A 180km-wide, submerged, multiringed impact
crater straddling the northwest coastline of the Yucatán Peninsula
in Mexico; it is named after a village located near its center (21° 20'
N, 89° 30' W).
Demise of the dinosaurs
The Chicxulub Crater is believed to be the result of the collision with
an asteroid measuring some 10 to 20km across.
The environmental effects that accompanied its formation are thought to
have been implicated in the mass extinction
at the end of the Cretaceous period,
about 66 million years ago, in which the last of the dinosaurs,
along with many other species, disappeared (see Cretaceous-Tertiary
However, there has been much debate about whether the Chixculub impact was
the main cause of the mass extinction or just a contributing factor.
A question of dates
In February 2004, an international group of scientists led by Gerta Keller,
of Princeton University, published results, based on a core sample, that
the Chicxulub crater predated the extinction of the dinosaurs by about 300,000
years. The authors argued that the impact did not wipe out the creatures,
rather two or more collisions might have been responsible. Keller and her
colleagues analyzed rock from their core using five separate indicators
of age, including fossil planktonic organisms
and patterns of reversals in the Earth's magnetic field.
The results suggested the crater was punched into the Earth around a third
of a million years before the dinosaurs disappeared from the face of the
planet. Keller and her team contended that their findings proved the Chicxulub
impact did not by itself trigger the extinction of the great beasts. Instead,
they argued that a cooling of the global climate shortly followed by a period
of greenhouse warming placed enormous stress on the dinosaurs. This warming
could have been triggered by carbon dioxide released by a massive eruption
of lava seen today in the Deccan traps of India. The Chicxulub impact occurred
during this warming period and, although the environmental effects were
severe, it did not cause the extinction of the dinosaurs. The team believed
a second impact, 300,000 years after the Chicxulub collision, finished off
the creatures. The structure of the sea bed beneath the Indian Ocean suggests
this second impact could have been there, Keller has indicated.
Not surprisingly, this controversial counter-hypothesis met with strong
opposition. In particular, it ran counter to other lines of evidence that
indicated that the Chicxulub crater formed at the K-T boundary. This evidence
includes once-molten material laid down at the K-T boundary in rocks from
Haiti which is similar to deposits from the Chicxulub crater. In addition,
debris thrown out by this collision gets thicker the closer one approaches
Chicxulub, like a trail pointing to the impact site. Additionally, an impact
the size of Chicxulub occurs on Earth about every 100 million years. That
two such impacts should occur within 300,000 years is statistically unlikely.
The debate settled?
In February 2013, an international team of scientists from the Berkeley
Geochronology Center and University of California, Berkeley, Vrije University
Amsterdam in the Netherlands, and the University of Glasgow, established
that the last of the dinosaurs died out 66,038,000 years ago – give
or take 11,000 years. This ties in closely with the date of the Chicxulub
impact and leaves little room for doubt that the collision play a major
role in the mass extinction. The team based their results on a study of
strata, containing traces of debris from the asteroid impact, which lie
close to the last fossil remains of dinosaurs.
signs of the Chicxulub crater
The illustration to the right is a photo of the Yucatan Peninsula taken
from the Space Shuttle and is one the very few pieces of surface evidence
that survives from the cataclysm. The 3- to 5-meter-deep and 3- to 5km-wide
trough traces weaknesses in the rock created by the space impactor. The
trough is visible today because of instabilities in the limestone
sediments that overlie the crater. The original offset would have been about
a kilometer or so; today it is just a few meters. The collapse of numerous
limestone caverns above the crater rim has resulted in an arcing chain of
sinkholes – also visible in the shuttle radar image. They are the
result of extensive erosion in the limestone. These cenotes, as they are
called, are several meters across and were in fact used by the Mayans to
make their sacrifices. Inside the crater the cenote population is less dense
than it is outside. Exactly why the crater (which is buried a few hundred
meters below surface) still has an effect on water flow is not clear.
|Shaded relief image of the northwest corner of the
Yucatan Peninsula generated from Shuttle Radar Topography Mission
data. Source: NASA/JPL
|A 3D map of local gravity and magnetic field variations
reveals the Chicxulub crater, now buried beneath tons of sediment.
This view is looking down at the surface, from an angle of about 60°.
Origin of the asteroid that made the Chicxulub crater
According to a study1 published in the journal Nature
in 2007, the Chicxulub crater was formed by an asteroid that had its origin
in a great collision that took place in the main
asteroid belt about 160 million years ago. The authors of the study,
William Bottke and David Nesvorny from the Southwest Research Institute,
Boulder, Colorado, and David Vokrouhlicky from Charles University, Czech
Republic, used computer modeling to show that a surge in asteroid strikes
on Earth in the last 100-200 million years was probably caused by the catastrophic
disruption of a 170km-wide asteroid by another space rock of less than half
its size. This collision resulted in a group of asteroids known today as
the Baptistina family. The computer simulation indicated that a number of
the original asteroids from this family found their way into the inner solar
solar system, resulting not only in the Chicxulub crater but also other
impact craters on Venus, Mars, and the Moon, including, probably, the 85km-wide
Tycho crater on the Moon 108 million
|Results published in 2007 suggest that the impactor
that wiped out the dinosaurs and other life forms on Earth 65 million
years ago can been traced back to a break-up event in the main asteroid
belt more than 100 million years earlier.
Chemical analysis of projectile material connected to the Chicxulub event
is also said to tie its impactor to the type of rocks that make up the Baptistina
A review of the paper by Bottke et al, in the same issue of Nature,
comments "It is a poignant thought that the Baptistina collision some 160
million years ago sealed the fate of the late-Cretaceous dinosaurs well
before most of them had evolved."
- Bottke, W. F., Vokrouhlicky, D., and Nesvorny, D. "An asteroid breakup
160 Myr ago as the probable source of the K/T impactor." Nature
449 , 48-53 (2007).
radar images (NEO program, JPL)
of Bottke et al's paper in Nature