Hydrogeologic and magmatic controls on phreatomagmatism at the La Garrotxa monogenetic volcanic field (NE of Iberian Peninsula)

Abstract

Phreatomagmatism caused by direct interaction of erupting magma and groundwater is a common phenomenon accompanying monogenetic volcanism. The controls on the type of phreatomagmatic phases and deposits that can be observed in such eruptions have been attributed to magma ascent rate, magma rheology, magma discharge, magma vesiculation, water availability, and the properties of the interacting aquifer (i.e., permeability, pore water pressure, recharge rate, etc.). The La Garrotxa monogenetic volcanic field displays a wide variety of eruption sequences and deposits among its 50 volcanoes, half of them including phreatomagmatic phases with very distinct characteristics. Using the characteristics of the phreatomagmatic deposits, including the lithology and content of the accidental lithic fragments, and the substrate stratigraphy and its hydrogeology through the analysis of available information on the water wells and boreholes of the area, it is evident that several aquifers with very different lithologic and hydraulic properties have interacted with the erupting magmas. Moreover, these same data show that all the phreatomagmatic explosions nucleated at a maximum depth of 200 m. These results suggest that pressure variations inside the eruptive conduit, caused by changes in the recharge rate from the magmatic plumbing system, have played a major role in controlling phreatomagmatism, rather than due to variation of the lithological and hydraulic properties of aquifers.