The current favorite, however, is the collision hypothesis , which is supported by an increasing mass of evidence. A large asteroid or comet, no matter what it's made of, packs quite a wallop. Consider an asteroid with a diameter of 10 kilometers. Its mass will be roughly a trillion tons. We know that impact craters exist on the Earth today - for instance, Meteor Crater in northern Arizona.
Erosion is very effective over the course of millions of years.
The crater corresponding to the Cretaceous impact wasn't discovered until this decade. Geologists working for the Mexican national oil company discovered rocks that had been shocked and melted, hidden away beneath the sediments of the Yucatan peninsula. Further investigation showed that the disturbed rocks formed a circle 180 kilometers in diameter. The circular structure was actually a buried impact crater. Iridium is a dense metal, like platinum and gold, which is very scarce in terrestrial rocks.