Every so often, nature decides to give us all a lesson in humility. The latest took the form of a minor volcano on a faraway island waking from a 200-year slumber for a few days - and plunging global air travel into chaos. The arguments over whether the disruption caused by the eruption of Eyjafjallakokul in Iceland was really necessary will rumble on for some time. But there is no doubt we have been served with a reminder of a shocking fact: we are all living on top of a colossal natural nuclear reactor, and bad things happen when it vents its waste into the atmosphere.
Every volcano is powered by nuclear energy, in the form of the radioactive decay of the uranium, potassium and thorium isotopes trapped inside the Earth at its formation. Around 20 volcanoes are active at any given time, pouring millions of tonnes of debris into the atmosphere. But every so often, one explodes with appalling violence: Vesuvius in 79AD, which killed around 25,000; Krakatoa in 1883, which took 36,000 lives; and the 1815 eruption of Tambora, which killed more than 70,000 and remains active in Indonesia to this day.
Unlike most other natural disasters, from hurricanes and tsunamis to earthquakes and even meteor impacts, there is nothing that can be done to prevent devastation by a volcanic eruption. The forces involved and the scale of their effects are simply too vast. When Tambora exploded, it blasted 100 billion tonnes of debris into the atmosphere, with global consequences. They included the notorious "year without a summer" of 1816, with plunging global temperatures, triggering what has been called the last great subsistence crisis in the western world.
Yet according to some scientists, life as we know it owes its very existence to volcanic eruptions. For without their unique powers, our planet might still be trapped in one of the greatest catastrophes to strike in its 4,500-million-year history. The extent of this catastrophe is only now becoming clear, with research published just last month pointing to its global reach. It adds to the mounting evidence that around 700 million years ago our entire planet was entombed in ice.
Known as the Snowball Earth hypothesis, the first hints of this global glaciation emerged in the early 1960s, with the discovery of ancient glacial deposits in several Arctic regions. Hardly a surprise, one might think - until the effects of continental drift are factored in. These showed that at the time the deposits were formed, they were not in the Arctic at all, but in the southern tropics. Quite why the Earth became cold enough to allow glaciers to exist so close to the equator was not clear. More puzzling still was the obvious fact that glaciers are no longer there. Climate scientists pointed to a feedback effect, in which the spread of ice across the Earth would make its surface ever more reflective, bouncing back ever more sunlight into space - and thus boosting the chilling effect. By the time glaciers formed at the tropics, it would be impossible to stop the entire Earth freezing over, and turning into a giant snowball. And once in that state, our planet would stay stuck in it indefinitely - unless something intervened to break the impasse.
What that "something" might be was identified around 20 years ago by Joe Kirschvink, a professor at the California Institute of Technology. While the surface of the planet might have been entirely icebound, underneath it the colossal natural nuclear reactor rumbled on regardless. And just like today, every so often it would vent some of its seethingly hot "waste" via volcanoes either under the sea or ice cap.
The direct melting effect of the volcanoes would not be enough by itself; far more important was the "greenhouse effect" produced by the carbon dioxide belched out during the eruptions. This would have produced the required global warming to free the Earth from its snowball state. Indeed, according to proponents of the Snowball Earth theory, volcanoes may have come to our planet's rescue as many as three times, around 770 million, 750 million and 650 million years ago.
Such dramatic scenarios tend to get short shrift among geologists, who are not exactly renowned for leaping aboard new bandwagons. They notoriously took decades to accept the idea that our planet could be subject to continental drift or cosmic impacts. Sure enough, the Snowball Earth theory has received a pretty cool reception over the years - though not without reason. Ever more sophisticated climate models have had ever more difficulty getting the entire Earth to freeze over. Put simply, the oceans act like huge storage heaters, mopping up the sun's heat and spreading it around the globe via ocean currents. At worst, researchers now think, the Earth may have been covered by a thin layer of ice. More likely it was more like a slushball, they argue, with plenty of areas for life to survive on its surface.
Last month new evidence emerged. A team led by Francis Macdonald at Harvard University has been studying rock samples found in the remote north-western Yukon Territory of Canada, and found clear evidence of glaciation. As with the original evidence for Snowball Earth, this hardly sounds like an earth-shattering discovery - until, again, one takes into account continental drift. Analysis of the samples shows that they are 716.5 million years old, and back then they were at sea level just 10 degrees from the equator.
According to the team, whose findings appear in the leading journal Science, this is the first evidence that around 700 million years ago the Earth really was covered in ice down to very low latitudes - and that it persisted for at least five million years. Perhaps it was at this point that the seething nuclear reactor beneath us intervened. Volcanoes trapped beneath the ice would then have burnt their way through to release warming carbon dioxide into the air, triggering a thaw.
And if you want some idea of what such an ice-melting, planet-saving volcano looks like, you can see one right now in the picture at the top of this page. Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England