Fuel, loose and fossil-free

NEARLY two millennia ago, Pliny the Elder, a Roman chronicler of natural history, spotted flames seeping from bare rock at a site that is probably Yanartas, in modern Turkey. It was an extraordinary find, though not for reasons that Pliny could have guessed. The gas that fuelled the flames, and fuels them still today, comes from a source that geologists would once have decried as implausible, if not altogether impossible. But that source is turning out to be a significant one.

The overwhelming majority of the methane extracted commercially and 90% of what is in the air is biotic—that is to say, it comes from the decomposition of the stuff of life. However, natural gas can also form abiotically, as a result of chemical reactions in a kind of volcanic rock called peridotite. Add to such rock the heat and pressures of depth, throw in a splash of water, and eventually out comes methane.