Iridium is a very rare element in the Earth's crust, but is found in anomalously high concentrations (around 1000 times greater than normal) in a thin worldwide layer of clay marking the boundary between the Cretaceous and Paleogene periods 66 million years ago. This boundary marks a major extinction event, including extinction of the dinosaurs along with about 70% of all other species. In 1979 the physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel were the first to link the extinction to an extraterrestrial impact based on the observation that iridium is much more abundant in meteorites than it is on Earth. During 1979 dozens of newspapers and magazine articles presented the original Alvarez hypothesis on the basis of only a talk at the American Geophysical Union meeting (Washington, May 1979) and accompanying abstract cited here.

(Thanks to Juan Weiss for this reference and its interpretation.)

Following up on No. 14206, the authors stated that the same excess iridium areas were found in two different areas of W. Europe and in New Zealand, and posited that “the anomalous iridium concentrations at the C [retacous]-T[ertiary] boundary is best interpreted as indicating an abnormal influx of extraterrestrial material.” They suggested that this was produced by an asteroid strike on the earth that formed an impact crater, and that some of the dust sized material about 60 times the object’s mass would eject and inject the stratosphere with pulverized dust containing iridium which would then spread around the globe. This dust from the explosion would have blocked sunlight for a long time and suppressed photosynthesis, and as a result most food chains would have collapsed, and extinctions resulted. Luis Alvarez calculated the size of the asteroid needed to produce the catastrophic impact -- an asterioid with a diameter of about 10 plus or minus 4 kilometers at the entry velocity that meteorites hit earth. They pointed out that “an asteroid of 10 km. diameter is twice the typical oceanic depth and this would produce a crater on the ocean bottom, and pulverized rock could be ejected.” They estimated that no terrestrial vertebrate heavier than about 25kg. would have survived the extinction. Finally they stated that the crater resulting from such a collision must be found to validate their hypothesis, and “there is about a 2/3 probability that this object fell in the ocean.”

(Thanks to Juan Weiss for this reference and its interpretation.)