In dawn’s early light it was dim and cool on the porch. But my usual stroll across the lawn to the edge of Grand Canyon was blocked by four large animals grazing, as if the lawn was theirs not mine.
Words cannot express the human reaction to Grand Canyon, which slashes this way and that, almost drunkenly, for 277 miles across a region that late in the game people named Arizona.
The early morning air on the canyon’s South Rim, 7,000 feet above sea level, is dry, fresh and cool. The few of us out to watch the dawn are reverent. The weak light filtering across the eastern horizon gradually whitens the sky. In front of us, over the little wall along the rim, the vast inky depths are still in night. Of course the sun has risen every day during the 6 million years it took for this chasm to be carved by the river that humans later named the Colorado. But to me this dawn seems fleeting and personal.
The El Tovar Hotel, with this porch and fine overlook, was built in 1905 when the Atchison, Topeka and Santa Fe Railroad extended the first spur to Grand Canyon. But from nowhere along the canyon’s rims, not even here, can one get a full view of “Earth’s greatest geological showcase,” in the words of the National Park Service’s visitor guide. In broad daylight looking east I can discern buttes ten miles away and make out the North Rim for another ten miles; but the canyon starts at least 70 miles beyond my seeing. When I look west, the nearest canyon walls hide another 200 miles' worth of cliffs and buttes. From this porch to the opposite edge a condor would fly ten miles. But to my dazed vision the North Rim might as well be Canada.
In its vertical dimension Grand Canyon teaches us the most. The river has cut through the plateau down to 1,800 million year-old rock formations, going back one-fourth of Earth’s history. Each new geologic layer was deposited over millions, tens of millions, or hundreds of millions of years. Then conditions changed and new processes overlaid different rock, with different fossils, because life was evolving too.
Faced with eons of time, creating such sublime beauty, a visitor can feel at peace. But a voice nags in my mind: in what time will humans remake the atmosphere with warming and the seas with more acidity and heat? Will human-induced change start a new geological age?
Consider the past, as I’m doing, helped by a book from the park shop, “An Introduction to Grand Canyon Geology” by L. Greer Price. The rock layer supporting me, the hotel and the animals—which turned out to be young bighorn sheep—are the first to be lit along the rim at sunrise. These rocks are gray and lumpy and called the Kaibab Formation; they formed on average 270 MYA (million years ago). (For this and later dates see Fig. 1.)
How can the top be so old? J. Michael Timmons of the New Mexico Bureau of Geology and a Grand Canyon geologist, later told me that, after the Paleozoic Era when the Kaibab layer ends, more rock was deposited as much as 2 km thick. The remnants of this regional rock layer are found in southern Utah and Colorado. But since this rock was deposited the region was pushed upwards, so in some places the new layer eroded away. Such missing layers are called “unconformities” by geologists, who decode what most likely happened from intact formations elsewhere.
As I walked alone on the dim rim path, the sun lit a thick tan layer a bit farther down. About 275 MYA the land was a desert of sand. Over 5 to 10 million years, the dunes cemented into rock called Coconino Sandstone.
As sunlight crept deeper into the canyon, suddenly a ribbon of high cliffs became a vivid coral red, like stained-glass. This is Redwall Limestone which lines the walls and buttes throughout. It was deposited in the middle Paleozoic on average 340 MYA. The limestone formed from the calcified shells and exoskeletons of marine organisms which proliferated because the ocean then was near the Equator and had warm, light-filled waters.
Below the Redwall layer I learned to detect (in good light) thin bands of Tonto Group rocks, laid down from about 525 through 505 MYA, as the ocean transgressed across land and became deeper. Near the beginning of this time, called the Cambrian, the land here was a coastal beach; so the sands deposited had beach-type features; this layer is called Tapeats Sandstone. By the end of Cambrian time, shallow seas covered much of the area depositing yellowish, mottled Muav limestone. In between, the transition from a beach environment to a deeper sea is marked by colorful sands and mudstones called Bright Angel Shale.
The so-called Great Unconformity—a gap of 1200 million years of missing material—occurs where Tonto Group rests directly on Vishnu basement rocks. “But in parts of Grand Canyon, we have rocks that fill part of that gap,” said Timmons. These are Grand Canyon Supergroup rocks. They're visible as broad, round plateaus that seem to push the canyon walls outward. The youngest were deposited about 740 MYA. Below these the gorge narrows because the Vishnu igneous and metamorphic rock is much harder.
I hardly believe I’m tossing off references to 1200 million years, or 5 to 10 million years. Timmons, the geologist, was sympathetic to my problem imagining geologic time. “The sharp contacts between layers that we see in Grand Canyon appear to be instantaneous events. But in reality, these transitions take time to occur, on the scale of thousands but probably not millions of years.”
Do geologists know of shifts in Grand Canyon rock as rapid as 200 years? That would be the time scale now, when the steady rise of carbon dioxide in the atmosphere is making Earth’s oceans more acid.
Timmons replied: “The rock record can show extreme instantaneous events, such as volcanoes. We also have found the record of common events like the back and forth of tides.” As for future ocean deposition, “it’s hard to tell what the record will look like. We just know that the rates of change are unmatched in the geologic record.”
The Intergovernmental Panel on Climate Change in 2007 suggested sea levels would rise by less than 0.6 meter by 2100. But since then, many scientists forecast greater rise. New Scientist in its July 4 issue quotes a likely rise of 0.8 to 1.4 meters by 2100, plus at least another meter by 3000 due solely to thermal expansion of the oceans. Even as the sea rises—starting a new layer of marine deposits where it transgresses present day land—the chemistry of ocean water will be different, changing some marine life forms that fall to the ocean bottom. (See my article “Acidifying oceans remake Earth and our food supply.”)
But most people who talk about climate change seem unable to consider 91 years ahead to 2100.
The media have stopped running the “hockey stick chart” popular in the global warming literature of the 1990s. It showed a J–shaped curve, for atmospheric concentrations of carbon dioxide equivalent, rising literally off the chart by 2200, assuming we go on emitting at present rates. (We’re still on this curve.)
It was momentarily depressing to think that, since humans are remodeling the oceans and marine life, we’re engineering a new layer of ocean floor rock that will become the mark of our passing. But then, maybe even “our” layer of rock will be wiped out and all our work on Earth will vanish in a discontinuity.
People may be speechless at Grand Canyon because it shows human irrelevance. We’re only passing through, much as I, having sampled this vista, will leave the porch and hotel tomorrow after breakfast.
And the bighorn sheep, which ignored me as I sidled by to reach the path along the rim? They evolved about 2.5 million years ago—even as the river was cutting this canyon. Their ancestors got to North America 600,000 years ago, whereas we Homo sapiens sapiens are 200,000 years young.
Even the sheep have more rights to the place.
© Deborah Shapley August 2009
Links to Ron Blakey Paleogeological Maps (Northern Arizona University)