Time lords

摘要

Geologists have come to an international agreement about the timeline of Earth's history. But the results are not quite set in stone, as John Whrtfieid discovers. Silurian, Devonian, Triassic: the names seem as solid and permanent as rocks themselves. But in fact, like fashions in hair or hem-length, the geological divisions of our planet's timeline are prone to change. Geologists argue over what defines the boundaries between these periods, and disagree over exactly how old they are. The end of the Jurassic period, for instance, has wobbled by more than 30 million years since it was dated in the 1930s (refs 1,2). This summer, the first fully revised geological timescale for 15 years will be published, helping to clear up some of the confusion. The new scale will help Earth scientists speak to one another in a common language, and the dates will help to answer questions about what caused various mass extinctions and changes in climate. The project will even have an impact on oil companies, which base their exploration programmes in part on estimates of the age of rocks. r