High-temperature deformation of tuffs at explosive volcanoes a field and structural investigation

Some explosive volcanoes generate pyroclastic density currents that carry searing-hot glass particles across landscapes to deposit as welded ignimbrites[1]. Some are so hot that the particles agglutinate in seconds, and the deposit deforms as a ductile fluid prior to cooling. This is rheomorphism[2]: it occurs at most volcanic provinces and generates spectacular ductile deformation structures from microscopic to outcrop-scale, including sheath folds and a variety of shear fabrics and kinematic indicators[2,3]. The project will distinguish early structures that record the transport direction of the density current from those caused by subsequent hot downslope slumping. The structural analysis will be complemented by innovative high-temperature deformation experiments[4] of hot volcanic glasses to quantitatively constrain strain rates and controlling factors. This will yield important new insights on some of the most destructive volcanic phenomena known (high-temperature super-eruptions).

This project is an opportunity for a student with strong structural fieldwork skills to apply the latest methods to some spectacular volcanic rocks. Results will benefit future scientists who interpret cataclysmic high-temperature events from the rock record. The student will gain expertise in volcanology, structural geology, laboratory experimentation and fieldwork skills.

Methodology

Detailed structural mapping of refolded rheomorphic ignimbrites at carefully selected sites in southern Italy and Yellowstone-Snake River province USA, will determine the spatial and temporal relationships of different types of structures and microfabrics: some record palaeocurrent direction, whereas others develope later and record local topographic slopes. Fieldwork results will be integrated with micro-structural analysis using Leicester’s SEM and micro-CT scanner, and high-temperature deformation experiments at the volcanic deformation laboratory at Liverpool to constrain shear rates and help constrain the physical and chemical parameters that control the deformation of the silicic glasses. This novel approach will generate much-needed information about ignimbrite emplacement temperatures and the time-scales of hot deformation. By analysing pristine volcanic glass from ignimbrites of contrasting welding intensity and rheomorphism, the study will shed light on the relationship between magmatic temperature, as determined from phenocryst pairs, and the intensity of deformation. The results will have important implications for how we interpret conditions during transient catastrophic events (super-eruptions) on Earth and other terrestrial planets.

Possible Timeline

Year 1

Supervised fieldwork in southern Italy to determine the relationship of different types of deformation structure to flow direction vs. local topographic slope. Orientated sample collection for SEM and micro-CT analysis at Leicester. Image processing of initial scans to quantify fabrics. Presentation at UK conference.

Year 2

Supervised and independent fieldwork in USA (Snake-River-Yellowstone) for structural mapping and sampling a range of tuffs with contrasting rheomorphic behaviour. Write-up for publication and conference presentation. First experiments to determine conditions that favour welding and ductile shear. Conference presentation. Start writing 1st paper.

Year 3

Final fieldwork to sample for microfabric analysis and experimentation. Last experiments, and integration of results for writing-up.