Sulfur as a binding agent of aggregates in explosive eruptions

Understanding the mechanisms that govern ash aggregation is of critical importance in volcanology. Aggregation reduces the residence time of ash (≤2 mm) in the atmosphere strongly, by enhancing the sedimentation of finer-grained material generated during explosive eruptions. To date, experimental studies have focused on the expectation that water provides the strongest bonds between particles to form spherical to oblate aggregates (typically less than or equal to a few mm, occasionally up to several cm) preserved in pyroclastic deposits. Under water-rich conditions, individual accreted particles rarely exceed 1 mm. In pyroclastic density current deposits produced during the 1982 eruption of El Chichón, Mexico (which emitted 7.5 Tg of SO₂ in the atmosphere), aggregate particles one to several millimeters, strongly cemented by a S-rich film, are common. They exhibit similarities with aggregates found in sulfur cones at Poás volcano, Costa Rica. We propose that sulfur is the binder between the silicate grains. Such a binding capacity implies a relatively fluid behavior of sulfur such as might be expected in the low viscosity temperature regime just above its melting point. If so, then the explosive ejection of sulfur during eruptions, combined with its ability to act as a cement for particles >2 mm, would imply that size fractions of lapilli can be efficiently removed from eruptive clouds a few kilometers from the vent. Such an aggregation mechanism would have important implications for pyroclast dispersal models and hazard assessment.