Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits

Ash aggregation is a common phenomenon in particle-laden environments of volcanic eruption plumes and pyroclastic density currents. Many of these initially fragile aggregates gain sufficient mechanical strength to remain intact after atmospheric transport and deposition. Several processes contribute to ash aggregate stability, including electrostatic and hydrostatic bonding, ice formation, and cementation by salt precipitates. Here, we compare leachate chemistry from aggregates from a variety of eruption and sedimentation conditions, ranging from dry magmatic eruptions with immediate deposition, to eruptions through seawater. The leachate data shows that the broad window of opportunity for aggregation and aggregate break-up may be used to qualitatively constrain suspended ash concentration and its temporal evolution. We show that aggregation rate and aggregate stability largely depend on the availability of external water and salt source. In particular, high humidity and extensive salt precipitation in seawater environments, such as during the Surtseyan eruptions, promote high aggregation rates and aggregate stability, with accordingly accentuated proximal deposition and aggregate concentration in the deposits. On the other hand, low humidity and salt concentrations during dry magmatic eruptions promote less aggregation and more efficient aggregate break-up, explaining the rarity of aggregates in the deposits. These results have strong implications for the ash budget in volcanic plumes and associated models of plume dispersal and related hazards.