Supercontinent: Ten Billion Years in the Life of Our Planet (13 page)

So in fact the winds within the cells spiral around inside them, like the rifling inside a gun barrel. And because these helical convection currents are wound in opposite directions either side of the Equator (coiling to the right in the north and left in the south thanks to the Coriolis effect), they give rise at surface to the famously reliable trade winds, beloved of sailors. The trade winds just north of the Equator blow from the north-east because the Coriolis effect deflects winds travelling south (completing their return leg to the Equator) to the right (i.e., the west). Contrariwise, below the Equator, the south-east trades blow from that quarter because these winds are deflected to the left (the west again) as they travel north.

At higher latitudes than the tropics, the surface winds of the second great cell blow from the south-west in the Northern Hemisphere (
bringing
Britain its rain from the Atlantic) because those convection currents become deflected to the right, veering westerly. Above the southern
tropics
, winds that would be blowing back towards the South Pole (i.e., northerlies) are deflected to the left, backing westerly.

What does all this mean for reconstructing vanished
supercontinents
? To some extent, no matter where the continents lie, the prevailing winds between the Equator and the tropics, and between the tropics and the pole, have always blown, and will always blow, in pretty much the same direction. These winds will be wet in the same places, and dry in the same places. Falling air will create high pressure; rising air will create lows. It’s simple – it’s physics.

The way in which the atmosphere then interacts with them creates
different environments, which the geologist can diagnose by looking at fossils, and the rocks that contain them, and by comparing this
evidence
with organisms and sedimentary environments around us today.

But then the distribution of land and sea comes into play, and snarls up this simple convecting system. Think of how, joining Laurasia, the northern landmass to Gondwanaland cut off the
equatorial
currents and plunged Gondwana into a deep ice age. The distribution of continents clearly affects the way the oceans’ gyres work, and in much more unpredictable ways than the unchanging and imperturbable Polar, Ferrel and Hadley Cells of the atmosphere. Moreover, the monsoon is entirely dependent on the distribution of land and sea, the heat differential between them, and seasonal
temperature
differences across the Equator. These elements prevent the atmospheric circulations from perfectly overlaying an unchanging pattern of climatic stripes upon the shifting continents.

Children’s encyclopaedias never fail to include an explanation of onshore and offshore breezes; the former created during the day when the sun heats the land, and the latter at night when the land cools off quickly, leaving the sea warmer. Monsoons are in a way similar, but writ large, operating at continental scales over annual rather than daily cycles, and large enough to disrupt entire climate zones.

The word ‘monsoon’ comes from the Arabic
mausin
, meaning ‘season’, and refers to winds that change from one part of the year to another. However, the common usage of ‘monsoon’ is for heavy rain associated with the summer monsoon winds of Asia, blowing ashore off the Indian Ocean.

In India, for example, monsoon rains arrive in early summer. The
winds blow onshore from the south-west, though these are actually the
south-east
trade winds of the subequatorial Hadley Cell, being pulled off course by hot air rising over the baking heart of India. The Himalayas and Tibetan Plateau intensify this process, by introducing hot air much higher in the atmosphere.

The moisture-laden winds, which otherwise would have made
landfall
in the Horn of Africa, find themselves yanked back on themselves (the Horn receives its rains either side of the summer monsoon season, when the trades go back to normal). As the winds rise up over India’s great plateau, they are forced to drop their moisture; but as the rain
condenses
out, a runaway effect is created because condensation releases yet more heat, the ‘latent heat’, which pays back the extra energy needed to evaporate the water in the first place (which is why fountains feel cool: the evaporation they induce absorbs heat from the surroundings).

To create a monsoon, therefore, all you need is a strong heat
contrast
between land and sea, and a source of moisture-laden air. At times in the Croll–Milankovich cycle when seasonality is most
pronounced
(say, a Southern Hemisphere Summer coinciding with the Earth’s closest approach to the Sun) any monsoon in that region will be enhanced. And that is the sort of cyclicity that geologists expect to find when looking at the rocks laid down through many such cycles over thousands of years.

Computer climate models for a Pangaean Earth show that by far the greatest portion of global rainfall in that time was convective, and took place at the Equator – and hence almost entirely over the ocean, Panthalassa and its reef-fringed embayment, Tethys. As the great ‘C’ of Pangaea drifted slowly northwards, coming to straddle the Equator more symmetrically, the huge baking landmasses now sitting just north and south of the Equator exerted a gigantic effect upon the distribution of rainfall.

Tethys, embraced by the supercontinent, was a warm ocean. An
equatorial current flowed directly into its maw, concentrating heat and nutrients gathered from Panthalassa and introducing massive amounts of moisture into the atmosphere above it. However, the huge land areas north and south of the gulf would have set up Northern Hemisphere Summer monsoons on Tethys’s northern coast, and a Southern Hemisphere Summer monsoon on its southern flank. Climate modellers believe that this effect dwarfed even the biggest modern monsoon, and have dubbed it ‘megamonsoon’. Also, Pangaean mountain belts were probably among the mightiest ever seen on Earth. Those bordering Tethys’s northern coasts would have
mirrored
the enhancing effect of the modern Himalayas on the modern Asian summer monsoon, making the megamonsoons even more so.

In conjuring these vanished worlds back into being in such great detail, geologists use two forms of uniformitarian reasoning. They project physical constants back into the past (adjusting for secular change, such as the Sun’s slowly increasing energy output) because physical laws do not change with time. And they interpret sediments in the light of what is known, by inspection, of sedimentary processes and environments around us today.

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.

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.