Distributed volcanic fields are a hardly-studied form of volcanism within continental plates. They are characterised by a large number of volcanoes and maars spread over an area of 1,000 to 10,000 km². In most cases, each of these volcanoes is active only once; subsequent eruptions then occur at a different location. In order to better assess the danger posed by this type of volcanism, it is important to map the magmatic system from the mantle depth to the upper crust and to map reservoirs where magma can accumulate and rise, potentially leading to individual eruptions.

To investigate these questions, the Helmholtz Centre for Geosciences GFZ, together with universities and earthquake services from Germany and Luxembourg, conducted a large-scale seismological experiment in the volcanic fields of the Eifel. More than 500 seismic stations were used in combination with acoustic sensors along a 64-kilometre-long, unused fibre-optic cable. The experiment, the largest of its kind ever conducted in Germany, enabled station distances of less than two kilometres in some cases. This made it possible for the first time to carry out high-resolution subsurface investigations directly beneath the volcanoes of the Eifel. Teams of scientists from the GFZ and the participating partners have now published their initial findings. These confirm previous assumptions about the structure and condition of the volcanoes in the Eifel, but also reveal some unexpected findings.

High-resolution tomographic images of the subsurface show for the first time the location, position and depth of the magma reservoir that caused the eruption of Laacher See 13,000 years ago. The main phase of the eruption, which lasted only a few days, was highly explosive and produced massive ash and pyroclastic avalanche deposits near Mendig, south of  Laacher See. Until now, the size and depth of the magma chamber beneath Laacher See could only be inferred indirectly from studies of the Mendig tephra layers. Seismic tomography shows an anomaly in seismic velocities beneath Laacher See at a depth of up to 10 kilometres, significantly deeper than previously assumed. Surprisingly, the anomaly does not dip vertically downwards, but slants towards the Neuwied Basin, where most of the microearthquakes in the Vulkaneifel region are concentrated.

With the help of the extraordinary data set, more than a thousand microearthquakes were located in one year. Most of these quakes occur along a narrow, vertical zone between Ochtendung and  Laacher See. However, there are also earthquake clusters that occur in the peripheral areas of the seismic velocity anomalies. This surprised the scientists, as it could indicate an increased temperature in these areas.

‘The strong reflections of seismic waves at layer boundaries in the upper and lower crust beneath the Neuwied Basin are also unusual,’ says Torsten Dahm, head of the Eifel Large N Experiment. ‘The strength of the reflections indicates that fluids have accumulated in these layers. Whether these are magma or magmatic fluids has not yet been clarified and will be investigated using improved evaluation methods.’