Continental bombardment

One of the reasons science is so useful is that our intuition about how things work is so often wrong.

A good example is the classic physics thought-experiment about taking two bullets -- one loaded in a gun that has the barrel perfectly horizontal, the other one held in your hand at the same height.  You fire the gun over level ground, and simultaneously let go of the bullet.  Which hits the ground first?

It seems like they should take different amounts of time; the one shot from the gun is traveling much farther, for one thing.  Most people think because of that, the dropped bullet would hit the ground first.  In fact, you undoubtedly know that (omitting the effects of air resistance or uneven terrain), the two bullets hit the ground at precisely the same time; Isaac Newton showed that the horizontal and vertical components of velocity are completely independent of one another.  It doesn't matter that the shot bullet is traveling rapidly in the horizontal direction; it and the dropped bullet have exactly the same vertical acceleration, namely 9.8 meters per second per second downward, starting from rest.  Thus they take exactly the same amount of time to hit the ground.

I was reminded of another example of this by some cool new research (which I will get to presently) I ran across yesterday.  It has to do with geology, namely, what the crust and mantle of the Earth are like.  It seems common-sensical that the surface of the Earth is uniformly cool and rocky, and the interior (judging by volcanoes) is molten; and while that isn't wrong in a broad-brush sort of way, what it misses is that there's a big difference between the rocks currently under your feet and the rocks at the bottom of the deep ocean.  Continental crust is thick, and extends both upwards into the air and downward into the mantle, a little like an iceberg; the rocks that make up the continents are, on the whole, lighter than oceanic crust, which is thin, brittle, and dense.  So the continents are literally floating in the liquid rock of the upper mantle.

This, of course, is what gives rise to plate tectonics; those iceberg-like blobs of floating rock we call continents, and the thin, heavy slabs of deep oceanic crust, jostle around on the magma of the upper mantle, colliding, pulling apart, shifting, and subducting (one piece going underneath another), and that gives rise to most of the geologic processes you've heard about.

But here's where we run into a fascinating question; why is the chemistry of continental rock (and thus its density) so different than oceanic rock?

[Image licensed under the Creative Commons Eric Gaba (Sting - fr:Sting), Tectonic plates boundaries detailed-en, CC BY-SA 2.5]

A piece of research out of Curtin University (Australia), published this week in Geology, suggests a surprising answer: the material that makes up the cratons -- the large, stable blocks of rock that form the nuclei of continents -- is extraterrestrial in origin.

Chris Kirkland, lead author of the study, was looking at the age of rocks in cratons around the world, and found something curious; their production seemed to occur at (roughly) two hundred million year intervals.  The formation of these blocks of rock coincide with the points at which the Solar System was passing through an area of dense stars in the spiral arm of the Milky Way as it orbited the Galactic Center.

"From looking at the age and isotopic signature of minerals from both the Pilbara Craton in Western Australia and North Atlantic Craton in Greenland, we see a similar rhythm of crust production, which coincides with periods during which the Solar System journeyed through areas of the galaxy most heavily populated by stars," Kirkland said.  "When passing through regions of higher star density, comets would have been dislodged from the most distant reaches of the Solar System, some of which impacted Earth.  Increased comet impact on Earth would have led to greater melting of the Earth’s surface to produce the buoyant nuclei of the early continents... Linking the formation of continents, the landmasses on which we all live and where we find the majority of our mineral resources, to the passage of the Solar System through the Milky Way casts a whole new light on the formative history of our planet and its place in the cosmos."

Of course, we've known for a while that all of the rock on Earth ultimately came from the coalescence of asteroids as the Solar System formed; but it's weird to think that the rock we're currently sitting atop may have been thrown at us by the near passage of other stars to our Sun as the entire Solar System hurtled its way around its host galaxy.  Whether Kirkland's claim will bear out under scrutiny, I don't know; but what's certain is that the methods of science has opened our eyes to a myriad processes that would have been entirely opaque to our so-called common sense.  Yes, scientists do get it wrong sometimes; they're fallible, and can misinterpret data or get hung up on their biases just like anyone.  But only science provides a protocol for catching and fixing those mistakes.

So it may not be perfect -- but for getting near to the truth, science really is the only game in town.

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Diamonds in the rough

Sure, diamonds are pretty and sparkly and rare and valuable, but do you know how they form?  Because that's honestly the coolest thing about them.

Diamonds are found in geological formations called kimberlite pipes.  This is a structure shaped like a long, narrow ice cream cone, extending downward into the Earth (how far downward we'll get to in a moment), and characterized by some rocks and minerals you usually don't find lying around -- chromium-rich pyrope garnets, forsterite, and various types of ultramafic (low-silica igneous) rocks that break down to a very specific kind of clay.  Jewel hunters long ago figured out that diamonds were likely to be found in association with these rocks and minerals, and used those as indicators of where to look -- such as the diamond-rich Kimberly region of South Africa (which gave its name to kimberlite), a couple of spots in Greene and Indiana Counties, Pennsylvania, and the Udachnaya area of Siberia.

[Image licensed under the Creative Commons Rob Lavinsky, iRocks.com – CC-BY-SA-3.0]

All of that's just background, though.  Here's the cool part, if (like me) you like things that are big and powerful and scary and can kill you.

Geologists discovered more or less simultaneously that the composition of kimberlite pipes is consistent with magma found in the (very) deep mantle, and that known kimberlite pipes extend a (very) long way down.  The best models indicate that the eruption that forms them starts on the order of four hundred kilometers below the surface of the Earth, making it the deepest known volcanic feature.

No one knows what triggers the eruption to begin.  It seems to be a rare occurrence, whatever it is.  Fortunately.  Because once it starts, and the magma moves upward through the mantle, the drop in pressure makes dissolved gases bubble out, rather like popping the cork off a bottle of champagne.  This speeds up the movement, which lowers the pressure more, so more gas bubbles out, and so on and so forth.  Also -- gases expand as the pressure drops, so the higher it rises, the more volume it displaces.

The result is what's called a diatreme.  What seems to happen is that with no warning, there's a Plinian eruption -- the same sort that destroyed Pompeii and Herculaneum -- but moving at supersonic speeds.  Imagine what it must look like -- from a distance, preferably -- everything is calm, then suddenly a several-kilometer-wide chunk of land gets blown up into the stratosphere.  The conical hole left behind fills with material from the deep mantle (thus its odd composition by comparison to other igneous rocks).  Give it a few million years, and weathering results in the characteristic clay found in a typical kimberlite.

So what's all this got to do with diamonds?

Well, in the intense heat and pressure of the eruption, some of the carbonate ions in minerals in the magma are reduced to elemental carbon, and that carbon is compressed to the point that its crystalline structure changes to a hexoctahedral lattice.  The result is a transparent crystal that looks nothing like the soft, black, powdery stuff we picture when we think of carbon.  (Further illustrating that bonding pattern is everything when it comes to physical properties.)

The reason all this comes up is a discovery described in a press release from the University of British Columbia that I found out because of a friend and loyal reader of Skeptophilia.  Kimberlite pipes are not only unusual, they differ from each other, so the composition of each acts as a geological fingerprint.  So when a UBC geologist named Maya Kopylova tested samples of a kimberlite on Baffin Island, she found that its composition was inconsistent with the rocks of the nearby geological province -- the nearest rocks it matched were in Labrador, almost two thousand kilometers away.

This was sufficient to identify it as part of the North American craton, a (relatively) stable piece of continental crust that currently extends from eastern Canada, through southern Greenland, and over to Scotland.  (It was torn into chunks when the Mid-Atlantic Rift Zone formed on the order of two hundred million years ago, breaking up what was the supercontinent of Pangaea.)

How a chunk of a billion-year-old craton ended up two thousand kilometers away is uncertain, but it does give us a lens into how the continents have shifted during geologic history.  "The mineral composition of other portions of the North Atlantic craton is so unique there was no mistaking it,"  Kopylova said.  "It was easy to tie the pieces together.  Adjacent ancient cratons in Northern Canada—in Northern Quebec, Northern Ontario and in Nunavut—have completely different mineralogies...  Finding these 'lost' pieces is like finding a missing piece of a puzzle.  The scientific puzzle of the ancient Earth can’t be complete without all of the pieces."

Cool, too, that the discovery was made using remnants of what is very likely to be one of the most unpredictable and violent geological events on Earth.  (Okay, the formation of igneous traps is worse.  But still, kimberlites should surely come in second.)  The universe never ceases to fascinate me, and I'm always struck by the fact that no matter how much you know, there's always more to find out.

More, too, to worry about.  Although considering the current state of affairs, a supersonic volcanic eruption might actually lighten everyone's mood.

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Any guesses as to what was the deadliest natural disaster in United States history?

I'd speculate that if a poll was taken on the street, the odds-on favorites would be Hurricane Katrina, Hurricane Camille, and the Great San Francisco Earthquake.  None of these are correct, though -- the answer is the 1900 Galveston hurricane, that killed an estimated nine thousand people and basically wiped the city of Galveston off the map.  (Galveston was on its way to becoming the busiest and fastest-growing city in Texas; the hurricane was instrumental in switching this hub to Houston, a move that was never undone.)

In the wonderful book Isaac's Storm, we read about Galveston Weather Bureau director Isaac Cline, who tried unsuccessfully to warn people about the approaching hurricane -- a failure which led to a massive overhaul of how weather information was distributed around the United States, and also spurred an effort toward more accurate forecasting.  But author Erik Larson doesn't make this simply about meteorology; it's a story about people, and brings into sharp focus how personalities can play a huge role in determining the outcome of natural events.

It's a gripping read, about a catastrophe that remarkably few people know about.  If you have any interest in weather, climate, or history, read Isaac's Storm -- you won't be able to put it down.

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