The Wuhan coronavirus has served as a vivid reminder of how quickly our whole world can change. It has brought to public consciousness great plagues and epidemics of the past, but I have seen no mention of the truly monstrous life changer of geologic history. Virus particles commonly are of the order of 20 to 300 nanometers in diameter (20 nm = 0.0000008 inches). What happens when the ecosystem is invaded by a particle 10 to 15 km (6 – 9 miles) in diameter traveling some 36,000 miles per hour? The energy released has been compared to 10 billion simultaneous Hiroshima A bombs exploding.
I am, of course, referring to the Chicxulub (say “cheek shoe lube”) meteorite impact of 66.043 +/- 0.11 Ma credited with the demise of the great dinosaurs and the rise of mammals. This monumental event nicely illustrates philosopher Will Durant’s quote: “Civilization exists by geological consent, subject to change without notice.”
When the tektite subject comes up, I am often asked whether Chicxulub produced any tektites. The answer is assuredly yes, but nearly all that are presently known are under a millimeter in diameter. In many places scattered widely over the earth, the Cretaceous-Tertiary stratigraphic boundary is marked by a thin layer, sometimes only a few cm thick, rich in tiny spheroids and famously anomalous in iridium. Some of the microtektites themselves contain anomalous iridium.
The Chicxulub microtektites are typically altered to secondary devitrification products, most often palagonite and smectite clays. Glass is geologically metastable and seldom survives more than a few tens of millions of years. Surprisingly though, there has been a recent discovery of pristine glass K-T boundary spheroids on Gorgonilla Island, offshore of Colombia. In that instance, the spherule layer is about 2 cm thick and is quite pure spheroids. In appearance, it is reminiscent of caviar! The island is not open to the public, and I have no specimens of this spectacular material.
Most Chicxulub microtektites are perfect spheres, but ovals, teardrops, dumbbells and other typical tektite
morphologies are occasionally present. Where actual glass persists, lechatelierite is reported and water contents are low at <0.021 weight percent. Researchers have differentiated several “types”, one of which is thought to consist of condensate droplets. Other types reflect differences in target rock lithologies, which included terrestrial volcanics and evaporites. The latter may have given rise to peculiar volatile gas releases that contributed to the extinction event.
Most of the Beloc spheroids are now hollow shells of smectite clay, sometimes with internal calcite encrustations.
Imagine a hailstorm of glass beads that rained down over much of the earth, followed by dust settling from a darkened sky that persisted for months, challenging all life on earth. Something like 75% of all plant and animal species then on earth met extinction in this event and its aftermath. This is the tale told by a couple inches of glass spheroids in the stratigraphic history book of planet earth.