Iridium is very rare. The best place to find it is in meteorites. In 1980 a team led by physicist Luis Alvarez and his son the geologist Walter Alvarez discovered sedimentary rock layers containing unusually high amounts of iridium. These rock layers occurred worldwide and were dated to 66 million years ago, a time known as the Cretaceous-Paleogene boundary.
During this time a mass extinction event took place on the earth. Many forms of life were wiped out—most famously the dinosaurs. The Alvarez duo proposed that an asteroid impacted the earth and that event and its subsequent “nuclear winter” were the primary cause of the extinction. They got this idea because iridium is present in higher concentrations in astronomical (off-earth) bodies than it is in earth rocks. In a remarkable bit of scientific inference they proposed that evidence of a cosmic collision—namely a crater—should be found to support the hypothesis. Sure enough the Chicxulub crater in the Yucatán peninsula was discovered about ten years later. It was ultimately mapped out and found to contain the appropriate geological signatures suggesting its impact was about 66 million years ago, right on the Cretaceous-Paleogene boundary.
These days the Alavarez hypothesis is widely accepted as the best description of this remarkable extinction event. I got to hear Walter Alvarez give a talk once. He taught a bunch of science teachers some plate tectonics then treated us to an open, free-ranging discussion of his and his father’s work. We asked lots of questions and wound up with a great lesson on induction and the general nature of scientific inquiry. I still have my notes on plate tectonics but I put my pencil down for the rest of it. Too bad, I wish I could remember more about that afternoon!
Iridium is very hard and dense and is alloyed with platinum or osmium to make strong, corrosion resistant parts and tools for specialized industrial and electronic applications. Iridium crucibles, for example, are use in high-temperature crystallography. Only about six metric tons of iridium are produced annually. The very scarcity of iridium in the earth’s surface is what led to the Alvarez’ interest in it when they found it in high concentrations. Their persistence in trying to make sense of those deposits led to their bold claims about an extraterrestrial cause for the mass extinction of life on earth. It’s a crazy notion that an asteroid killed the dinosaurs and yet we pretty much take it for granted today! Even if a better hypothesis some day replaces the Alvarez story theirs is such an interesting piece of scientific detective work that opened up new avenues of thinking about evolution that we’ll likely be talking about it for decades.
The Williamette meteorite was found in Oregon and is now on display in NYC at the American Museum of Natural History. It contains 4.7 ppm of iridium which is almost 5000 times greater than the normal crustal abundance of 0.001 ppm.