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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