K-T wallop: through the looking glass.

K-T wallop: Through the looking glass 

Scientists who believe a meteorite slammed into Earth 66 million years ago are racking up considerable evidence in their favor, making it increasingly difficult to relute the theory that a catastrophic impact played some role in killing off the dinasaurs and many other forms of life at the end of the Cretaceous period.

This month, geologists report finding talltale evidence of such an impact: rounded pieces of glass in a Haitian rock layer dating to the time of the Cretaceous-Tertiary (K-T) boundary. The glass sphrerules, which range up to about 6 millimeters in diameter, have almost entirely crystal-free structures -- an important feature because it allows scientists to determine how they formed. Glass drops created when meteorites strike Earth typically do not developed crystals, whereas those formed during volcanic eruptions always contain some crystals, note Haraldur Sigurdsson of the University of Rhode Island in Narragansett and his colleagues, who describe their discovery in the Feb. 7 NATURE. The Haitian finds therefore support the theory that an impact, rather than intense volcanic eruptions, caused the K-T extinctions.

These specimens are among the oldest known glasses on Earth; such delicate structures usually don't survive for tens of millions of years. In the same issue of NATURE, Jan Smith of the Free University in Armsterdam calls the Haitian fragments the "smoking gun" in the K-T drama.

The composition of the new finds reveals clues to where the meteorite or comet actually crashed, suggests that ground zero was a region of continental crust covered by a layer of limestone-like sediments. This kind of geologic fingerprints fits with several structures proposed as the K-T impact site, including a round feature on the northern end of the Yucatan peninsula, identified last year as a leading candidate (SN: 11/17/90, p.319).