Geology on the habitable edge

kw: book reviews, nonfiction, geology, plate tectonics

A formative experience of mine took place in a wilderness area north of Twenty Lake Basin in the Sierra Nevada mountains. The second session of Summer Field Camp was held there, for six weeks. What could we accomplish there that could not be done in the suburbs? To do geology you have to go where the rocks are…that is, where it is easy to get to the rocks. Where I live, near the Pennsylvania-Delaware border, you'd typically have to dig or drill 50-100 feet to find anything approaching "rock" as we know it: The Columbia Formation consists of loose to poorly consolidated (that is, cemented) sand with sparse fossils of Cretaceous dinosaur bone. You need to go a lot deeper to get to actual "bedrock".

Thus, a dozen other Geology students and I spent half a summer in high mountains, among lovely scenery, because the bedrock, mostly granite and limestone, was right there at the surface. We could walk up to it and hammer off chunks to take back to the "library" tent and study. We were interested in the intersection between the limestone and the granite, studying "cooked" rocks called skarn.

To study bigger problems you need to go to places even more remote. In A Wilder Time: Notes From a Geologist at the Edge of the Greenland Ice, William E. Glassley weaves a narrative of discovery around four field seasons, each about a month long, along the Arfersiorfik Fjord in western Greenland. The camp area is at or near the little white arrow I placed at the middle of this image. The map pin is on Tunertooq Island, where significant evidence was discovered by the author and his colleagues.

He and two colleagues were working to gather evidence that the area had been a continental suture in the deep past, around two billion years ago. The deformed rocks in the area look very similar to other areas of mountain-building, but are so much older that some geologists wonder if it is possible. The short answer is, "hard rocks", what we call igneous and metamorphic rocks, form primarily when continents collide and thrust softer materials ("soft rocks" such as sandstone and limestone) deep into the crust and mantle. They are later brought to the surface by various mechanisms of plate tectonics, where erosion eventually exposes them.

Plate tectonics describes the movements of the crust of the Earth over time. The "plates" are large portions of crust, including thicker continental crust and thinner oceanic crust; there are 8 major plates and about 20 smaller ones. They are in constant motion, but the rates are slow and imperceptible without instruments: 10cm/year or less, averaging 4-5 cm/year. That is just slightly faster than the rate fingernails grow. But give it time: If the Earth had only two continents, and they had separated some time in the past and were moving first away from one another, at a rate of 5cm/yr each, but later toward another as they each circled halfway 'round, how long would it take until they collided? This is equivalent to asking how long it would take one continent to circle the Earth at a rate of 10 cm/yr. The circumference of Earth, 40,000 km, is 4 billion cm, so the time would be 400 million years. That implies that the crust beneath the oceans is formed and then consumed on a time span of a few hundred million years. Indeed, the oldest sections of oceanic crust are no more than 200 million years old (except for a small portion of older crust, ~300 million years of age, that was preserved in the Mediterranean basin).

New material is added to the oceanic crust of tectonic plates at divergent margins, AKA mid-ocean ridges. Iceland rides atop one of these ridges, which is why it is so volcanic. Where plates move toward one another, one or the other will be pushed downward and (mostly) consumed into the mantle beneath. Such convergent margins are also volcanic, such as the "ring of fire" around the Pacific Ocean. The volcanic activity is evidence of the energies involved in the convergence. Where the convergence brings together two major continents, you get mountain uplift. The Himalayas are still growing as India presses into the Eurasian plate. In the roots of mountain belts, remnants of the collided plates, including bits of oceanic crust, remain to mark the suture zone.

Greenland, where you can get to the rocks (most of it is under a mile or more of ice), has large areas of strongly folded rock, similar to that seen in the Alps, the Himalayas, and the Rockies. These are understood to mark the continental collisions that produce each mountain chain. The Appalachian mountains, including the area shown here in central Pennsylvania, are understood to be the roots of a mountain chain that stood tall 300 million years ago, but is now eroded to these remnants. By comparison, the Rocky Mountains were formed during the Laramide orogeny, between 80 and about 40 million years ago; the Alps began forming about 65 million years ago, and the process is presently winding down; and the Himilayas began forming about 40 million years ago. Each such mountain range has buried beneath it a suture zone where two continents collided.

The folded rocks in western Greenland are about 2,000 million (2 billion) years old. There is still some controversy among geologists about whether plate tectonics operated that early, or if it did, whether it worked the same way as it has in the past half billion years or so.

When I was a graduate student of Geology in the early 1980's, in one class we were asked how we would determine whether plate tectonics had operated in the early Precambrian, prior to about 1.2 billion years ago. I didn't do well on the assignment, and received my only C grade. Dr. Grassley and his Danish colleagues would have received an A+. They not only figured out how to do so, they went and did it, though it took a couple of decades. I think it no spoiler to report that the field seasons described in A Wilder Time led to a much better understanding that the Arfersiorfik Fjord area does indeed include a continental suture zone.

The book is in three parts, describing first the breaking down of old concepts, then accepting ignorance and becoming open to new ideas, and finally the beginnings of integration as a broader understanding emerges. The author stresses several times that our biology constrains us to awareness of only a tiny fraction of what the Universe has to offer. We "see" within one octave of a span of nearly infinite radiative wavelengths; we hear a wider range of sound frequencies, but most animals can hear sounds we cannot; we can bear only a narrow range of temperatures without severe damage; and so forth.

I was doubly compelled and fascinated by the book. I thoroughly enjoyed the geological material, of course. Even more, the author writes with a rare lyric intensity. He sparsely limns the emotions and impressions evoked by the harsh landscape. And sometimes it is not so harsh. One day he knelt and lay flat, to see the outline of a ptarmigan and her chicks hiding in plain sight atop the tundra:

I was suddenly awash in layers of sweet flower scents. As I rested lightly on the surface, the smell of dozens of blossoms I hadn't noticed engulfed me. Arctic poppy and white Arctic bell-heather were interspersed among mountain sorrel, hairy lousewort, purple saxifrage, and mountain avens. I was awash in a botanical sea, carried into an unexpected world.

Upon arising, he found none of the scents could be discerned more than a few inches above. He realized that the bird and her young would live among these scents:

A world of perfumes would cloak the hatchlings and saturate their feathers, becoming a sensory background to the birds' accumulating experience of living…

If "poets" who like to write free verse could write it like Dr. Glassley, I'd read more free verse.