Deep impact

Tektites are curious, glassy blobs of rock, from millimeters to centimeters in diameter.  At first thought to be similar to obsidian (volcanic glass), formed when silica-rich lava cools too quickly to form crystals, it soon became apparent that tektites were something else entirely.  They have strangely pitted surfaces, are often teardrop-shaped, and (once such studies became possible) they were found to have an entirely different chemistry than obsidian.  Most puzzling was the fact that tektites are most often found in circumscribed geographical regions nicknamed "strewnfields" -- which usually were nowhere near recently-erupted volcanoes.

It wasn't until the 1920s that geologist Franz Eduard Suess proposed the theory now accepted today, and coined the name tektite (from the Greek τηκτός, "molten").  Tektites form when a meteorite strikes the Earth, liquefying the rock on the surface upon impact.  The molten rock is thrown outward from the blast site, creating the circular or elliptical "strewnfield" -- and explaining why the blobs thus created don't match the chemistry of igneous rock.  Their composition is different depending on the nature of the rock at the location where the meteorite struck.

[Image licensed under the Creative Commons James St. John, Indochinite tektite (Pleistocene, 783-803 ka; Australasian Tektite Strewn Field, southeastern Asia) 4, CC BY 2.0]

So, you'd think once Suess said, "These are formed when a bigass rock slams into the ground" (I paraphrase him slightly), finding the crater where the thing landed would be easy, right?  Just draw a circle around the strewnfield and then look in the middle?

Wrong.

There's a relatively recent strewnfield -- on the order of 790,000 years old, which is a snap of the fingers, geologically speaking -- that is abso-freaking-lutely huge.  It extends from southern China to Antarctica (going north-south) and from the floor of the middle of the Indian Ocean to Micronesia (going west-east).  And that's just where the tektites have been definitively identified.  By some estimates, the Australasian strewnfield might cover thirty percent of the Earth's surface.

But the location of the crater proved elusive.  Part of it is that the center of the strewnfield is in Southeast Asia, which is (mostly) impenetrable jungle, and in places the terrain is so steep and rugged as to be nearly impassable.  But despite the difficulties, geologists have finally located the crater, and also determined why it wasn't obvious despite how recently it occurred.

The Australasian meteorite struck a spot in Laos that already had an active volcano.

The heat from the impact did two things -- flung blobs of molten rock all over the place (the tektites geologists later found in the strewnfield), and also triggered a massive eruption, producing a large enough lava flow to fill in and bury the crater.

[Map from Sieh et al.]

What I find most astonishing about all this is that the impact of this gigantic rock, only 790,000 years ago, didn't cause climatic chaos and a resulting extinction event.  Our relatives, Homo erectus, were living and apparently thriving in southern China both during and after the impact, and seem to have been none the worse for the event.  (If some of them were in Laos, they were probably deep-fried; but given that there was an active volcano there anyhow...) 

I wonder if the reason for the relatively low environmental impact had to do with the geology of the place the meteorite hit, which was primarily made of basalt and other hard igneous rocks.  The Chicxulub strike, 66 million years ago, was devastating not only because it was so big, but because it hit a formation of shallow marine limestone, which literally vaporized on impact, creating a shock wave of superheated water vapor and carbon dioxide that incinerated everything within a radius of a thousand kilometers.  There has to be more to it than simply size; the two weren't that different, an estimated two kilometers in diameter for the Australasian impact and between ten and twelve for Chicxulub.

Whatever the reason was for the difference, it's a good thing for us, because another Chicxulub-type event 790,000 years ago, and we'd very likely not be here.

In any case, it's pretty cool that we can use the splash patterns of molten debris to identify the location of a meteorite impact almost eight hundred thousand years after it happened, despite the fact that the whole thing was filled in with lava and overgrown by jungle.  Further underscoring my bafflement over how anyone can not find science amazingly cool.

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Snapshot of a very bad day

Some of you have probably bumped into articles in the last week or so about a phenomenal discovery in paleontology -- a fossil bed in North Dakota that may have been created the day the Chicxulub Meteor Strike occurred, 66 million years ago.  This single event is thought to have flash-fried everything in the southern half of what is now North America, changed climates worldwide, and was the death blow to the dinosaurs, with the exception of the lineage that led to modern birds.

The deposit contains exquisitely preserved remains of a variety of fish, plants, dinosaurs, and mollusks.  The gills of the fish contained huge numbers of tektites -- tiny spheres of glass formed during a meteorite collision and ejected into the atmosphere.  The impact is thought to have caused a magnitude 10 earthquake (almost unimaginable to me), which took the shallow ocean that crossed what is now the central United States and "agitated it like a washing machine" -- creating a seiche, a standing wave like the sloshing of water in a giant bathtub.

The seiche caused the repeated exposure and inundation of shallow regions, and while exposed, the stranded animals were subjected to a rain of tektites and other debris thrown up by the collision.

"This is the first mass death assemblage of large organisms anyone has found associated with the K-T boundary," Robert DePalma, curator of paleontology at the Palm Beach Museum of Natural History in Florida and a doctoral student at the University of Kansas, said in a press release.  "At no other K-T boundary section on Earth can you find such a collection consisting of a large number of species representing different ages of organisms and different stages of life, all of which died at the same time, on the same day."

One of the fish from the Hell Creek fossil bed

"The seismic waves start arising within nine to ten minutes of the impact, so they had a chance to get the water sloshing before all the spherules (small spheres) had fallen out of the sky," said Mark Richards, professor emeritus of earth and planetary science at the University of California - Berkeley.  "These spherules coming in cratered the surface, making funnels — you can see the deformed layers in what used to be soft mud — and then rubble covered the spherules.  No one has seen these funnels before...  You can imagine standing there being pelted by these glass spherules.  They could have killed you."

It's amazing to think that if these scientists are correct -- and the consensus amongst paleontologists is that they are -- we're seeing a remnant of a catastrophe initiated at a single moment in time.  The simulations of what happened are astonishing enough:

But somehow, to see the remains of animals that were directly killed by the collision, who were there when it happened, gives it an immediacy that is stunning.

So this is cool enough, right?  But what makes it even more personal for me is that one of the researchers who has worked the Hell Creek fossil bed, and was a co-author of the paper...

... is Loren Gurche, who is a former student of mine.

I distinctly remember Loren's contributions to my AP Biology class -- whenever the topic was prehistory, I always deferred to his greater knowledge.  Even then, when he was in 11th grade, he clearly knew way more paleontology than I did, or probably, than I ever would.  The presence of a true expert enriched both my experience and the other students', and it's thrilling to see that he is making significant contributions in a field about which he is so deeply passionate.

So the whole thing is doubly cool for me to read about.  I'm looking forward to more discoveries by Loren and the team he's working with, although it must be said it'll be hard to top this one.  This snapshot of one of the worst disasters ever to strike the Earth is the find of a lifetime.

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This week's Skeptophilia book recommendation combines science with biography and high drama.  It's the story of the discovery of oxygen, through the work of the sometimes friends, sometimes bitter rivals Joseph Priestley and Antoine Lavoisier.   A World on Fire: A Heretic, an Aristocrat, and the Race to Discover Oxygen is a fascinating read, both for the science and for the very different personalities of the two men involved.  Priestley was determined, serious, and a bit of a recluse; Lavoisier a pampered nobleman who was as often making the rounds of the social upper-crust in 18th century Paris as he was in his laboratory.  But despite their differences, their contributions were both essential -- and each of them ended up running afoul of the conventional powers-that-be, with tragic results.

The story of how their combined efforts led to a complete overturning of our understanding of that most ubiquitous of substances -- air -- will keep you engaged until the very last page.

[Note:  If you purchase this book by clicking on the image/link below, part of the proceeds will go to support Skeptophilia!]