Earth is rather like an onion, made up of a series of concentric shells or layers. On the top is a crust, averaging 7 kilometres thick under the ocean and 35 kilometres thick in continents. That sits on the hard, rocky lithosphere at the top of the mantle, and below it is the softer asthenosphere. The upper mantle extends to a depth of about 670 kilometres, the lower mantle goes down to 2,900 kilometres. Below a thin transition layer comes the liquid outer core of molten iron and a solid iron inner core about the size of the Moon. But it is not a perfect onion. There are horizontal differences within layers, variations in thickness of layers, and, we now know, continuous exchange of material between layers. Where our planet departs from the perfect onion model is where most off the interest and excitement in modern geophysics lies, and where we can find the clues to the processes that drive the system.



4. The main ‘onion’ layers in a radial section of the Earth.





Lava lamps


Do you remember those lava lamps of the 1960s and numerous later revivals? They make a good model for the processes at work within the Earth. Whilst they are switched off, a layer of red gloop sits at the bottom of a layer of transparent oil. But turn on the lamp and the filament in its base warms the red gloop so it expands, becomes less dense, and begins to rise in elongated lumps to the top of the oil. When it has cooled sufficiently, it sinks back down. So it is in the Earth’s mantle. Heat from radioactive decay and from the Earth’s core drives a sort of heat engine in which the not-quite-solid rock of the mantle slowly circulates over billions of years. It is this circulation that drives plate tectonics, causes the continents to drift, and triggers volcanoes and earthquakes.





The rock cycle


At the surface the results of that heat engine beneath our feet and the solar furnace above our heads meet and drive the rocks full circle. Mountain ranges lifted high by mantle circulation and continental collisions are weathered down by solar-powered wind, rain, and snow. Chemical processes are at work as well. Oxidation by the atmosphere and chemical dissolution by acids from living organisms and dissolved gases help to break down the rocks. Large quantities of carbon dioxide can dissolve in rainwater to make a weak acid that causes chemical weathering, turning silicate minerals into clay. These remains are washed back down to estuaries and oceans where they form new sediments, eventually to be scooped up into new mountain ranges or carried back down into the mantle for deep recycling. The whole process is lubricated by water incorporated into the crystal structure of minerals. This rock cycle was first suggested in the 18th century by James Hutton, but then he had no idea of the depths and the time scales over which it occurs.

So far we have glimpsed just the surface of our amazing planet. Now we will dig deeper into the rocks and into time.





Chapter 2

Deep time


Space is big, really big . . . You may think it’s a long way down the street to the chemists’ but that’s just peanuts to space.

Douglas Adams, The Hitchhiker’s Guide to the Galaxy



The world is not only large in its spatial dimensions. It also extends almost unimaginably far back in time. It is impossible to get a full grasp of the concepts and processes at work in geology without an understanding of what writers John McPhee, Stephen Jay Gould, and Henry Gee have referred to as deep time.

Most of us know our parents, many remember our grandparents. Only a few have met great grandparents. Their youth lies more than a century in our past, a time which seems alien to us with our vastly different scientific understanding and social structure. Just a dozen generations back, England was ruled by Queen Elizabeth I, motorized transport and electronic communication was undreamt of, and Europeans were exploring the Americas for the first time. Thirty generations takes us a thousand years back, before the Normans invaded Britain. It is also before continuous written records are likely to be able to trace our direct ancestry. We may be able to tell from archaeology and genetics roughly who our ancestors were at this time and where they might have lived but we cannot be certain. Fifty generations ago, the Roman Empire was in full swing. And 150 generations back, the Great Pyramid of Ancient Egypt had not been constructed. About 300 generations takes us back to the Neolithic in Europe at a time when the last Ice Age had only just ended and simple agriculture was the latest technological revolution. It is unlikely that archaeology can reveal where our ancestors were living at that time, though comparisons of our maternally inherited mitochondrial DNA may indicate the broad region. Add another zero to the year and we have gone back 3,000 generations to 100,000 years ago. At this time, we cannot trace separate ancestry of any living racial group. Mitochondrial DNA suggests that there was a single maternal ancestor of all modern humans in Africa not long before. But, in geological time, this is still recent.

Ten times older at a million years and we start to lose track of the modern human species. Another factor of ten and we are looking at the fossil remains of early ape ancestors. This far back it’s impossible to point even to a single species and say with certainty that amongst these individuals was our ancestor. Multiply by ten again and, 100 million years ago, we are in the age of the dinosaurs. The ancestor of humans must be some insignificant shrew-like creature. A thousand million years ago and we are back amongst the first fossils, maybe before even the first recognizable animals. Ten billion years ago and we are before the birth of the Sun and solar system, at a time when the atoms that today make up our planet and ourselves were being cooked in the nuclear furnaces of other stars. Time is indeed deep.

Now think again of the changes that can take place in a few generations. Historical time is trivial compared to the age of the Earth, yet a few centuries have seen many volcanic eruptions, cataclysmic earthquakes, and devastating landslides. And think of the relentless progress of less devastating changes. In 30 generations, parts of the Himalayas have risen by maybe a metre or more. But at the same time they have eroded, probably by more than this. Islands have been born, others washed away. Some coasts have eroded back hundreds of metres, others have been left high and dry. The Atlantic has widened by about 30 metres. Now multiply all these comparatively recent changes by factors of ten or a hundred or a thousand, and you are beginning to see what can happen over geological ‘deep time’.





Flood and uniformity


Humans have noticed fossil remains since prehistoric times. There are ancient stone tools that appear to have been chipped so as to show off a fossil shell. The fossilized stem of a giant cycad was placed in an ancient Etruscan burial chamber. But attempts to understand the nature of fossils are comparatively recent. The science of geology arose primarily in Christian Europe where beliefs based on biblical stories made it unsurprising to discover the shells and bones of extinct creatures high up in mountainous regions: they were the remains of animals that perished in the biblical flood. Even granite, it was suggested by so-called neptunists, was precipitated from an ancient ocean. The idea of extreme acts of God such as the flood helped people to imagine that the Earth had been shaped by catastrophes, and this was the generally accepted theory until the end of the 18th century.

In 1795 the Scottish geologist James Hutton published his now famous Theory of the Earth. The much quoted though paraphrased summary of its message is that ‘the present is the key to the past’. This is the theory of gradualism or uniformitarianism, which says that if you want to understand geological processes you must look at the almost imperceptibly slow changes occurring today and then simply trace them out through history. It was a theory developed and championed by Charles Lyell, who was born in 1797, the year Hutton died. Both Hutton and Lyell tried to put religious beliefs in events such as the creation and the flood to one side and proposed that the gradual processes at work on the Earth were without beginning or end.





Dating creation


Attempts to calculate the age of the Earth came originally out of theology. It is only comparatively recently that so-called creationists have interpreted the Bible literally and therefore believe that Creation took just seven 24-hour days. St Augustine had argued in his commentary on Genesis that God’s vision is outside time and therefore that each of the days of Creation referred to in the Bible could have lasted a lot longer than 24 hours. Even the much quoted estimate in the 17th century by Irish Archbishop Ussher that the Earth was created in 4004 BC was only intended as a minimum age and was based on carefully researched historical records, notably of the generations of patriarchs and prophets referred to in the Bible.

The first serious attempt to estimate the age of the Earth on geological grounds was made in 1860 by John Phillips. He estimated current rates of sedimentation and the cumulative thickness of all known strata and came up with an age of nearly 96 million years. William Thompson, later Lord Kelvin, followed this with an estimate based on the time it would have taken the Earth to cool from an originally hot molten sphere. Remarkably, the first age he came up with was also very similar at 98 million years, though he later refined it downwards to 40. But such dates were considered too recent by uniformitarianists and by Charles Darwin, whose theory of evolution by natural selection required more time for the origin of species.

By the dawn of the 20th century, it had been realized that additional heat might come from radioactivity inside the Earth and so geological history, based on Kelvin’s idea, could be extended. In the end, however, it was an understanding of radioactivity that led to the increasingly accurate estimates of the age of the Earth that we have today. Many elements exist in different forms, or isotopes, some of which are radioactive. Each radioactive isotope has a characteristic half-life, a time over which half of any given sample of the isotope will have decayed. By itself, that’s not much use unless you know the precise number of atoms you start with. But, by measuring the ratios of different isotopes and their products it is possible to get surprisingly accurate dates. Early in the 20th century, Ernest Rutherford caused a sensation by announcing that a particular sample of a radioactive mineral called pitchblende was 700 million years old, far older than many people thought the Earth to be at that time. Later, Cambridge physicist R. J. Strutt showed, from the accumulation of helium gas from the decay of thorium, that a mineral sample from Ceylon (now Sri Lanka) was more than 2,400 million years old.

Uranium is a useful element for radio dating. It occurs naturally as two isotopes – forms of the same element that differ only in their number of neutrons and hence atomic weight. Uranium-238 decays via various intermediaries into lead-206 with a half-life of 4,510 million years, whilst uranium-235 decays to lead-207 with a 713-million-year lifetime. Analysis of the ratios of all four in rocks, together with the accumulation of helium that comes from the decay process, can give quite accurate ages and was used in 1913 by Arthur Holmes to produce the first good estimate of the ages of the geological periods of the past 600 million years.

Some radioisotopes used for dating





The success of radio-dating techniques is due in no small way to the power of the mass spectrometer, an instrument which can virtually sort individual atoms by weight and so give isotope ratios on trace constituents in very small samples. But it is only as good as the assumptions that are made about the half-life, the original abundances of isotopes, and the possible subsequent escape of decay products. The half-life of uranium isotopes makes them good for dating the earliest rocks on Earth. Carbon 14 has a half-life of a mere 5,730 years. In the atmosphere it is constantly replenished by the action of cosmic rays. Once the carbon is taken up by plants and the plants die, the isotope is no longer replenished and the clock starts ticking as the carbon 14 decays. So it is very good for dating wood from archaeological sites, for example. However, it turns out that the amount of carbon 14 in the atmosphere has varied along with cosmic ray activity. It is only because it has been possible to build up an independent chronology by counting the annual growth rings in trees that this came to light and corrections to carbon dating of up to 2,000 years could be made.





The geological column


Look at a section of sedimentary rocks in, for example, a cliff face and you will see that it is made up of layers. Sometimes annual layers corresponding to floods and droughts are visible. More often, the layers represent occasional catastrophic events or slow but steady sedimentation across hundreds of thousands or even millions of years, followed by a change of environment leading to a layer of slightly different rock. In the case of a really deep section of ancient rock, such as that seen in the Grand Canyon in Arizona, hundreds of millions of years of deposits are represented. It is a natural human instinct to divide up and classify things, and sedimentary rock with its many layers is an obvious candidate. But, when viewing a spatially narrow cliff face of flat layers, it’s easy to forget that the layers are not continuous around the world. The entire globe was never covered by a single shallow ocean depositing similar sediments! Just as today, there are rivers, lakes, and seas, deserts, forests, and grasslands, so in ancient times there was a panoply of sedimentary environments.



5. The main divisions of geological time (not to scale). Ages (on the right, in millions of years before present) are those agreed by the International Commission on Stratigraphy in 2000.

It was an English civil engineer, William Smith, who, in the early 19th century, began to make sense of it all. He was surveying for Britain’s new canal network and started to realize that rocks in different parts of the country sometimes contained similar fossils. In some cases the rock types too were the same, sometimes only the fossils were similar. This enabled him to correlate the rocks in different places and work out an overall sequence. As a result, he published the first geological map. Once the dates were added in the 20th century, and the rocks correlated between different continents, it was possible to publish a single sequence of layers representing periods of geological time for the whole world. The geological column we know today is the product of many techniques, refined over the years and agreed by international collaboration.





Extinctions, unconformities, and catastrophes


It became clear that some of the changes in the geological column were bigger than others, and these provided convenient places to divide the geological past into separate eras, periods, and epochs. Sometimes there was a sudden and significant change in the nature of the rocks across such a boundary, indicating a major environmental change. Sometimes there was what is known as an unconformity, a break in deposition, caused, for example, by a change in sea level so that either deposition stopped or the layers were eroded away before the column continued. They are often also marked by major changes in fauna, represented by fossils, with many species becoming extinct and new ones beginning to arise.

A few intervals in the geological record stand out for the severity of the extinctions across them. The end of the Cambrian period and the end of the Permian period were both marked by the extinctions of around 50% of families and up to 95% of individual species of marine invertebrates. The extinctions that marked the late Triassic and late Devonian saw the loss of about 30% of families and, slightly smaller at 26%, but the most recent and the most famous, is the mass extinction at the end of the Cretaceous period 65 million years ago. That so-called K/T boundary is famous not only because it saw the extinction of the last of the dinosaurs but also because there is good evidence for the cause.





Threat from space


The first suggestion, by Walter and Louis Alvarez, that the extinction might be due to an astronomical impact at first received little scientific support. However, they soon discovered that sediments in a narrow band at that point in the geological column were enriched in iridium, an element abundant in some types of meteorite. But there was no sign of an impact crater of that age. Then evidence began to emerge, not from the land but from the sea just off the Yucatan Peninsula of Mexico, of a buried crater 200 kilometres across. There is evidence of debris from a much wider area. If, as is calculated, it marks the point where an asteroid or comet, maybe 16 kilometres across, hit the Earth, the results would indeed have been devastating. Apart from the effects of the impact itself and the tsunami that resulted, so much rock would have been vaporized that it would have spread round the Earth in the atmosphere. At first it would have been so hot that its radiant heat would have triggered forest fires on the ground. The dust would have stayed in the atmosphere for several years, blocking out sunlight, creating a global winter, and causing food plants and plankton to die. The sea bed at the impact site included rocks rich in sulphate minerals and these would have vaporized, leading to a deadly acid rain when it washed out of the atmosphere again. It is almost surprising that any living creatures survived.





The menace within


It was once hard to understand how any mass extinctions could have occurred. Now, there are so many competing theories that it is difficult to choose between them. They mostly involve severe climate change, whether triggered by a cosmic impact, changing sea levels, ocean currents and greenhouse gases, or a cause from within the planet such as rifting or major vulcanism. It does seem that most of the mass extinctions we know coincided at least approximately with major eruptions of flood basalts. In the case of the late Cretaceous, it was the eruptions that produced the Deccan Traps in western India. There has even been a suggestion that a major asteroid impact caused shock waves to focus on the other side of the Earth, triggering eruptions. But the times and positions do not seem to line up well enough to prove that explanation. Whatever the reason, the history of life and of the planet has been punctuated by some catastrophic events.





Chaos reigns


We can all remember climatic events that stand out, say over the last decade, as the worst winter, flood, storm, or drought. Take the record back for a century and the likelihood is that an even bigger one will stand out. Authorities often use the concept of a ‘100-year’ flood in planning coastal or river flood defences; they are designed to withstand the sort of flood that only happens once a century. It’s likely to be more severe than the sort which happens only once a decade. But, if you extend the same idea to a thousand years or a million years, there is likely to be one that will be bigger still. According to some theorists, that is likely to be true of anything from floods, storms, and droughts to earthquakes, volcanic eruptions, and asteroid impacts. Over geological time we had better watch out!





Deeper time


The list of geological periods that is often shown in books goes back only about 600 million years to the start of the Cambrian period. But that ignores 4 billion years of our planet’s history. The trouble with most Pre-Cambrian rocks is that they are, as Professor Bill Schopf of the University of California puts it, fubaritic – fouled up beyond all recognition. The constant tectonic reprocessing of the Earth from within, and the relentless pounding of weather and erosion from above, mean that most of the Pre-Cambrian rocks that survive at all are deeply folded and metamorphosed. But on most clear nights you can see rocks that are more than 4 billion years old – by looking up at the Moon rather than down at the Earth. The Moon is a cold, dead world with no volcanoes and earthquakes, water or weather to resurface it. Its surface is covered with impact craters, but most of those happened early in its history when the solar system was still full of flying debris.

The Pre-Cambrian rocks that do survive on Earth tell a long and fascinating story. They are not, as Darwin had supposed, devoid of the traces of life. Indeed, the end of the Pre-Cambrian, from about 650 to 544 million years ago, has yielded a rich array of strange fossils, particularly from localities in southern Australia, Namibia, and Russia. Prior to that there seems to have been a particularly severe period of glaciation. The phrase ‘snowball Earth’ has been used, conveying the possibility that all the world’s oceans froze over. Inevitably, that would have been a major setback for life, and there is scant evidence for multicellular life forms before this. But there is abundant evidence for microorganisms – bacteria, cyanobacteria, and filamentous algae. There are filamentous microfossils from Australia and South Africa that are around 3,500 million years old, and there is what looks like the chemical signature of life in carbon isotopes in rocks from Greenland that are 3,800 million years old.

During the first 700 million years of its history, the Earth must have been particularly inhospitable. There were numerous major impacts far bigger than that which may have killed the dinosaurs. The scars of this late heavy bombardment can still be seen in the great Maria basins on the Moon, which are themselves giant impact craters filled with basalt lava melted by the impacts. Such impacts would have melted much of the Earth’s surface and certainly vaporized any primitive oceans. It is possible that the water on our planet today came from a subsequent rain of comets as well as from volcanic gases.





Dawn of life


The early atmosphere of Earth was once thought to have been a mixture of gases such as methane, ammonia, water, and hydrogen, a potential source of carbon to primitive life forms. But it is now believed that strong ultraviolet radiation from the young Sun would have broken that down quickly to give an atmosphere of carbon dioxide and nitrogen. No one yet knows for certain how life began. There are even claims that it may have had an extra-terrestrial origin, arriving on Earth in meteorites from Mars or beyond. But laboratory studies are beginning to show how some chemical systems can begin to self-organize and catalyse their own reproduction. Analysis of present-day life forms suggests that the most primitive are not the sort of bacteria that scavenge organic carbon or that use sunlight to help them photosynthesize but those that use chemical energy of the sort that is found today in deep-sea hydrothermal vents.

By 3,500 million years ago, there were almost certainly microscopic cyanobacteria and arguably primitive algae – the sort of thing we see today in pond scum. These began to have a dramatic effect. Using sunlight to power photosynthesis, they took in carbon dioxide from the atmosphere, effectively eating the blanket that, by the greenhouse effect, kept the Earth warm when the power of the Sun was weak. This may be what led ultimately to the late Pre-Cambrian glaciation. But long before that, it resulted in the worst pollution incident the world has known. Photosynthesis released a gas that had not existed on Earth before and which was probably toxic to many life forms: oxygen. At first, it did not last long in the atmosphere but quickly reacted with dissolved iron in sea water, resulting in thick deposits of banded iron oxide. Almost literally, the world went rusty. But photosynthesis continued, and free oxygen began to build up in the atmosphere from about 2,400 million years ago, paving the way for animal life that could breathe the oxygen and eat the plants.





Birth of Earth


About 4,500 million years ago there was a great cloud of gas and dust, the product of several previous generations of stars. It began to contract under gravity, perhaps boosted by the shock waves from a nearby exploding star or supernova. A slight rotation in the cloud accelerated as it contracted and spread the dust out into a flattened disc around the proto-star. Eventually, the central mass, mostly of hydrogen and helium, contracted sufficiently to trigger nuclear fusion reactions at its core and the Sun began to shine. A wind of charged particles began to blow outwards, clearing some of the surrounding dust. In the inner part of the nebula, or disc, only refectory silicates remained. Further out, the hydrogen and helium accumulated to form the giant gas planets Saturn and Jupiter. Volatile ices such as water, methane, and nitrogen were driven still further out and formed the outer planets, Kuiper belt objects, and comets.

The inner planets – Mercury, Venus, the Earth, and Mars – formed by a process known as accretion which began as particles bumped into one another, sometimes splitting, occasionally joining together. Eventually, the larger lumps developed sufficient gravitational attraction to pull others to them. As the mass increased, so did the energy of the impacts, melting the rocks so that they began to separate out, with the densest, iron-rich minerals sinking to form a core. The new Earth was hot, probably at least partially molten, from the impacts, from the energy released by its gravitational contraction, and from the decay of radioactive isotopes. It is likely that many radioactive elements in the pre-solar nebula had been created not long before in supernovae explosions and would still have been radioactively hot. So it is hard to see how there could have been liquid water on the surface initially, and it is possible that the first atmosphere was mostly stripped away by the force of the solar wind.





A chip off the block


The formation of the Moon had long been a mystery to science. Its composition, orbit, and rotation didn’t fit with the idea that it had split off from the young Earth, formed alongside it, or been captured whilst passing it. But one theory does now make sense and has been convincingly simulated in computer models. It involves a proto-planet about the size of Mars crashing into the Earth about 50 million years after the formation of the solar system. The core of this projectile would have merged with that of the Earth, the force of the impact melting most of the Earth’s interior. Much of the outer layers of the impactor, together with some terrestrial material, would have vaporized and been flung into space. A lot of that collected in orbit and accreted to form the Moon. This cataclysmic event gave us a companion which seems to have a stabilizing effect on the Earth, preventing its rotation axis swinging chaotically and thus making our planet a more amenable home to life.





Chapter 3

Deep Earth




The surface of the Earth is covered by a relatively thin, cold, hard crust. Beneath the oceans it is about 7 or 8 kilometres thick; in the continents, 30 to 60 kilometres thick. At its base lies the Mohorovicic discontinuity or Moho, a layer which reflects seismic waves, probably as a result of a change in composition to the dense rocks of the mantle beneath. The lithosphere, the complete slab of cold, hard material on the Earth’s surface, includes not only the crust but the top of the mantle as well. In total, the continental lithosphere may be 250 or even 300 kilometres thick. It thins under the oceans, as you approach the mid-ocean ridge system, down to little more than the 7-kilometre crust. The lithosphere is not a single rigid layer, however. It is split into a series of slabs called tectonic plates. They are our principal clue as to how the deep Earth works. To understand what’s going on, we must probe beneath the crust.





Digging deep


Only 30 kilometres away from us lies a place we can never visit. If the distance was horizontal it would just be an easy bus ride away, but the same distance beneath our feet is a place of almost unimaginable heat and pressure. No mine can tunnel that deep. A proposal in the 1960s to use ocean-drilling techniques from the oil exploration industry to drill right through the ocean crust into the mantle, the so-called Moho project, was ruled out on grounds of cost and difficulty. Attempts at deep drilling on land on Russia’s Kola Peninsula and in Germany had to be abandoned after about 11,000 metres. Not only was the rock difficult to drill, but the heat and pressure tended to soften the drill components and squeeze the hole shut again as soon as it was drilled.





Messengers from the deep


There is one way in which we can sample the mantle directly: in the outpourings of deep-rooted volcanoes. Most of the magma that erupts from volcanoes comes from only partial melting of the source material, so basalt, for example, is not a complete sample of mantle rock. It does, however, carry isotopic clues to what lies beneath. For example, basalt from some deep-rooted volcanoes, such as that in Hawaii, contains helium gas with a high ratio of helium 3 to helium 4, as the early solar system is believed to have had. So this is thought to come from a part of the Earth’s interior that is still pristine. The helium gets lost in volcanic eruptions and is slowly replaced by helium 4 from radioactive decay. The basalt in ocean ridge volcanoes is depleted in helium 3. This suggests that it is recycled material that lost helium gas in earlier eruptions and does not come from so deep in the mantle.

Violent volcanic eruptions do sometimes carry in their magma more direct samples of mantle rocks. These so-called xenoliths are samples of mantle rock that have not been melted, just carried along in the flow. They are typically dark, dense, greenish rocks such as peridotite, rich in the mineral olivine, a magnesium/iron silicate. Similar rock is sometimes found in the deep cores of mountain ranges which have been thrust up from great depths.





Slow flow


The magnificent medieval stained-glass windows of Canterbury Cathedral can tell us something about the nature of the Earth’s mantle. The windows are composed of many small panes of coloured glass in a leaded frame. If you look at the sunlight filtered through the panes, you will notice that some of them are darker at the bottom than at the top. This is because the glass flows. Technically, it’s a super-cooled fluid. Over the centuries, gravity has made the panes slowly sag so that the glass is thicker at the bottom. Yet, to the touch, or, heaven forbid, a hammer blow, the glass still behaves as a solid. A key to understanding the Earth’s mantle is the realization that the silicate rocks there can flow in the same sort of way, even though they do not melt. In fact, the individual mineral grains are constantly re-forming, giving rise to the motion known as creep. The effect is that the mantle is very, very viscous, like extremely thick, sticky treacle.





A planetary body scan


The clearest clues to the internal structure of the Earth come from seismology. Earthquakes send out seismic shock waves through the planet. Like light being refracted by a lens or reflected by mirrors, seismic waves travel through the Earth and reflect off different layers within it. Seismic waves travel at different velocities depending on how hot or soft the rock is. The hotter and therefore softer the rock, the slower the wave travels. There are two main types of seismic wave, primary, or P, waves, which are the faster and thus the first to arrive at a seismograph, and secondary, or S, waves. P waves are pressure waves with a push-pull motion; S waves are shear waves and cannot travel through liquids. It was by studying S waves that the molten outer core of the Earth was first revealed. Detecting these seismic waves on a single instrument would not tell you much, but today there are networks of hundreds of sensitive seismometers, strung out around the planet. And every day there are many small earthquakes to generate signals. The result is a bit like a body scanner in a hospital, in which the patient is surrounded by X-ray sources and sensors and computers use the results to build up a 3D image of her internal organs. The hospital version is known as a CAT scan, standing for Computer Assisted Tomography. Its whole-Earth equivalent is called seismic tomography.

The global network of seismographs is best at seeing things on a global scale. It will reveal the overall layering in the mantle and changes in seismic velocities due to high or low temperature on scales of hundreds of kilometres. There are also more closely spaced arrays, originally set up to detect underground nuclear tests, and they, together with new arrays being deployed by geophysicists in geologically interesting regions, have the potential to see structure deep in the mantle on a scale of a few kilometres. And it seems that there is structure on every scale. The clearest things in these whole-Earth body scans are the layers. Below 2,890 kilometres, the depth of the liquid outer core of our planet, S waves will not pass. But several features stand out within the mantle. There is, as we’ve mentioned, the Mohorovicic discontinuity at the base of the crust, and another at the base of the hard lithosphere. The asthenosphere beneath is softer so the seismic velocities are slower. There are clear layers 410 kilometres down and 660 kilometres down, with another less distinct layer after about 520 kilometres. At the base of the mantle is another, probably discontinuous, layer called the D” or D double prime layer, which varies from nothing to about 250 kilometres thick.

Seismic tomography also reveals more subtle features. Essentially, colder rock is harder so seismic waves travel through it more quickly than they do through hotter, softer rock. Where old, cold ocean crust dives beneath a continent or into an ocean trench, reflections from the descending slab reveal its passage down into the mantle. Where Earth’s hot core bakes the underside of the mantle, it appears to soften and rise in a huge plume.

The mantle is full of mysteries which at first sight seem to be contradictory. It is solid yet it can flow. It’s made up of silicate rock which is a good insulator, yet somehow about 44 terawatts of heat finds its way to the surface. It’s hard to see how that heat flow could happen through conduction alone, and yet, if there was convection, the mantle would be mixed, so how could it show a layered structure? And how could ocean volcanoes erupt magma with a different mix of tracer isotopes than that believed to exist in the bulk of the mantle, unless there are unmixed regions or layers? Resolving these mysteries has been one of the prime areas of geophysics in recent years.





A diamond window on the mantle


Some of the best clues have come from understanding the nature of the rocks down there. To find what the rocks are like deep within the Earth, you have to replicate the fantastic pressures down there. Amazingly, that’s possible with just your finger and thumb. The trick is to get hold of two good, gem-quality diamonds, cut in what jewellers term ‘brilliant’ cuts, with a tiny, perfectly flat face at the apex of each. Mount them face to face with a microscopic rock sample between the two, and turn a little thumbscrew to force the faces tighter together. The force gets so concentrated between the tiny diamond anvils that it’s possible to create pressures more than 3 million times atmospheric pressure (300 gigapascals), just by turning the screw. Because the diamonds are conveniently transparent, the sample can be heated by shining a laser in, and viewed with a microscope and other instruments. This can literally be a window on what rocks are like deep in the mantle.

Professor Bill Bassett was studying a tiny crystal in a diamond anvil one day in his lab at Cornell University. Nothing much had happened when he’d increased the pressure, so he decided to go for lunch. As he was leaving, he heard a sudden ‘crack’ from the anvil. Certain that one of his precious diamonds had broken, he rushed back and looked down the microscope. The gems were OK, but the sample had suddenly transformed into a new, high-pressure crystal form. It was what is known as a phase change: the composition remains the same but the structure changes, in this case into a more dense crystal lattice.

We know from the composition of xenoliths that at least the upper mantle is made of rocks such as peridotite, rich in the magnesium and iron silicate mineral olivine. Put a tiny sample of this between diamond anvils and turn up the pressure and it goes through a whole series of phase changes. At a pressure of about 14 gigapascals, equivalent to a depth in the mantle of about 410 kilometres, olivine transforms into a new structure called wadsleyite. At 18 gigapascals, 520 kilometres down, it changes again, adopting the structure of ringwoodite, a form of the mineral spinel. That then changes at 23 gigapascals, corresponding to 660 kilometres down, into two minerals, perovskite and a magnesium iron oxide mineral called magnesiowüstite. You’ll notice that the phase changes happen at precisely the depths at which seismic waves can be reflected. So perhaps the layers indicate a change in crystal structure rather than composition.





A double boiler?


The 660-kilometre layer, the division between the upper and lower mantle, is a particularly strong feature and the focus of vigorous debate between those who think that the entire mantle is circulating in a huge convection system and those who think that it is more like a double boiler with separate circulating cells in the upper and lower mantle and little or no exchange of material between them. Historically, geochemists tend to favour the double structure as it allows for chemical differences between the layers, whereas geophysicists prefer whole-mantle convection. Present indications are that both might be right, in a compromise solution in which whole-mantle circulation is possible but difficult. Data from seismic tomography would seem at first to favour the double boiler idea. The seismic scans reveal where slabs of subducted ocean crust sink down towards the 660 km anomaly. But they do not seem to pass through it. Rather, the material spreads out and seems to collect at that depth, for hundreds of millions of years. But further scans show where it can break through like an avalanche and continue on through the lower mantle almost to the top of the core.

In June 1994, Bolivia was shaken by a powerful earthquake. It did little damage because its focus was so deep – about 640 kilometres. But at that depth, the rocks should be too soft to fracture. This is a region where a slab of old ocean crust from the Pacific is sinking down beneath the Andes. What must have happened is that a whole layer of rock underwent a catastrophic phase change into the denser perovskite structure. That seems to be necessary before it can sink down into the lower mantle. The explanation solves the mysteries of mantle layering and deep earthquakes at one go.

But there is much that still needs explaining. For example, the slab of ocean crust that is subducting below the Tonga trench in the Pacific is passing into the mantle at about 250 millimetres per year, far too fast for its temperature to even out. Material would reach the base of the upper mantle in just 3 million years and its low temperature should be obvious if it pools there or extends into the lower mantle. But there is no evidence for such a slab. One theory is that not all of the olivine converts into higher-density minerals, making the old slab neutrally buoyant in the upper mantle. The combination of cool temperature and mineral content would give it a seismic velocity very similar to other mantle material, so it would not show up easily, just as a layer of glycerine does not show up well in water. There is indeed tantalizing faint seismic evidence for such a slab deep below Fiji.



6. The basic circulation in the Earth’s mantle and how it is reflected in lithospheric plate motions and plate boundaries. For clarity, motions are simplified and the vertical scale of the lithosphere is greatly exaggerated.





Message in a diamond


Diamond is the high-pressure form of carbon. It can only form in the Earth at depths of over 100 kilometres, sometimes well over this. Isotope ratios in diamonds suggest that they often form from carbon in subducted ocean crust, maybe carbonate from ocean sediments. Sometimes, there are tiny inclusions of other minerals within a diamond. It is not a feature that is popular among gem stone dealers, but it is just what geochemists are searching for. Minute analysis of those inclusions can tell the long and sometimes tortuous history of the diamond’s formation and passage through the mantle.

Some of the inclusions are of a mineral called enstatite, which is a form of magnesium silicate. Some researchers believe that it was originally magnesium silicate perovskite and comes from the lower mantle. Their evidence comes from the observation that it contains only one-tenth as much nickel as would be expected in the upper mantle. At lower mantle temperatures and pressures, nickel gets taken up into a mineral called ferropericlase, which is also a common inclusion of diamonds, leaving very little nickel left in magnesium silicate perovskite. In a few cases, the inclusions are rich in aluminium which, under upper mantle conditions, is locked up in garnet. And some inclusions are iron-rich, suggesting that they might have originated very deep in the mantle, close to the core mantle boundary. These deep diamonds also have a different carbon isotope signature, believed to be characteristic of deep mantle rock rather than subducted ocean lithosphere. Estimates of the age of diamonds and the rock that surrounds them suggests that some have had a very long and tortuous passage through the mantle that may have taken them more than a billion years. But it is convincing evidence of at least some transfer between the lower and upper mantle.

Almost as fascinating as the diamonds themselves is the rock in which they are found. It’s called kimberlite after the South African diamond-mining town of Kimberley. The rock itself is a mess! Apart from the diamonds, it contains a whole range of angular lumps and pulverized fragments of different rocks; a so-called breccia. It is volcanic and tends to form the carrot-shaped plugs of ancient volcanic vents. It is hard to determine its exact composition because it contains so much pulverized debris from its passage through the lithosphere, but the original magma must have been mostly olivine from the mantle together with an unusual amount of volatile material now in the form of mica. If it had found its way up slowly from the mantle, we would not have diamonds today. Diamond is unstable at pressures found less than 100 kilometres underground and, given time, would dissolve in the magma. But kimberlite volcanoes did not keep it waiting. It is estimated that the average speed of material through the lithosphere was about 70 kilometres per hour. The widening neck of the vent as it approaches the surface suggests that volatile material was expanding explosively and the surface eruption speed could have been supersonic. As a result, all the rock fragments collected on the way up have been quenched, frozen in time, so that they represent samples from deep in the lithosphere and even the mantle.





The base of the mantle


Recent analysis of seismic data from around the world has revealed a thin layer at the base of the mantle, the D″ layer, up to 200 kilometres thick. It is not a continuous layer but seems more like a series of slabs, a bit like continents on the underside of the mantle. This could be regions where silicate rocks in the mantle are partly mixed with iron-rich material from the core. But another explanation is that this is where ancient ocean lithosphere comes to rest. After its descent through the mantle, the slab is still cold and dense so it spreads out at the base of the mantle and is slowly heated by the core until, perhaps a billion years later, it rises again in a mantle plume to form new ocean crust.

Clues to the deep interior of the Earth also come from measuring tiny variations in day length. Our spinning planet is gradually slowing down due to the pull of the moon on the tides and to the rising of land compressed by ice in the last Ice Age. But there are other even smaller variations of a few billionths of a second. Some may be due to atmospheric circulation blowing on mountain ranges like wind on a sail. But there is another component which seems to be caused by circulation in the outer core pushing on ridges in the base of the mantle like ocean currents pushing on the keel of a ship. So there may be ridges and valleys like upside-down mountain ranges on the base of the mantle. There seems to be a great depression in the core beneath the Philippines that is 10 kilometres deep, twice the depth of the Grand Canyon. Bulging up beneath the Gulf of Alaska is a high spot on the core; a liquid mountain taller than Everest. Maybe sinking cold material indents the core, while hotspots bulge up.





Super plumes


Although much hotter, the perovskite rock of the lower mantle is much more viscous than upper mantle rocks. Estimates suggest that it is 30 times more resistant to flow. As a result, material rises from the base of the mantle in a much slower, broader column than the plumes which characterize the upper mantle. It behaves, in very, very slow motion, rather like the blobs of gloop in a lava lamp. It may well be true that, although some material circulates through the entire mantle, there are also smaller convection cells that are confined to the upper mantle. Convection cells in experimental systems tend to be about the same width as they are deep and, in some parts of the world at least, the spacing of plumes of mantle material seem to match the 660-kilometre depth of the upper mantle.





How the Earth melts


What goes down must come back up again. As plumes of hot mantle rock slowly rise towards the crust, the pressure on them drops and they begin to melt. Scientists can recreate what happens using great hydraulic presses to squeeze samples of artificial rock, heated in furnaces. It’s not the entire rock that melts, only a few per cent; producing magma that is less dense than the rest of the mantle and so is able to rise up rapidly to the surface and erupt as basalt lava. How it flows through the remaining rock was another great mystery. It turns out to be down to the microscopic structure of the rock. If the angles at the corners of the little pockets of melt that form between rock grains were large, the rock would be like a Swiss cheese; the pockets would not interconnect and the melt couldn’t flow out. But those angles are small and the rock is like a sponge, with all the pockets interconnecting. Squeeze the sponge and the liquid flows. Squeeze the mantle and the magma erupts.





Free-fall


When he saw an apple fall, Isaac Newton realized that the force of gravity was pulling objects towards the centre of the Earth. What he did not know was that apples fall slightly faster in some parts of the world than others – not that it is a difference you normally notice, nor could you easily measure it with apples. But you can with spacecraft. The secret of flying, according to Douglas Adams’ Hitchhiker’s Guide to the Galaxy, is to fall but forget to hit the ground. That’s roughly what a satellite does. It’s falling freely, but its speed keeps it in orbit. The stronger gravitational pull of a region of dense rock will make satellites speed up. Over a region of lower gravity, they will slow down. By tracking the orbits of low satellites, geologists can build up gravitational maps of the Earth beneath.

When geophysicists compared gravity maps of the surface of the Earth with seismic tomography scans of its interior, they had a surprise. You might expect that cold, dense slabs of ocean crust would result in an excessive gravitational pull because of their higher density, whilst a plume of hot mantle rock rising upwards would be less dense and cause a gravity low. That reality is the opposite way around. The effect is especially pronounced over southern Africa, where a huge plume of hot mantle appears to be rising, and around Indonesia, where cold slabs are sinking. Brad Hager of the Massachusetts Institute of Technology came up with an explanation. The mantle super-plume under southern Africa is causing a huge part of the continent to rise up, higher than you would expect were it simply floating on a static mantle. Southern Africa, he estimates, is elevated by about 1,000 metres above where it would naturally float on the mantle, and this excess uplift of rock causes the gravity high. Similarly, the subducting ocean lithosphere beneath Indonesia is dragging the surrounding surface down behind it, creating a gravity low and resulting in a general rise of sea level compared to the land. Clement Chase, now at the University of Arizona, realized that other broad gravity anomalies corresponded to the ghosts of past subduction. A long band of low gravity that passes from Hudson Bay in Canada, over the North Pole, through Siberia and India, and on to the Antarctic seems to mark a series of subduction zones where ancient sea floor has plunged back into the mantle over the last 125 million years. What was thought to be a rise in sea level which submerged most of the eastern half of Australia about 90 million years ago may have been caused by the continent drifting over an ancient subduction zone that tugged at the region as it passed over, lowering land by more than 600 metres.





The core


We have no direct experience or samples of the Earth’s core. But we do know from seismic waves that the outer part of it is liquid and only the inner core is solid. We also know that the core has a much higher density than the mantle. The only material that is dense enough and sufficiently abundant in the solar system to make up the bulk of the core is iron. Although we do not have samples of the Earth’s core, we do have pieces of something that’s likely to be similar, in iron meteorites. Though not as common as stony meteorites, they are easier to spot. They are believed to come from large asteroids in which an iron core separated out before they were smashed by bombardment early in the history of the solar system. They are mostly made of iron metal but contain between 7% and 15% of nickel. Often, they have a structure of intergrown crystals of two alloys, one containing 5% nickel, the other about 40% nickel, in proportions that give the bulk composition.

An iron core must have formed in the Earth by gravitational separation from the silicate mantle when the new Earth was at least partially molten. As the layers separated, so-called siderophile elements such as nickel, sulphur, tungsten, platinum, and gold that are soluble in molten iron would have separated with them. Lithophile elements would have been held back by the silicate mantle. Radioactive elements such as uranium and hafnium are lithophile, whereas their decay products, or daughters, are isotopes of lead and tungsten so would have been separated out into the core at its formation. That consequently reset the radioactive clock in the mantle at the time the core formed. Estimates of the age of mantle rock put that separation at 4.5 billion years ago, about 50 to 100 million years after the ages of the oldest meteorites which seem to date from the formation of the solar system as a whole.





The inner core


The centre of the Earth is frozen. Frozen at least from the viewpoint of molten iron at the incredible pressures down there. As the planet cools, solid iron crystallizes out from the molten core. Present understanding of the electrical dynamo that generates the Earth’s magnetic field requires a solid iron core, but the planet may not have had one for its entire history. There is evidence of the Earth’s past magnetic field locked into rocks throughout the Phanerozoic. But most Pre-Cambrian rocks have been so altered that it is difficult to measure any original magnetism. So the only estimate of the age of the inner core comes from models of thermal evolution of the core as the Earth slowly cools. It’s the same sort of calculation that Lord Kelvin performed in the late 19th century to estimate the age of the Earth from its rate of cooling. But now we know there is additional heat from radioactive decay. The latest analysis suggests that the inner core began solidifying somewhere between 2.5 and 1 billion years ago, depending on its radioactive content. That may seem a long time, but it implies that for billions of years of its early history, the Earth was without an inner core and perhaps without a magnetic field.

Today, the inner core is about 2,440 kilometres across, 1,000 kilometres smaller than the Moon. But it is still growing. The iron is crystallizing at a rate of about 800 tonnes a second. That releases a considerable amount of latent heat, which passes through the liquid outer core, contributing to the churning of the fluid within it. As the iron or iron-nickel alloy crystallizes out, impurities within the melt, mostly dissolved silicates, separate out. This material is less dense than the molten outer core, so it rises through it in a steady rain of perhaps sand-like particles. It probably accumulates on the base of the mantle like a sort of upside-down sedimentation, collecting in upside-down valleys and depressions. There are seismic hints of a very low velocity layer at the base of the mantle that this upward sedimentation could explain. The sandy sediment would trap molten iron just as ocean sediment traps water. By holding iron within it, the layer provides material that can magnetically couple the magnetic field generated in the core with the solid mantle. If some of this material rises in super-plumes to contribute to flood basalts on the surface, it could explain the high concentrations of precious metals such as gold and platinum in such rocks.





Magnetic dynamo


From the surface, the Earth’s magnetic field looks as if it could be generated by a large permanent bar magnet in the core. But it is not. It must be a dynamo, with the magnetic field generated by electrical currents in the circulating molten iron of the outer core. Faraday showed that if you have an electrical conductor, any two out of electrical current, magnetic field, and motion will generate the third. That is the principle on which all electrical motors, generators, and dynamos work. But in the case of the Earth, there are no external electrical connections. Somehow both the currents and the field are generated and sustained by the convection currents in the core. This is what is called a self-sustaining dynamo. But it must have needed some sort of kick-start. Perhaps that came from the Sun’s magnetic field before the Earth had one of its own.

The magnetic field on the Earth’s surface is relatively simple, but the currents in the Earth’s core that generate it must be far more complex. Many models have been proposed, some of which, such as the idea of a rotating conducting disc, are purely theoretical. A model that best accounts for the field we see involves a series of cylindrical cells each containing spiral circulation produced by the combination of thermal convection and the Coriolis forces generated by the Earth’s rotation. One of the strangest features of the Earth’s magnetic field, as we will see in more detail in the next chapter, is that it reverses its polarity at irregular intervals, typically of a few hundred thousand years. At other times there can be periods of up to 50 million years without a reversal. Evidence of the strength of the field trapped in individual volcanic crystals suggests that the field might have been stronger than it is today during such non-reversing periods, or superchrons. The magnetic field is not precisely aligned with the Earth’s axis of rotation. At present, it is inclined at about 11 degrees to the Earth’s rotation axis. But it hasn’t always stayed there. In 1665 it was almost true north, then wandered off, reaching 24 degrees west by 1823. Computer models cannot explain it exactly, but suggest that the dynamo itself is fluctuating chaotically. Most of the time, coupling with the mantle damps out the effects, but sometimes they get so great that the field flips. What is not clear is whether the reversals happen virtually overnight or whether there are thousands of years in which the field wanders wildly or virtually disappears. If the latter is the case, it would be bad news not only for navigation by compass but for life on Earth generally as it would be exposed to more hazardous radiation and particles from space.



7. A possible model for the generation of the Earth’s magnetic field. Convection currents in the outer core spiral due to Coriolis forces (ribbon arrows). That, and electrical currents (not shown) produce the magnetic field lines (black arrows).

There have also been attempts to model what goes on in the Earth’s core by experiment. This is not easy since it requires a large volume of electrically conducting fluid circulating with sufficient velocity to excite a magnetic field. It was achieved in Riga by German and Latvian scientists who used 2 cubic metres of molten sodium contained in concentric cylinders. By propelling the sodium down the central cylinder at 15 metres per second, they were able to generate a self-exciting magnetic field.





Taking the Earth’s temperature


The deeper you go, the hotter it gets, but how hot is the middle of the Earth? The answer is that, at the boundary between the molten outer core and the solid inner core of the Earth, the temperature must be at exactly the melting point of iron. But the melting point of iron under those incredible pressures will be very different from its value on the surface of the Earth. To find out what it is, scientists must recreate those conditions in their laboratories or calculate them from theory. They’ve tried two different practical methods: one using tiny samples squeezed between diamond anvils, the other using a giant multi-stage compressed gas gun to compress samples just for an instant. Because of the difficulties in achieving such incred