A Wilder Time Page 14

  View through a microscope.

  Zircon is a mineral mainly composed of zirconium, silicon and oxygen. It is a resilient phase, stable at most temperatures and pressures experienced by rocks in the middle and deeper levels of Earth’s crust. It, too, is tough—it can travel tens or hundreds of miles in streambeds, pounded on and scraped by boulders and cobbles, resisting abrasion through the uniquely powerful bonds of its crystal lattice.

  Because of its unique combination and arrangement of the atoms that determine its structure, zircon also readily accepts traces of uranium, which virtually all rocks possess. This simple fact makes it one of the most profoundly important minerals for reconstructing the history of the crust, especially in places where tectonic activity has heated and compressed the rock under multiple extreme conditions. The uranium in zircon slowly decays radioactively at a constant rate, disintegrating into lead, thorium, and helium that accumulate over time. Hence, the ages of rocks can be determined if the concentrations of these elements are measured—zircon as geological clock.

  To obtain the zircon for dating, part of the sample must be crushed and sieved until the minute zircon crystals can be separated out. A suite of the grains is then mounted on an epoxy disk, polished so the interiors of the grains are exposed, and then analyzed. Inevitably, complexities arise. When the zircon crystals are viewed at high magnification, it is immediately obvious that they are often not homogeneous. They commonly contain zones that look much like tree rings, which surround the central interior core of the crystal—a record of changing conditions in which new zircon grew over old. Often the thickness of these growth bands is only a few millionths of an inch or less—these currently remain unreadable and cannot be dated.

  But even though we are not capable of resolving the finest growth rings in zircons, our analytical techniques applied to broader bands allowed us to obtain hundreds of individual dates, making it possible to construct in more detail than ever before the evolution recorded in the landscape’s bedrock.

  SOME OF THE SAMPLES WE SELECTED for dating were from the flowing, plastic forms we found at the eastern end of Tunertoq Island. Poring over the data with colleagues, we discovered how unexpectedly old some of the rocks were. Cores of numerous zircons turned out to be nearly 3,400 million years old, older than virtually the entire terrain on the other side of the shear zone. This implied that an ancient continental body extended farther north but not to the south. As such, those rocks defined one boundary of the vanished ocean.

  Around those old cores were younger zircon rings, many dating an event about 2,750 million years ago. That age corresponds to a major convulsion seen in old continents around the world, as well in the rocks south of the shear zone. The significance of this remains unclear, although it seems to indicate a time at which much of the mass of continents on the planet bubbled out of the mantle. Such evidence establishes a correspondence between what was occuring in this terrain and processes happening elsewhere—a common denominator demonstrating that this part of Greenland is typical continental crust.

  Cutting through these old rocks was a dike with homogeneous zircons that formed 1,805 million years ago, which turned out to be a defining time for the region—the time in which the continents were colliding.

  Farther to the east, the rocks forming the huge igneous massif recognized by Kalsbeek and his coworkers in 1987 are now well dated, based on zircons we collected as well as those of a similar age obtained by a few other researchers. The measured ages prove there was active plate movement and Andes-like volcanism throughout that region between 1,875 and 1,980 million years ago.

  An active volcanic system that lasts for 100 million years helps constrain, through simple arithmetic, the size of the ocean that must have been consumed. We know that today the rate at which ocean crust descends into subduction zones is usually between one and five inches per year. If we assume a low rate for the convergence of the plates back then, the amount of crust consumed would be about three thousand miles, which is almost the distance between New York City and Lisbon, Portugal. In other words, the ocean that the pillow basalts floored may have been about the size of the North Atlantic today.

  But is the age of the pillow basalts consistent with being part of an ocean basin that was active between 1,875 and 1,980 million years ago? Using the same analytical techniques, we dated the pillow basalts and found them to be at least 1,895 million years old, which shows they likely were part of that long-vanished ocean floor.

  Other samples collected in the shear zone where deformation and metamorphism were concentrated provided a variety of ages, but all of them within the range of 1,720 to 1,820 million years ago. This 100-million-year duration for the complete mountain building cycle is consistent with what is known for similar types of collisional mountain-belts, such as the Himalayas and Alps, both of which are still active and have millions of years to go before they cease. The Himalayan system began forming sixty million years ago, the Alps are at least thirty million years old.

  Before the collision. At about 1,890 million years ago, continents that were involved in the collision, as well as the subduction zone and volcanic system that were the main active plate tectonic elements at the time, would have been arranged, schematically, as shown. The ultrahigh-pressure (UHP) region would have been just below the area where the rising magma bodies are coming off of the down-going pillow basalts and peridotites, and the high-pressure (HP) rocks would have been above that region.

  A new interpretation. A schematic drawing of a cross section through the mountain system that formed when the collision of continents had nearly finished, about 1,720 million years ago. The arrows indicate movement along major faults that were rooted in the shear zones we now recognize. The darkly shaded continents would have been joined together before the collision. The oldest continental rocks that compose the northern continent are to the left of the Nordre Strømfjord shear zone. Metasediments and other rocks are indicated by the thin wavy and folded lines. This figure is a modified version of a model originally drawn by Kai Sørensen.

  A time line of the most significant events preserved in West Greenland geology. The ages are mainly based on dates obtained from zircons. The length of each bar encompasses the majority of ages that define each event. The activation of the Nordre Strømfjord shear zone (NSSZ) took place during the last few million years of the collision. For reference, the earth formed about 4,560 million years ago, and the oldest preserved continental fragments on earth are about 4,100 million years old.

  The ability to date the mineral grains in our samples provides a chronology upon which the journey of a rock can be detailed. Using that approach, a three-dimensional model through time can be constructed for the history of the terrain.

  The singed-hair rock that we examined with microscopes and discovered was filled with garnets and olivines and spinels contained a startling history of burial at a depth of at least forty miles, an HP metamorphic environment. Up to that time, none of us had imagined that any of the rocks in this region had traveled more than fifteen miles down. We wrote reports and published papers and looked at more samples in the basement archives of Aarhus University, seeking confirmation that such rocks were not enigmatic anomalies.

  The UHP samples were found during those explorations in the archives. For months, we examined thousands of samples that had been collected over decades by a small cohort of faculty and students working on master’s and doctoral degrees on Greenland geology. Out of all those samples, we found two that preserved evidence of the same very deep burial. The samples came from sites tens of miles farther to the west of where we had been working, but along the same belt of unusual rocks, and along the northern edge of the Nordre Strømfjord shear zone. The samples from both of those sites had identical characteristics. One sample, ironically, had been collected by Kai when he and Fleming Mengel, a student of his, had worked in the region nearly forty years before. Kai didn’t remember collecting it. The other sample came from a site near Gie
secke Sø and had been studied in the late 1960s by Steen Platou, who was working on a graduate degree at the time. Those samples became the core of a small collection that proved that fragments of the region had, indeed, been pushed to extraordinary pressures, surviving a round-trip circuit to depths greater than 150 miles. They have become the oldest known samples that preserve evidence of travel into a deep subduction zone, the place where the ocean floor descends hundreds of miles into the mantle when tectonic plates meet. Prior to these discoveries, no direct evidence existed that such plate tectonic-driven processes occurred any further back in time than 900 million years ago. These samples pushed that age limit back to at least 2,000 million years.

  AT THE TIME WE DISCOVERED THE SAMPLE from Steen Platou’s field area, he was a retired farmer, living on the outskirts of Århus, Denmark. We visited him at his farmhouse, went over his notes and maps, and talked to him about what he remembered of the place. Eventually, we all realized the only smart thing to do was go with him to his old field area. So, in the summer of 2012, we returned to the place Steen had explored on his own, an area no one had visited since he last worked there in 1969. He was now in his early seventies. We spent days slowly walking through the terrain, with him as our guide. He smiled often, smoking his pipe, obviously enjoying the return to his old haunts. One afternoon toward the end of our stay, he proudly pulled up his shirt to show us that his belt no longer fit—he had lost so much weight hiking miles in that wilderness that it was too big and there were no holes left for the buckle clasp.

  Steen died a few months after his trip with us. He was happily working on the maps we needed to assemble our data when he suffered a stroke. The samples he had long ago collected, and the ones we collected with him on our last trip, formed the core of evidence documenting a unique history. The data and samples thoroughly supported the argument of Kalsbeek and his coworkers that there once had been a volcanic system on top of a Greenlandic version of the Andes.

  We found the suture zone that demarcated the boundary between continents and the remnant pieces of the ocean floor that had once separated those landmasses. Through that work, and related studies by others, the Nagssugtoqidian shear zone is unequivocally established as the last major deformation at the end of the grinding collision between ancient continents, a fault system much like those active systems emanating from the Himalayas today. And contained within this system are the rare vestiges of rocks carried 150 miles down into the mantle and returned to the surface. They are the oldest known record of an entire terrain on the surface of the world that had descended to such depths—the exposed remnant of the earliest known plate tectonics and subduction. It was Steen who had found them.

  WE RIDE THE BACK OF A TIDAL FLOW, coursing among the rocks along the shore, which is glittering in the rainbow light of comb jellies. John turns us farther out into the current. The gneisses and schists, indeed every lithic form cradling the flow, sing of a past we revel in. In the wake of our boat is the shape of the future. The flexing pontoons respond to each crest and trough, slowing, accelerating, a brief sideways shove, a chance to splay water.

  Since our last expedition, polar bears have been sighted in our field area in the summer—a place they never were before. To satisfy the needs of our funding agencies, we would have to carry rifles for our protection if we were to return when they are there.

  In my mind I again see the dissolving landscape of tundra tufts resting on boulders in that encroaching bay from years ago. I watch reindeer bones decay, ice melt, new surfaces appear. Despite its inevitable dissolution and change, what wildness remains will forever quietly, irresistibly beckon.

  SETTLEMENTS AT THE EDGE OF WILD PLACES are the punctuation points in nature’s wilderness stories. By providing a human element, they shape emotion and responses, affecting the texture of place. Because they are at the edge of what is inhabitable, they define what it means to exist in harmony with untouched landscapes; the wisdom contained in such places is deep.

  Early one morning, John and I walk the streets of Aasiaat, looking for the house of one of the people who will help with our logistics. He is Inuit, an older resident whose home is on a small knoll overlooking Disko Bay. Draped on the roof is the drying pelt of a reindeer he recently killed; hanging from the second-floor window frames of the modest house are strips of curing reindeer meat. Huskies are tied out back to their respective little houses. The sled they will pull in the coming winter stands upright next to them, its gracefully curving white runners arcing toward the sky.

  As we approach the front door, a strange sound moves through the air. Multitoned, the pitch slowly changing from low to high and back again, the song roils up from the bay. I turn and look out over the ice-studded water, but all I see is white-speckled blue, a shimmering, placid reflection of sky. Just as we get to the front porch, the bay surface erupts in three massive waves, out of which the huge gaping mouths of humpback whales slowly rise. The sonorous noise has stopped, replaced by the sound of rushing water draining through baleen. They are feeding, the song the mechanism they use to herd and concentrate the small sea life they live on.

  When we finish our business, we head back to the seamen’s hotel we are staying at to meet up with Kai. The road we walk goes by the beach that runs along the small harbor. We stop at a small cluster of white canvas booths where fish and seal meat are sold by a few of the local fishing people. As we look over the flatfish, fjord cod, Arctic char, and some fish I do not know, a small outboard roars into the harbor, throttles down as it approaches the beach, then slowly glides onto the sand. A large man in yellow chest-high waders steps out of the boat and hauls out long, thick strips of deep red seal meat. We watch him bring them to one of the booths and negotiate with the Inuit woman standing behind a table. After a brief exchange, she makes room for his goods among the display of fish she has laid out. He heads back to the little boat and returns with slabs of blubber. Once he has been paid, he walks back to the skiff, pushes out into the bay, and pulls the chord on the outboard engine. As it roars to life, he, standing, slowly maneuvers it through the moored boats in the harbor, then opens the throttle and roars away, disappearing behind a headland.

  This is a traditional scene, an old way of commerce and sustainable coexistence with wildlife and wild land that has changed little over hundreds of years. But the catch of cod is diminishing, whales are harder to find, the migratory routes of the reindeer are more difficult to discover, and the seal population is now out of balance with its ecosystem. What was once a strenuous but consistent existence is now threatened.*

  But this is not a situation unique to Greenland; on every continent, wilderness is being consumed, and the people who have depended on it, living at its fringe and within its embrace, are forced to relinquish what they cherish. With infinite hubris, the modern world is imposing the consequences of its industrial avarice on lifestyles it knows nothing of. The moral bankruptcy of the rationalizations for the destruction of wilderness and the people who live in harmony with it is staggering. That many are, in fact, angry and seeking ways to mitigate impacts is heartening, but the pushback is formidable. The moral outrage we all should feel seems meager against the economic juggernaut.

  Compounding the consequences of this economic monstrosity is the diminished role wilderness has in our everyday lives. Seldom is it mentioned on the news, rarely is it considered in politics, and its presence on social media is almost nil. In his profoundly influential “Wilderness Letter” in 1960, Wallace Stegner wrote:

  [Wilderness] is good for us when we are young, because of the incomparable sanity it can bring briefly, as vacation and rest, into our insane lives. It is important to us when we are old simply because it is there—important, that is, simply as an idea.

  The message of this letter is withering, but its urgency is more powerful today than ever.

  Humanity is grounded in community, which requires cooperation and shared experiences. As politics and economic self-interests bulldoze through the wo
rld and wilderness recedes, we risk losing access to our own essential wildness. Whether through direct experience or through poetry, art, or song, we must share and celebrate the wild so that it may be saved. The lives lived there—of all species—are worthy of our recognition and respect, the land, our awe, art, and dreams.

  *F. Karlsen, 2009. Management and Utilization of Seals in Greenland. The Greenland Home Rule Department of Fisheries, Hunting and Agriculture. 28 pages.

  Glossary

  anorthosite: A rock formed from magma that is composed of small amounts of orthopyroxene and mainly contains plagioclase, a mineral rich in calcium, sodium, aluminum, and silicon. It is believed to be a common rock at the bottom of continents.

  Bugt: A Danish word for an ocean bay.

  country rock: The main bulk of lithologies that compose a terrain. Also used to describe the rocks into which magmas invade.

  defile: A valley or pass in a terrain with significant topographic features.

  fjord: An inlet, often with high, sheer bounding walls, formed where a glaciated valley is invaded by the sea.

  foehn: A strong, warm wind that forms on the downslope side of a topographic feature. Originally applied to meteorological phenomenon in the Alps, the name is now also applied to winds that form on the downslope side of major ice sheets, such as the Greenland ice cap.

  fractionate, fractionation: A process of separation. In scientific applications, the term is usually applied to situations in which one material, such as a solid or gas, separates out of another material, such as a liquid.

  gneiss: A metamorphic rock that has experienced high temperatures and pressures and contains layers of differing mineralogy. A gneiss usually displays banding seen as different-colored layers. A gneiss can be formed from virtually any kind of rock (igneous, sedimentary, metamorphic) that is sufficiently heated and sheared.