Describe two pieces of evidence that support the hypothesis that an asteroid impact was responsible for the iridium anomaly.

Evidence for the Asteroid Impact Hypothesis

Impact Crater

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.

Rare Metal

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.

Melted Rock

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.

Fractured Crystals

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.

Fossil Record

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.