Thursday, January 29, 2009

The January Accretionary Wedge: Seeing the Future in a Picture

This month's Accretionary Wedge Carnival is being hosted by BrianR at Clastic Detritus, and involves speculation about the future of the earth in the context of geologic events and processes. It is a delightfully open-ended idea, and one that I have used as a final exam question in my historical geology classes. After all, historical geology is an attempt on our part to understand the past history of the planet by observing present-processes (the simple definition of uniformitarianism, a founding principle of the science). But a part of the value of understanding the past and present is the ability to project into the future, and when I ask students to "predict" the future, it is an excellent gauge of their understanding of the basic principles of the class. Did they really understand the concept of geologic time? Do they understand the context of human existence with the vast flow of geologic time? Can they relate long-term changes of climate over periods of thousands or millions of years with climate change in a human context of a century or so? Will our activities leave a mark in the geologic record?

Speculating about the future often is rather anthropomorphically-based: it's an ego trip. Will we as humans be around to see and observe the results of our present-day choices over a geologically extended period? I decided that would be the direction of my post...not whether we would be here so much, but would anyone else figure us out?

I wrote in my last post about the mining of coal at Black Mesa on the Hopi and Navajo reservations in northern Arizona, and I chose one of my favorite chance pictures of the place to decorate this post. I was off to a late start on a 500-mile drive, and was only halfway to where I was going by the time the sun set and behind me I saw a bright light that proved to be the rising full moon. There weren't too many choices for parking and framing a picture, so I got what I got, with fences, telephone poles, and highway traffic (that I cropped out). The cliffs of Black Mesa form the horizon. The picture is a nice gathering of the elements for my speculation and musings that follow.

The moon is a remote dead planet, remaining essentially unchanged for the last two billion years, aside from the occasional meteorite impact, and the footprints of a rather small number of humans. But it touches us through the tides, and as a source of light in the night-time. I am told that it is moving farther away from the earth at a rate of a few inches per year. So the tides will continue and the moon will rise in the sky on a regular basis for a long time. Some things don't change very much, even over millions of years.

The atmosphere, the prism through which we observe the cosmos, contains nitrogen, oxygen, argon, and small amounts of carbon dioxide, methane and water vapor. The nitrogen is largely unreactive, aside from being transformed by bacteria into a usable form for plant life. The oxygen is very reactive, and plays an active part in the weathering of the earth's surface; most children are taught (and maybe even remember) that animals consume oxygen and give off carbon dioxide, while plants consume the carbon dioxide and give off oxygen. Even if a giant catastrophe, a mass extinction event, were to disrupt the carbon cycle, it eventually reach an equilibrium state and the atmosphere would achieve a relative balance over time. Thus, I don't expect the atmosphere to change much, with the exception of carbon dioxide and methane. As many are coming to realize, we have been doing a mass planet-wide experiment that answers the following question: if we take a significant proportion of all the hydrocarbons produced by earth processes over the last 400-500 million years, and pump them into the atmosphere in a period of 200 years, what will happen to the climate? Results of the experiment are pending; a preliminary report will ready in a few decades or centuries

The fences, the telephone poles, the asphalt highway are all ephemeral monuments to our existence that will fade within a few decades or centuries. I'm not sure that the fast-food restaurants, motels and houses of the nearby town of Kayenta will even come close to outlasting the work of the Ancestral Puebloan people who built Chaco Canyon and Mesa Verde (I am reminded of the permit for my tract home in California that listed the expected existence of my house as being 40 years; yikes, I used up half of it already!). Is there anything that we will leave as a mark of our passing?

The strip mine on Black Mesa might be something. In 40 years or so, the miners have moved mountains, in the most literal sense of the word. In a sense though, they are hiding the results of their labor as they back fill the coal pits to return the land to the original configuration. The geologists of some future species might detect our presence because of the disruption of the sedimentary sequence, since the coal will be missing, and the waste rock will be a jumbled chaos. There may be some symmetry in the pattern of the chaotic rock, with straight lines and corners representing the edges of the strip mine. But that will only be possible for a few more tens of thousands of years.

In the fullness of time, the entire mesa will be disappearing, along with all signs of human existence. The entire region sits at an elevation of 6,000-7,000 feet above sea level, and is being vigorously eroded by the Colorado River and its tributaries. The massive dams at Lake Powell and Lake Mead will matter little in the long run. Filled with sediment in decades, and removed by erosion within a few thousand or tens of thousands of years, the dams cannot hold back the massive movement of sediment from the plateau to the sea. There have been dams before, made of lava flows, and they could not hold back the flowing rivers for long. In a few million years the entire region will have hundreds or thousands of feet of sediment removed and the only remaining clues of our existence there will be downstream somewhere.

There will be floodplain deposits preserved in parts of the world that will contain deposits of concrete and metal, and even human bones, much as the dinosaurs and other terrestrial creatures have been preserved. I have no doubt that a layer will be discovered that represents a mass extinction event wherein a vast number of organisms disappeared forever, including the mammalian megafauna, and perhaps many oceanic species, especially at the top of the food chain, such as many kinds of sharks and whales. Will we be identified as the cause of the catastrophe? Who knows? Someone or something might figure it out. Or maybe not; it might be another of those mysteries that nag and bother the psyches of those who study the earth and it’s past.

I'm going to close with a story from my youth. If you have stayed with my narrative for this long without laughing or sighing in frustration, well bear with me because you may laugh now. I figured it out once...I figured out as a 12-year-old why the dinosaurs disappeared from the earth (I'm old enough to remember when birds were not dinosaurian species in the eyes of paleontologists).

I had learned how dinosaurs lasted for many millions of years, and that humans had existed for only a few thousand as a dominant species. It occurred to me that perhaps intelligence evolved very late in the Cretaceous, literally within the last million years of the period, and technology evolved in the last few thousand years. The dinosaurs built cities and roads and planes and bombs, and in the blink of an eye they obliterated themselves and most of the ecosystem they occupied in a vast war. So complete was their destruction that no sign remained of their cities and technology. And the world started over without them....

Such are the musings of a child of the Cold War.....

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