Chapter 1 Introduction and key concepts

Abstract

AbstractPyroclastic density currents are inhomogeneous mixtures of volcanic particles and gas that flow according to their density relative to the surrounding fluid (generally the atmosphere) and due to Earth's gravity. They can originate by fountain-like collapse of parts of an eruption column following explosive disintegration of magma and rock in a volcanic conduit, or from laterally inclined blasts, or from hot avalanches derived from lava domes. They can transport large volumes of hot debris rapidly for many kilometres across the ground and they constitute a lethal and destructive volcanic hazard. Ground-hugging pyroclastic density currents produce a buoyant counterpart, known as a phoenix cloud or co-ignimbrite ash plume, which can carry ash and aerosols into the stratosphere and so cause significant climatic perturbation. Most processes within pyroclastic density currents are impossible to observe and so are commonly inferred from the associated deposits.Deposits of pyroclastic density currents have been generally categorized, according to lithology and sedimentary structure, as ignimbrites, pyroclastic surge deposits and block-and-ash flow deposits. Ignimbrites typically are pumiceous and ashrich. They predominantly comprise a poorly sorted mixture of pumice and lithic lapilli supported in a matrix of vesicle-wall-type vitric shards and crystal fragments. They may be loose and uncompacted, or partly to entirely densely indurated. Some show evidence of hot deposition (e.g. > 550°C). They generally form low-profile sheets or fans, which can cover areas as large as 45 000 km2, and they tend to bury or partly drape pre-eruption topography with marked thickening into topographic depressions. Ignimbrites can