The story of the Scablands

After my rather cataclysmic posts about geology last week, which looked at the volcano off the Greek island of Thera and the fault just waiting to rupture near the Pacific Northwest, one of my friends and a long-time loyal reader of Skeptophilia asked me if I'd ever done a post on the "Channeled Scablands."  I told him I'd mentioned it once or twice, but always in passing.

So as befits a catastrophe so big it beggars belief, I thought a more thorough look was warranted.

There's a bizarre bit of terrain in what is now eastern Washington and Oregon that goes by the rather horrid-sounding name my friend referenced, and if you ever fly over it, you'll see why.  It's formed of teardrop-shaped pockets of relatively intact topsoil surrounded by gullies floored with bare rock.  The terrain has the look of what a shallow stream does to a sandy beach as it flows into sea, only on a gargantuan scale:

[Image licensed under the Creative Commons DKRKaynor, Channeled Scablands, CC BY-SA 4.0]

Then there's Dry Falls, in the upper Grand Coulee Basin, which even has a plunge pool basin at its foot... but almost no water:

[Image licensed under the Creative Commons © Steven Pavlov / http://commons.wikimedia.org/wiki/User:Senapa / CC BY-SA 3.0, Dry Falls (Washington), CC BY-SA 3.0]

Geologists figured out pretty quickly that the entire terrain was sculpted by a huge amount of running water.  But the problem is, the entire area is a desert, and apparently has been for a long while.

So where'd all the water come from -- and where did it go?

The answer turned out to be the Missoula Megaflood -- a tremendous flood (thus the name) that occurred eighteen thousand years ago, and which is right up there with Thera and the Cascadia Subduction Zone on our list of things that are big and scary and can kill you.

What apparently happened is that during the last ice age, a glacial dam formed across the northern Idaho Rockies, blocking the outflow of what are now the Columbia, Snake, Okanogan, Pend Oreille, and Spokane Rivers.  As the climate warmed, the ice melted, but all that water had nowhere to go, so it backed up into an enormous lake -- called Lake Missoula -- that covered a good bit of what is now western Montana. 

As the ice age ended, the warming continued.  Eventually this caused the ice dam to collapse, and all that water drained out, sweeping across what is now eastern Washington, and literally scouring the place down to bedrock. 

What made the flood even worse was a phenomenon called isostasy.  We're used to thinking of the tectonic plates as moving back and forth, more or less parallel to the Earth's surface, but what is less obvious is that they can also move up and down -- perpendicular to the surface, like ice cubes bobbing in a glass of sweet tea.  These chunks of the Earth's crust are actually floating in the semi-solid mantle beneath them, and the level they float is dependent upon how heavy they are, just as putting heavy weights in a boat makes it float lower in the water. 

Well, as the Cordilleran Ice Sheet melted, that weight was removed, and this caused the flat piece of crust underneath it to lift upward on its eastern edge.

The whole western corner of the United States tilted toward the Pacific Ocean.  It's like having a full bowl of water on a table, and lifting one end of the table. The bowl will dump over, spilling out the water, and it will flow downhill and run off the edge -- just as Lake Missoula did.

The result was a colossal flood that at its peak was traveling at an estimated one hundred kilometers an hour.  Dry Falls was, at that point, an enormous waterfall five times the width of Niagara Falls, with a flow rate ten times higher than all of the rivers in the world combined.

[Nota bene: This sort of isostatic tilt is still going on today, most notably underneath Great Britain.  During the last ice age, Scotland was completely glaciated; southern England was not.  The melting of those glaciers has resulted in isostatic rebound, lifting the northern edge of the island by ten centimeters per century.  The problem is, the whole country is connected (however a lot of Scottish people might wish otherwise), so the entire island is tipping like a teeter-totter.  The tilt is pushing southern England downward, and it's sinking, at about five centimeters per century.  Fortunately, there's no giant lake waiting to spill across the country.]

These kinds of megafloods aren't uncommon, usually during the transition between a glacial and an interglacial period.  Another place this happened is not far from where I live -- around thirteen thousand years ago, there was an ice dam across what is now the St. Lawrence River, blocking the main outflow from the Great Lakes.  This backed all that water up into a huge lake called Lake Agassiz, which encompassed all of the Great Lakes and then some.  It was the same story as Lake Missoula; as the climate warmed, the ice dam collapsed, and a large percentage of the water drained out into the North Atlantic.  Some climatologists think that this may have been responsible for the Younger Dryas event, when the warming trend in North America suddenly reversed, the temperatures dropping in only a few decades by an estimated 7 C -- because all that fresh water temporarily shut down the North Atlantic Meridional Overturning Circulation, stopping the Gulf Stream in its tracks and sending eastern North America, Greenland, and western Europe back into the deep freeze for over a thousand years.  (The link between the draining of Lake Agassiz, the Younger Dryas, and the shutdown of the NAMOC is far from settled, however.)

This sort of thing, besides being fascinating in its own right, always makes me think of the people who talk about Earth being "fine tuned" for human habitation.  When you start looking into geology, climatology, and astronomy, you realize how tenuous our existence actually is.  That we've done as well as we have, despite all the natural calamities, is impressive, but our continued survival is hardly guaranteed.  We'd better start understanding the past history of our home, including the violent and uncomfortable bits -- lest our own reckless and heedless actions trigger a catastrophe we're ill prepared to deal with.

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

Analysis of a cold snap

Almost exactly 12,800 years ago -- and yes, we know it to that degree of accuracy -- there was a sudden plunge in the global temperature.

It's known as the "Younger Dryas" event, after the steppe wildflower Dryas octopetala, which only grows well when the conditions are very cold in winter.  The proxy records (bubbles in ice cores, patterns of glaciation, and types of pollen found in ice and sediments -- such as the aforementioned Dryas) are all in good agreement that in only ten or twenty years, the temperature in the Northern Hemisphere plunged, in some places by as much as 6 C.

[Image licensed under the Creative Commons Bjoertvedt, Dryas octopetala IMG 5641 reinrose reinsdyrflya, CC BY-SA 3.0]

That may not seem like very much, but six degrees is huge.  In fact, the word that comes to mind is "catastrophic."  The glaciers that had been receding -- this is, or at least was, an interglacial period -- suddenly began to extend their reach.  The cold period didn't abate for over a thousand years, with enormous impact on the humans around at the time.  The Younger Dryas correlates with the collapse of two of the dominant cultures, the Clovis civilization of North America and the Natufian culture of the Middle East.

But what could cause such a sudden and calamitous change in the temperature?

For years, the culprit was thought to be Lake Agassiz, a colossal freshwater lake that encompassed all five of the Great Lakes (and a lot more square milage as well), which was held back by an ice dam across what is now the Saint Lawrence Seaway.  As the temperature warmed -- remember, interglacial period -- the dam became unstable and finally collapsed, causing a humongous (I'm running out of words for "big," here) outflow of cold fresh water into the North Atlantic.  The result was a drastic slowing of the North Atlantic Meridional Turnover, which powers the Gulf Stream and keeps the Northeastern United States, Great Britain, Iceland, and Scandinavia at least reasonably warm.  The Turnover is caused by saline water (which is denser) sinking south of Iceland, and when the ice dam collapsed and the lake drained, the entire North Atlantic was covered by a sheet of water that was too fresh to sink.  The result: a slowdown of the circulation, and a return of glacial conditions.

Another, more far-fetched possibility is that the Earth got blasted by the shock wave of a supernova in the constellation Vela.  There is good evidence that the Vela supernova was coincident with the beginning of the Younger Dryas -- but connecting this to the drop in temperature is a bit of a stretch for most climatologists.

Recently, a third option has been gaining strength, and that's the fallout from the impact of a comet or meteor.  Here, the idea is that the debris thrown skyward by the impact blocked sunlight and caused a drop in temperature.  The impact hypothesis just got a boost last week with a paper in Scientific Reports, about a microanalysis of sediments from a place called Abu Hureyra that show good evidence of being flash-fried 12,800 years ago.

The sediments were collected decades ago, because the site itself was drowned when the Taqba Dam was put in place in 1970.  Archaeologists figured they better get what they needed from Abu Hureyra before the waters rose, and that included samples of everything they could get their hands on.  And an analysis by a team led by Andrew Moore of the Rochester Institute of Technology found that the bits of rock and other debris from the site dating to -- you guessed it, 12,800 years ago -- were coated with melted glass, indicative of a temperature of at least 2,200 C.

"To help with perspective," said James Kennett of University of California-Santa Barbara, who co-authored the paper, "such high temperatures would completely melt an automobile in less than a minute."

So the impact hypothesis is sounding more and more plausible.  What this kind of research always brings home for me, though, is how fragile the Earth's climate balance is.  Climate change deniers like to point out that there have been climatic ups and downs in the past, and the Earth has recovered; what they seldom add is that those ups and downs often resulted in mass extinctions.  So sure, the temperature rebounded after the Abu Hureyra collision.

A fat lot of good that did for the Pleistocene megafauna, such as mastodons, dire wolves, North American camels, and gomphotheres -- a bizarre North American elephant relative.  The humans didn't do much better; the ones who didn't get cooked and/or flattened by the impact very likely starved to death because of the mass die-off of plants and animals in the years following the collision.  How many made it through the bottleneck, and became our direct ancestors, is unknown, but it probably wasn't many.

So that's our cheery scientific discovery of the day.  A meteor impact triggering temperatures hot enough to melt glass, followed by a shower of debris and a drastic drop in global temperature.  I'd like to think this would be a cautionary tale, showing the effect one event can have on the climate, but at this point I know better.  We've pretended that everything is hunky-dory and ignored the scientists so far, so we'll be fine, right?

Of course right.

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

This week's Skeptophilia book-of-the-week is brand new: Brian Greene's wonderful Until the End of Time.

Greene is that wonderful combination, a brilliant scientist and a lucid, gifted writer for the scientifically-inclined layperson.  He'd already knocked my socks off with his awesome The Elegant Universe and The Fabric of the Cosmos (the latter was made into an equally good four-part miniseries).

Greene doesn't shy away from difficult topics, tackling such subjects as relativity, quantum mechanics, and the nature of time.  Here, Greene takes on the biggest questions of all -- where the universe came from, how it has evolved and is evolving, and how it's going to end.

He begins with an observation that as a species, we're obsessed with the ideas of mortality and eternity, and -- likely unique amongst known animals -- spend a good part of our mental energy outside of "the now," pondering the arrow of time and what its implications are.  Greene takes a lens to this obsession from the standpoint of physics, looking at what we know and what we've inferred about the universe from its beginnings in the Big Bang to its ultimate silent demise in the "Heat Death" some billions or trillions of years in the future.

It's definitely a book that takes a wide focus, very likely the widest focus an author could take.  And in Greene's deft hands, it's a voyage through time you don't want to miss.

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

Confidence, impacts, and ice ages

One of the most common misunderstandings about science by laypeople centers around the concept of degree of confidence.

This misunderstanding can be summed up that "all unproven hypotheses are equally likely."  You hear it with lots of loopy ideas -- that (for example) because we don't have strong evidence one way or the other regarding the existence of an afterlife, it's on the same footing as other phenomena for which we have no direct evidence, such as dark matter, time travel, and the claim we've been visited by extraterrestrials.

Another way this shows up is the dismissive, "all of this could be proven wrong tomorrow" attitude toward science.  The fact that new discoveries have on occasion overturned what we thought we understood is taken as evidence that all of science is on thin ice, that it's all equally tentative.  But this rests on a serious misapprehension about the reliability of evidence.  It's true that, as Einstein allegedly put it, "one experiment could prove wrong" either the Second Law of Thermodynamics or our understanding of the mechanisms of quantum entanglement; but the first is extremely unlikely (the Second Law is one of the most extensively-tested scientific principles known, and there has never been a single exception found to it) while even the physicists would admit the second is a possibility (we're still elucidating the idea of quantum entanglement, and new and intriguing data is being added to our understanding on nearly a daily basis).

This frustrates people who like to have certainty, or at least like to be able to say with confidence that something isn't possible.  I ran into an especially good example of this just yesterday when I was reading an article about the Younger Dryas, a mysterious climatological reversal that occurred 12,900 years ago and lasted only 1,200 years -- a mere blip on the geological time scale.  What happened was that during a period when the Earth was warming, in only a few decades the average temperature of the Earth dropped by an average of four degrees Celsius, enough to put most of the Northern Hemisphere back in the deep freeze.  (The event is named after a plant, Dryas octopetala, which only grows in extremely cold places, and which became common across Europe and North America through the duration of the temperature drop.)

[Image licensed under the Creative Commons Opioła jerzy, Dryas octopetala a4, CC BY-SA 3.0]

Of course, presented with such a conundrum, the first question that comes up is "Why did this happen?"  There are three main hypotheses:

• As the Earth was warming up after the last major glaciation, a huge freshwater lake that had piled up behind an ice dam was suddenly emptied when the dam collapsed.  This lake, nicknamed "Lake Agassiz," emptied out through what are now the St. Lawrence and Mackenzie Rivers, and caused a slowdown (or complete cessation) of the thermohaline circulation.  Put simply, this is the engine that powers the Gulf Stream, which brings warm water northward and keeps the northeastern United States and most of western Europe relatively temperate.  When the flood occurred, the north end of the thermohaline circulation became too fresh to sink, and the whole system ground to a halt, propelling us into another ice age.  It was only after a thousand years had passed, and the lake water had adequately mixed with the ocean water, that the circulation rebooted and things warmed back up.
• 12,900 years ago, the Earth was hit by an object from space -- probably either a comet or a meteorite -- and that collision flash-burned a significant fraction of the vegetation in northern North America.  The debris and ash blocked sunlight, cooling down the surface of the Earth and halting the warm-up we'd been experiencing in its tracks.  Eventually the ash settled out, the forests regrew, and the climate restabilized, but that took several centuries.
• A supernova in the constellation of Vela created a burst of radiation that destroyed the Earth's ozone layer and killed most of the Earth's megafauna, including mammoths, mastodons, dire wolves, and several species of temperate-climate camels, rhinos, and hippos.  The gamma radiation striking the atmosphere caused a cascade of chemical reactions that disrupted the balance of nitrogen-containing compounds (such as nitrous oxide and nitrogen dioxide), and this caused a sudden and drastic temperature drop.

Each has some points in its favor.  The ice-dam proponents argue that the temperature drop wasn't as fast as you'd expect from something catastrophic like a collision or supernova, and that in fact the extinctions that occurred were in species that had already been declining for millennia.  Scientists supporting the impact hypothesis were buoyed by the discovery of a previously-unknown crater in Greenland -- but they've been unable to pinpoint its age any more accurately than "some time between three million and twelve thousand years ago."  The supernova enthusiasts point to the existence of "black mats" -- thin layers of the remnants of anaerobic organisms -- as evidence that something drastic happened to the atmosphere at the beginning of the Younger Dryas, and samples taken from it do seem to have skewed nitrogen content.  (This same evidence is considered support for the impact hypothesis, because there have been "microspherules" -- tiny spheres of melted and refrozen metal -- found in some of those boundary layers.)  But the black mats in different locations seem to date from different time periods, with only three of the thirteen studied being coincident with the Younger Dryas event.  And most of the black mats studied don't contain microspherules.

So the argument is still out there.  As far as my own opinion, I can only say that I'm neither a paleoclimatologist nor an astrophysicist, so am unqualified to weigh in (and my opinion wouldn't mean much anyway).  It seems like the dam collapse model is the one that currently has the most support, but -- like all science -- new information could tilt us toward one of the others.

Why does this come up with regards to our confidence in scientific models?  Not only because it's a great example of competing explanations and the fact that good scientists are willing to entertain the possibility of alternate solutions to the conundrums they study.  The idea for this post came to me because of another twist on the Younger Dryas -- this one from noted wingnut Graham Hancock, who says that the Younger Dryas event not only inconvenienced the camels and dire wolves, it also wiped out an advanced technological civilization...

... which gave rise to the myth of Atlantis.

So this is what I mean about levels of confidence.  No, we haven't been able to rule out two of the three models for the cause of the Younger Dryas with any real certainty.  But the fourth idea -- that whatever caused the event also destroyed Atlantis -- has nothing, not a shred of evidence, to support it.  As the brilliant skeptic Jason Colavito put it:

[R]egardless of whether a comet hit, the existence (or non-existence) of the comet implies nothing about the existence of Atlantis any more than it would unicorns or leprechauns. 

It remains a point of astonishment that the bones of megafauna that supposedly died in the comet strike turn up with regularity, but every human being and all of the buildings, tools, and material possessions of the lost Atlantis-like civilization were blasted clean off the face of the Earth, without a single trace remaining.  I have trouble imagining that a sloth can manage to have its bones preserved for all time, but not a single outpost of Atlantis had even a single bolt or screw remain.

As do I.  All unproven assertions are not on an equal footing.  And that's really the point of Ockham's Razor, isn't it?  The fastest way to winnow down competing ideas is to see which ones require you to make the most ad hoc assumptions.  And I'd put any of the three scientific explanations I mentioned above ahead of Hancock's assertion that the Younger Dryas event destroyed the lost civilization of Atlantis.

I'm perfectly willing to stay in uncertainty, indefinitely if need be, in the absence of convincing evidence one way or the other.  But in the case of explanations that require us to stretch credulity to the snapping point, I have no problem saying, "Nope.  That one isn't true."

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

This week's Skeptophilia book recommendation is an entertaining one -- Bad Astronomy by astronomer and blogger Phil Plait.  Covering everything from Moon landing "hoax" claims to astrology, Plait takes a look at how credulity and wishful thinking have given rise to loony ideas about the universe we live in, and how those ideas simply refuse to die.

Along the way, Plait makes sure to teach some good astronomy, explaining why you can't hear sounds in space, why stars twinkle but planets don't, and how we've used indirect evidence to create a persuasive explanation for how the universe began.  His lucid style is both informative and entertaining, and although you'll sometimes laugh at how goofy the human race can be, you'll come away impressed by how much we've figured out.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]