Eifel maars: Quantitative shape characterization of juvenile ash particles (Eifel Volcanic Field, Germany)

• For the first time fractal analyses have been applied to quantitatively characterize the shape of juvenile maar particles.
• Small-scale fractal dimensions of West Eifel maar particles resemble fractal dimensions of typical magmatic particles.
• Fractal dimensions of East Eifel maar particles (Eppelsberg) coincide with dimensions of phreatomagmatic end-member particles.
• Microscopic and quantitative particle shape analyses suggest magmatic fragmentation for West Eifel maar eruptions.
• Equivalent analyses indicate that the studied East Eifel maar deposit resulted from explosive water-magma interaction.

The Eifel region in western central Germany is the type locality for maar volcanism, which is classically interpreted to be the result of explosive eruptions due to shallow interaction between magma and external water (i.e. phreatomagmatic eruptions). Sedimentary structures, deposit features and particle morphology found in many maar deposits of the West Eifel Volcanic Field (WEVF), in contrast to deposits in the East Eifel Volcanic Field (EEVF), lack the diagnostic criteria of typical phreatomagmatic deposits.The aim of this study was to determine quantitatively the shape of WEVF and EEVF maar ash particles in order to infer the governing eruption style in Eifel maar volcanoes. The quantitative shape characterization was done by analyzing fractal dimensions of particle contours (125–250 μm sieve fraction) obtained from Scanning electron microscopy (SEM) and SEM micro-computed tomography (SEM micro-CT) images.The fractal analysis (dilation method) and the fractal spectrum technique confirmed that the WEVF and EEVF maar particles have contrasting multifractal shapes. Whereas the low small-scale dimensions of EEVF particles (Eppelsberg Green Unit) coincide with previously published values for phreatomagmatic particles, the WEVF particles (Meerfelder Maar, Pulvermaar and Ulmener Maar) have larger values indicating more complex small-scale features, which are characteristic for magmatic particles. These quantitative results are strengthening the qualitative microscopic observations, that the studied WEVF maar eruptions are rather dominated by magmatic processes. The different eruption styles in the two volcanic fields can be explained by the different geological and hydrological settings found in both regions and the different chemical compositions of the magmas.