External vs. internal forcing mechanisms in generating maar volcanoes
Final Report Abstract
Our suspicion in the proposal, based on our previous studies, that the deposits of the dominating nephelinitic Westeifel maars differ significantly from those of non-nephelinitic maars, has been strengthened. 1. Lack of clearcut criteria for pervasive water-quenching of the magma, the most fundamental criterion recognized over the past 4 decades for magma-water interaction. Evidence for external water is mainly restricted to the basal 1.2 m section and to the presence of dust films on particles in higher sections. 2. Common lava agglutination in ash-lapilli-bomb-sized clasts is fundamental evidence for low-viscosity magma having been the pervasive physical state of magma during the fragmentation of magma and countryrock, although the latter was likely fragmented by complex heating and water-liberation in the explosion system. 3. The inferred high CO2-concentration in the nephelinitic magmas must have been the major propellant of the magma and low pressure CO2 expansion was most likely also the dominating fragmentation factor. Various types of evidence for high CO2-concentration in the Westeifel nephelinite magmas have been inferred by us for several decades, is petrologically reasonable and has not been questioned in the literature. The escape of CO2 appears to be extremely pervasive in the Westeifel. 4. The abundance of dense mantle/cumulate rock fragments („nodules“), some exceeding several decimeters in all Westeifel maar deposits, indicates very high rise velocity of the ascending gas-liquid-particle mixture (>10.9 m/s), not unlikely driven by exsolving CO2. These rock fragments comprise various types of true ultramafic mantle-type rocks as in the Meerfelder Maar, undeformed magmatic cumulates with rare fragments of mantle rocks as in Pulvermaar, Weinfelder/Gemündener and other maars of the Pulvermaar group or sundry variously heated and recrystallized lower crust lithologies as in Oberwinkeler and partly Weinfelder/Gemündener maars. 5. On balance, explosive eruptions of highly SiO2-undersaturated magmas such as nephelinites appear to be governed principally by high CO2-concentration/exsolution. However, given that the near-surface environment of eruptive sites most everywhere is likely H2O-rich, a complex interaction external H2O/CO2 represents a plausible eruptive scenario.
Publications
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(2014) Vulkane der Eifel. Aufbau, Entstehung und heutige Bedeutung. Spektrum Verlag, Heidelberg. 2nd ed. pp 1-159
Schmincke H-U
