In the mid-1970s, the geologist Walter Alvarez was in the Italian city of Gubbio studying two layers of limestone dating from the time of the dinosaur extinction. The lower layer was rich in fossils, but the upper layer had virtually none at all. Between the two layers was a one centi- metre thick deposit of clay. If only Alvarez could work out how long it took to deposit the clay, then he would be able to get a handle on the time taken to kill off the dinosaurs.
It was at this point that Alvarez had a remarkable idea. Every day our planet is struck by millions of meteorites - not giant rocks, but tiny granules. This continual dusting from outer space contaminates everything, and Alvarez realised that he could work out how long it took to create the clay layer by measuring the amount of meteorite dust within it. The more meteorite dust, the longer it took to form the clay, the longer it took to wipe out the dinosaurs.
In fact, measuring ancient meteorite granules is impractical, so instead Alvarez set about measuring the amount of iridium in the clay - meteorites are the only source of iridium on earth. He expected to find iridium at levels of about 0.1 parts per billion, but instead the amount was one hundred times greater. Alvarez had set out to look for a gentle smattering of iridium to measure the duration of the extinction, but instead he discovered a whopping dollop of the stuff, thereby indicating that a massive meteorite was the cause of the extinction.
Although scientists have since discovered the meteorite's impact crater in the Yucatan Peninsula in Mexico, one mystery still remains. The fossil record suggests that the dinosaurs were dying off before the iridium layer was formed, in other words before the meteorite hit the earth. As a result, some geologists have put forward an alternative theory. They argue that a spate of volcanic eruptions in India 65 million years ago was responsible for kick-starting the demise of the dinosaurs. The scale of eruption was far greater than anything experienced in human history.
The beauty of the volcano theory is that the Earth has witnessed more than one mass extinction, and each of them can be linked to a massive contemporaneous volcanic eruption. On the other hand, it has been much harder to link a meteor crater to each extinction.