Evidence for the Asteroid Impact Hypothesis
This 150-kilometer-wide crater lies just off the Yucatan peninsula. Scientists
calculate that it was blasted into Earth by a 10-kilometer-wide asteroid or comet
traveling 30 kilometers per second -- 150 times faster than a jet airliner.
Scientists have concluded that the impact that created this
crater occurred 65 million years ago. The date corresponds perfectly to the date
of the dinosaur extinction.
The metal iridium, which is similar to platinum, is very rare on Earth's surface
but is more common in asteroids and in molten rock deep within the planet.
Scientists have discovered levels of iridium 30 times greater
than average in the Cretaceous/Tertiary (KT) boundary, the layer of sedimentary
rock laid down at the time of the dinosaur extinction.
These pieces of once-molten rock, called impact ejecta, are evidence of an explosion
powerful enough to instantly melt bedrock and propel it more than a hundred miles
from its origin.
Ranging in size from large chunks to tiny beads, impact ejecta
are common at or near the Cretaceous/Tertiary (KT) boundary, the geological layer
that defines the dinosaur extinction.
These crystals, often called "shocked quartz," show a distinctive pattern of
fracturing caused by high-energy impacts or explosions.
Some scientists maintain that the fracture pattern in these
quartz crystals could only have been caused by a massive asteroid or comet impact.
The pattern is prevalent in quartz found at or near the Cretaceous/Tertiary (KT)
boundary, the geological layer deposited at the time of the extinction.
A gradual decline in the number of dinosaur species would likely mirror an equally
gradual cause of their ultimate extinction. Conversely, a sudden "now you see them,
now you don't" end to the dinosaurs implies a catastrophic cause. Depending on
location and interpretation, the fossil record seems to say different things.
Some paleontologists see evidence in the fossil record that
dinosaurs were doing quite well prior to the end of the Cretaceous -- that they
were in no way declining in abundance when the impact occurred.