Tumulus development on lava flows: insights from observations of active tumuli and analysis of formation models

Here, we use observations of active flows along with detailed morphometric field measurements of more than 70 tumuli on flows at Mount Etna (Italy), Kilauea, and Hualalai (US) volcanoes to constrain a previously published model that estimates the pressure needed to form tumuli. In an attempt to discover the nature and magnitude of pressure variations within active lava flow interiors, we then consider how tumuli differ from idealized circular plates. We incorporate observations of active tumuli and find that they may grow asymmetrically yet produce a symmetrical tumulus and can form where the flow path significantly changes direction. Bending models of clamped edges provide the most reasonable head estimates for the tumuli in our study. Tumulus formation requires the proper combination of cooling and effusion rate. If cooling is too extensive and effusion rate too low, the crust will provide too much resistance to bending. If cooling is too limited and effusion rates too high, crusts will not develop or have insufficient strength to resist fracture and subsequent breakouts. We do not find it surprising that tumuli are rarely found over well-established lava tubes that typically have rigid, walls/overlying crusts that exceed 2 m in thickness and provide too much resistance to bending. Silicic flows lack tumuli because the viscosity gradients within the flow are insufficient to concentrate stress in a localized area.