Deep time

I closed an earlier essay (That time of year) with a quote about the curious blend of terror and exhilaration that the contemplation of deep time can bring. On the one hand, we’re so tiny and so ephemeral in comparison with events that take millions or even billions of years, like the growth and erosion of mountains or the lifetimes of stars. On the other hand, here we are, able to understand something of these events despite the fact that we are so finite. That, to me, is one of the most amazing things about being thinking meat. As far as we know, we’re the only things on this planet capable of understanding events in the far distant past (and speculating about the far distant future).

Ptolemy expressed this feeling thus:

I know that I am mortal and the creature of a day; but when I search out the massed wheeling circles of the stars, my feet no longer touch the earth, but, side by side with Zeus himself, I take my fill of ambrosia, the food of the gods.

The night sky provides plenty of such ambrosia. However, the earth beneath our feet also offers suitable food for thinking meat in search of inspiration. When I visited Manhattan a couple of years ago, for example, one of the most compelling sights out of all that jam-packed island had to offer was a piece of rock in the American Museum of Natural History. Not a sapphire or a diamond (although the sapphires were spectacular), this particular rock came from a 2.7 billion-year-old banded iron formation in Canada.

It’s a big piece of beautiful rock with striking red and gray stripes. The earliest chapters of life on earth are bound up in this rock. When single-celled creatures began to release oxygen into the atmosphere as a by-product of photosynthesis, the oxygen reacted with iron and other minerals in the oceans to precipitate out into the gorgeous red colors in the rock. This oxidation went on for the better part of a billion years, until the atmosphere and the ocean were saturated with oxygen. From such humble beginnings, life proliferated into its present diversity, after leaving its mark on the rocks.

I remember seeing a show on PBS about John Denver, including some footage of him at the bottom of the Grand Canyon. A companion was telling him about the great age of the rocks he was looking at (like the banded iron formation, they’re some of the oldest on the planet). In response he leaned over and put both hands on the rocks, leaning forward and putting his weight into the contact with the unimaginably distant past. I know exactly what he felt like. The banded iron rock in the museum was not in a display case, and there were no signs telling people not to touch it, so I somewhat shyly reached out and put my hand on it.

I can’t even begin to really understand what a billion-year interval is like, or how long ago 2.7 billion years is, but with my hand on the rock, I tried. The volume of life involved is striking too; how many tiny single-celled creatures did it take to produce the oxygen that reacted with so much rock? The earth of that time is as alien to me as the surface of Mars, and yet I could reach out and put my hands on the relics of that time, and understand at least some of the story behind it.

There’s a rock in the Arizona-Sonora Desert Museum near Tucson that is equally impressive. It’s quite a respectable age (but not nearly as old as the banded iron) and is the result of more familiar processes. It’s an orangish sandstone, and it formed 800 million years ago in northern Arizona. On the surface of the rock, you can see ripple marks; the rock used to be sand near the shore of a huge inland sea. In addition to the ripples, there are dimples that were formed when raindrops fell on the wet sand. Another layer of sand covered the ripples and the rain marks, preserving them for all those millions of years. When I first saw the rock in 1987, it looked so similar to the rain-pocked and rippled sand I’d seen the previous summer on a rainy Florida beach that it stopped me in my tracks. Those raindrops had fallen 800 million years before, and here I was to see their traces.

It’s that feeling of knowing in some detail of long ago events that makes these ancient rocks so fascinating. When I visited the University of Utah several years ago, my friend Barbara, a geologist at the university, gave me a guided tour of the rocks on display outside the building where she works. One rock had fine layers of alternating light and dark materials, each layer a year’s deposition of sediments on the bed of a stream. One of the layers was much thicker than the others. “What about this one?” I asked. “That was a flood,” said Barbara. I can imagine the booming rush of water sweeping down its channel, carrying a huge load of mud and debris, and the living things either swept helplessly along, or buried in the mud, or maybe hunkered down waiting for it to pass, with perhaps land animals nearby scurrying to higher ground. Then I thought how all that motion and drama was reduced over time to a relic, a thicker layer in the rock that told a story to two hominids standing outside on a sunny winter day millenia later, talking about rocks. (Are any of the dramatic and heartrending geological or meteorological events of our time leaving behind traces in the rocks, traces that future humans are going to be able to identify from some calm vantage point after we’re all long gone?)

There are rocks that contain similar records of sediment deposition, except that they are daily rather than seasonal. These rocks were formed in coastal waters, and the patterns of extremely narrow layers in them represent material deposited during high and low tides (the high tide layers are slightly thicker). The Hindostan whetstone rocks of southern Indiana are 300 million years old, and they show the tides of the ancient inland sea. With care, the timing between the tides can be calculated (I heard an astronomer who studied these rocks say that until he started looking at them he had no idea tides were so complicated). The time between tides is related to the distance between the earth and the moon, which has increased over time. By analyzing these and similar but even older rocks, scientists can track the slow retreat of the moon from the earth and the slow lengthening of the day. In nineteenth century Indiana, these ancient records of time and tide were widely used as whetstones and gravestones, so the gravestones around here might record not just the passing of a single life, but also several months of tidal history from hundreds of thousands of years ago.

One of the more exotic events that left its traces on the earth goes back to Ptolemy’s heavens. A supernova explosion about three million years ago and one or two hundred light years away showered the earth with stellar debris, some of which was identified in sediments from the floor of the Pacific Ocean in 1999. This debris, in the form of atoms of an isotope of iron that could only have come from within a supernova, was the first such material identified on earth.

Of course the earth, the sun, and all that we can see in the solar system ultimately must have come from inside a star (Carl Sagan’s quote about us all being star-stuff was absolutely correct). But all the years of transformation since then have turned the star-stuff into the stuff of everyday life, which dulls the wonder of that knowledge most of the time. These atoms of iron, on the other hand, were fresh and relatively untouched from this distant catastrophic event. Distant as it was, this catastrophe might have affected earth’s climate and changed the course of human evolution, drying Africa out somewhat and sending our early ancestors out in search of wetter ground. This small sample of iron atoms is a tantalizing window into the history of our species and our planet.

Our brains weren’t shaped by any need to understand ancient floods that left their traces in the rocks, or to identify atoms that come from supernovae, or to contemplate the arid heights of northern Arizona and imagine a long-ago beach lapped by the waters of an inland ocean that stretches over what’s now prairie. It’s intriguing, when you stop to think about it, that we can do these things. The poignant awareness of our own brevity is balanced somewhat by our awareness of the deep past and the deep future. We can picture events and scenery that are far behind us in time, and this offers at least some consolation in the face of the sobering realization that our own role in the drama of the planet is minuscule and provisional.

(Originally posted November 30, 2005)

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