Large patches of glassy rock in the Chilean desert, probably created by an ancient exploding comet

PROVIDENCE, RI [Brown University] – About 12,000 years ago, something burned a large shard of the Atacama Desert in Chile with heat so intense that it turned the sandy soil into widespread plates of silicate glass. Now a research team studying the distribution and composition of these glasses has come to a conclusion about what caused the inferno.

In a study published in the journal Geology, researchers show that samples of the desert glass contain tiny fragments of minerals that are often found in rocks of extraterrestrial origin. These minerals closely match the composition of material returned to Earth by NASA’s Stardust mission, which tested the particles from a comet called Wild 2. The team concludes that these mineral collections are probably the remains of an extraterrestrial object – most likely a comet – which flowed down after the explosion that melted the sandy surface below.

“This is the first time we have clear evidence of glasses on Earth created by thermal radiation and wind from a fireball exploding just above the surface,” said Pete Schultz, professor emeritus at Brown University’s Department of Earth, Environmental and Planetary Sciences. “To have such a dramatic effect on such a large area, it was a really massive explosion. Many of us have seen boled fireballs streak across the sky, but those are small blips compared to this.”

Image or glass deposits
The glass deposits stretch along a 75-kilometer-long corridor in northern Chile.

The glasses are concentrated in spots across the Atacama Desert east of the Pampa del Tamarugal, a plateau in northern Chile located between the Andes Mountains to the east and the Chilean coastal area to the west. Fields of dark green or black glass occur within a corridor that stretches for about 75 kilometers. There is no evidence that the glasses could have been created by volcanic activity, Schultz says, so their origin has been a mystery.

Some researchers have claimed that the glass originated from ancient grass fires, as the region was not always desert. During the Pleistocene, there were oases of trees and grassy wetlands created by rivers stretching from mountains to the east, and it has been suggested that widespread fires may have burned hot enough to melt the sandy soil into large glassy plates.

But the amount of glass present along with several important physical properties make simple fires an impossible formation mechanism, the new research found. The glasses show signs of having been twisted, folded, rolled and even thrown while still in molten form. It is consistent with a major incoming meteor and air eruption explosion, which would have been accompanied by tornado force winds. The mineralogy of glass further casts serious doubt on the grass bonfire idea, Schultz says. Together with researchers from the Fernbank Science Center in Georgia, the Chilean Universidad Santo Tomás and the Chilean Geology and Mining Service, Schultz and colleagues performed a detailed chemical analysis of dozens of samples taken from glass deposits throughout the region.

The analysis found minerals called zircons that were thermally degraded to form baddeleyite. This mineral transition typically occurs at temperatures above 3,000 degrees Fahrenheit – far warmer than what could be generated by grass fires, Schultz says.

Microscope image of glass
Analysis of the samples revealed a mineralogy in accordance with a comet origin.

The analysis also showed collections of exotic minerals found only in meteorites and other extraterrestrial rocks, the researchers say. Specific minerals such as cubanite, troilite, and calcium-aluminum-rich inclusions matched mineral signatures from comet samples taken from NASA’s Stardust mission.

“These minerals are what tell us that this object has all the markings of a comet,” said Scott Harris, a planetary geologist at the Fernbank Science Center and study co-author. “Having the same mineralogy that we saw in the Stardust samples brought about in these glasses is a really powerful proof that what we are seeing is the result of a cometic air eruption.”

More work needs to be done to determine the exact age of the glass, which will determine exactly when the event took place, Schultz says. But the preliminary dating puts the effect just around the time that large mammals disappeared from the region.

“It’s too early to say whether there was a causal link or not, but what we can say is that this event took place around the same time as when we think the megafauna disappeared, which is exciting,” Schultz said. . “There is also a chance that this was actually seen by early settlers who had just arrived in the region. It would have been something of a show.”

Schultz and his team hope further research can help narrow the timing and shed light on the size of the impactor. So far, Schultz hopes this study can help researchers identify similar blasting sites elsewhere and reveal the potential risk that such incidents pose.

“There may be many of these explosive scars out there, but so far we have not had enough evidence to make us believe that they were really related to air eruptions,” Schultz said. “I think this site provides a template to help refine our impact models and will help identify similar sites elsewhere.”

Other authors of the study were Sebastian Perroud, Nicolas Blanco and Andrew Tomlinson.

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