unusually high concentrations of iridium, an element that is rare in the Earth's crust but relatively abundant in many meteorites. From the amount and distribution of iridium present in the 65 million year old "iridium layer, the Alvarez team later estimated that an asteroid of 10–14 kilometers must have collided with the earth. This iridium layer at the KT boundary has been found worldwide at 100 different sites. Multi directionally shocked quartz (coesite), which is only known to form as the result of large impacts or atomic bomb explosions, has also been found in the same layer at more than 30 sites.
