Geoscience Reference
In-Depth Information
the way back to the K-T boundary at 65 million years. They found
iridium levels above background only at the K-T boundary. As far as
we know, high iridium concentrations are exceedingly rare in terres-
trial rocks.
PREDICTION
3: Iridium anomalies will be associated with proven
meteorite impact craters.
The Alvarezes started with an iridium spike and inferred an impact;
it should be possible to move in the other direction as well. That is,
it should be possible to find a crater whose origin by impact is undis-
puted, predict where the corresponding iridium-enriched ejecta will
be located, and go find it. But since it is hard to detect terrestrial
craters in the first place, and since erosion will have removed some
ejecta layers, the absence of such a connection would not falsify the
Alvarez theory.
FINDINGS
Two craters have been found to have associated iridium-rich ejecta
layers. One is the 600-million-year-old crater at Acraman, South
Australia, whose ejecta deposit contains not only iridium but other
platinum group metals as well as gold.
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This ancient crater has been
so deeply eroded that only a multiringed scar remains. Its ejecta,
even though located more than 300 km away, can still be tied confi-
dently back to the crater. The other is the 40-km diameter, 143-
million-year-old Mjolnir crater, in the Barents Sea north of Scandi-
navia, which was detected through geophysical methods.
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A diligent
search led by a group of Norwegian geologists found its ejecta layer,
which contained both iridium and shocked quartz, in a core taken
30 km from the crater's center.
At first it may seem surprising that it is so difficult to connect
iridium-rich ejecta layers to their parent craters. But remember how
difficult it is to recognize terrestrial impact craters, and to find the
thin ejecta layers, in the first place. Comparing it to the search for a
needle in a haystack may be optimistic. In any case, the two exam-
ples prove the principle. As the science of crater detection improves,
other ejecta layers will be tied back to their parent craters.
PREDICTION
4: The boundary clay layer will generally be thin
and of worldwide distribution.
The immediate effects of a giant impact take place in minutes or
hours; the secondary ones may last for hundreds or at most a few
