386 PYROCLASTICS See Also Further Reading Biosediments and Biofilms Diagenesis, Overview Fossil Plants: Calcareous Algae Sedimentary Rocks: Chert Trace Fossils Garcia Ruiz JM, Hyde ST, Carnerup AM, et al (2003) Self assembled silica carbonate structures and detection of ancient microfossils Science 302: 1194 1197 PYROCLASTICS R J Brown, University of Bristol, Bristol, UK E S Calder, Open University, Milton Keynes, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Pyroclasts are formed by the explosive fragmentation of magma (molten rock) during volcanic eruptions (see Volcanoes) They are carried away from the vent by buoyant eruption plumes, extensive umbrella clouds, or by destructive ground-hugging pyroclastic density currents The largest explosive eruptions can produce pyroclastic deposits many 1000’s km3 in volume, which can be emplaced on a regional scale in a matter of hours The study of the physical characteristics of pyroclasts and pyroclastic deposits can reveal much about the dynamic processes involved during explosive eruptions and in pyroclast dispersal and deposition Recognizing, observing, and understanding pyroclastic deposits are vital first steps in assessing and mitigating volcanic hazard (see Engineering Geology: Natural and Anthropogenic Geohazards) This chapter summarizes the physical characteristics of the principal types of pyroclastic deposits and presents an introduction to their generation and emplacement mechanisms Generation of Pyroclastic Material Explosive fragmentation of magma during volcanic eruptions can occur by two main mechanisms The first involves the rapid exsolution of dissolved magmatic gases during rapid decompression events (magmatic eruptions), and the second results from the interaction of hot magma with external water sources (phreatomagmatic eruptions) Pyroclastic material can also be generated by rapid decompression and by autobrecciation processes during lava dome collapses Magma comprises three separate materials or phases: a viscous silicate melt (of varying composition), variable amounts of crystals (phenocrysts), and gas (volatiles) such as H2O, CO2, S, F, and Cl There is a general positive correlation between the silica content of a magma and the degree of explosivity (Table 1) However, it is the quantity and behaviour of the gas phases that are critically important in determining the eruption style, because it is the rapid expansion of gas during decompression that drives explosive volcanic eruptions Magma is stored at depth in magma chambers, under high temperatures and pressures Magmatic eruptions are preceded by an increase in pressure and volume in the magma chamber This is often attributed to the arrival of new magma into the chamber The upper parts of many magma chambers are thought to contain a small volume fraction of gas bubbles (vesicles) due to supersaturation with volatiles, and seismic disturbance of these pre-existing bubbles can also lead to increases in magma chamber pressure (Figure 1) Crystallization, which enriches the melt in volatiles, can also act as a trigger Once a critical point is reached, mechanical failure of the magma chamber roof occurs, allowing magma to rise, decompress, and exsolve gas in a runaway process (vesiculation) that can rapidly drive magma up the conduit at speeds of 200–400 m/s Vesicle growth is controlled by the volatile content and by the physical properties of the magma (diffusivity rate, density, viscosity, and surface tension) The diffusivity rate is particularly important, and controls the rate at which gas bubbles escape from the magma: where escape is fast (in hot basic lavas), eruptions tend to be effusive or weakly explosive, but where escape is inhibited by high viscosities and low diffusivity rates (in intermediate and rhyolitic magmas), the exsolution of gas can explosively, and very violently, disrupt the magma The expansion and coalescence of these bubbles forms a magma foam with radically different physical properties to that of the parent magma During ascent, this rising vesiculated magma is fragmented into discrete particles and transforms into a gas-particle mixture, which accelerates up the conduit and is discharged into the atmosphere (Figure 1) Phreatomagmatic fragmentation is driven by the volumetric expansion of external water after it has been rapidly heated by contact with magma This mechanism is not restricted by magma type or vent type and it encompasses a spectrum of eruption styles