Pleistocene explosions and pyroclastic currents in west-central Anatolia

The discovery of unexpectedly young tephra, with K-Ar dates of 1.50 +/- 0.1 8 and 1.38 +/- 0.13 Ma BP, in the Lake Egirdir area sheds new light on the time-span and magnitude of explosive volcanism in west-central Anatolia and on the physical behaviour of pyroclastic currents. The Golcuk maar near Is parta, widely thought to have ceased its activity in Pliocene time, was app arently still erupting with strong explosions in the Early Pleistocene. The Egirdir tephra, 34 km NE from the volcano, indicate deposition from powerf ul pyroclastic surges directed by the topography. The depositing currents w ere fully turbulent and carried unusually large lithic clasts, up to 2-3 cm in diameter, in suspension, which is incompatible with the existing physic al model for pyroclastic currents. Based on the tephra characteristics, a c onceptual model of the pyroclastic current's segmentation, or lateral dynam ic partition, is suggested and used further to explain the current's specif ic response to an encountered topographic barrier. The widely held concept of a pyroclastic current's Vertical separation, or sharp rheological decoup ling, is thought to describe a common secondary phenomenon.