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478 grams.   Impactite



Note from the MPOD Webmaster on 3-D Viewing:

I cannot fuse the stereo-pair images as presented. They are too large for my aged eyeballs to bring together. If you have the same problem, try this:

1) Click on the picture to view the full-resolution photo.

2) Click the 'Smaller' button 10-15 times to shrink the picture to the point where you can fuse the images.

3) Hold the mouse over the 'Bigger' button, fuse the pictures, and click the mouse while keeping the images fused to see how large you can go.

More Info on 3-D Viewing

Steve writes:

Pica Glass, like the 478-gram example shown in Photos 1 through 3, is a dark grey-green impact silicate; it’s found in several Late Pleistocene sediments along a 47-mile (75 km) strewn field near the town of Pica in northern Chile’s Atacama Desert. Thousands of pieces have been collected from five areas, each between one and three kilometers across (see Photo 4, modified slightly from an original graphic by Peter H. Schultz et al. – Pica is near the second circled area from the left).

Originally discovered in 2012, competing theories regarding Pica Glass’ formation occurred in 2016 and 2017. The first described a near-ground cometary explosion some 12,000 years ago (a time of rapid climate change in the Southern Hemisphere and just after the area became inhabited by proto-Archaic hunter-gatherers, who likely observed the event). The second theory, noting twigs and other plant material seen trapped in some pieces of Pica Glass, postulated that massive wildfires during exceptionally dry periods melted soil with accumulated plant material to create glass, similar to a 1961 Australian haystack blaze that produced 16 tons of hay-silica from 325 tons of pasture plants (unlike today, 12 millennia ago the Atacama Desert was populated with rivers and oases with trees and grassy wetlands).

But a 2021 paper described analyses of 70 thinly sliced Pica Glass samples that identified nickel-rich minerals, troilite, cubanite, and other exotic substances commonly found in meteorites but unrelated to the local rocks, as well as trapped extraterrestrial dust that strongly resembled carbon-rich particles like those seen in CI chondrites and the Comet 81P/Wild material collected during NASA's Stardust mission. Additionally, the team found embedded zircon crystals and baddeleyite (zirconium dioxide) that can only form when temperatures exceed 1670° C (3000° F) – typical of a meteorite impact, but twice that of the hottest terrestrial forest fires. These results confirmed the 2016 theory of Pica Glass’ origin, with Brown University professor Peter Schultz concluding “This is the first time we have clear evidence of glasses on Earth that were created by the thermal radiation and winds from a fireball exploding just above the surface”. (Like the 2016 paper, the 2021 researchers also describe a cometary airburst, but I’m not smart enough to understand how they decided on a comet versus a meteor as the near-impactor.)

As an interesting aside, the formation of Pica Glass occurred at a time not inconsistent with the Younger Dryas event – a suspected massive cosmic impact that caused major climate shift, megafaunal extinctions, and corresponding changes in human cultures (the Younger Dryas boundary layer is observed across Greenland, North America, Central America, parts of South America, most of Europe, and parts of the Middle East – it contains excess platinum, osmium, quench-melted glass, carbon spherules, high temperature minerals, nanodiamonds, and other materials consistent with an extraterrestrial impact event).

Often opaque but sometimes semi-translucent, Pica Glass is quite light, often frothy, and can exhibit folds, twists, rolls, sliding, shearing, and flowlines. Intense heat from the air-burst explosion and the supersonic winds accompanying it not only melted the surface soil and plants, but ejected melted material into the air that formed the myriad of shapes seen in typical examples. Rapid cooling forced hot gas bubbles to the surface to create the vesicles observed in most pieces. And the fact that there are five major, but separate accumulations of Pica Glass suggest the comet may have broken up into multiple pieces during its plunge through our atmosphere.

Photos 5 and 6 provide 3D crossed-eyes and red-cyan anaglyph views of the front of the piece in today’s post – in it, one can better see the complex features typical of this glass.

Photos 7 and 8 are 3D crossed-eyes and red-cyan anaglyph photomicrographs showing an area exhibiting an almost foamy texture (one can see in Photos 9 and 10 a frothy vesicle inside of a more traditionally glassier exterior).

Photos 11 and 12 are 3D crossed-eyes and red-cyan anaglyph photomicrographs of an area with a secondary kind of lighter material that is occasionally seen in small areas of this specimen.

And finally, Photos 13 and 14 are 3D crossed-eyes and red-cyan anaglyph photomicrographs of a tubular “tunnel” (right of center) that shows the material changing with depth (the bubbles within bubbles just under and to the left of center is kind of interesting too).

IMPORTANT NOTE: As Paul has pointed out in my prior posts with 3D images, it is easier to start with them scaled as small as possible, then increase the size while maintaining focus. This advice is most applicable to the crossed-eyes 3D images, but it also helps with the red-cyan anaglyphs having significant depth.