The Alvarez group therefore suggested that iridium was scattered worldwide from a cloud of debris that formed as an asteroid struck somewhere on Earth. Computer models suggest that if such an asteroid collided with Earth, it would pass through the atmosphere and ocean almost as if they were not there and blast a crater in the crust about 100 km across. The iridium and the smallest pieces of debris would be spread worldwide by the impact blast as the asteroid vaporized into a fireball. The glass is formed as the target rock is melted in the impact, blasted into the air as a spray of droplets, and almost immediately frozen. Over geological time, the glass spherules may decay to clay.
Shocked quartz is formed when quartz crystals undergo a sudden pulse of great pressure.
It is only a few millimeters thick, but in total it contains more than a cubic kilometer of shocked quartz in North America alone. All this evidence implies that the K-T impact occurred on or near North America, with the iridium coming from the vaporized asteroid and the shocked quartz coming from the continental rocks it hit. The K-T impact crater has now been found. The structure is about 180 km across, one of the largest impact structures so far identified with confidence on Earth. That chemistry could have been generated by melting together a mixture of the sedimentary rocks in the region.