A Crack in the Edge of the World (11 page)

But what of the time
before
Pangaea—what of the time before the cutoff period of 300 million years that is commonly ascribed to the
convection-current-ordained fragmentation of the Pangaean supercontinent? How does one account for the existence and the placing of rocks that are very much older than this—such as the ancient rocks of Greenland and on the shores of Hudson Bay? How do juxtapositions such as those found in Angmagssalik and inland from the Blosseville Coast—rocks not 50 million years old sitting atop rocks that are more than 3,000 million years old—all fit into the picture?

The pace of geological research is alarmingly impressive these days, and it may well be that by the time this appears in print new knowledge will have been uncovered and new models drawn. But since the end of the last century information has come to light that now suggests the existence of a great extended family of supercontinents that popped into being long before Pangaea, and that these very ancient rocks were part of it. An entire new taxonomy has had to be invented, and names have been given to an entirely new gathering of bodies that is believed to have existed in the world long, long before the making of the fragments of today's plates. Parts of those early worlds exist as ancient echoes in today's Greenland, lying deep below the rocks that more modern processes have created to lie above them.

It is now possible to imagine in increasingly realistic terms what took place at the very beginning of the planet's history. It all used to be the purest of speculation; now it has a growing ring of authenticity.

First things first. It is generally accepted that the earth formed when, under the bonding influence of gravitational force, an enormous liquid or solid mass coalesced out of a myriad aggregation of space-borne components that were drawn together some 90 million miles from the star we call the sun, some 4,550,000,000 years ago. The very hot liquid-metal core and the hot liquid-plastic mantle of metals and silicate magma became, in due course, differentiated from each other, also under the pervasive influence of gravitational pull. And, after about 100 million years of gradual cooling, something approaching a stable and solidified scum formed on the surface of the still-boiling or simmering planet. (The scummy part of the crust that is involved in tectonics is these days more generally called the lithosphere, and the plastic layer at the top of the mantle, the part that lies
above an important line that is recognized only by the high priests of physics, is known as the asthenosphere.)

It was at this point in the planet's history that the earth's eggshell-like crust, which was slowly forming on the surface from this cooling scum, began to stop doing what up to that point it was prone to do, and that is to keep on remelting itself. For eons it kept sinking back into the mantle just a few millennia after it had formed, utterly wrecking itself in the process—and then it would pop up out of the molten ocean of lava and be reborn in a totally different guise. Instead, all of a sudden, large chunks of crust were staying afloat, more or less permanently. In cooling, the crust was forming itself into rocks that would themselves be permanent—if only the external forces permitted them to remain at the surface and did not try to drag them or push them down toward the heat again.

As they slowly cooled, some of these rocks-to-be separated themselves out, according to perfectly understandable laws of physics: The lighter materials of the scum rose to the surface, the heavier ones passed downward in one enormous fractionating column—a little like the Skaergaard, though over infinitely longer periods of time and under very different physical conditions. The lighter materials generally formed themselves into those rocks we now call granites—the coarse-
grained
rocks that tend to be prettily light in color as well as in constitution. The heavier fractions created layers of rocks like basalt and diorite and gabbro, which were darker and tended to sag downward under the force of gravity, forming sloughs, whereas the granites tended to form uplands. The darker and heavier slabs lay sluglike and low on the earth's surface, and in time they began both to accumulate and to accommodate water that fell from the skies; over many millions of years, this resulted in the creation of oceans. Dark rocks underlay the seas; granites made up the new continents. And this law of basic igneous geology has remained a verifiable truth ever since.

The new crust, as it spread and wafted itself around the surface of the sphere, also became cracked, as cooling crusts of clinker and furnace slag are wont to do, and the plates, or rafts, or slabs of floating or sagging clinker that were then formed between the cracks began to
swirl about, thanks to the currents of terrifyingly hot material that were (as they still are today) upwelling and sinking back underneath. No doubt the slaggy scum came under the influence of other forces: There was gravity, there were great gyrations in the planet's magnetism, there was its spinning motion, the occasionally too-close-for-comfort proximity of the moon and other planets, and the tilting and wobbling of the earth's own axis of rotation. The third planet from the sun, it must be remembered, is in geological terms a comparatively small ball of material, subject to all manner of kinetic and thermal influences; and the first continent-in-the-making was turned this way and that for millions of years, as it struggled gamely to get a grip on itself and remain more or less in place on the ever-changing molten mantle that underlay it.

And finally, about 3,000 Ma,
*
it emerged as a fully fledged entity, sizable and solid and stable enough to be given a new name, to be classified as something else entirely. Enough of the crustal material floating about had now gathered itself together. Scores of islands of granitic material, floating on a basement of darker rock, had agglomerated, like raindrops on a windshield, to produce ever-larger accretions, which themselves met and married and did so again and again—until, midway through that period of geological times now called the Archaean, they combined to form one very large body, one covering sufficient of the earth's otherwise still-molten surface to be classified as, and called, a continent.

Those with fanciful imaginations might say it was shaped rather like a bird, an albatross with outstretched and enormous wings. It was small, compared to the immensity of the earth—it seems to have been
about 5,000 miles from birdlike wingtip to wingtip and maybe no more than a thousand miles from north to south across the thickness of its bird body. It seems to have lain close to the notional equator of the early earth, a little to the western side of where the meridian would eventually be.
*
And then it broke up, and its granitelike rocks were, in the fullness of geologic time, scattered to the four winds; they have since spread themselves liberally all over the planet. Gigantic amassments of rock from this strange little protocontinent are visible, and perfectly recognizable, in places like Zimbabwe, southern Australia, central India, and Madagascar.

This frail-looking, tiny, and delicate thing is in truth the
fons et origo
of everything that is solid and habitable about our earth today. Which is why, when it was named, shortly before the twentieth century was ending, it did not take much of an effort of mind or spirit to decide to christen it, most appropriately, the continent of Ur.
^†

However, the supposed creation of Ur prompts a question: If Ur is 3,000 million years ago, how can it be that the rocks of Greenland and around Hudson Bay are 3,500 and 3,850 million years old respectively? Why were they not a part of Ur?

The answer is that for some reason—and at the time of this writing it is still an unfathomed reason—the first aggregation of small pre-continental bodies occurred in the planet's Southern Hemisphere. Since the current configuration of the world has by far the greater amount of its landmass to the north of the equator, this presents something of a poser. But the evidence is unassailable: It is very clear that a number of modest-size rock masses had formed and become more or less stable in a variety of places around the world; in particular,
some 850 million years before the formation of Ur, there was a significant quantity of small bodies of granite floating around in the Northern Hemisphere, thousands of miles away from where Ur would eventually form. But none of these northern bodies had met up with any of their neighbors and massed to form a continent-size body. That did not happen north of the equator until about 2,500 Ma—500 million years after Ur was created in the south and fully 1,350 million years after the rocks themselves had been created out of that crystallizing and fractionating column of cooling magma.

The first continent that is believed to have existed in the north has in recent years been christened Arctica. It is a gathering of granite islands that includes the very rocks of Greenland and Hudson Bay that we have been considering; it includes also a vast amount of material of what would later become Siberia; it has a smaller body of very old rock that in billions of years would become Wyoming; and it enfolds hundreds of thousands of square miles of what would later be northern and northwestern Canada.

That said, it needs to be noted that a small plume of national chauvinism intrudes at this point into the story. Canadian geologists have long claimed that this conglomeration of granites and other very old and stable “shield” rocks that exists in the Canadian North and Northwest was already large enough by 2,500 Ma to be called a continent in its own right. The most typical granites of this region occur in and around Kenora, a town on the Lake of the Woods close to the border between Ontario and Manitoba. Much earlier research showed that the Kenora series of rocks displayed evidence of a major episode of ancient mountain building, a so-called orogeny, which had taken place all over Canada, as well as in Wyoming, the Dakotas, and the Outer Hebrides of Scotland (geology knowing no national boundaries, of course, and the distances between these “places” of yesterday having no relation whatsoever to the distances that we know of today: Wyoming and Scotland lapped up so close to each other then as to be one place, making the very concept of “place” more than a little surreal). In recognition of the importance of the Kenoran rocks and the Kenoran Orogeny, Canadians have proudly christened the huge body
that they suppose to have existed Kenorland,
*
and they think of it as having a presence quite as valid and provable as that of Ur and of Arctica. Non-Canadians are not so sure, however, and wonder whether it is much more than a piece of an enormous and very ancient jigsaw puzzle.

The world became steadily more complicated as time wore on. There were two further coalescences about 500 million years later on, when the continents now known as Baltica and Atlantica emerged, also then in the Northern Hemisphere. Baltica held much of what is now northern Europe as far south as today's Ukraine. Atlantica, on the other hand, encompassed what is today's West Africa, Congo, Guyana, Brazil, and the region around the river Plate, all landmasses that would eventually shift south of the equator.

The world was now possessed of four continents—or five, if Kenorland is counted—Ur, Arctica, Baltica, and Atlantica, which sound as if they come from the title of a short story by Borges. It seems that these bodies, all of them massive, cool, and quite stable platforms, probably represent the totality of continental material that would be on the fledgling earth for a very long while.

And, having been fully made, these bodies then began a complex dance—a dance that, in the phrase of some fascinated geophysicists who are brave enough to mix metaphors, seems an accordion-like process, in which continents clang into one another, sometimes joining up, then separating, most colliding and separating once again for the remainder of their existence, right up to the present day. Knowledge of what happened and the sequence in which it did so is still ragged, and since the creations that resulted from the marriage of the various bodies all have new names, too, the whole
megillah
adds up to a delicious tectonic confusion. In essence, though, it seems to have unrolled itself approximately thus:

First, about 1,800 million years ago, or maybe a little before, Arctica (which, it will be remembered, contained Siberia, Canada, Greenland, a bit of Wyoming, and the majestic monster called by some Kenorland) collided with Baltica, which held a great deal of what is now far northern Europe down to the Ukraine, and the two subsumed for good measure a small and noncontinental part of what is now the Antarctic. The resulting supercontinent has been called Nena (or Nuna to some, who claim it as an Inuit word that means “the land around us,” as in the recently formed Canadian province of Nunavut). Nena then collided with and promptly gobbled up Atlantica and became a truly vast body, which its namers (Americans) have seen fit to call Columbia (or, according to some others, Hudsonland). This enormous continent enfolded all of the world's continental blocks for about 400 million years, then broke asunder once more around 1,300 Ma, whereupon its parts—behaving for the next 300 million years or so with the accordion-like back-and-forth squeezing and pulling apart that seems to have marked the world's early progress—in due course collided with a re-formed Atlantica and a reinvigorated Ur and created yet another extrasupercontinent that has been called Rodinia, after the Russian word for “homeland.”

Rodinia then stayed stable, despite being so immense, for the next 300 million years—from about 1,000 Ma to 700 Ma, or the period known as the late Precambrian, when there was a fairly healthy amount of multicellular life to be found on the planet (fossils of which are scattered about the world's outcrops today, in patterns that indicate where Rodinia had been stitched together). But then, and one hesitates to say
once again
, it fractured into a number of slightly different constituent parts—Laurentia, East and West Gondwana, bits of Ur, bits of Atlantica, all of which careened across the millions of square miles of crust before finally reassembling themselves into yet another body, which has been named Pannotia (which essentially means “all southern continents”). Another breakup, another reconciliation—and then, with some parts eventually drifting back together about 550 million years ago, and after a quarter of a billion more years of miniseparations and microdivorces, all were rejoined in an unholy tectonic
matrimony almost exactly 280 million years ago, forming what we now know, with all the familiarity of its relative chronological closeness to us (a mere 250 million years, after all), as Pangaea.