Mind the gap

In 1869, explorer John Wesley Powell did the first systematic study of the geology of the Grand Canyon.  As impressive as it is, the Grand Canyon's not that complicated geologically; it's made of layers of sedimentary rock, most of them relatively undeformed, one on top of the other from the oldest at the bottom to the newest at the top.  A layer cake of billions of years of Earth history, and a wonderful example of the principle of superposition -- that strata form from the bottom up.

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 fifty kilometers 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.

However, a 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]

When uplift occurs, erosion increases, as water and wind take those uplifted bits, grind them down, and attempt to return them to sea level.  And massive scale uplift results in a lot of rock being eroded.

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.

****************************************

The geological time slip

The geologists have lost about a billion or so years, have you noticed?

This wee time slip has to do with the sedimentary strata best observed in the Grand Canyon, which showed a peculiarity noticed -- but not explained -- by explorer John Wesley Powell in 1869.  Powell knew about sedimentary rock formation and the Principle of Superposition -- the rather common-sensical idea that in undisturbed strata, the lower layers were formed first.  (It's like building a layer cake -- it'd be rather tricky to build it from the top down.)

In Powell's case, he noticed what looked like a jump in rock types and stratum structure, where because of fault lines and angles of tilt it seemed like there was a gap in the rock record.  What neither Powell nor anyone else knew at the time was how big this gap -- which Powell called "the Great Unconformity" -- was.

It represents the loss of about a billion years of accumulated rocks.

The Arroyo Penasco Formation showing the Great Unconformity, Montezuma, New Mexico [Image is in the Public Domain]

That by itself would be enough to suggest that whatever caused the Great Unconformity, it was not some local effect, and it didn't take much surveying to confirm this.  In fact, the same jump has been seen in just about every place there's rock of that age (right around the boundary between the Cambrian and Precambrian eras), most notably in the St. François Mountains of Arkansas and at Siccar Point on the east coast of Scotland.

In each case, there is 1.4 billion year old rock (mostly granite, rhyolite, and schist), and the layer immediately above it dates to around 500 million years ago.

In between -- nothing.

It wasn't until 1910 that the magnitude of this bizarre gap was fully appreciated.  In Cambrian Geology and Paleontology, geologist Charles D. Walcott wrote:

I do not know of a case of proven conformity between Cambrian and pre-Cambrian Algonkian rocks on the North American continent.  In all localities where the contact is sufficiently extensive, or where fossils have been found in the basal Cambrian beds or above the basal conglomerate and coarser sandstones, an unconformity has been found to exist.  Stated in another way, the pre-Cambrian land surface was formed of sedimentary, eruptive, and crystalline rocks that did not in any known instance immediately precede in deposition or origin the Cambrian sediments.  Everywhere there is a stratigraphic and time break between the known pre-Cambrian rocks and Cambrian sediments of the North American continent.

But what on Earth could tear down a billion years' worth of strata -- all over the world, more or less simultaneously (if you can call anything that had a duration of a billion years "simultaneous")?  Scientists believe that these missing layers represent something on the order of six to eight vertical kilometers of rock.

Some new research has indicated a possible trigger -- and perhaps the mechanism involved.  Back around the beginning of the gap, all of the continents of the Earth had slammed together to form one huge supercontinent.  This was pre-Pangaea, the one most people will think of; this was Rodinia, a colossal land mass that lasted from the late Precambrian Era to right about the beginning of the Cambrian, at which point rifting took over and the continents separated into a new configuration.

Here's what seems to have happened.  When Rodinia formed, the force of the collisions pushed a lot of rock skyward.  We're seeing exactly the same thing happen today in the Himalayas; Mount Everest is sedimentary rock that was once at the bottom of the ocean, but the collision between India and the main part of Asia scooped it up like a huge plow and raised enormous mountains.  This same process occurred during the formation of Rodinia, but on a global scale as all of the world's land masses collided.

But by the beginning of the Cambrian, a huge amount of that rock was gone, eroded away.  What could cause erosion on that scale?

It seems like the likeliest explanation is worldwide glaciation.  The late Precambrian has been called the "Cryogenic Period" -- from Greek words meaning "ice-forming" -- as well as the perhaps more vivid moniker of the "the Snowball Earth."  The shoving of the Precambrian rocks aloft created steep topography (again, just like in the Himalayas today), so any erosive forces, whether ice or liquid water, would have that much more gravity-driven force to grind it down.

As a biologist, what I find even cooler is that that breakup of Rodinia, which coincided with the thawing of the Snowball Earth, was also the beginning of a huge diversification of life on Earth, something that has been nicknamed the Cambrian Explosion.  I don't think it's a reach to hypothesize that these two events were connected.

So do we owe the current biodiversity -- and, by our extension, our own presence here -- to a process that erased every trace of a billion years of sedimentary rock layers?

I find it fascinating how everything is connected, and that even after a couple of centuries of intense study, there are still mysteries out there to solve.  The unconformities in our own knowledge are still huge, but unlike the one in the Grand Canyon, aren't immediately obvious.  Filling in these gaps inevitably opens up new questions, enough that scientists will never run out of new areas to explore.  As Socrates said, over two thousand years ago, "If I am accounted wise, it is only because I alone realize how little I know."

*******************************

One of the most enduring mysteries of neuroscience is the origin of consciousness.  We are aware of a "self," but where does that awareness come from, and what does it mean?  Does it arise out of purely biological processes -- or is it an indication of the presence of a "soul" or "spirit," with all of its implications about the potential for an afterlife and the independence of the mind and body?

Neuroscientist Anil Seth has taken a crack at this question of long standing in his new book Being You: A New Science of Consciousness, in which he brings a rigorous scientific approach to how we perceive the world around us, how we reconcile our internal and external worlds, and how we understand this mysterious "sense of self."  It's a fascinating look at how our brains make us who we are.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Mind the gap

In 1869, explorer John Wesley Powell did the first systematic study of the geology of the Grand Canyon.  As impressive as it is, the Grand Canyon's not that complicated geologically; it's made of layers of sedimentary rock, most of them relatively undeformed, one on top of the other from the oldest at the bottom to the newest at the top.  A layer cake of billions of years of Earth history, and a wonderful example of the principle of superposition -- that strata form from the bottom up.

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]

When uplift occurs, erosion increases, as water and wind take those uplifted bits, grind them down, and attempt to return them to sea level.  And massive scale uplift results in a lot of rock being eroded.

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!]

The fluid Earth

I've always had a fascination for maps, and when I was a kid spent many happy hours perusing a huge old world atlas my parents owned.

I remember how impossibly exotic a lot of those places seemed.  Some of them, too, seemed awfully oddly-shaped.  I remember being struck, for example, by the peculiar contour of the island of Celebes (now known as Sulawesi):

Map from The Birds of Celebes and the Neighouring Islands (1898) [Image is in the Public Domain]

What on earth gave the place its strange shape?  I was years away from finding out about plate tectonics, seafloor spreading, and continental drift -- this would have been 1968 or so, and the seminal paper by Fred Vine and Drummond Matthews confirming the truth of plate tectonics had only been published five years earlier, so the idea had yet to make its way into elementary school science classes.

The first inkling I had that the current map of the world was only the latest of a myriad configurations that Earth's land masses had taken was when I found out about marine fossils on the top of Mount Everest and tropical fossils in Antarctica in a book I had on prehistoric life.  Everything was shifting around, apparently, in some mysterious fashion, and the familiar maps from my parents' atlas would have been completely incorrect in the past.  For example, India broke off from what is now Madagascar, sliding across the ocean on its piece of plate, and rammed into Asia only fifty or so million years ago -- which may seem like a long time, but at that point the dinosaurs had already been extinct for fifteen million years (as I always feel obliged to add, except for birds).

Still, I didn't know much in the way of details.  When I took two geology courses in graduate school, however, I hit the idea head-on, including the now-familiar idea of Pangaea -- that there was a time when all of the continents were joined into one enormous land mass.  Even more mindblowing was the fact that this wasn't the only time this had happened -- the accretion and disintegration had occurred at least three or four times before, each time ending when rifts formed and forced the place apart.

The traces of these repeated hookups and breakups are still with us.  In fact, one was just announced in a paper recently in Geology, by a team led by Adam Nordsvan of Curtin University, in which evidence was uncovered that a piece of Australia -- the region of Georgetown in far northeastern Queensland -- was actually geologically related not to the rocks immediately adjacent to it, but to rocks in (of all places) Canada.

The Canadian (or Laurentian) Shield is one of the oldest relatively unaltered blocks of rock on the Earth, of Precambrian age -- on the order of three and a half billion years old -- so to a geologist, they're pretty distinct from the geology of the nearby Mount Isa formation, which is only half that old.  (I realize how ridiculous it is to use the word "only" to describe something 1.8 billion years old, but I'm trying to think like a geologist, here.)

The coolest thing is that the piece of Canada left behind in Australia wasn't from the most recent continental pile-up, which occurred on the order of three hundred million years ago, nor even the one before that.  The most likely time that Canada and Australia were joined together was three supercontinents ago, when all the Earth's land masses were fused around a billion years ago into a huge clump called Rodinia:

[Image is in the Public Domain]

So apparently when that rifted apart, around 750 million years ago, a chunk of Canada decided to split off and ended up (literally) on the other side of the world.

The whole thing is pretty cool.  I'm still fascinated by maps in general, and thinking about what the world was like when Antarctica was in the tropics of the Northern Hemisphere and the equator cut across what is now Labrador will never fail to spark my imagination.  Add to that the bizarre thought (to me, at least) that at that point, all living things were confined to the oceans -- there was not a bug, not a worm, not so much as a sprig of moss anywhere on land, the whole place was completely devoid of life -- well, it brings to mind the line from Contact about a universe empty of all life except for us being an "awful waste of space."

Fortunately for us, though, at that point the conquest of dry land was right around the corner.

"Only" three hundred million years later.

I'll end with the prescient lines from Alfred, Lord Tennyson, penned in 1849, long before continental drift was even considered:

There rolls the deep where grew the tree.
O Earth, what changes hast thou seen?
There where the long road roars has been
The stillness of the central sea;
The hills are shadows, and they flow
From form to form, and nothing stands,
They melt like mists, the solid lands,
Like clouds, they shape themselves and go.

***********************

This week's Skeptophilia book of the week is brand new; Brian Clegg's wonderful Dark Matter and Dark Energy: The Hidden 95% of the Universe.  In this book, Clegg outlines "the biggest puzzle science has ever faced" -- the evidence for the substances that provide the majority of the gravitational force holding the nearby universe together, while simultaneously making the universe as a whole fly apart -- and which has (thus far) completely resisted all attempts to ascertain its nature.

Clegg also gives us some of the cutting-edge explanations physicists are now proposing, and the experiments that are being done to test them.  The science is sure to change quickly -- every week we seem to hear about new data providing information on the dark 95% of what's around us -- but if you want the most recently-crafted lens on the subject, this is it.

[Note: if you purchase this book from the image/link below, part of the proceeds goes to support Skeptophilia!]