Seasonal pressure drops trigger sudden Katla floods

Sudden floods from beneath Mýrdalsjökull glacier are more consistent with geothermal activity than small eruptions, geologist Magnús Tumi Guðmundsson told mbl.is. He said the floods occur at the same time of year and without the earthquake activity that typically precedes volcanic eruptions.

Guðmundsson, a professor of geophysics at the University of Iceland, presented research on four floods from Mýrdalsjökull over the past 70 years at a Road and Coastal Administration research conference on Friday. The most recent was in 2024.

He said the floods occur within the same four-week period each year, when water pathways open into Katla’s caldera and pressure at the glacier bed falls. If the geothermal system has heated sufficiently, the pressure drop can cause intense boiling. Steam then rises beneath the glacier, melting large amounts of ice and sending water out in a sudden flood. A magma intrusion weeks or months earlier could contribute to the heating, he said, but there is no evidence to confirm that.

As surface ice melts in spring, channels form beneath the glacier and allow water to drain, lowering pressure at the bed. By well into summer, the pressure change appears to reach the caldera, and geothermal water can emerge beneath ice cauldrons. Major floods are uncommon, Guðmundsson said, because usually only a small part of the geothermal system is at boiling point.

After the 2024 flood, additional flow meters were installed closer to the glacier to detect floods sooner. Guðmundsson said there had previously been no clear warning signs. The seasonal pattern and absence of the precursory earthquakes seen before known eruptions support a geothermal explanation, he said.