The explosive origin of Pulvermaar in the West Eifel Volcanic Field (WEVF): H2O or CO2?
Palaeontology
Final Report Abstract
It was the aim of the project, to check the formation of a particular type of touristically very attractive volcano, the maars of the Eifel with a new perspective. Steiner (Trier) in 1820 described the Pulvermaar, the prototype maar, as consisting of broken-up country rock and Lavakugeln. This was a major discovery because it highlights the conflict with the current interpretation that developed through the past 50 years. The phreatomagmatic model of maar formation postulates magma rise into the upper 200- 300 below the surface, heating of groundwater causing the massive steam explosions that break the country rock forming walls of rock fragments surrounding maar craters. In this initial study focusing on the nature of the nodules we recognized at least two types of Lavakugeln in the ca. 12 m thick presently exposed Pulvermaar deposits. One type is up to 35 cm across and consists of almost completely crystallized alkali amphibolite/pyroxenite. All nodules are well-rounded and all are covered by lava rinds up to 3 cm thick that consist of microscopic spheres suggesting deposition as lava spray. Another type of large nodule consists of metamorphic country rock (gneisses etc), also well rounded and surrounded by lava skins. This nodule population reflects a remarkably violent and highly turbulent environment. Fragments of country rock were broken off their parent bodies, tumbled very violently to form rounded nodules that were subsequently covered by several layers of lava spray. All this must have occurred at high temperature and at about 20 km crustal depth as shown by CO2-inclusions in clinopyroxene crystals. Temperatures of this environment must have exceeded 1000 o C. A second type of noduls, called pellet by us, comprises subround lapilli mostly <5-10 cm in diameter that consists of agglutinated shreds of lava and that are also surrounded by very thin lava coatings judging from the few thin sections sofar examined. A very important aspect of the pellets is their abundance. While they present <50 vol.% in the lower half of the exposed deposits, their abundance is > 50 vol % in the upper ca. 6 m thick bedded deposits. In other words, volcanic lapilli make up a major volume of the exposed Pulvermaar deposits. Clearly, magmatic/volcanic processes played a major role in the formation of Pulvermaar. In the early 1970ies, maars were commonly interpreted - in the Eifel and elsewhere - as formed by gas explosions. Because convincing criteria for this interpretation were lacking, the phreatomagmatic model quickly superseded the magmatic model. However, geologists working in provinces of very alkalic and carbonatitic rocks such as in central Italy and Brazil are still postulating that maars form by CO2-explosions A major aspect of the volcanic clasts of the Pulvermaar and other maars of the closely spaced Daun and Gillenfeld maar clusters is their melilite nephelinitic chemical composition. Magmas of this composition have been shown experimentally to contain very large amounts of CO2 which can be liberated in vast amounts during their passage through the crust. It is our preferred interpretation at this early stage of the investigation that CO2 liberation has been a major factor in releasing large volumes of CO2 representing the major factor in generating Lavakugeln and leading to formation of Pulvermaar. We published our findings in Bulletin of Volcanology 2025.
Publications
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Origin of maar clusters at the type locality Eifel (Germany): H2O or CO2?. Bulletin of Volcanology, 87(3).
Schmincke, Hans-Ulrich; Sumita, Mari; Chakraborty, Sumit & Hansteen, Thor H.
