Overview

How explosive caldera volcanoes erupt and collapse is essential to understand some of the most catastrophic events at the Earth’s surface. But research is hindered at modern volcanoes because most of the critical features, such as caldera faults, eruption conduits, vents, and caldera fill deposits are concealed1.  However, in rare cases geological uplift and erosion can beautifully exhume such features, allowing direct examination. Scafell caldera in the English Lake District is one of the best exposed, exhumed and dissected explosive caldera volcanoes in the world2. It has attracted interest internationally because it provides astonishing access to caldera fill ignimbrites, caldera floor faults, eruption vents and post-climactic lava extrusions2,3. It therefore offers unique insights into the internal workings of these complex systems. Of particular interest is how subsidence shifts with time, and how vents activate and migrate as caldera floor faults propagate and dilate.

Scafell is one of a several overlapping (nested) calderas that generated explosive super-eruptions in the Lake District. The project will clarify the relationships between the large eruptions and their subsidence structures and vent systems. Logging and sampling of sections will be followed by geochemical analysis to characterise the outflow ignimbrites and relate them to their source volcanoes and vents. The project will reveal where successive subsidence events occurred, and how vent locations and caldera faults shifted with time.

Explosive volcanic caldera spectacularly exhumed by uplift and glacial incision, English Lake District

Methodology

Using a combination of detailed fieldwork and analytical geochemistry, the products of individual explosive eruptions in the English Lake District will be characterised and correlated. Drawing on existing maps, and adopting an approach developed by the Leicester group in the Snake River–Yellowstone region of USA4, whole-rock trace-element ratios will be used to identify and distinguish between a number of super-eruptions: the utility of this method has just been tested in the Lake District with very promising results.

Graphic-style logging through the varied pyroclastic succession will be used to establish local stratigraphies, unit thicknesses, and provide the context for rock sampling.

Ignimbrites and fallout layers will be sampled for geochemical analysis.

Detailed geological mapping of selected hillsides will be required to establish the geological relationships.

Geochemical analysis by XRF and LA-ICPMS at Leicester will be used, together with field evidence to define and characterise individual eruption-units, and to correlate between distal and near-source exposures. Optical microscopy will help in interpretations.

Vent sites and silicic domes/intrusions will be mapped and chemically analysed to determine unit source locations.

Thickness variations of correlated units will be used to estimate run-out distances, successive eruption volumes and related areas of subsidence.

Training and Skills

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

You will be trained in field (at Tenerife and in the Lake District) and laboratory analysis of pyroclastic successions including geological mapping, logging, interpreting, and sampling of ignimbrites, ashfall tuffs, and lavas. Training will be given on geochemical analysis (XRF, LA-ICPMS and EMP) correlation of tephra units regionally. By the end of the project you will have developed specialist expertise in physical volcanology and geochemistry of supervolcanoes.

You will join a thriving research community within the Volcanoes, Tectonics and Mineral Resources Research Group at the University of Leicester. By the end of the project you will have gained a set of skills ideal for a career in geology, physical volcanology or igneous petrology.

Timeline

Year 1: The project will start in 2018 with supervised fieldwork to familiarise with the well-exposed pyroclastic successions. In December, training at Tenerife will be on interpreting pyroclastic deposits. The student will start a second field season in early Summer, 2019, characterising a sampling successions, followed by sample preparation and XRF analysis. Interpret initial data and participate in a national conference (VMSG).

Year 2: Fieldwork will to identify, map-out and sample potential vent sites and outflow sheets. Samples will be prepared and geochemically analysed, and the data used to correlate units. You will present preliminary results at a national conference, and there will be opportunity to draft a first publication. Final fieldwork in Summer to solve problems and extend the correlations further into new regions.

Year 3: Complete the laboratory work, and develop interpretations of individual eruptions. Estimate the geographical extents and volumes of eruptions. Geological synthesis to assess how vent positions migrated, and how areas of subsidence shifted with time. Write-up international publications and thesis, and present findings at an international conference.

Partners and collaboration (including CASE)

Professor Michael Branney has extensive experience on large explosive eruptions around the world, and is a leading international expert in the interpretation of pyroclastic successions, with an emphasis on super-eruptions, calderas and ignimbrites. He has first-hand experience mapping and interpreting UK calderas.

Prof Jan Zalasiewicz is a stratigrapher and mapper.

Dr Tiffany Barry is an igneous geochemist with 25 years experience investigating basaltic and evolved volcanic successions of ignimbrites and flood basalts from Antarctica to central Asia. She oversees the analytical laboratories at Leicester.

 

Further Details

Please contact Professor Michael Branney, University of Leicester (mjb26@le.ac.uk).    

This project would suit an energetic geology student proficient in independent hillwalking, geological mapping, and camping in remote upland terrain. You will be able to drive, and have a facility for geochemistry, and a keen interest in volcanology, stratigraphy and structural geology.