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SIGNS of embryonic volcanoes that could erupt hundreds of millennia from now have been detected for the first time.

The Cheb basin in the Czech Republic was last volcanically active about 300,000 years ago. Over the past 12 years, Karin Br盲uer from the Leipzig-Halle Centre for Environmental Research in Germany and her colleagues have recorded a significant increase in the ratio of two helium isotopes in the gas bubbling up from mineral springs in the area. This ratio is known to be higher in the Earth鈥檚 upper mantle than the crust, so the increase implies that melt from the upper mantle has started to intrude into the crust, a process that could eventually form fresh volcanoes (Geophysical Research Letters, vol 32, p L08303).

Rising magma could also explain why the Cheb basin is prone to 鈥渟warm鈥� earthquakes 鈥� thousands of low-magnitude earthquakes occurring within just a few months. Until now it has not been clear what causes the swarms; most earthquakes occur at the edges of tectonic plates, but these occur in the middle. 鈥淲e think that the extra fluid pressure from magma intrusions could be responsible,鈥� says Br盲uer.

But residents of the Cheb basin needn鈥檛 worry just yet. 鈥淓ruptions on the surface are unlikely for another few hundred thousand years,鈥� she says.