Geoscience Reference
In-Depth Information
explained. Thus, to the Officer-Drake school, volcanism tied together
all the facts remarkably well.
There was no denying that they were onto something. After the
eruptions of Krakatoa in 1883, Mount St. Helens in 1980, and
Pinatubo in 1991, the dust and sulfur injected into the atmosphere
shaded the earth enough to cause average world temperature to
drop, though by only one or two degrees. And remember that it was
the report of the darkening effects of the Krakatoa eruption that set
Luis Alvarez on the trail of meteorite impact in the first place. It
would be the height of irony were Krakatoa now to be used to
defeat his theory.
V
OLCANIC
I
RIDIUM
The picture of iridium anomalies as uniquely diagnostic of mete-
orite impact began to cloud in the mid-1980s, lending additional
credence to the volcanism theory. The chemistry of aerosols (sus-
pensions of fine solid or liquid particles in gases) emitted from
Kilauea Volcano in Hawaii had been under investigation by scien-
tists from the University of Maryland.
5
Although for five years they
detected no iridium, aerosols from the 1983 eruption unexpectedly
contained up to 10,000 times as much iridium as the Hawaiian
basalts. Officer and Drake pointed out that the iridium in the air-
borne particles was "comparable to concentrations associated with
meteorites."
6
The picture quickly clouded further: High iridium levels were
discovered in particles emitted by a volcano on the remote island of
Reunion in the western Indian Ocean
7
and in the ejecta of silicic
volcanoes on Kamchatka.
8
Iridium levels as high as 7,500 ppt, com-
parable to the K-T levels, were found in layers of volcanic dust
buried in the Antarctic ice sheet.
9
Thus, contrary to the view that
prevailed when the Alvarez theory was first introduced, as the
1980s progressed it began to appear that certain volcanic processes
can concentrate iridium and in amounts approaching K-T boundary
levels.
In 1996, Frank Asaro, an original member of the Alvarez team,
and Birger Schmitz of the University of Gothenburg in Sweden,
reported iridium measurements in a number of ash deposits, includ-
ing some near the K-T boundary.
1
0
They confirmed the discovery
that some types of explosive volcanism produce ash with up to
7,500 ppt iridium, but said that by comparing the levels and ratios
of various chemical elements in volcanic rocks and meteorites, it is
