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 rocks drawn down

Imagine yourself standing on the shoreline, somewhere on the Earth, three billion years ago.

If you're picturing swamps and tree ferns and dinosaurs, you're way off.  The first trees wouldn't appear for another 2.6 billion years or so; the first dinosaurs we know about were 150 million years after that.  Three billion years ago there probably weren't even any eukaryotes -- organisms with complex cells containing organelles and nuclei, such as ours, as well as those of plants, fungi, and protists -- anywhere on Earth.  It's likely there wasn't much oxygen in the atmosphere, either.  This is before the "Great Oxidation Event," when photosynthesizing cyanobacteria reached a population sufficient to dump huge quantities of oxygen into the atmosphere, dooming most of Earth's living things (but simultaneously setting the stage for the rise of aerobic organisms such as ourselves).

So an accurate picture of what you'd experience: land that is nothing but an enormous expanse of bare rock and sand, devoid of a single living thing; murky water containing a soup of organic compounds, generated by the reducing atmosphere and frequent lightning storms; and unbreathable air mostly made of nitrogen, methane, carbon dioxide, carbon monoxide, sulfur dioxide, and ammonia.

I can just hear Mr. Spock saying, "The planet appears to be entirely inhospitable to life, Captain."

It's hard to imagine that our lush, verdant, temperate world evolved from that, but it did.  Consider, too, that the continents weren't even remotely in the same positions as they are now.  Where I sit writing this, in upstate New York, I'd have been about at the same latitude as I am now -- maybe a little bit farther south, about thirty degrees north.  But that's by far the exception.  See where you'd be on this map between about 2.5 and 1.5 billion years ago, when all of Earth's land masses were fused into a supercontinent named Columbia.  [Nota bene: this is not Pangaea.  This is two supercontinents before Pangaea.]

[Image licensed under the Creative Commons Alexandre DeZotti, Paleoglobe NO 1590 mya-vector-colors, CC BY-SA 3.0]

This was, in fact, not long after the continents formed.  We have continents because there are two basic kinds of rocks in the Earth's crust: felsic rocks, which are rich in silica, low in iron, and relatively lightweight; and mafic rocks, which are the opposite.  Most of the continental land masses are made up of felsic rocks (like granite and rhyolite), so they float in the denser rock of the mostly-mafic upper mantle.  (It's hard to imagine something as gigantic and heavy as a continental rock mass floating, but that's what it does.)  About three billion years ago was when there was sufficient separation of felsic and mafic chunks of crust that we started to see continental cratons form, and these blocks have been so stable thereafter that they're basically the same land masses we have today (albeit much cut apart and rearranged).

The reason this comes up is the discovery of evidence of what might have been one of the Earth's earliest megaquakes.  It occurred in what is now South Africa, part of the Kalahari Craton (as you can see from the map above, it'd have been in the northeast corner of the Columbia Supercontinent, at about the current latitude of Oslo, Norway).

The Barberton Greenstone Belt is one of the oldest relatively undisturbed chunks of rock in the world, and the current study, which was published two weeks ago in the journal Geology, suggests that it shows evidence of an overturned layer of chert that formed from a humongous underwater landslide of the type we see with megathrust earthquakes.  This, the researchers say, is the smoking gun that plate tectonics was already up and running three billion years ago -- that the reshuffling of continental blocks still going on today started not long after the blocks themselves formed.

The Barberton Greenstone Belt, about 350 kilometers east of Pretoria, South Africa [Image credit: Simon Lamb, Victoria University]

We think of the Earth as unchanging, don't we?  "Solid as a rock" is close to a cliché.  And yet, as we've seen, everything shifts, melts, moves; life comes and goes, evolves and falls to extinction; even the continents beneath our feet break up and recombine.  It's been going on for billions of years, and will continue for billions more.  The whole thing puts me in mind of Percy Shelley's evocative poem "Mont Blanc," which seems a fitting place to end:

Yet not a city, but a flood of ruin
Is there, that from the boundaries of the sky
Rolls its perpetual stream; vast pines are strewing
Its destin’d path, or in the mangled soil
Branchless and shatter’d stand; the rocks, drawn down
From yon remotest waste, have overthrown
The limits of the dead and living world,
Never to be reclaim’d.  The dwelling-place
Of insects, beasts, and birds, becomes its spoil;
Their food and their retreat for ever gone,
So much of life and joy is lost.  The race
Of man flies far in dread; his work and dwelling
Vanish, like smoke before the tempest’s stream,
And their place is not known.  Below, vast caves
Shine in the rushing torrents’ restless gleam,
Which from those secret chasms in tumult welling
Meet in the vale, and one majestic River,
The breath and blood of distant lands, for ever
Rolls its loud waters to the ocean-waves,
Breathes its swift vapours to the circling air.

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

Desert world

Following hard on the heels of yesterday's post, about whether we'd be able to tell if there'd been a technological civilization on Earth tens or hundreds of millions of years ago, today we have a new study in Nature projecting the configuration of the continents 250 million years in the future -- and what that might portend for life.

We've known about the movement of the continents since geologists Harry Hess, Frederick Vine, and Drummond Matthews conclusively demonstrated in the early 1960s that convection currents in the mantle were dragging the tectonic plates along and shifting the positions of pieces of the Earth's crust relative to each other.  Of course, we might have figured all that out thirty years earlier if we'd just listened to poor Alfred Wegener, who proposed what he called "continental drift" to explain such observations as the near perfect fit between the eastern coastline of the Americas and the western coastline of Europe and Africa.  But because such an idea ran counter to accepted geological model of the time, Wegener was laughed out of academia -- literally.  He ended up taking off to Greenland to do paleoclimatological studies of the ice cap, and froze to death in November of 1930, never finding out that he'd actually landed on the truth.

In any case, what all this means is that the current configuration of continents and oceans is only the latest in a continuously shifting tableau, and it won't be the last.  Because we now have a pretty good idea of the motions within the known fault lines, we can run the clock forward and find out where things are likely to be in the future.

And the picture, unfortunately, doesn't look all that great.

The Atlantic Ocean, currently widening, will begin to close up, and by 250 million years in the future all that will be left of it will be two shallow landlocked seas.  Almost all the Earth's land surface area will have coalesced into a single supercontinent, which geologists have nicknamed "Pangaea Ultima" -- a misnomer, as "ultima" means "last" and this isn't the first time this has happened, and it won't be the last.  The thing is, the projection is that this gigantic land mass will be aligned along the equator, one of three factors that are projected to make this a hot time for land-dwelling organisms on planet Earth -- the other two are the carbon dioxide released because of the widespread volcanism predicted to take place as everything smashes together, and the fact that by then, the astronomers are telling us the Sun is going to be 2.5% brighter than it is now.

The geologists are making dire predictions about what this will do to terrestrial life on Earth -- mass extinction being the gist of it.  

"It does seem like life is going to have a bit more of a hard time in the future," said Hannah Davies, a geologist at the GFZ German Research Centre for Geosciences in Potsdam, who co-authored the study.  "It’s a bit depressing...  There have been extinction events in the past, and will be extinction events in the future.  I think life will make it through this one.  It’s just kind of a grim period."

Well, okay, it'd be grim if you took the species around today (humans included) and teleported them into Pangaea Ultima.  None of us would last very long.  But I'm not quite as pessimistic as Davies is about life in general.  This change to a planet dominated by deserts -- something more like Arrakis or Tatooine than the lush and verdant planet we now have -- won't happen overnight, and it's sudden change that usually triggers mass dieoffs.  Sure, it's likely that there will be a whole different suite of species than there is now, but hell, we're talking about 250 million years, so that was going to happen anyhow.

Give species time to adapt, and they do.  As Ian Malcolm put it, "Life... uh... finds a way."

Now, whether we (or more accurately, our descendants) are amongst those species that make it that far remains to be seen.  Very few species survive for 100 million years, much less 250.  But honestly, right now I'm more concerned with whether we'll get our comeuppance for our rampant pollution, out-of-control resource use, and burning of fossil fuels in a hundred years; let the hundreds of millions of years take care of themselves.

So that's our latest look at a future that really isn't as depressing as the scientists are claiming it is.  Although it's a little sobering to think that our descendants could be the Jawas and Tusken Raiders of the future Earth.  Those things are freakin' creepy.

And don't even get me started about Sandworms.

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

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!]