Picture of the Day

Touching the demise of the dinosaurs? This is the famous outcrop at Gubbio, Italy, where the Alvarez team discovered the iridium anomaly that led to the modern development of the asteroid hypothesis of the extinction of the dinosaurs and other Cretaceous creatures. The layers of limestone above and below the clay layer behind my hand contain very low amounts iridium, on the order of 6-8 parts per trillion, but the clay layer itself is over 3,000 ppt. Iridium is a relatively important constituent of asteroids, but is generally rare in the crust of the earth. Similar anomalies around the world suggest a global event. The issue, of course, is not settled, as other hypotheses explain the iridium as a result of deep mantle source volcanism, and uncertainties exist as to whether the layer actually represents the extinction event.