Supercontinent: Ten Billion Years in the Life of Our Planet (28 page)

How thick the ocean crust can be is governed by the volcanoes at mid-ocean ridges. The more active they are, the thicker the resulting crust (remember Iceland). On the other hand, the thickness of
continental
crust is a product of mountain-building processes, combined with the innate strength of continental rocks, which imposes an upper limit. When continents collide they build thick crust that extends both down into the mantle and up into the atmosphere. But there is a point above which mountains cannot rise, set by the limit of the rocks’ mechanical strength. Beyond a certain weight and height, mountains are not mechanically strong enough to support themselves.

Bradley Hacker, Professor of Geological Sciences at the University of California, Santa Barbara, recently spent time investigating the Tibetan Plateau (which dates back 13.5 million years and is like a ‘bow wave’ to the collision of India with Asia). The Plateau affects weather worldwide. It plays a powerful role in creating the monsoons of India and Asia, for example, and has a global cooling effect on climate that may have helped tip the world into its current ‘icehouse’ regime shortly after it began to rise.

Although his was not the first contribution on this subject, what
Hacker confirmed was that although the crust thickens in the area of the collision, after a certain amount of thickening it weakens and spreads apart. He reported his findings in the journal
Nature
in 2001: ‘Consider stacking pats of butter on top of one another. Imagine that stacking each pat … also generates heat, so that a thicker stack of butter is hotter than a thin stack.’ In the case of the Earth, heat generated by radioactive decay within the rocks builds as the crust piles up, making the thickened crust weaker. Ultimately the rocks reach a certain maximum height and begin to flow outward. As Hacker concluded, ‘There is a balance between the strength provided by the thickening of the crust and the weakness caused by heating from all that material.’ The Tibetan Plateau is in a steady state. Currently standing at five kilometres tall, it will not get any higher.

For similar reasons of dynamic balance (and not including their very earliest days) the continents’ average thickness has not changed very much through most of geological time. However, a pre-Rodinian world with much thicker
oceanic
crust would have been a very different place. All the ocean basins above the thick crust would have been much shallower. The difference between average continental height and average ocean floor depth, or ‘continental freeboard’ as it has been called, would have been much lower than now. Just as
The Book of Urantia
appears to have ‘predicted’, one billion years ago does indeed seem to have been an ‘age of increased continental emergence’. Until the worldwide orogeny that created Rodinia, the amount of continental crust that poked above sea level would have been much smaller than it has been since. This is especially likely when we realize that, the mantle being very much hotter, more of the Earth’s total water would have been in liquid form.

But as Rodinia formed, things were changing. The oceanic crust, responding to falling mantle temperatures, began to approach present-day thickness (about six to seven kilometres). Increased continental
freeboard exposed more rock to the atmosphere, with a resultant increase in weathering on land. The chemical breakdown of rock materials sucked yet more carbon dioxide out of the atmosphere as it was converted to bicarbonate and carried away into the oceans in solution.

Because there was now more exposed land, seasonality also became more important on Earth than ever before, because land areas are much more susceptible to seasonal variation in the power of sunlight. Greater seasonality, combined with the return of more nutrients to the seas (as a result of enhanced weathering) further improved the oceans’ organic productivity, which in turn led to even more carbon dioxide being swabbed from the atmosphere (just as even more algae could trap even more of it in even more lime mud).

The Earth’s climate was reaching a threshold: a ‘tipping point’. Rodinia’s eventual break-up, on top of all these cooling factors, would precipitate the greatest climatic catastrophes ever to afflict our planet.

Within this cooling world Rodinia seems to have sat astride the Equator, leaving the planet’s poles free of land, a rather rare event in Earth history. The stage was set. Rodinia gave way to the radiogenic heat building up beneath it, and started to fragment. Massive igneous provinces erupted, their dust and ash blocking out heat from the Sun, which by this point in its evolution was about 6 per cent weaker than today.

Supercontinents are arid because moisture cannot reach their interiors; but on smaller continental blocks this situation is reversed. After supercontinent fragmentation, more rain tends to fall on more land, and rock weathering speeds up. Because the continental fragments were then sitting entirely within the tropics, weathering rates were particularly high. What is more, the newly erupted basalt provinces were especially susceptible to chemical weathering.

So, as more rocks were weathered, even more carbon dioxide was removed from the atmosphere and delivered to the seas. The length of
coastline increased, as did the area of shallow shelf sea, providing even more habitat for stromatolite-forming algae to colonize.

As these progressive effects took hold, they drove the climate into colder and colder territory. Icecaps began to form and expand. Normally, when icecaps expand over continents lying at high latitude, they exert a negative feedback on the process because they cover up more rock, preventing weathering and leaving more carbon dioxide in the air to keep the planet warm. But that didn’t happen. With the continents far away from the poles, no brake was applied. As the icecaps crept Equatorwards to within about 25–30 degrees of latitude, they passed a point of no return. Earth was doomed to ten million years of icy stillness.

This point came because, at a certain coverage of brilliant ice and snow, the amount of heat reflected back into space became so high that the cooling process was unstoppable. The Earth system had no choice, the cooling effect had nowhere else to go but completion, encasing the whole surface of the planet in ice. This was how Lasagne World gave rise to Snowball Earth.

Once the planet was encased from pole to pole, the Earth system was frozen, literally and figuratively. Apart from rare nunataks of rock, the tallest mountaintops poking above the endless ice plain, the whole sunlit globe shone in only two colours: blue above and white below. On Iceworld there was no evaporation and no clouds anywhere. The hydrological cycle, in which water evaporates from the sea to be deposited as rain and snow on land and so returns in rivers to the sea once more, became restricted to the precipitation of the small amount of water that would ‘sublime’ from the ice surface: in other words, go straight from ice to vapour. And beneath their icy carapace, the seas became stagnant as interchange between them and the atmosphere ceased.

Yet deep below, far underneath ice, oceans and rocky crust, churning away irrespective of the catastrophe at surface, Earth’s planetary heat engine rolled on, driven by the imperative to dissipate its radiogenic heat. Beneath the sealed seas, volcanically active spreading ridges pumped their acidic, superheated, mineral-rich waters into the frigid water; while above the ice, volcanoes that poked their hot heads through the frozen veneer spewed their gases into the atmosphere.

And there they stayed. No rain washed these gases out of the air and into the seas, and carbon dioxide began building up. It is a fairly safe uniformitarian assumption that Neoproterozoic volcanoes gave off at least as much gas as volcanoes today. From this it can be calculated that after a snowball event lasting ten million years, levels of carbon dioxide in the atmosphere would have risen possibly as much as a thousandfold. Earth’s inner fire was about to save the world from the reign of the ice.

The end would have come suddenly. As the greenhouse effect kicked in, temperatures swung wildly upwards, to perhaps as much as 50°C at the ocean surface. Evaporation began again, further enhancing the greenhouse, since water vapour is one of the most powerful greenhouse gases of all. The water cycle now went into overdrive; torrential rainfall washed carbon dioxide out of the atmosphere, creating acid rain that landed on the newly exposed land surface (strewn with glacial rock flour) and so dissolved its minerals even more quickly. The reborn rivers returned huge quantities of bicarbonates to a sea already saturated by ten million years’ worth of volcanic carbon dioxide pumped into it by submarine volcanoes.

Massive limestones were then deposited in shelf seas worldwide: seas that were also progressively deepening as the liberated water of the ice sheets filled up the ocean basins. Carbonates precipitated out on the seabed, depositing thick limestones directly above glacial deposits, with no sign of a time-break. Some of these limestones
contain crystals of the calcium carbonate mineral aragonite, which normally only precipitates today from supersaturated pore-fluids as microscopic, needle-like crystals. Aragonite crystals that formed on the Neoproterozoic seabed took on gigantic dimensions as giant fans, some as tall as a man.

The ice-break of a snowball would have been Earth’s most dramatic spring. The gradual release of the last Ice Age’s grip 10,000 years ago must have been as nothing to the chaos that prevailed as the Neoproterozoic icecaps retreated. Persistent winds of over 70
kilometres
per hour blew over much of the Earth’s surface as a result of the vast air-pressure differences between the thawing tropics and the polar caps. These winds were not passing storms but lasted for years, causing huge oscillation ripples to form in sediments accumulating as deep as 400 metres: nowadays far beneath the reach of even the biggest storm waves. The ocean-surface waves generated by these global hurricanes probably exceeded seventeen metres in height, over huge tracts of sea. Below, meanwhile, the dissolved volcanic iron built up during the snowball finally oxidized and precipitated, and Banded Iron Formations suddenly made a comeback after a hiatus of a billion years.

Can all this really have happened? The geological evidence – glacial deposits at low latitudes, directly overlain by limestones, and the reappearance of BIFs – all fit the hypothesis. Moreover, they make sense of phenomena that have long been seen as ‘anomalies’, giving the Snowball Earth model immensely persuasive explanatory power.

As early proponents of continental drift had found, ‘old-fashioned’ geological evidence is qualitative, and often open to several possible interpretations. Powerful hypotheses, especially when they are
‘non-uniformitarian’
in the sense that they have no precise modern analogue on the planet today, are at once exhilarating and disturbing.
Doubters mutter that a powerful ruling theory is driving interpretation. Could the whole snowball story be no more than a ‘selective search after facts’? What of ‘multiple working hypotheses’?

We have already seen some of the many different isotopes of carbon, the element of life, and how their different atomic weights lead them to behave differently in the natural environment. Carbon 12 is the common form, but there is an unusual heavy isotope, carbon 13, which gains its extra unit of mass by having an extra neutron in its nucleus. The carbon that comes out of volcanoes (mostly as carbon dioxide) contains both isotopes in a well-known ratio. But any carbon that has been involved in
life
processes has a different signature, because photosynthesis, where everything begins, prefers carbon 12. Living tissues therefore contain lower than average amounts of
¹³
C; but conversely, limestones (principally calcium carbonate) that form at times when life is thriving have above average values of
¹³
C because they are made out of the carbon left over in the environment after life processes have taken their share.

If you test the carbon-isotope ratios in limestones immediately underlying and overlying the snowball’s glacial deposits, remarkable changes are revealed, bringing impressive support to the snowball model. When pre-snowball limestones were laid down, life was thriving; so
¹³
C values begin high but drop steeply as the contact with the glacial deposits approaches. This says that life was shrinking back with the onset of snowball conditions, leaving more
¹³
C around in the seawater to be incorporated in limestones. For ten million years or so that the snowball lasted, no limestones were laid down. However, the first limestones deposited after the snowball – the ‘cap carbonates’ – remain low in
¹³
C because life had yet to recover. Then, gradually,
¹³
C values rebound as resurgent life in the recovering shallow seas fractionated ever more
¹²
C into living tissues.

Between 710 million and 580 million years ago, the snowball cycle
happened twice, possibly three and (some say) even four times during this never-to-be-repeated interval in our planet’s history, as the supercontinent Rodinia split apart. So why did they stop happening? Why did snowballs not happen, for example, during our most recent glaciation, commonly known as ‘the’ Ice Age, which ended between ten and twelve thousand years ago?

The reason lies in the fact that, unique among supercontinents, Rodinia seems to have straddled the Equator, meaning that the world had no land at either pole. Never has that coincidence of low greenhouse gases, weak Sun and tropical concentration of landmasses come about again. Since the end of the Neoproterozoic, despite ice ages aplenty, none has ever gone to anything approaching a snowball. Since the break-up of Rodinia, the safety catch has been back on. Iceworld was finished.

In structuring the story as I have, however, I have taken one particular route through a mass of scientific evidence treading a line of stepping stones across a torrent of argument. The Snowball Earth hypothesis remains controversial and contested.