Abstract A remarkable variety of pyroclast types is produced in explosive volcanic eruptions, each reflecting the many factors controlling the eruption energy and eruption sequence, including viscosity, gas content, and phenocryst content of the magma. Fragmentation during a volcanic eruption and the resulting pyroclast characteristics can also be linked to external water affecting the eruption process. Volcanic ash characteristics can be used to infer the eruption type and emplacement processes.

Most phases of silicic lava dome growth have some associated explosive activity. Tephra produced during this activity have depositional characteristics, grain sizes, and grain shapes that reflect different mechanisms of dome growth and destruction. It is therefore possible to interpret the explosive history of a dome through study of adjacent tephra deposits even though the dome may no longer be present. Five stages of dome growth and their associated tephra deposits are considered here. (1) Crater formation before extrusion of a dome, including phreatic, phreatomagmatic (ph-m), and Plinian pumice eruptions, produces a tephra sequence at the base of a dome consisting of deposits rich in accidental lithic clasts from crater walls, overlain by beds of fine-grained tephra and coarse-grained pumice. (2) Magma pulses during dome growth (ph-m, in part) produce tephra consisting of mixtures of juvenile pumice and clasts derived from the partly solidified dome. (3) Ph-m interaction between new magma and a water-saturated dome produces uniform tephra consisting of angular clasts of dome lava. (4) Explosive eruptions that follow collapse of a gravitationally unstable dome produce tephra that consists of angular, partly pumiceous clasts of dome lava which fragment due to expansion of metastable water after release of confining pressure. (5) Posteruptive destruction of the dome by phreatic eruptions results in pyroclasts consisting of fine-grained, hydrothermally altered clasts derived from dome lavas. Major kinetic processes before explosive dome eruptions are the relatively slow diffusion of magmatic volatiles from magma to fracture planes and foliations within the dome, and the relatively fast diffusion of meteoric water into magma by mechanical mixing. These basic processes control most explosive activity at domes in cases of either expulsion of new magma or collapse of an unstable dome.