December 18th, 2020, by Tim Radford Tavurvur, Papua-New Guinea: Volcanic eruptions may go a long way to explain mass extinctions. Image: By Richard Bartz, via Wikimedia Commons Life on Earth has been through mass extinctions before − every 27 million years. Blame it on celestial clockwork. Instead, the planet and the solar system could be caught up in some deadly astronomical cycle. They argue that every 27 million years, a high proportion of land-dwelling species − birds, mammals, reptiles and amphibians − disappear from the fossil record at around the same time. And this disappearance seems to coincide, again according to geological evidence, with devastating eruptions of volcanic lava and violent asteroid collisions that would have had the effect of darkening the skies, lowering the temperature, depleting the ozone layer, then stimulating a greenhouse effect and starting extensive fire and acid rain.