Atacama desert glass, also known as Pica glass, is an impactite (impact glass) found along a broad stretch of the Atacama Desert in northern Chile, near the town of Pica. The glass occurs as twisted, folded, and vesicular masses of dark green to black silicate glass, with individual pieces reaching up to 50 centimeters across. The glasses are concentrated in patches along a corridor approximately 75 kilometers (47 miles) long.
Origin
The glass is believed to have been formed by a cometary airburst event during the late Pleistocene, approximately 12,000 to 14,000 years ago. According to a 2021 study published in Geology by Schultz et al., the glass was generated when one or more cometary bodies exploded at low altitude just above the Earth's surface. The intense thermal radiation and supersonic winds from the fireball fused surface sediments into molten glass, which was then twisted, folded, and rolled before quenching. Evidence includes mineral decompositions (such as zircon transforming to baddeleyite) indicating temperatures exceeding 1,670 °C (3,000 °F), dynamic emplacement textures, and the presence of extraterrestrial mineral assemblages (including Ni-troilite, cubanite, calcium-aluminum-rich inclusions, and perovskite) that closely match material collected from Comet Wild 2 by NASA's Stardust mission.
Scientific debate
An alternative hypothesis, published in Earth and Planetary Science Letters in 2017 by Roperch et al., proposed that the glasses resulted from intense natural grass fires in paleowetlands, citing radiocarbon evidence suggesting at least two distinct thermal events. However, the 2021 study by Schultz et al. argued against this interpretation on the basis of the extreme temperatures required, the presence of meteoritic debris within the glass, and the dynamic emplacement textures incompatible with stationary grass fires.
Significance
The Atacama desert glass represents the first clear evidence of glasses on Earth created by the thermal radiation and winds from a cometary fireball exploding just above the surface, as opposed to a direct impact forming a crater. The timing of the event coincides with the disappearance of Quaternary megafauna in South America and with the arrival of early human populations (proto-Archaic hunter-gatherers) in the region, though a causal link has not been established. The glass is also known as Pica glass, named after the nearby town of Pica.
