Geoscience Reference
In-Depth Information
Smit, Walter Alvarez, and others attribute the two layers to two tra-
jectories of impact explosion. Immediately after impact, the explo-
sive fireball, composed mostly of superheated vapor and mineral
dust, was lofted high above the stratosphere. Like the volcanic dust
from Krakatoa, it took several months to settle back to the earth;
arriving last, it became the upper layer. The coarser material, ejected
next from the crater, traveled on ballistic trajectories that did not
carry it to high altitudes. Thus it settled out first and became the
lower layer. The presence of the double layer only in North America
was yet another indication that the crater is located there.
The upper stratum shown in Figure 13 is the K-T clay layer on
which we have focused so far—the one marking the boundary at
Gubbio and the other sites outside North America. It occurs sand-
wiched within a variety of marine and nonmarine sediments that
were deposited at greatly differing rates. Yet regardless of how rapid
the rate of sedimentation of the rocks on either side, the clay layer
always has the same thickness: 2 mm to 4 mm. This can only mean
that it was deposited independent of normal sedimentation processes
and at a much faster rate than that of any of the surrounding sedi-
ments.
3
Comparison with the settling rates of atmospheric dust sug-
gests that deposition of the upper layer took place over only two to
three months.
M
ANSON
In proposing that the target was within 3,500 km of Montana, Bevan
French went on to identify two possible craters, one of which was
the Manson structure in Iowa. Because it is covered by 30 m to
90 m of glacial debris, Manson is not visible at the surface. Such hid-
den geological structures are detected using geophysical techniques
that rely on magnetism, gravity, and seismic waves. For example,
rocks with more magnetic minerals than average produce a positive
magnetic anomaly; those that are more dense give rise to positive
gravity anomalies. Using such methods, geophysicists can tell a great
deal about rock structures that they cannot see; indeed, this is the
way they discover buried structures that may contain oil. The geo-
physicists confirmed that the Manson structure is about 35 km in
diameter, smaller than the predicted size of a crater resulting from
the impact of a 10 km projectile. On the other hand, Manson is of
late Cretaceous age and the basement rocks in the area are granitic.
Although Officer and Drake concluded from their analysis that
