Geoscience Reference
In-Depth Information
of Rutgers, who wrote a fine topic on the Late Devonian mass ex-
tinction, believes that
three
impacts occurred, and indeed, several
craters do date to this part of geologic time.
3
The Siljan crater in
Sweden is the most promising; it is also of interest because it was the
focus of the deep-earth methane hypothesis of Thomas Gold of
Cornell University. Gold convinced himself, and then the Swedish
Power Board, that the impact of a large asteroid would produce frac-
tures that would tap deep-seated reservoirs of gases, among them
methane, which then might be produced in commercial quantities.
The Swedish Power Board drilled the Siljan structure but found no
methane.
THE JURASSIC-CRETACEOUS
Impact craters have several ways of escaping notice. They may hide
beneath seafloor sediments, arctic ice, flood basalt flows, and younger
sediments. Tectonic plates may carry them down to oblivion. Or,
more mundanely, erosion may obliterate them. But recently yet
another hiding place has been discovered.
In the southwestern part of Africa lies the great Kalahari Desert.
Until the advent of four-wheeled drive vehicles, travel in the Kalahari
was next to impossible as the region is covered by a layer of sand
100 m thick. In the early 1990s, aerial gravity and magnetic surveys,
of the kind done in the search for oil-bearing structures in the Yuca-
tan, revealed a nearly circular structural dome buried beneath the
Kalahari sands near the South African town of Morokweng. Drills
sent down into the underlying bedrock brought back melt rock
containing an iridium anomaly and shocked quartz. Although at first
the structure appeared to be 70 km wide, further data analysis sug-
gests a diameter of 340 km; if so, it would be even larger than Chic-
xulub. Zircons from the Morokweng melt rock give an age of 145
million years.
4
The accepted age of the boundary between the Juras-
sic period and the overlying Cretaceous (see Figure 2, page 8), at
which, according to Sepkoski, 38 percent of species became extinct,
is also 145 million years.
THE PERMIAN-TRIASSIC
The "mother of mass extinctions," the Permian-Triassic, is a critical
case. It marks such a vast change in the history of life that, like the
K-T, it not only separates two geologic periods, but two great eras—
the older Paleozoic and the younger Mesozoic (see Figure 2, page 8).
But the Permian-Triassic extinction is much the larger—96 percent
of species are estimated to have expired then, compared with 70
