Spatter, fountains and plumes: the fragmentation of basaltic magma

Overview

Basaltic eruptions form a spectrum of activity from gentle effusion of lava flows, through spattering, lava fountaining, and mild explosions, to violent, sustained explosions that produce a lofting ash column. The nature of the eruptive products is determined by the eruption style and, in turn, the products determine the nature of the hazard posed by an eruption.

All but the most gentle basaltic eruption styles produce clastic material, but the processes by which basaltic magma is fragmented or disaggregated into clasts (spatter, scoria, ash) are poorly understood. The central goal of this project is to investigate the physical and textural characteristics of basaltic clasts, infer the processes that form them, and link those processes to the physics of eruption.

The fragmentation of basaltic magma may involve both ductile and brittle processes. Ductile disaggregation produces clasts with shapes that record fluidal processes: Pele’s hair, tears and limu for the smaller clasts; spatter and spindle bombs for the larger clasts. By contrast, brittle fragmentation produces clasts that are angular and fractured: ash for the smaller clasts; scoria for the larger clasts.

The different types of erupted clast reflect differences in the physical processes operating in the conduit, and in the eruption fountain or column above the vent. Interpreting differing particle morphologies in terms of the differing in-conduit and in-column processes that form them will provide a valuable new tool for understanding and reconstructing eruption processes across the spectrum of basaltic eruption styles.

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Image Captions

Figure 1: a lava fountain during the 2014 Holuhraun eruption, Iceland.
Figure 2: SEM imagery of natural (l) and synthetic (r) Pele’s tephra
Figure 3: brittle/ductile processes captured in hot glass during pilot work

Methodology

The study will combine field volcanology and sampling, laboratory analysis of eruptive products, and experimental fragmentation of natural and synthetic silicate melts. Field work will focus on the products of Kilauea (Hawaii), which span the effusive to fountaining eruption styles, and the products of Etna (Italy) which span the strombolian to plinian eruption styles. Sample collection will be guided by published work on these eruptions, so that samples are confidently associated with different phases and styles of the relevant eruptions. Scanning electron microscopy (SEM) will be used to analyse the textures of eruption products, focussing particularly on surface and fracture morphology, building on recent work focussed on Pele’s hair (Porritt et al., 2012; Cannata et al., 2019). Compositions will also be analysed to determine the physical properties (particularly rheology) of the samples. These data will be used to associate specific textural features and abundances with different eruption styles.

Natural and synthetic silicate melt compositions will be analysed using extensional rheometry up to failure, at a range of temperatures and strain rates. Brittle and ductile failure regimes will be mapped out in terms of the physical properties of the melts, and the conditions of stress and strain, building on theory from the mathematical literature (Eggers and Villermaux, 2008). SEM analysis of the experimental products will be used to compare textures with natural products. Larger scale processes, such as growth and bursting of large bubbles, will be investigated in collaboration with the National Glass Centre (University of Sunderland), using synthetic glass compositions and industrial/artistic glass blowing and manipulation techniques. This will allow behaviour at the micro-scale to be identified with macro-scale processes, relevant to in-conduit and in-column conditions.

Results will be synthesized to produce a framework for mapping eruption processes to clast morphologies, and vice versa.

Project Timeline

Year 1

Fieldwork in Hawaii, investigating field presentation of eruptive products spanning effusive to fountaining activity. Textural and compositional analysis of Hawaiian samples. Possible output linking clast morphology to processes of formation for ductile disaggregation.

Year 2

Fieldwork on Etna, investigating field presentation of eruptive products spanning strombolian to plinian activity. Textural and compositional analysis of Etnean samples. Rheometry of basalts from both field sites. Possible output linking clast morphology to processes of formation for brittle fragmentation.

Year 3

Laboratory experiments with analogue materials, including high-temperature synthetic glass in collaboration with the National Glass Centre. Synthesis of results for ductile and brittle fragmentation.

Year 3.5

Possible output synthesising formation of basaltic clasts across the ductile-brittle transition. Thesis write-up

Training
& Skills

The student will be trained in:
• Field analysis of products of basaltic eruptions across the effusive to explosive spectrum
• Laboratory techniques, including experimental design, scaling and dimensional analysis, and safe working practices
• X-ray tomography and textural analysis, rheometry
• Glass hot-shop working practices
The student will be embedded in the Durham Volcanology Group, a vibrant and collaborative group of 7 staff and around 20 postdocs and graduate students. Co-supervision will link the student to the BGS Volcanology Team who research volcanic processes, hazards and risks, and work on applied science through engagement with policy and decision makers. The project links to two joint US/UK NSF-NERC-funded research projects: one focussing on disequilibrium processes in basaltic eruptions; the other on the 2018 Kilauea Lower East Rift Zone eruption) which will provide unparalleled opportunities for international collaboration.

References & further reading

• Cannata, CB et al., 2019. First 3D imaging characterization of Pele’s hair from Kilauea volcano (Hawaii). Scientific reports, 9, 1711.
• Porritt, LA et al., 2012. Pele’s tears and spheres: Examples from Kilauea Iki. Earth and Planetary Science Letters, 333, 171-180.
• Eggers, J & Villermaux, E.,2008. Physics of liquid jets. Reports on progress in physics, 71, 036601.

Further Information

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