Base surges and their deposits: Capelinhos and Taal Volcanoes

Volcanic base surges from Taal volcano (1965, 1966) and Capelinhos volcano (1957–1958) showed all transitions from: (1) small radial surges of nearly pure white steam puffing out from the base of tall, vertically rising eruption columns charged with abundant black particles, to (2) much larger, dark gray, particle-laden base surges fed by subsidence of the eruption column. Radial dispersal of volcanic base surges occurs essentially in three stages: (1) Surges of white steam with only a few solid particles spread immediately from the base of the emerging eruption column as water vapor concentrated along its periphery escapes and condenses; (2) Black plumes of solid particles shoot radially on ballistic trajectories from the walls of the rising eruption column as it is torn apart by internal steam bursts; and (3) The turbulent mixture of solid particles, water, and air tumbles en masse to the ground, and much of it flows away on the heels of the steam surge and incorporates it. Deposits from the particle-laden surges consist mainly of chilled sideromelane accumulated in characteristically cross-bedded dunelike layers with poor sorting. These alternate with air-fall layers, many of which are rich in accretionary lapilli. At Taal, cross bedding developed from the deposition of dunelike bed forms that were eroded and buried by the debris of the next passing base surge. The dune shapes and form of internal cross bedding resemble antidunes that develop in high-flow regimes on sandy beds of sediment-laden streams, and from underwater density currents simulated in flumes. The cohesiveness of base-surge deposits, shown by bedding sags and mud-plastered walls and trees, is a major factor in preserving the antidune bed forms, but is not a factor in their initial development.