Probing the explosivity of scoria cone eruptions with GPR: The example of Cerro Negro

Summary Scoria cones are often assumed to grow by the accumulation of individual particles transported ballistically in Strombolian type eruptions. Alternative models of cone formation suggest that, for more explosive, violent Strombolian eruptions, fallout from the eruption column also contributes significantly to edifice construction. Currently, in the absence of direct observations, distinctions between normal Strombolian and more explosive events are based on inference of eruption characteristics from mapping of tephra fall deposits. However, medial to distal tephra fall deposits erode rapidly, often leaving behind only the edifices. A tool that is capable of delineating differences between low-energy Strombolian deposits and higher energy violent Strombolian deposits from investigation of the preserved scoria edifice is therefore necessary. Ground-penetrating radar imaging of Cerro Negro, an active basaltic scoria cone in Nicaragua, has revealed details of cone deposits at depths of up to 12 m. The record of the 1992 eruption, which was observed to be violent Strombolian, shows quantifiable differences from normal Strombolian near-vent facies, including reflections in the downwind profile that are continuous for hundreds of meters and through the slope break. This radar record enables researchers to differentiate between tephra fallout and ballistically emplaced deposits, suggesting ground-penetrating radar imaging may be useful in distinguishing eruptive style in older scoria cones, where the medial to distal tephra deposits are eroded or buried.