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Morphometric analyses are based on multiparametric data sets that describe quantitatively the shapes of objects. The stochastic nature of fracture formation processes that break up magma during explosive eruptions yields mixtures of particles that have highly varied shapes. In volcanology, morphometric analysis is applied to these mixtures of particles with diverse shapes for two purposes: (1) to fingerprint tephra from individual eruptions and use the fingerprints to distinguish among tephra layers and determine their extents and (2) to reconstruct eruption processes, by linking particles formed by known fragmentation processes in experiments with particles from natural pyroclastic deposits. Here, we review the most commonly adopted statistical techniques for morphometric analysis of pyroclasts. We provide sets of objects with different shapes, along with their morphometric data, in order to demonstrate and illustrate the methods. They can be used not only for addressing the processes of fragmentation during explosive eruptions, but also for the characterization of other types of solid particles with complex morphologies. [ABSTRACT FROM AUTHOR]

The 1963–1967 shallow-to-emergent eruption in Iceland's Vestmannaeyjar earned a place in the history of volcanology by creating the island of Surtsey while under close observation of volcanologist Sigurdur Thorarinsson (Sigurður Þórarinsson in Icelandic). This is an example of what is now called Surtseyan volcanism, and it included explosive and effusive phases from multiple vents that formed the island of Surtsey itself, as well as one fully subaqueous pyroclastic edifice and two additional, but ephemeral, islands. Sigurdur Thorarinsson identified tephra jetting and continuous uprush as characteristic types of subaerial explosive activity of Surtseyan volcanism. Subaerial cone-forming deposits of Surtseyan volcanism are typically poorly sorted, with fine-grained beds rich in sideromelane ash fragments, punctuated by larger, ubiquitously composite bombs, whereas deposits sampled by coring deep into the submarine edifice include fines-poor horizons dominated by vesicular coarse sideromelane ash. Here, we present new textural data and highlight the diversity of pyroclasts and microtextures from Surtsey (Surtur I and Surtur II) and its satellite vents (Surtla, Syrtlingur and Jolnir), in the context of Surtseyan volcanism. We used several sample sets. Some were collected during the 3.5-year long eruption and were conserved in the Icelandic Natural History Museum, including one sample from the core drilled into Surtsey in 1979. Other samples were collected during more recent field campaigns on Surtsey Island. In closing, we discuss the implications of this diversity for the range of activity and products produced by Surtsey. [ABSTRACT FROM AUTHOR]

Havre caldera volcano's deep (~1 km) submarine eruption in July 2012 produced an ~80 km long vapour plume and ~400 km2 pumice raft at the sea surface. The vapour plume indicates focused delivery of heat to the surface and indicates that the eruption formed some kind of narrow thermal plume. In 2015 during an NSF-funded research cruise we observed and sampled the eruption's deposits in situ with the remote operated vehicle Jason. High-resolution bathymetry was obtained with the autonomous underwater vehicle Sentry, providing a LiDAR-quality map with 1-m resolution of the entire caldera including the multiple vent sites of the 2012 eruption. A surprising product of the eruption is extensive fine and very fine ash, forming part of a multi-layered deposit with local contributions from effusive sources. The main ash layers show no clear thinning or fining trends across the whole of the caldera floor and shoulders, implying wide dispersal. Since significant fractions of this ash show a texture different to the pumice it is unlikely that these grains have been produced by abrasion of the raft pumice. Analysis of the ash will help us determine the driving mechanisms for ash production, which have implications for the eruption dynamics, and for rates and mechanisms of heat transfer to the ocean. There are specific, identifiable energy costs to the process of fragmentation, but energy expended in fragmentation greatly enhances rates of heat transfer by forming additional surface area. The effect of the latter is well known for subaerial eruptions, in which ash production strongly controls generation of buoyant plumes. For Havre's deep-sea eruption, we have conducted heat-transfer and fragmentation experiments by remelting products of the 2012 eruption. Both dense rock from lava domes, and seafloor-collected pumice, were remelted. Physical data from experiments has been combined with 2D and 3D shape analyses of both lab-generated volcanic particles and natural volcanic ash to reveal the dominant ash generation mechanisms involved in the 2012 Havre eruption. Experimental setups and applied analysing techniques will be presented. Preliminary results indicate that magma-water interaction, and specifically fuel-coolant interaction, probably induced, was important in generating most of the ash. This result rests primarily on characteristics of fractal dimension of natural and experimental particles, as assessed using high-resolution (sub-micron) micro-CT imaging and statistical T-tests of significant numbers of analysed particles. The result is interesting in the context of other analyses of the Havre 2012 products, which did not address ash formation, and which concluded that it was a non-explosive high-rate effusive eruption. [ABSTRACT FROM AUTHOR]