ABSTRACT Field studies of tephra-fall deposits traditionally use the density of juvenile pyroclasts to determine vesicularity of the host magma at the point of fragmentation. A range of pyroclast sizes between 16 and 32 mm has commonly been chosen for this purpose. Larger pyroclasts outside this range may undergo postfragmentation vesiculation due to slow cooling of the interior of the clasts, while smaller pyroclasts may be too small to represent accurately the distribution of the largest vesicles. The assumption of this method, of course, is that the 16–32 mm size range is representative of the fragmented magma. We explore, in detail, variations in density over a size range of 4–128 mm from Unit 2 pyroclasts of the 1.8 ka Taupo eruption and make inferences about the roles of postfragmentation vesiculation and secondary breakage of pyroclasts. We find (1) there is a clear threshold for onset of postfragmentation vesiculation at >32 mm, and (2) there are broken small pieces of the largest pyroclasts in the sample that artificially skew the density distribution for smaller size fractions. We constrain uncertainty associated with vesicularity measurements and offer best-practice recommendations in the hope of improving consistency of field sampling and laboratory processing of pyroclast populations for vesicularity studies.

Abstract This collection of papers presents recent advances in the study of deformation mechanisms and rheology and their applications to tectonics. Many of the contributions exploit new petrofabric techniques, particularly electron backscatter diffraction, to help understand evolution of rock microstructure and mechanical properties. Papers in the first section (lattice preferred orientations and anisotropy) show a growing emphasis on the determination of elastic properties from petrofabrics, from which acoustic properties can be computed for comparison with in-situ seismic measurements. Such research will underpin geodynamic interpretation of large-scale active tectonics. Contributions in the second section (microstructures, mechanisms and rheology) study the relations between microstructural evolution during deformation and mechanical properties. Many of the papers explore how different mechanisms compete and interact to control the evolution of grain size and petrofabrics. Contributors make use of combinations of laboratory experiments, field studies and computational methods, and several relate microstructural and mechanical evolution to large-scale tectonic processes observed in nature.