Geoscience Reference
In-Depth Information
argue that Chicxulub may be as large as 300 km.
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At 170 km, it
would be one of the largest craters yet discovered on the earth; at
300 km, Chicxulub would be one of the largest impact basins
formed in the inner solar system in the last 4 billion years. Since a
300-km crater would have released far more energy and been far
more destructive to life than one just over half as wide, measuring
the true size of Chicxulub is crucial to understanding the extinc-
tion. Drilling, geophysical studies, and topographic work of the kind
being done by Kevin Pope and Adriana Ocampo of the Jet Propul-
sion Laboratory in Pasadena should soon resolve this dispute.
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The distribution of impact ejecta of various sorts can be used to
learn more about how Chicxulub formed. Why, for example, is the
shocked quartz in the North American sites only in the upper of the
two impact layers, and why does more of it exist to the west than to
the east of Chicxulub? Walter Alvarez, planetary-impact specialist
Peter Schultz of Brown, and their colleagues are addressing both
questions by studying the trajectory of the impactor and the mech-
anism of fireball creation and ejecta formation.
Note in the gravity anomaly map that the structure appears
breached on the northwest side, an effect sometimes observed in
craters on other bodies in the solar system. Peter Schultz believes
that the K-T impactor was 10 km to 15 km in diameter and was
descending toward the northwest at an angle of about 30 degrees
from the horizon. According to Schultz, an impact from that angle
and direction would explain the observed distribution pattern of
shocked quartz, tektites, and iridium.
How important to the K-T event was the particular geology
of ground zero in the Yucatan? Had the target area not been com-
posed of limestones and sulfate deposits, would the extinction have
been so great? Understanding the importance of the target rock types
is critical to comparing Chicxulub with other potential impact-
induced mass extinctions.
TOPOGRAPHY
Kevin Pope, a consulting geologist now associated with the Jet
Propulsion Laboratory, and two colleagues, came close to being the
first to "rediscover" Chicxulub. He and Charles Duller of NASA
were examining Landsat high-altitude satellite photographs, trying
to determine the relationship between the ancient Mayan sites on
the Yucatan Peninsula and the location of surface water deposits.
They soon noticed a set of small ponds, which they later found went
by the Mayan name of
cenotes,
that were arranged along the arc of
an almost perfect circle. What could cause a set of small ponds to
