Volcanic activity is not merely a subject for scientific study; it has profound implications for the people living in the areas near volcanoes. Of particular interest are when a volcano will erupt and what the nature of the eruption will be. Predicting the time of an eruption is hard. Nevertheless, by monitoring earthquakes and other signals, scientists can sometimes give local officials a few days’ notice.

To foresee how an eruption could develop, scientists run computer models that require input information about the interior of the upper part of the volcano, the so-called edifice. Conventional techniques—based, for example, on the propagation of waves through Earth (seismology) or measurements of gravitational fields (gravimetry)—can be used to investigate internal structures such as magma chambers that lie deep under a volcano. However, their spatial resolution is no better than a few hundred meters, so the classic methods cannot identify possible conduits of eruptive...

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