On the emplacement of ignimbrite in shallow-marine environments

The emplacement of primary pyroclastic flow deposits on the sea floor can occur (1) by nonturbulent, dense flows that conserve their heat under water, or (2) because the flow (either turbulent or nonturbulent) temporarily pushes back the shoreline. The feasibility of the second mechanism has been investigated theoretically and experimentally using a simple analogue system. The experiments involved sustained (turbulent) saline currents of different densities flowing into a sectorial tank of ambient fluid that was either lighter or denser than the current. The currents in all cases displaced the entire layer of ambient fluid up to a distance ( R), irrespective of whether they were denser or lighter. R is analogous to the shoreline displacement distance up to which hot ignimbrite can be emplaced by a pyroclastic flow entering the sea. By varying flow rate, density contrast, and depth of the ambient fluid layer, a simple equation for R is found that is in good agreement with the experimental data. The effect of seawater boiling on shoreline displacement was also investigated by experiments using currents of carbonate solution that produced gas when flowing into a layer of ambient acid. The distance R was not reduced in these experiments. The equations show that pyroclastic flows of about 10 km 3 or more are capable of pushing back the sea at least a couple of kilometres and possibly more in areas of extensive shallow water, enabling hot ignimbrite to be laid down on the seabed at depths corresponding to a few tens of metres. Shoreline displacement is thus a feasible mechanism for the emplacement of welded ignimbrite in shallow-marine settings. Emplacement of subaerially erupted ignimbrite below storm wave base is not ruled out by the calculations, but requires a combination of high discharge rate, sustained eruption, low density contrast between flow and water, steep bathymetric gradient, and small island volcano. Application of the model to the 1883 Krakatau eruption suggests that seawater displacement may explain the occurrence of primary ignimbrite on the seabed 10 km northwest of the volcano.