Submarine pyroclastic deposits: Implications for pyroclastic flow/seawater interactions and volatile discharge during explosive volcanic eruptions

We have studied the submarine and subaerial pyroclastic deposits of an island arc volcanic center in order to investigate pyrolastic flow/seawater interactions. During the 1883 eruption of Krakatau volcano, approximately 12.4 km$\sp3$ of dense rhyodacite magma was erupted largely as pyroclastic flows. Over 78% of the tephra generated was deposited on the sea floor within a 15-20 km radius of the volcano as a result of the entrance of subaerially generated pyroclastic flows into the shallow marine environment. A paleomagnetic investigation of juvenile and accidental pyroclasts sampled from the submarine deposits indicates that the material was emplaced on the sea floor at temperatures above 500$\sp\circ$C. High subaqueous emplacement temperature implies minimal mixing between pyroclastic flows and seawater.

Two primary pyroclastic facies have been recovered in SCUBA cores that sampled the 1883 submarine deposit. The most common facies is a massive textured, poorly sorted, mixture of pumice and lithic lapilli to block sized fragments in a silty to sandy ash matrix. This facies is indistinguishable from the 1883 subaerial pyroclastic flows based on grain size and component abundances. Subaqueous deposition of the massive facies was by deceleration and sinking of a highly concentrated, deflated component of the pyroclastic flows. A less common, primary facies consists of well sorted, planar laminated to low-angle cross-bedded, vitric enriched, silty-ash. The laminated facies most likely was deposited from low concentration turbidity currents. The Krakatau deposits are the first documented example of true submarine pyroclastic flow deposition from a historic eruption.

A petrologic investigation of Krakatau 1883 erupted products indicates a range in melt composition of 13 weight % SiO$\sb2$, from andesite to rhyolite, with rhyodacite magma constituting approximately 90%. Subordinate amounts of gray dacite (4%), and andesite (1%) are present. Evidence for magma mixing consists of banded pumices, melt inclusions more mafic than whole rock compositions and disequilibrium phenocrysts. Melt inclusions indicate pre-eruption volatile content in rhyodacite and gray dacite of 3-4.5 wt. %. Total estimated sulfur discharge from erupted magma is 2.7 $\times$ 10$\sp{12}$ g S, and estimated chlorine discharge is 9.2 $\times$ 10$\sp{12}$ g Cl.