Overview

Project Highlights:

  • Uniquely combines fieldwork & experimentation to shed new light on hot flow-deformation.
  • Detailed structural mapping at European and US volcanoes.
  • First micro-CT investigation of rheomorphic fabrics, plus innovative high-temperature experiments on shear rates of volcanic glasses.

Many explosive volcanoes generate vast pyroclastic density currents that carry searing-hot glass particles across landscapes. These deposit welded ignimbrites1. Some are so hot that the particles agglutinate in seconds, and the deposit continues to deform as a ductile fluid prior to cooling. This is rheomorphism2: it occurs in most volcanic provinces and generates spectacular ductile deformation structures from microscopic to outcrop-scale, including sheath folds, extensional lineations and a variety of shear fabrics and kinematic indicators2,3. This project will distinguish early structures that record the transport direction of the density current, from later structures caused by subsequent hot downslope slumping. For the first time, this structural analysis will be complemented by innovative high-temperature deformation experiments4 of hot volcanic glasses to quantitatively constrain strain rates and controlling factors. This will yield important new insights into some of the most destructive volcanic phenomena known (high-temperature super-eruptions).

This project presents an opportunity for a numerate student with good structural geology and fieldwork skills to apply the latest structural methods to some fascinating surface rocks. The results will benefit scientists who interpret cataclysmic high-temperature events from the rock record.

Welded tuffs are found at many large volcanic provinces worldwide and record peculiarly devastating eruptions1. They resemble folded glassy mylonites, and can be investigated by a variety of structural geology methods.

Methodology

Detailed fieldwork and structural mapping of highly folded rheomorphic ignimbrites at carefully selected sites in Italy, Canary Islands and Yellowstone-Snake River province USA, will determine the spatial and temporal relationships of different types of structures and microfabrics: some form during initial welding and record palaeocurrent direction, others develop later during local downslope flowage. 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. What physical and chemical parameters control the deformation of the silicic glasses - is it temperature, composition or shear rate? 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 catastrophic events (super-eruptions) on Earth and other terrestrial planets, where similar materials are found.

Training and Skills

CENTA students will benefit from 45 days training throughout their PhD including a 10 day placement. In the first year, students will receive training as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes. 

The student will interact within a lively research group of PhD students at Leicester. Supervisors will provide hands-on training in modern physical volcanology, structural mapping of complex deformed rocks, quantitative analysis of microfabrics, and the use of high-temperature experimental apparatus to study the rheology and timescales of ductile deformation of volcanic glasses. Training will be provided on scientific writing, conference presentation, and publication.

Timeline

Year 1: Supervised fieldwork on Pantelleria to determine the relationship of different deformation structures to flow direction vs. 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.

Year 3: Final fieldwork to sample for microfabric analysis and experimentation. Last experiments, and integration of results for writing-up.

Partners and collaboration (including CASE)

Professor Mike Branney (Leicester) is a physical volcanologist and leading expert on ignimbrite emplacement and rheomorphism. He will provide hands-on field training in field volcanology and structural techniques. Dr David Holwell (Leicester) has expertise in 3D textural analysis and will provide training on Leicester’s micro-CT scanner and image processing. Prof Yan Lavallee (Liverpool) will provide expertise on laboratory experimentation. This combination will place the student in a competitive position for state-of–the-art research in this popular discipline.

Further Details

Any questions? Please contact Mike Branney at the Department of Geology, University of Leicester: mjb26@le.ac.uk