A temperature jump of 400°C at a depth of 2 km—it’s possible in Krafla to cook a steak!

5 February 2026

This study explains the heat transfer between a magma reservoir and the surrounding rock in the high-enthalpy geothermal exploration area of Krafla, Iceland. Thermomechanical numerical models explain the sudden jump observed during drilling in 2009: the gradual partial melting of the crust over 35 years, in contact with magma injected at over 900°C, generates convection and a jump of 400°C in just 15 meters. This result will make it possible to better anticipate future drilling in similar areas.

When you want to use the heat produced by magmatic rocks near volcanoes for heating, you have to drill. This high-enthalpy geothermal energy is common in Italy, Iceland, New Zealand, and Japan. The Krafla volcanic complex in northern Iceland erupted between 1975 and 1984 (the “Krafla Fires”) and 35 years later, two boreholes were drilled to assess the geothermal potential associated with this event, targeting a depth of 4 km based on geophysical data from the time: However, at a depth of 2 km, the drill encountered lava, recording a sudden increase of 400°C in less than 20 meters; it broke, but a few rock samples were brought to the surface and provided additional information. How can such a temperature be maintained at such a shallow depth?

Conceptual diagram of the volcanic context of Krafla, Iceland: the 3D view shows the magma reservoirs at a depth of 2 km, the IDDP-1 and KJ-39 boreholes drilled in 2009, and the heat anomalies detected at greater depths (in blue). On the left, the enlarged image shows the partially molten host rock (in yellow, location of the thermal jump) that convects between the initial magma injection (in red) and the cold crustal host rock (in gray).

We developed a numerical model in which we assumed that a pocket of magma at over 900°C and 100 to 300 m thick (too small to be detected) had been injected into the cold surrounding rock. We simulated how this very hot magma could have gradually heated the surrounding rock, and we show that in 35 years, the rock itself began to melt enough to convect over a thickness of several tens of meters. This convection phenomenon evens out the heat, as opposed to the “classical” conduction that characterizes the thermal gradient in a normal crust, producing rapid circulation cells like water boiling in a pan. At this point, the temperature gradient at the transition between the cold, conductive domain and the hot, convective domain jumps abruptly from 400°C in a few tens of meters. It will take several decades for this entire area to cool down and return to a stable thermal state.

The numerical solver developed (openFoam platform) simulates the flow between immiscible fluids and takes into account variations in viscosity, density, heat capacity (etc.) during the partial melting of rocks. These magmas have very low viscosities and require high-resolution calculation meshes of around 10 meters to accurately reproduce the heat transfer involved. We achieved this through cases running on the CALMIP regional computing cluster over a period of more than three months.

Contacts GET: Muriel Gerbault, Anastassia Borisova

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