Explosive Eruptions

Explosive volcanic eruptions can produce large sequences of pyroclastic deposits that reveal eruptive intensities, durations, and significant changes in eruptive styles.

Samuel uses microanalysis and numerical modelling to show how measurements of clast densities, microtextures, grain size distributions and componentry reveal information about explosive volcanic eruptions and transitions to effusive behaviour.

Transitions in eruptive styles

Numerical modelling and analysis of clast microtextures can be used to tell us about changes in volcanic eruptive styles. In particular, identifying changes in vesicle and crystal growth, and the development (or closing) of permeable pathways in magma can result in shifts from explosive-to-effusive or effusive-to-explosive activity.

In previous research, a transition from high-intensity clast-producing behaviour to low-intensity lava dome effusion could be achieved through lateral outgassing of magmatic fluids from the conduit into the surrounding crust. Newer research (in prep.) is expanding this modelling to assess how saline and metal-rich fluids may leave volcanic systems to produce ore and brine deposits soon after eruption.

Samuel has also been involved in more recent efforts to assess effusive-explosive transitions at La Soufriere volcano (St. Vincent and the Grenadines) and Pululahua Dome Complex (Ecuador) through use of ash grain size distribution and componentry analysis.

Strain and decompression of shallow magma

Vesicle and microlite number densities in erupted pyroclasts and lavas can be used as geospedometers for the ascent of magma through a volcanic conduit. Past research analysed vesicles and microlites from pumice blocks to assess magma decompression rates in a shallow conduit prior to eruption.

Further work from this research used 2D and 3D textural analysis of elongate vesicles (in tube pumice) to assess strain conditions across the profile of a volcanic conduit. Large cumulative strain in volcanic conduits can produce very elongate tube vesicles, but flaring (opening) of a conduit near the vent can reduce strain, permitting bubbles to retain a more spherical shape upon eruption.

Modelling of strain rates within a conduit can also reveal the depths (or lack) of magma fragmentation prior to eruption. In a previous study of the 2012 Havre submarine eruption, strain rates were found to be too low to result in magmatic fragmentation in the conduit. Instead, magma erupted into the water in a high eruption-rate "effusive" manner, where magma then broke up in the water column.

Density and vesicle connectivity of pyroclasts

Additional research within explosive eruptive products re-evaluated appropriate clast sizes for accurate measurements of density (vesicularity) within pumice. Secondary vesicle growth post-fragmentation was more apparent in clast sizes larger than 32mm. This study also evaluated how pumice clasts should be carefully considered for analysis based their external faces (broken during transport/deposition or an original magmatic fragmentation surface).

The connectivity of vesicles within pumice and lava (measured through gas pycnometry) can also reveal information about the flotation potential of clasts within water, how quickly clasts may cool, or how gases may permeate through (or become trapped) in lava structures.