A Crack in the Edge of the World
Page 8
The expedition that we took to the Blosseville Coast, as it was called, of East Greenland was just one such search for evidence: We were there to examine the basalts in a very specific way, to try to establish if there was indeed anything about their makeup that might suggest, conclusively, that their position had shifted in the 30 million years since they had poured and oozed and erupted out of the ground. So we spent several weeks taking core samples of the rocks, which we looked at back in the laboratories in Oxford. When we compared the direction of the magnetism in the tiny crystals of hematite, which had frozen themselves into the basalt like microscopic compasses at the very moment they were laid down, we proved, in what came to be our own private (if small) Eureka! moment in the affirmation of tectonic plate theory building, that the Atlantic Ocean had indeed become fifteen degrees of longitude wider at this point* during the 30 million years since the basalts were laid down. Greenland had moved westward during those years—whether it had shifted or drifted, been shoved or pulled, or accelerated away we could not tell. But those little magnets told us incontrovertibly that it had moved, and the way was now left open to others to explain why.*
There were other Greenland rocks nearby that were fascinating, too, and that remain a subject of much study still (in a way the basalts do not, as their story is now familiar to everyone in the geological community). These were located about a hundred miles farther south, close to the community of Angmagssalik: They are to be found in what is generally known as the Skaergaard Layered Igneous Intrusion, and constitute one of the more remarkable geological phenomena known.
When they were first spotted from the deck of a ship, all on board thought, seeing the horizontal layers that were such a prominent feature, that they were sediments, like sandstones, with bands showing traces of the history of the period when they were deposited. But when geologists clambered ashore they quickly discovered that the rocks were not sedimentary at all but in fact volcanic—which was shocking, to say the very least. And these volcanic rocks had the outward appearance of having been laid down, instead of having been spurted out of a volcano, which was something that had never been seen before. Fieldwork began in earnest, and eventually it was discovered that the layering had been the result of a pod, or a lens, of trapped volcanic magma having been allowed, thanks to a coincidence of unusual circumstances, to cool very slowly indeed—to cool so slowly as to allow all the heavier crystals and minerals that formed during the cooling to sink downward, and all the lighter minerals and crystals to float upward, until the moment when the rock froze, or solidified, and layers were locked into place for all eternity. There are, for example, places where crystals of chromium salts have floated down to one particular place in the pluton—the tear-shaped body of rock, a mile or so in circumference—to form a layer of chromium so solid and metallic and thick that when hit by a hammer it rings.
Geologists from the world over visit the Skaergaard—and its two sister intrusions, one known as the Stillwater, in western Montana, and the other in the bushveld in South Africa. To see it is a compulsion common to many, to be there, to marinate oneself in its millions of unsolved chemical and physical mysteries. For an igneous geologist—the kind of scientist whom I have long thought of as an inquirer into a science of an elemental purity, a figure largely unbothered by the baser concerns of commerce—there is perhaps no more hallowed a place than the Skaergaard. Reputations have been won and lost there, theories have advanced and retreated like the glaciers that spill down to the sea. To the outside world the place is little known (and it was very little known indeed back in the sixties), but today if one whispers names like Kangerlugssuaq and Basistoppen and Aputiteq and Mount Wager (named after the Oxford professor, mine for a while, who first worked on it) in certain corners of university common rooms around the world, men in beards and wearing oiled-wool sweaters and corduroys will nod knowingly and smile.*
Curious and interesting though the Skaergaard is, its relevance to the North American Plate and, by extension, to the mechanics of the San Francisco Earthquake might seem just a little tenuous. It is not tenuous at all, however, and for two reasons.
The Skaergaard, it turns out, sits at the base of a four-mile-thick layer of basalts; during the millions of years that the Atlantic has been opening, untold trillions of cubic feet of lava have spewed from vents in what was then the center of Iceland, depositing on each spreading side layers of basalt that are miles and miles thick. But it is not quite so important for this story to know what lies on top of the Skaergaard. More important is what the intrusion itself lies on—and it so happens that it rests on top of two layers of rock, each of which, crucially, is of much, much greater age.
It sits first on a massively thick layer of sediments, filled with fossils, that was laid down in Cretaceous times, as much as 145 million years ago. These Cretaceous sediments in turn sit on a basement of rocks—highly contorted schists and gneisses, formerly made up of an entire spectrum of more recognizable sedimentary and igneous rocks but now hopelessly contorted under the influence of intense pressure, extreme heat, and eons of time—that are so ancient that some scholars of radiochemical-dating techniques believe they are the oldest in the world. It was the rocks of the Isua Formation in West Greenland that had, until 2002, been positively dated at 3,500 million years old, thus claiming the record; the fact that more recent studies have said a group of rocks at Porpoise Cove on Hudson Bay in Canada is 350 million years older still—before that the earth seems to have been an inchoate blob of hot gases and slithery supermelt—does not make the central question posed by the rocks’ existence any less relevant.
And that question is: How is it that such very old rocks lie cheek by jowl with an array of rocks of relatively recent age? What mechanism allowed the world’s youngest country of any extent—which is what Iceland is—to stand directly beside an island that has within its geological suite some of the oldest rocks on the planet? How did it happen—and why, for that matter?
THE WORLD BEYOND UR
Ever since 1915, when the quietly charming and tragically misunderstood German Alfred Wegener—a meteorologist, an explorer, and a pipe-smoking theorist of the first water—proposed the idea that the continents had not always been where they are today, there has been a grudging familiarity abroad with five hitherto entirely unfamiliar words: Panthalassa, Gondwanaland, the Tethys, Pangaea, and Laurasia. All of these words entered the language at Wegener’s urging, though the first had been invented in the late nineteenth century by an Austrian polymath named Eduard Seuss, who thought continents floated and sank, popping up out of the abyssal dark on the command of some heavenly genie. Seuss, regarded by his supporters as an early seer of the science of geology, wrote an impenetrable four-volume book, The Face of the Earth, setting out his theories.
Wegener’s basic notion, vaguely familiar in a back-of-the-mind kind of way today, held that there had at some stage on our planet been the one huge supercontinent, Pan-gaea, which was surrounded by the vast proto-sea, Pan-thalassa. Part of this continent then foundered or split up or in some other exotic way broke apart to leave behind an immense scattering of bodies of land, of which Gondwanaland*—an assemblage that contained today’s India, Madagascar, Australia, southern Africa, and Patagonia—is the best known, together with a sea that surrounded the remaining continental islands and that Seuss was moved to call, after an ancient Greek sea giant, the Tethys.
This basic model has survived ever since. The birth of the plate tectonic theory has done little to dampen the enthusiasm for the story of Pangaea, and it is now reckoned with some certainty that such a supercontinent did indeed exist between about 200 and 300 million years ago. Its plates did indeed break up more or less as Wegener surmised and, in doing so, formed mountain ranges and oceans, many of which still exist today. Movement is still going on: The Atlantic widens, Australia shifts northward, plates hurtle slowly toward Alaska, the Pacific folds and buckles itself beneath California. This much is certain: Its processes are c
omplex and its effects dramatic and often catastrophic.
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 m
et 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.