Characteristics of rootless cone tephra emplaced by high-energy lava–water explosions

The effects of external water during hydrovolcanic eruptions are difficult to quantify; however, research investigating the explosive interactions between surface lava flows and water has provided new insights into explosion dynamics. Previous work has focused on relatively low-energy lava–water explosions from a 100-m-diameter cone in the Rauðhólar cone group in Iceland. For comparison, we study ejecta derived from higher-energy explosions at the 400-m-diameter Pu‘u Kīholo rootless cone in Hawai‘i, one of the largest rootless cones on Earth. This comparative approach elucidates the effect of lava–water mixing conditions and range of explosivity on ejecta characteristics. Utilizing grain size analysis, we found that Pu‘u Kīholo beds typically have median grain sizes at the cone rim that are smaller than − 3 ϕ (8 mm) and are on average finer-grained than those of the Rauðhólar cone. Ash-sized grains, the most energetically significant ejecta size fraction, typically make up > 20% of bed ejecta, with a maximum of 90%, both greater proportions than found in beds of the Rauðhólar cone. Utilizing grain morphology, crystal texture, and surface feature analysis, we found that the abundance of ash-sized grains associated with high heat transfer rates (i.e., active particles) increases at Pu‘u Kīholo as explosivity increases, which is an important consideration for energy modeling. This dataset also includes ejecta from the finest-grained beds, and therefore highest-energy lava–water explosions, studied to date.