Illuminating the Internal Structure of Two Monogenetic Volcanoes by ERT (La Garrotxa Volcanic Field, Spain)

Eruptive activity of individual monogenetic volcanoes usually lasts few days or weeks. This short lifetime is not ever synonym of simplicity in terms of dynamics and structure. The construction of monogenetic cones is rarely witnessed from the beginning to the end and conditions for observing their internal structure are hardly reached. We provide high-resolution electric resistivity tomograms (10 m electrode spacing) of three monogenetic cones from north-eastern Spain. Details of their structure that could not be observed through other geophysical methods are revealed. We compare our results to geological observations, interpreting their underground continuation. With a maximum depth of exploration of about 100 m, we provide images of almost the entire edifices, highlighting the relationships between Strombolian and phreatomagmatic deposits in two multi-phases edifices. A main observation is a column of lower resistivity at the centre of the Puig d’Adri Strombolian cone, which we interpret as the eruptive conduit. This method provides information on the past volcanic dynamics of the monogenetic volcanic fields in general, which has real implications for the forecast of future activity.