A zone of partially molten rock extends beneath several volcanoes in Washington state, USA, according to a study published online this week in Nature Geoscience. If confirmed by additional methods, this could be one of most widespread magma-bearing areas of continental crust discovered thus far.

Graham Hill and colleagues measured the electrical conductivity in the rocks under the northern Cascade Mountain range. Their data confirmed that there was a widespread layer of material with low conductivity below the range. They also found that narrow fingers of this material rise towards the surface, below the Mount St. Helens and Adams volcanoes.

As molten rock has a lower electrical conductivity than solid rock, the researchers suggest that there is a zone of partially molten rock that has pooled in the continental crust. According to Hill and colleagues, the small fingers probably indicate areas where the molten rock is moving up towards the magma chambers of the volcanoes, feeding future volcanic activity.