Formation of an 'a'a lava delta: insights from time-lapse multibeam bathymetry and direct observations during the Stromboli 2007 eruption

Five consecutive multibeam bathymetries collected before, during, and after the 2007 Stromboli eruption, combined with visual inspections, allowed us to document the morphological evolution of an 'a'a lava-fed delta and to reconstruct the main processes acting during its submarine emplacement. The 2007 Stromboli delta extended down to 600-mwater depth and covered an area of 420x103m2, with a maximum thickness of 65 m and a total estimated volume of 7x106 m3, i. e., three times larger than its subaerial counterpart. The lava delta grew mainly through the emplacement of discrete lobes about 50-150 m in size. Lobes were fed from point sources along the paleoshoreline, and their subaqueous pathways seem to be mainly controlled by the submarine morphology, with flows mostly filling in depressions left by previous lobes. The main controlling factors on the lobe morphology and thickness are the effusion rates and the preeruption morphology, i. e., the geometry and gradients of the basal surface. Data also shows that sudden slope failure of portions of the submarine delta may occur simultaneously with accretion, implying that a significant part of the delta material can be transported to greater depths by submarine gravity flows. The present study is relevant for future monitoring and hazard assessment during the growth of active lavafed deltas as well as for a better interpretation of ancient volcaniclastic successions inland.