SOMEWHERE beneath the small Icelandic coastal town of Grindavik, a river of molten rock is on the move. A series of earthquakes over recent weeks, plus satellite images showing the ground changing shape, have stoked fears that a nearby volcano is about to erupt.

Iceland’s government has ordered an evacuation of the town’s more than 3,000 residents. Nobody can say what will happen next; if hot magma hits seawater, there could be explosive consequences, including a scaled-down repeat of the kind of ash cloud that accompanied the Eyjafjallajökull eruption in 2010.

The unfolding drama is testament to the powerful geological processes heaving unseen beneath our feet — and to the limitations of science when it comes to calculating how these forces will play out. That renders policymaking around natural hazards difficult, including deciding when to issue evacuation orders and when to rescind them.

Iceland sits at the seam separating the North American and Eurasian tectonic plates, making it a seismic and volcanic hotspot. The movement is happening underneath the southern Reykjanes Peninsula, where the country’s international airport and touristfriendly Blue Lagoon are located. The earthquakes began on October 25 but intensified last week, with large, steaming fissures appearing in local roads. The Icelandic Meteorological Office declared a “significant likelihood