Explosive Eruption Column Dynamics for Crystal-Rich Magmas

Many deposits of large-volume explosive eruptions involved magma with ∼25–50 vol.% of lapilli-size crystals. Numerical modeling shows that such abundances of coarse crystals in an erupting mixture significantly influence eruption column dynamics compared to eruptions dominated by fine ash. In high gas content eruptions that would otherwise produce buoyant plumes/columns, the abundant crystals can cause chaotic fountains wherein waves of particles collapse from time-varying locations in the eruptive jet. Crystal-rich eruptions with intermediate gas content can generate stable fountains and pyroclastic currents under conditions that would produce a buoyant plume in fine ash-only eruptions. Some conditions produce mixed behaviors involving low-altitude fallout of crystals while a buoyant fine-ash plume rises above. Crystal-rich eruptions, particularly large ones, are likely to proceed directly to fountaining and ignimbrite deposition without the canonical buoyant eruption plume and basal fall unit, explaining field observations.