A 250-million-year shutdown of volcanic activity which is thought to have occurred early in Earth鈥檚 history may be what turned the planet into a glacier-covered snowball. It could also have helped give rise to our oxygen-rich atmosphere.
Previous studies have noted that very little volcanic material has been dated to between 2.45 and 2.2 billion years ago, but it was widely assumed the gap would vanish as more samples were dated. Now an analysis of thousands of zircon minerals collected from all seven continents indicates that the gap may be real after all. Zircons provide a record of past volcanic activity, as the date they were formed can be calculated from the radioactive isotopes they contain.
The failure of so many samples from all over the world to fill the gap suggests there was a major slowdown in the planet鈥檚 volcanic activity during this period, says of New Mexico Tech in Socorro, who led (Earth and Planetary Science Letters, ). 鈥淰olcanism didn鈥檛 shut off, but it became much, much less widespread during this time.鈥�
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The lull could be tied to a pause in the motion of tectonic plates, which drives much of Earth鈥檚 volcanic activity, Condie says. Computer simulations suggest this motion, which now takes place continuously, would have been intermittent early in Earth鈥檚 history, when the planet鈥檚 interior was hotter and less viscous, so less able to drag the plates.
The lull may in turn be a major factor behind a suspected 鈥渟nowball Earth鈥� event between 2.4 and 2.3 billion years ago, when much of the planet is thought to have been covered with ice (New Scientist, 2 December 2006, p 14). With no new carbon dioxide being spewed from volcanoes, its concentration in the atmosphere would have declined, leading to global cooling.
The lull could also be behind the rise in atmospheric oxygen that is known to have taken place around 2.4 billion years ago (New Scientist, 17 January, p 10). Prior to the lull, any oxygen produced by marine microorganisms was consumed in reactions with iron in the ocean. With no fresh volcanic material to replenish the iron, oxygen would have been free to build up in the atmosphere.
This in turn could have further cooled the Earth by removing another powerful greenhouse gas from the atmosphere. Methane is thought by some to have been relatively abundant in Earth鈥檚 early history, helping to keep the planet warm at a time when the sun was much dimmer than it is now. But it would have been scrubbed away by the oxygen that was building up in the atmosphere.
Mark Harrison of the University of California, Los Angeles, says the idea of a lull is plausible, and agrees it would have had major climate effects. 鈥淚t鈥檚 intriguing, but it鈥檚 going to be hard to demonstrate a smoking gun,鈥� he says. Alternatively, the lull could merely be an illusion that has arisen because volcanic material from this period has not been well preserved, Harrison warns.
