Explosive Deep Water Basalt in the Sumisu Backarc Rift

Subaqueous Explosive Eruption and Welding of Pyroclastic Deposits

Silicic tuffs infilling an ancient submarine caldera, at Mineral King in California, show microscopic fabrics indicative of welding of glass shards and pumice at temperatures >500 degrees C. The occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water, which would cause chilling. Intracaldera progressive aggradation of pumice and ash from a thick, fast-moving pyroclastic flow occurred during a short-lived explosive eruption of approximately 26 cubic kilometers of magma in water >/=150 meters deep. The thickness, high velocity, and abundant fine material of the erupted gas-solids mixture prevented substantial incorporation of ambient water into the flow. Stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general, both above the vent and along any flow path, may be the main process giving rise to buoyant-convective subaqueous eruption columns and attendant fallout deposits.

Fallout of Pyroclastic Debris from Submarine Volcanic Eruptions

Volcanic fallout deposits on land, being widespread and accessible for study, have received much attention and have revealed a great deal about subaerial eruption mechanisms. In contrast, virtually nothing is known about equivalent deposits produced by submarine volcanoes, despite the probable abundance of such material in today's oceans and in accreted volcanic arc terrains. Many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns. Experiments on water-saturated pumice and pieces of rock (lithics) show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters: pieces of pumice may be five to ten times as large as codeposited lithic fragments. Similar material, erupted into the air and deposited on land, displays less well-developed bimodality; pumice diameters are generally two to three times as large as associated lithics. Submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain.