The extinction event

Dinosaur fossil: why did dinosaurs become extinct?
The extinction event
In the 1970s, guess work turned into real scientific research thanks to a team at the University of California, Berkley. Geologist Walter Alvarez was examining rock strata in Guppio, Italy when he came across a layer of clay between the Cretaceous and Tertiary eras called the K-T boundary. The Cretaceous rock was full of fossils but rock from the Early Tertiary era had none. This brown-black stratum was formed at the time of the K-T mass extinction event that represented the end of the dinosaurs.
Alvarez took a handful of this K-T boundary clay to his father, the late Noble physicist Luis Alvarez, for analysis. With the help of nuclear chemists Helen Michels and Frank Asaro, they discovered that it contained an abnormally high quantity of the element iridium. This element is very rare in the crust of the Earth, but levels of iridium at the K-T boundary were 200 times higher than usual. Where had it come from? Extraterrestrial bodies such as comets and asteroids are rich in iridium, and to the Alvarez team, this clay bore the distinctive signature of asteroid dust. In 1980, Luis Alvarez and his son proposed that the dinosaur's demise was caused by an asteroid impact.
Since that time, geochemists have discovered iridium anomalies at the K-T boundary at over 100 places around the world. The boundary rock in these sites contains other unmistakable signs of a meteor impact. The scarred rocks show signs of being physically altered, fractured, melted and deformed beneath the intense heat and force of an explosion. For example, shocked minerals such as quartz are common in K-T boundary rock. These form under a sudden pulse of extremely high pressure. Small glass spherules are also present, remnants of superheated melt rock at the impact site, which were blasted as droplets that fell from the sky hundreds of miles around.
An asteroid that left its wounds around the world would have been huge. But where did it hit? The search for a crater site was on.