However, Powell also noted something rather peculiar. It's called the Great Unconformity. In geologic parlance, an unconformity is a break in the rock record, where the layer below is separated from the layer above by a gap in time when either no rocks were deposited (in that location, at least), or the rocks that were laid down were later removed by some natural process. At that stage in the science, Powell didn't know when exactly the Great Unconformity occurred, but it was obvious that it was huge. Something had taken away almost a billion years' worth of rocks -- and, it was later found out, that same chunk of rock was missing not only at the future site of the Grand Canyon, but across most of North America.
It was an open question as to why this happened, but one leading hypothesis was that it was massive glaciation. Glaciers are extraordinarily good at breaking up rocks and moving them around, as I find out every time I dig in my garden and my shovel runs into the remnants of the late Pleistocene continental glaciation. At that point, where my house is would have been under about thirty meters of ice; the southern extent is the Elmira moraine, a line of low hills thirty miles south of here, left behind when the glaciers, pushing piles of crushed rock and soil ahead of them like a backhoe, began to melt back and left all that debris for us gardeners to contend with ten thousand years later.
There was a time in which the Earth was -- as far as we can tell -- completely covered by ice. The Cryogenian Period, during the late Precambrian, is sometimes nicknamed the "Snowball Earth" -- and the thawing might have been one contributing factor to the development of complex animal life, an event called the "Cambrian explosion," about which I've written before.
The problem was, the better the data got, the more implausible this sounded as the cause of the Great Unconformity. The rocks missing in the Great Unconformity seem to have preceded the beginning of the Cryogenian Period by a good three hundred million years. And while there were probably earlier periods of worldwide glaciation -- perhaps several of them -- the fact that the Cryogenian came and went and didn't leave a second unconformity above the first led scientists away from this as an explanation.
Now, a new paper in Proceedings of the National Academy of Sciences, written by a team led by Francis Macdonald of the University of Colorado - Boulder, has come up with evidence supporting a different explanation. Using samples of rock from Pike's Peak in Colorado, Macdonald's team used a clever technique called thermochronology to estimate how much rock had been removed. Thermochronology uses the fact that some radioactive elements release helium-4 as a breakdown product, and helium (being a gas) diffuses out of the rock -- and the warmer it is, the faster it leaves. So the amount of helium retained in the rock gives you a good idea of the temperature it experienced -- and thus, how deeply buried it was, as the temperature goes up the deeper down you dig.
What this told Macdonald's team is that the Pike's Peak granite, from right below the Great Unconformity, had once been buried under several kilometers of rock that then had been eroded away. And from the timing of the removal -- on the order of a billion years ago -- it seems like what was responsible wasn't glaciation, but the formation of a supercontinent.
But not Pangaea, which is what most people think of when they hear "supercontinent." Pangaea formed much later, something like 330 million years ago, and is probably one of the factors that contributed to the massive Permian-Triassic extinction. This was two supercontinents earlier, specifically one called Rodinia. What Macdonald's team proposes is that when Rodinia formed from prior separate plates colliding, this caused a huge amount of uplift, not only of the rocks of the continental chunks, but of the seafloor between them. A similar process is what formed the Himalayas, as the Indian Plate collided with the Eurasian Plate -- and is why you can find marine fossils at the top of Mount Everest.
[Image is in the Public Domain]
Thus the missing layers in the Great Unconformity.
"These rocks have been buried and eroded multiple times through their history," study lead author Macdonald said, in an interview with Science Daily. "These unconformities are forming again and again through tectonic processes. What's really new is we can now access this much older history... The basic hypothesis is that this large-scale erosion was driven by the formation and separation of supercontinents. There are differences, and now we have the ability to perhaps resolve those differences and pull that record out."
What I find most amazing about this is how the subtle chemistry of rock layers can give us a lens into the conditions on the Earth a billion years ago. Our capacity for discovery has expanded our view of the universe in ways that would have been unimaginable only thirty years ago.
And now, we have a theory that accounts for one of the great geological mysteries -- what happened to kilometer-thick layers of rock missing from sedimentary strata all over North America.
John Wesley Powell, I think, would have been thrilled.
This week's Skeptophilia book of the week is six years old, but more important today than it was when it was written; Richard Alley's The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Alley tackles the subject of proxy records -- indirect ways we can understand things we weren't around to see, such as the climate thousands of years ago.
The one he focuses on is the characteristics of glacial ice, deposited as snow one winter at a time, leaving behind layers much like the rings in tree trunks. The chemistry of the ice gives us a clear picture of the global average temperature; the presence (or absence) of contaminants like pollen, windblown dust, volcanic ash, and so on tell us what else might have contributed to the climate at the time. From that, we can develop a remarkably consistent picture of what the Earth was like, year by year, for the past ten thousand years.
What it tells us as well, though, is a little terrifying; that the climate is not immune to sudden changes. In recent memory things have been relatively benevolent, at least on a planet-wide view, but that hasn't always been the case. And the effect of our frantic burning of fossil fuels is leading us toward a climate precipice that there may be no way to turn back from.
The Two-Mile Time Machine should be mandatory reading for the people who are setting our climate policy -- but because that's probably a forlorn hope, it should be mandatory reading for voters. Because the long-term habitability of the planet is what is at stake here, and we cannot afford to make a mistake.
As Richard Branson put it, "There is no Planet B."
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]
No comments:
Post a Comment