In response to rapid decompression, porous magma may fragment explosively. This occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles. This occurs at a critical pressure difference between the bubbles and the surrounding magma. In this study we have investigated this pressure threshold necessary for the fragmentation of magma. Here we present the first comprehensive, high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity, permeability, crystallinity, and chemical composition. We exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated. Experimentally, we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample. Our results show that the fragmentation threshold is strongly dependent on porosity; high porosity samples fragment at lower pressure differentials than low porosity samples. The fragmentation threshold is inversely proportional to the porosity.Of the other factors, permeability likely affects the fragmentation threshold at high porosity values, whereas chemical composition, crystallinity and bubble size distribution appear to have minor effects. The relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma.

[1] Explosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the Earth's crust. Knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption, and yet direct measurements are still a difficult task with largely variable results. Here we apply pioneering high-speed imaging to measure the ejection velocity of pyroclasts from Strombolian explosive eruptions with an unparalleled temporal resolution. Measured supersonic velocities, up to 405 m/s, are twice higher than previously reported for such eruptions. Individual Strombolian explosions include multiple, sub-second-lasting ejection pulses characterized by an exponential decay of velocity. When fitted with an empirical model from shock-tube experiments literature, this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses. These results directly impact eruption modeling and related hazard assessment, as well as the interpretation of geophysical signals from monitoring networks. Citation: Taddeucci, J

International audiencePore connectivity is a measure of the fraction of pore space (vesicles, voids or cracks) in a material thatis interconnected on the system length scale. Pore connectivity is fundamentally related to permeability,which has been shown to control magma outgassing and the explosive potential of magma duringascent in the shallowest part of the crust. Here, we compile a database of connectivity and porosityfrom published sources and supplement this with additional measurements, using natural volcanic rocksproduced in a broad range of eruptive styles and with a range of bulk composition. The databasecomprises 2715 pairs of connectivity C and porosity φ values for rocks from 35 volcanoes as well as 116products of experimental work. For 535 volcanic rock samples, the permeability k was also measured.Data from experimental studies constrain the general features of the relationship between C and φassociated with both vesiculation and densification processes, which can then be used to interpret naturaldata. To a first order, we show that a suite of rocks originating from effusive eruptive behaviour can bedistinguished from rocks originating from explosive eruptive behaviour using C and φ. We observe thaton this basis, a particularly clear distinction can be made between scoria formed in fire-fountains andthat formed in Strombolian activity. With increasing φ, the onset of connectivity occurs at the percolationthreshold φc which in turn can be hugely variable. We demonstrate that C is an excellent metric forconstraining φc in suites of porous rocks formed in a common process and discuss the range of φc valuesrecorded in volcanic rocks. The percolation threshold is key to understanding the onset of permeability,outgassing and compaction in shallow magmas. We show that this threshold is dramatically different inrocks formed during densification processes than in rocks formed in vesiculating processes and proposethat this value is the biggest factor in controlling the evolution of permeability at porosities above φc

No abstract

Explosive volcanic eruptions are commonly associated with intense electrical activity and lightning. Direct measurement of the electric potential at the vent, where the electric activity in the volcanic plume is fi rst observed, is severely impeded, limiting progress in its investigation. We have achieved volcanic lightning in the laboratory during rapid decompression experiments of gas-particle mixtures under controlled conditions, and recorded it using a high-speed camera and two antennas. We fi nd that lightning is controlled by the dynamics of the particle-laden jet and by the abundance of fi ne particles. The relative movement of clusters of charged particles generates the electrical potential, which is necessary for lightning. The experimental generation of volcanic lightning suggests that rapid progress can now be expected in understanding electrical phenomena in volcanic plumes to implement lightning monitoring systems and the forecasting of volcanic ash emissions.