Quench fragmentation is a non-explosive process that occurs when molten magma is super-cooled to glass upon contact with ambient water. This occurs when coherent lavas are erupted subaqueously, when they flow into water, when magma intrudes into water-saturated sediments, and when magma and water interact explosively during phreatomagmatism. Quench fragmentation also occurs alongside explosive phreatomagmatic activity. Although products of quench fragmentation (hyaloclastite sensu stricto) have been discussed qualitatively in the volcanological literature, compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass. Based on literature from materials engineering, we present a detailed review of the processes by which glass forms, the properties of glass, and the fracture mechanics that cause it to fragment non-explosively. We also consider how this can be applied to understanding the dynamics behind the volcanological processes of in-situ glass fragmentation during quenching in wet environments and phreatomagmatism. Important parameters for the occurrence of quench fragmentation are the temperature difference between the magma and the ambient water and how much the ambient water is superheated above its Leidenfrost temperature. The geometry of the lava or magma intrusion, the thermal conductivity and the thermal expansion are also of great importance. The resistance of the magma against fragmentation can be increased with the presence of crystals provided the thermal expansion of the crystals does not greatly exceed that of the glass; vesicles have the opposite effect, unless the magma is highly vesicular. This overview then provides a solid basis for further quantitative study of quench fragmentation and hyaloclastite formation.

This chapter contains sections titled: Introduction Stratigraphy And Facies Discussion Conclusions

The c. AD 1305 Kaharoa eruptive episode consisted of a complex sequence of basalt-triggered high-silica rhyolite eruptions from at least seven vents along an 8 km linear zone across Tarawera Volcano. Initial plinian eruptions from a summit vent spread Kaharoa Tephra southeast across the North Island, accompanied by phreatomagmatic explosions and pyroclastic density currents from vents opened on the northern flanks of the volcano. The early plinian phase was ended by extrusion of Crater Dome in the summit vent, with explosive activity migrating to two adjacent vents (Tarawera and Ruawahia) to the southwest and northeast. Renewed plinian eruptions, apparently from the northern vents, produced tephra falls dispersed northeast-northwest from the volcano. Extrusion of the three summit lava domes was accompanied by voluminous block-and-ash flows generated by collapse of the growing Ruawahia and Wahanga Domes, forming large fans to north, southeast, and northwest of the volcano.Calculation of Kaharoa lava volumes and comparisons with the extrusion rates of observed dome-building eruptions suggest a duration of c. 4 yr for the c. 4 km(3) Kaharoa eruptive episode. This estimate, of years rather than days or weeks, is significant for planning an effective response to a future similar rhyolite episode in New Zealand.

The Islands of Ponza and Gavi (western Pontine Archipelago, Italy) preserve parts of a subaqueously emplaced, high‐silica (75–77% SiO2) rhyolitic lava flow that overlies the margins of three older domes. Exposures of the lava alone cover 24 km2, and with a thickness of 150 m exposed in the cliffs, the preserved volume of the lava flow is 3.6 km3, and the original volume is likely to have exceeded 7 km3. The lava flow consists of coherent obsidian and several hyaloclastite facies, including in situ and clast‐rotated breccias and sandstones, which all exhibit gradational relationships. The coherent obsidian occurs as separate domains and is often flow banded. Although flow folds occur, the banding is generally subhorizontal, dipping gently to the NNE. The hyaloclastite, representing 90% of the lava by volume, has a diffuse layering varying from 0.5 to 2 m in thickness. This layering also dips gently to the NNE and is marked in places by alternation of some coherent horizons, coarse and finely fragmented in situ hyaloclastite, pumiceous hyaloclastite, and clast‐rotated hyaloclastite. The layering is laterally continuous and consistent in orientation over most of the 13 km length of Ponza: the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow. The resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones. The lava flow has been intruded and crosscut by rhyolitic bodies of various ages, including discordant, coherent to internally fractured contemporaneous rhyolite bodies, and late, penecontemporaneous dikes with glassy margins and alteration haloes.