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What is the primary evidence that a large asteroid impact is associated with the extinction of the dinosaurs?
a. a world-wide layer of iridium which appears to have been created about 65 million years ago
b. several giant craters all around the planet
c. a 180 degree reversal of the Earth's magnetic field at the end of the creteous period
d. no evident has been found, it is a completely unsubstantiated claim.
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The answer to the question is A. While it is true that there are large impact craters all over the world and the Earth's magnetic poles have reversed several times over the course of geological time, there is a layer of iridium that is present in a layer of sedimentary rock around the world from around 65 million years ago. Iridium is a naturally occurring element that is relatively rare on Earth but can be found on asteroids. If a massive asteroid crashed into the planet's surface 65 million years ago, iridium from the asteroid would have been released with the massive amount of dust and debris into the atmosphere. The particles would have been spread around the planet through air currents, thus leaving the fine layer of iridium in the rock strata.
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.
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