Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece

Abstract

The Kos Plateau Tuff (KPT) is dominated by a sequence of nonwelded rhyolitic ignimbrites, some of which were deposited more than 60 km from source on islands in the eastern Aegean sea, Greece. Palaeo-shoreline reconstruction at the time of the KPT eruption (161 ka) suggests that northward-directed pyroclastic flows travelled over an extensive landmass, although the proximal zone may have been sea. In comparison, flows directed to the southeast and east most likely crossed wide expanses of sea, depositing ignimbrite on islands and peninsulas. The ignimbrites include basal coarse lithic-rich breccias overlain by thick, massive pumiceous zones and generally have low abundances of fine ash. They are interpreted to have been deposited from pyroclastic flows that were relatively dilute and stratified, with lower, denser, lithic-rich zones and upper, expanded, pumice-rich zones. High-velocity inherited from the intensity of the eruption, combined with turbulence and low bulk density, favoured transport across water. The pyroclastic flows remained hot even after crossing 35 km of the sea. Heat capacity equations indicate that a depth of ~1 cm of water ingested over the outflow path would have cooled the flows by only a few degrees. The ignimbrites deposited prior to the eruption climax have distributions restricted by 300-m-high topographic barriers and they thicken in topographic depressions. These ignimbrites show significant decreases in grain size, proportion of lithic clasts and thickness with distance from source. In comparison, during the eruption climax, the coarsest, most voluminous and widespread KPT ignimbrite was deposited. This ignimbrite has relatively even thickness and uniform componentry and texture regardless of topographic relief or distance from source. Textural analysis indicates that crossing 35 km water to the southeast had little effect on the pyroclastic flow generated during the eruption climax, except for the loss of the coarsest (>20 cm) vent- and conduit-derived lithic clasts. Preferential loss of lithic clasts is related to the mechanism by which they were transported. On land, large, dense clasts could be saltated over the ground surface at the base of the pyroclastic flow. In the case of transport over water, these large, dense clasts sank and were permanently removed from the transport system. Outflow across water had a greater effect on the pre-climax pyroclastic flows. The ignimbrites formed at this stage are significantly thinner and slightly finer grained, and lack basal lithic breccias and lithic clasts >5 cm where the pyroclastic flows crossed the sea, reflecting the lower velocities of the flows. All KPT ignimbrites to the east of the source were deposited from pyroclastic flows that crossed >25 km of water. Ignimbrites in this sector are markedly finer grained, possibly as a result of blocking of coarser pyroclasts by a topographic barrier close to source.