Supercontinent: Ten Billion Years in the Life of Our Planet

by Ted Nield

  Working from several lines of evidence (including fossil magnetism in rocks, fossil animals and sediment types), geologists can determine where all the broken bits of Pangaea used to be and how they fitted together, giving a broad outline of the supercontinent. On to this outline, the ancient topography (young, high mountain belts like the Urals, older ones like the Pennines, the basins and plains) can be plotted. Those fossils and sediment types that give firm indications of climate – so-called ‘climate proxies’, such as glacial deposits or coals – can then be added to the picture. If the geologists have plotted and interpreted the rocks and fossils correctly, if the assumptions made about them by analogy with modern sediments and living things are correct, if the palaeomagnetists have got the continents in the right place, if the modellers have understood the palaeoclimate correctly, and if the computer model is truly reflecting the way energy balances between land and sea and the way oceans and the atmosphere exchange heat and moisture, then everything should fit perfectly and make sense. Needless to say, it rarely does, and this is what keeps it interesting.

  To objectify the process of deciding if the distributions really do make sense – to make it more ‘scientific’ – modellers compare the geological evidence (often referred to as the ‘ground truth’) with computer predictions generated by (more or less) the same sort of computer models used every day to generate weather forecasts. These massive programs attempt to mimic the complexity of the Earth’s climate system by breaking the hydrosphere and atmosphere down into layers and the geography of the Earth into manageable pixels 1.25 degrees square. With a supercomputer doing all the calculations, they re-create ancient water temperatures, winds, evaporation, cloud cover, storminess, snow depth, soil moisture, hurricanes, monsoons. The lost continents of science are brought to life partly in machines.

  Energy Balance Models look at the land–sea distribution and solve the thermodynamic equations that can give some idea of how hot the land was relative to the sea at different times of the year. Climate-predicting programs are called GCMs, General Circulation Models, and combine fluid dynamics with ancient geography to simulate the climatic response to the energy balance. GCMs that try to simulate the atmosphere are called AGCMs, while OGCMs treat the circulations of the ocean. In recent years these have been brought together in coupled ocean-atmosphere circulation models (OAGCMs). Researchers can tweak the parameters of these models – for example, to take account of the Sun’s lower energy output 250 million years ago, or to allow for different mixes of gases in the air at different times in the Earth’s history. They keep tweaking until the model matches the evidence – or exposes anomalies that merit closer inspection.

  Model predictions of the Pangaean megamonsoon have one major thing in common: all predict strong seasonality on and around the northern and southern shores of Tethys. And seasonality is something that geologists can look for, because pronounced seasons leave behind patterns in sediment sequences. Also, plotting particularly climate-sensitive rock types on a reconstructed map of the supercontinent will produce a pattern that – if the monsoon phenomenon is real – will not be perfectly zonal, as might result from the atmospheric cells alone. The monsoons will perturb this pattern, and the zones will depart from perfectly paralleling latitude.

  In orbit over Pangaea

  So, as the waters seep in, the Zechstein Sea fills and the drowned dunes release their sudden frothy exhalations, let us avoid the possibility of being surprised by a gorgonopsian, the top predator of its time, and soar through the air in which no bird has ever flown, up through the circulating atmosphere to the edge of space, and look down upon the latest (but not the last) supercontinent.

  Below the bands of cloud, Pangaea sits within the globe like the ‘C’ in a copyright symbol. The curve of land encloses a great sea – an inland ocean, the Tethys – whose east-facing opening to the global ocean Panthalassa is partially obstructed by a number of small island subcontinents covered in dense jungle, much like Borneo or Sulawesi today. One day these microcontinents will drift north and collide with the northern limb of Pangaea to form much of what is now China. But for the time being they are the only major land areas not accreted to the supercontinent, which is just now at or about ‘maximum packing’. Mountain building is still taking place along the northern shore of Tethys, which is fringed by a long mountain range created by the subduction of Tethyan ocean floor and the occasional accretion of those small continental fragments, waiting like ships outside harbour. This line of mountains already includes the older, northern ranges of the great mountain belt most people refer to collectively as the Himalayas: the Tien Shan and Nan Shan mountains.

  The northern limb of Pangaea, stretching from Siberia, the Urals, Europe, Greenland and North America to the future Pacific rim, is known as Laurasia and was formed when the Ural Mountains were raised in the collision of North America with Europe and Siberia. This towering young belt bisects Laurasia north–south, to the northern Tethys shore. To the west the Hercynian (and beyond them the older Caledonian) mountains stand proud; but they are much older and (thanks to millions of years of erosion) already less pronounced. Between these ranges a finger of sea reaches south from the Boreal Ocean and feeds a growing area of water, the Zechstein Sea, that will soon spread south and east to cover much of future central Europe, bringing moisture to the heart of the great northern deserts. On the other side of the Pennines another inland sea, the Bakevellia Sea, fills a basin that mirrors the shape of the modern Irish Sea.

  From its desolate western shore, the lone and level sands are interrupted only by the Appalachians, the US continuation of the Caledonian range, before vast stretches of sandy and rocky desert extend for thousands of kilometres towards what is now much of central and western America, where shallow ephemeral shelf seas and massive reef complexes mirror, on a much larger scale, what is happening in northern Europe.

  To the south, where the supercontinent narrows to its equatorial waist, the Hercynian mountain system cuts inland, rising to four kilometres above sea level and marking the suture between the continental blocks of North America and North Africa. The range separates Laurasia from Pangaea’s southern lobe, Gondwanaland. At its western end, where it meets the longitude-parallel Panthalassan coast, it turns south, defining the coastline of the future South America: the early Andes. At their southern tip these mountains touch also the Cape of South Africa, skirt Antarctica, and run up the western coast of Australia before coming to an end on the south-eastern extremity of Tethys, and so completing our round-Pangaea trip.

  Gondwanaland is a much more ancient entity than Laurasia, and many traces of the older suturing events that brought it together can be seen in the remnants of much older mountain ranges, one of which runs between eastern South Africa, Antarctica and the eastern coast of India, passing through ‘Gondwana Junction’, where those three future separate continents now touch. There, in 250 million years’ time, when the old sutures have opened up again, thousands of miles of ocean will have squeezed into the crack, and the Vivekananda Memorial will stand on a rocky islet of charnockite at India’s Land’s End, staring out across the sea to its vanished neighbours.

  Forests of the polar night

  As Pangaea has moved steadily north through the Permian, the South Pole has all but slipped into the sea. The great continental icecap that had existed for many millions of years since the Carboniferous has finally melted away completely, releasing the last of its cargo of mud and boulders the size of men. Unique to Gondwanaland, dense forests of Glossopteris trees, standing up to twenty-four metres tall, the shape of Christmas pines and growing a thousand to the acre, fringe the southern coasts of Tethys and stretch inland to within twenty degrees of the pole.

  These forests of the polar night withstand two seasons: one of feeble light and one of unremitting dark. Today’s world has no equivalent of this eerie ecosystem. Their growth rings show that each summer these trees grow frenetically. Those nearer the coast are lashed by megamonsoon ra
ins roaring in from Tethys, the thick cloud further weakening the feeble sunshine raking the latitudes at the bottom of the world. And as the brief growing season comes to an end, and the orbital progress of the leaning Earth draws the sun in its undulating course daily closer to the horizon, the tongue-like leaves turn wild and fall on thick beds of countless others on the sodden forest floor. The sun dips further, finally no longer peeping above the ending line, and all growth ceases for six months without prospect of a dawn.

  Leaves that will one day lie fossilized beside the frozen body of Captain Scott fall into the anoxic peat. The great Permian coals store up the Sun’s ancient energy like a battery, waiting for release in power stations and steel mills.

  These coal-producing forests occupy a climate zone designated ‘tropical everwet’ and, according to the occurrence of coals at this time, this zone extended from about midway along the southern coast of Tethys, across the island archipelagoes standing in the great gulf’s mouth, to the northern shore’s eastern promontory, and then back west, ending not far short of where the Ural mountains join the coastal cordillera. Oil source rocks and coral reefs cluster here, bearing testimony to the high organic productivity of the Permian tropics.

  Around the reef-fringed Tethys, only rarely does this everwet zone give ground, and then mostly to ‘tropical summerwet’ conditions that also prevail across the mountainous mid-section of equatorial Pangaea and extend only a little way east along each shore of the great embayment.

  Tropical summerwet is too dry for coals, and none is found today in places that were once situated here. But coal can, and does, form beyond the everwet tropics. It is a common misconception that all coal forms in steaming swamps like the Amazon or Congo basins of today. The main requirement for coal formation is a high water table that prevents plant matter from decaying. So, if that can be combined with high productivity of plant matter, coals can also form in cool and warm-temperate climate zones. This was particularly true over Gondwanaland, clothed with its unique Glossopteris forests, growing amid the lakes and valleys of the sodden, recently deglaciated southern continent. But plant remains (not abundant enough to make coals, though significant enough to create tantalizing ‘floral localities’) also extend around the shallow seas running south from the northern Boreal Ocean – like the Zechstein.

  Despite these exceptions the main signature of Pangaea is one of almost unremitting aridity. The continent is too large for the moisture of the oceans to reach its interior; the late-Permian atmosphere, richer in carbon dioxide by perhaps five times the modern level, holds in the heat of the weaker Sun. In the parched heart of the northern and southern lobes of Pangaea summer temperatures soar over 45°C, while at polar latitudes they fall in winter below –30°C. South of the equatorial mountains, salt flats, gypsum playas and dune fields link the west coast of Pangaea, across the whole of the landmass that is now split into North America, North Africa and Arabia, to the southern shore of Tethys. North of it, desert; from shining, reef-fringed western shores bordering Panthalassa, all the way to the towering Urals – interrupted only by ephemeral, evaporating seas, recently filled and soon to teem with rich, spiny shellfish adapted to their bitter, hypersaline waters.

  Further north, around the Boreal Ocean and its embayments, under the northerly storm track, the prevailing westerlies bring moisture in from Panthalassa, just as they do today from the Pacific to the boreal rainforests of moss-curtained pinestands of Washington State and British Columbia. But that moisture is soon spent and cannot penetrate far inland, so these conditions give way rapidly eastward to cool and finally cold temperate zones along the chilly, but ice-free, roof of the end-Permian world; a silent tundra shore, where soon some of the largest volcanic eruptions in Earth history will devastate hundreds of thousands of square kilometres, burying them in kilometres-thick lava piles and nearly bringing the whole story to an end, even before the first dinosaur stalked the planet.

  World reborn

  Since Alfred Wegener first pieced it back together in 1912, Pangaea continues to be reborn in the minds of Earth scientists – and their computers – as a living and ( just about) breathing world; a unique place with many lessons to teach us about how our planet’s climate works. It is the supercontinent about which we will always know most, because it is not long gone; its sediments are everywhere; our modern oceans contain a magnetic road map that helps us reconstruct it from its shattered remnants. Pangaea gave us much of our coal; Tethys laid down most of our oil and gas; evaporites formed in its shelf seas gave us nearly all our salt, on which almost all our chemical industries were established. The Zechstein Sea even gave us the fabric of the Palace of Westminster, and the treacherous landscape of Ripon. It even gave us dinosaurs, Alice and the rabbit hole.

  Pangaea was the first lost supercontinent that actually existed to be imagined by the human mind. In one sense, it is still and always will be a fantasy; but one constrained by uniformitarianism – not Lyellian, but one that truly takes full account of the importance of the rare event in geological time. Thus the human imagination is held within the fruitful confines of method. You can see the effect of this in every academic reconstruction of Pangaea. Ever since Wegener himself, whose didactic purpose made them necessary, the outlines of the present continents are always made clear, embodying the claim of this supercontinent to its objective reality.

  But what of older supercontinents? What of the supercontinent that broke up to give us Pangaea? And the one before that? Compared with Pangaea, those lost worlds seem truly lost. As with all geological evidence, the older it is, the less of it survives, the more mangled it has become and the harder it is to interpret. It is all but impossible to picture them – to see oneself standing on them – as you can with Pangaea. They have their magical names, which lend them reality of a sort despite the fact that, for some, even their very existence remains controversial. About Rodinia, Pannotia, Columbia, Atlantica, Nena, Arctica or distant Ur, the mists of time gather ever more thickly.

  7

  WORLD WARS

  At a specified time the earth can have just one configuration. But the earth supplies no information about this. We are like a judge confronted by a defendant who refuses to answer, and we must determine the truth from circumstantial evidence …

  ALFRED WEGENER

  Freikörperkultur

  A white-backed vulture, circling high over the empty central deserts of Namibia one day in 1940 would have seen something odd going on at the bottom of one of the rocky gorges of the Kuiseb River, which drains the Khomas Hochland west of Windhoek. Unusually for a Namibian river, the Kuiseb does not peter out into the Namib Desert but runs into the South Atlantic at Walvis Bay, the major port along Namibia’s beautiful but forbidding Atlantic coastline.

  There has been a river valley at Kuiseb for perhaps thirty million years, though the present gorge is as little as two million years old, dating from the beginning of the last Ice Age, when global sea levels fell dramatically. Today, for most of the time, there is no water in the Kuiseb River, but the size of some boulders, the smoothing of the rocks high along its banks, or the occasional telltale tangle of logs and brushwood, lodged way up in the cliff, speak of the river’s terrible force when rains finally come to Khomas Hochland. Yet, luckily for the game, and the occasional bushman trekking by, water often persists in isolated pools on the valley floor, even through the dry intervening months and years.

  At any time such water holes might play host to a troupe of zebra, encircled by a cloud of hoof-kicked dust; or to a lone gemsbok, his dark, ribbed horns sweeping upright as he dips his head to drink in the still heat. Light-coloured tracks lead off from the pool in all directions. All around the water the trampled dust lies like buff flour. There is no patch that does not bear a hoof print. An animal reek, from the spoor and urine with which the visiting beasts pollute their source, hangs in the air. But on that day in the first year of the Second World War, the hole has human visitors.

  Two naked young Ge
rman geologists are wading through its tepid, greenish water, catching carp with a makeshift fence-net made out of two bed sheets sewn together with a pair of string underpants and stiffened with tamarisk twigs. One of these intrepid hunters is Dr Hermann Korn; the other – owner of the sacrificed pants – Dr Henno Martin. Both are on the run.

  The two fugitives, who had both studied in the ancient University of Göttingen in Germany, had rejected the rise of fascism in their own country and emigrated to the protectorate of South West Africa, a former German colony. They earned their living on water exploration projects and together got to know the geology of this remote country. But not remote enough; before the growing tide of war, the two men, fearing internment by the South African Mandatory Government, hatched a plan. They would escape, to live a Robinson Crusoe existence in the desert they had come to love; and on 25 May 1940, with a stolen truck full of essential provisions, their dachshund Otto, an air rifle, a pistol and some ammunition, they set off for the wilderness.

  Their desert sojourn, a constant battle for survival fought over water, food and the many dangers of the desert and isolation, lasted two years. It was described in a book known in English as The Sheltering Desert, which Martin wrote for his wife and published in 1956, ten years after Hermann Korn was killed in a road accident.