Subaerial Pyroclastic Density Currents (Pyroclastic Flows and Surges)

Explosive eruptions can produce a spectrum of pyroclastic density currents (pdcs) which we describe in this chapter. Pdcs are potentially the most destructive of all volcanic phenomena, due to their high velocities, the large distances that some can travel, the large volume of volcanic debris they can carry, and their high temperature. Pyroclastic flows include pumice/scoria and ash flows, blast flows, and block and ash flows. Pumice/scoria and ash flows are characterised by variably vesiculated pumice and scoria and finer ash and result from partial or wholesale gravitational collapse of initially buoyant explosive eruption columns or directly from a boil-over fountain of the pyroclastic mixture directly from the vent(s). In some cases, column collapses are preceded or accompanied by pyroclastic fallout, but some extremely large volume pyroclastic flow systems, especially those derived from very large, crystal-rich magma reservoirs that source major caldera collapse events and explosive super-eruptions, seem to originate from ring fractures rather than point source vents and immediate eruption column or fountain collapse, as soon as the eruption begins. Pumice/scoria and ash flows can flow from <1 to >100 km from source, and range in volume from <1 to >1000 km3. Deposits, called ignimbrites, range from felsic to mafic in composition, and they are massive to diffusely stratified, non-welded to largely welded. They are generally poorly sorted, and include variable proportions of accessory lithic clasts, including significant breccia horizons. Surface area and volume of ignimbrites are related and reflect variable mass eruption rates and eruption durations, which can be used to distinguish between column collapse and caldera collapse associated ignimbrites. Lateral blast flows originate from sector collapse of parts of high-relief stratovolcanoes, or the margins of high-relief lava domes, which are very polymictic, and can flow several tens of km. Block and ash flows are relatively small in volume, monomictic, lack accessory lithics, originate from lava dome collapse and have limited flow distances (<15 km). Pyroclastic surges range from vent-centric base surges derived from phreatomagmatic-phreatic-hydrothermal explosions with limited flow distances, to marginal, co-pyroclastic flow surges that can continue to propagate for the length of pyroclastic flows. Although we focus on deposit facies characteristics to interpret the flow dynamics of pdcs, post-depositional effects are also considered, particularly welding, which is a common, although not universal characteristic of ignimbrites, but absent or rare in block and ash flow deposits, and absent in blast flow and base surge deposits.