Supercontinent: Ten Billion Years in the Life of Our Planet (25 page)

When geologists hit upon the notion of constraining their dreams of the past in terms of processes observed operating today, it made geology a science. Although debate has raged ever since about whether those processes always operated with the same
intensity
at all times in the long history of the Earth, the method still held together. But it only held for those youngest, lightly scraped and overwritten pages of the great palimpsest that was open to geologists of the nineteenth century and much of the twentieth: namely, the 542 million years since the beginning of the Cambrian Period, when the age of animals (and eventually plants) with easily fossilizable bodies dawned, heralding complex life’s long march – or random walk – through increasing complexity.

But with radiometric dating came the shocking realization that this segment only represented about the last 12 per cent of Earth history; and that, in the conventional stratigraphic tables of the time, the tiny unregarded plinth of complex rocks labelled ‘Precambrian’ on which the geological column rested, actually contained nearly all the time that had elapsed since the planet formed. It was like digging a well,
only to find what you had thought to be solid bedrock giving way into a black, bottomless, unsuspected cavern, loud with a vast and terrifying silence.

 

An old-fashioned stratigraphic table, dating from 1898. The basal part labelled ‘Precambrian’ actually contains 88% of Earth history. Taken from Charles Lapworth’s
Intermediate Text-Book of Geology
(Blackwood & Sons). From the collection of the author.

As geologists now looked for ways to decipher the rare and often badly damaged pages of the Precambrian chapter, they began to realize something else deeply shocking. They began to see that there were things in the deepest places of Earth history for the unlocking of whose secrets the present no longer provided the key. True, up to a point the old tools still worked; after all, a poorly sorted conglomerate full of mud and cobbles and boulders the size of a man were still probably dumped by glaciers. (However, the same difficulties and controversies would attend their interpretation in the late twentieth century as in the mid-nineteenth, the only difference being that now the arguments were more sophisticated.)

But the problem went deeper than just interpreting the meaning of particular rock types. The Precambrian world that the old tools and other new tools such as isotope analysis revealed was not the familiar, endlessly cycling Huttonian or Lyellian world, ringing to what Thomas Hardy described as ‘the full-fugued song of the universe unending’: a world with no beginning, offering no prospect of an end. By contrast the vast spans of Precambrian time were dominated by progressive, secular processes that had wrought permanent change upon the evolving Earth system.

The more geologists thought about it, the more reasonable this began to seem. Just like the life of a human being, Earth’s growing years left their indelible marks upon her; and yet despite her difficult early traumas, by middle age she was leading a much more stable, routine, almost (but not quite)
predictable
life. She had reached a time in her life in which it was almost impossible to conceive of things ever being that radically different. Indeed, if things were so radically different then from now, perhaps they were
too
different for geologists to build a scientific picture of the Precambrian world? If the rule of the present could no longer be used to measure out the ancient world, what price the scientific method? How could a geologist’s imaginings of these very different times be constrained?

But all was not lost. Geologists turned first to the immutable laws of physics and chemistry; and in addition they found something new: the emerging techniques of computer modelling. Using these new approaches, John Sutton and Janet Watson’s dream of opening up the Precambrian gradually came to be realized. It was the final confirmation that the uniformitarian visions of Lyell and Hutton did not, and could not, do full justice to Earth’s chequered past. Moreover, as the idea that human activity might be affecting the Earth System became familiar to followers of current affairs, the whole question of how the climate works (a question rooted in how it evolved into its current state since the birth-time of the Earth) lent Precambrian geology a sudden relevance, even urgency. The purest of pure science, this expedition to an alien planet whose curiosity-driven mission directive had been drawn up without the slightest idea of practical application, suddenly moved politically centre-stage. For the tale of the Precambrian has proved to be a litany of terrible climate disasters, all of them brought about – or at least hastened – by life itself, and the Supercontinent Cycle.

Erasmus Darwin (1731–1802), Charles’s eccentric, versifying, visionary ancestor, in his epic poem
The Temple of Nature
wrote: ‘Organic Life began beneath the waves.’ In 1871 his grandson, on the other hand, would speculate, in a letter to the Director of Kew Gardens, the explorer-botanist Sir Joseph Hooker:

So who was right? Did life start in the seas, or in a little lightning-struck dish of lukewarm primeval soup? Today’s leading theory about how life came to planet Earth suggests that the older Darwin, on this occasion, came nearer the mark. Life, it now seems likely, originated not in a superficial pond, but deep below the waves on the gloomy floor of the Earth’s early oceans. What is more, it did so long before even the continents were fully formed and set sail across the globe. It isn’t only Hugh MacDiarmid’s stones that are ‘one with the stars’: life is, too.

The Earth began to accrete from a disc of space debris around 4.7 billion years ago, in a hellish birth-time colourfully referred to by some geologists as the ‘Hadean’ eon, though this name is not recognized by the International Commission on Stratigraphy, the body that decides such things. It prefers instead the rather more prosaic name ‘Eoarchean’ for the earliest, bottomless section of the Earth’s life story: the true and final plinth of the modern stratigraphic column below which there was simply no Earth.

As it vacuumed space debris orbiting the young Sun, the Earth gradually heated up. Gravitational energy from incoming bolides was converted into thermal energy. Deep within, the iron and nickel in the mixture separated out from the molten rocky materials and sank into
the planet’s core, where it still remains, an eternal but querulous dynamo driving (and occasionally flipping) the Earth’s magnetic field. Up above, and for perhaps as long as 500 million years, our planet was a cratered, volcanic spaceball, sporadically molten, dark, sterile; blasted by solar wind, flayed by ultraviolet light, too hot for oceans, too hostile for life.

Although Lord Kelvin was quite wrong about how old the Earth could be because he assumed it had just cooled by radiating heat into space from its original molten state, it is true that our planet was a very much hotter place four billion years ago. This was partly due to bombardment and gravitational heating; but it was also due to the much greater abundance at that time of highly radioactive isotopes.

Remember that all radioactive decay series eventually end up in stable isotopes, or at least in longer-lived and much less radioactive ones. Shortly after the solar system formed and the rocky planets coalesced from space junk, Earth’s nuclear reactor burned much hotter than today, just as radioactive waste, which will eventually become harmless, is at its hottest when it is fresh.

Because the young planet needed to dissipate maybe five times as much internal heat, the mantle convection systems deep below its crust would have been smaller and more active than today’s. Therefore, with greater production of volcanic material at surface, and faster movements among smaller plates, the number of spreading and subduction zones would have been greater. Moreover, the crust that they formed (and consumed) would also have been much thicker: somewhere between twenty-five and fifty kilometres. This has led geologists like Eldridge Moores to ask when this non-uniformitarian change from thick to thin oceanic crust took place, what the environmental implications of that change would have been, and whether it happened gradually, or more suddenly.

The crust of the newly accreted Earth would have been everywhere of the same composition (roughly speaking) as modern ocean floor, simply because this is the basic stuff of the Earth. Lighter rocks, which float high as the continents of today, had to be distilled from that crude material by the fractionation of lighter elements. Thus the silica and aluminium compounds, identified as ‘SiAl’ by Eduard Suess, had to be separated from the heavy ones, made predominantly out of silica and magnesium (‘SiMa’). So the continents cannot always have been the same size as today: they had to grow. The oceans, too, may well have been more voluminous than today because the hotter mantle could contain less water, chemically locked up in minerals, than it can today. Some scientists think there may have been twice as much water at surface, making the early Earth a truly panthalassic water-world.

An example of how the process of continent formation might have started can be seen today on Iceland. By processes of partial melting and melt extraction in the system of conduits under the island, magmas of granitic (classic SiAl) composition are being formed. This is why, although Iceland is always portrayed as a black, basaltic place befitting its position on a mid-ocean ridge, in fact up to 10 per cent of its rocks are of light, granitic type.

Iceland sits astride a hot, hyperactive stretch of the Mid-Atlantic Ridge, and has been forming for a mere sixteen million years or so. Just like the early crust of the Earth, because the amount of volcanic material erupted there is higher than average, Icelandic crust is twice as thick as normal ocean crust, which all helps the lighter ‘continental’ type rocks to separate out as the cooling magma circulates in the plumbing below. Because of their lower density, sialic rocks – once created – would then remain at the surface, gradually coalescing as they jostled and fused by continuous minor collisions forming the
protocontinents
.

As the Earth cooled further, oceans began to condense, hydrological cycles of evaporation and precipitation began to operate and the erosion and deposition of sedimentary rocks could really begin. The earliest evidence of erosion comes from rocks over four billion years old, in the form of those amazingly robust and persistent microscopic mineral grains that John Joly saw at the centre of his pleochroic haloes: zircons.

As we have seen, when rocks undergo melting, and elements of differing atomic weight separate out between solid and liquid phases – a process called fractionation – different isotopes of the elements (despite their identical chemical properties) behave differently according to their slightly different weights and measures. Some prefer life in the melting pot, while others tend to remain in the solid. If continental crust is continually fractionating from the Earth’s primitive material, isotope ratios within the different rocks generated will gradually come to differ from average, or ‘bulk Earth’, values. Thus, even in one single, tiny grain of zircon, distinctive fractionated isotope ratios remain as the telltale signature of early crustal processes that generated suites of continental rocks of which those tiny crystals are today the only surviving remnants. Truly, a whole world in a grain of sand.

Mark Harrison at the Australian National University in Canberra and his co-workers have been studying zircons that were eroded, reincorporated and then sealed within younger rocks about 4.4 billion years ago. These rocks come from the Jack Hills in Western Australia, and themselves constitute one of the oldest pieces of continental crust on the planet. Harrison and his colleagues have tested these grains for two isotopes of the element hafnium (Hf ).

The zircons contain very low concentrations of another element, lutetium, whose radioactive isotope
¹⁷⁶
Lu decays to hafnium; so the researchers believe that the ratios of
¹⁷⁶
Hf to
¹⁷⁷
Hf which they find
in these grains are very close to the primitive ratios that prevailed when they formed – that is, at the original zircon-containing rock’s absolute age, determined independently using uranium-lead dating methods. Those ratios are characteristic of fractionated sialic crust. What this suggests is that continents were not only forming, but even being eroded and their detritus redeposited, within as little as 200 million years of the Earth’s accretion: that is, between 4.4 and 4.5 billion years ago.