Assessing the rock record of hazardous pulsatory pyroclastic density currents

Overview

Pyroclastic density currents (PDCs) are hot, density-driven flows of gas, rock and ash generated during explosive volcanic eruptions, or from the collapse of lava domes (e.g. Sparks, 1976; Fisher, 1979; Branney and Kokelaar, 2002; Cas et al. 2011). They pose a catastrophic geological hazard, and have caused >90 000 deaths since 1600 AD (Auker et al. 2013). Improved understanding of PDCs will enable us to better understand the explosive eruptions that generate them, improving our preparedness for future volcanic events. However, these deadly hazards are rarely observed up close and are difficult to analyse in real-time. To understand the flow dynamics of density currents we must use models and interpretations of their deposits (e.g. Smith N and Kokelaar, 2013; Rowley et al. 2014, Williams et al. 2014, Sulpizio et al. 2014; Lube et al. 2019, Pollock et al 2019, Smith 2018, 2019).

Interpreting the rock record at volcanoes is the primary way in which volcanologists assess the hazards that the volcano poses to local communities. However, the rock record of pyroclastic density currents is incomplete – currents can pass over the landscape without depositing and they can even erode their own deposits. Sometimes the sedimentary structures within these deposits are difficult to interpret. But, deciphering the structures in that deposit enables us to understand the evolution of an eruption; was there a single, large sustained current, or a series of discrete currents, possibly prior to a climactic caldera collapse? Did multiple currents traverse different areas of the surrounding landscape, or was there a focussed zone of activity?

The number of PDCs generated during an eruption has typically been interpreted by stratigraphic evidence for a cessation in flow that defines discrete “flow-units” (e.g. Brown and Branney 2013). However, in a study where sufficient exposures were available for comparison (Smith N, 2012) it was found that different numbers of flow-units can be recorded in proximal and distal exposures, demonstrating that waxing and waning (“unsteadiness”) along a current’s run-out can create a contradictory picture of flow-units in different locations. Thus, there remains a question on the use of stratigraphic markers to define numbers of discrete PDCs.

This PhD will use field observations and laboratory experiments to explore how changes in PDC dynamics are recorded in volcanic stratigraphy, and how this information can be used to better reconstruct eruptive activity. The work will focus on how variations in current steadiness, mass flux, particle size and substrate can impact current run-out and the deposition of flow-units.

 

An example showing how multiple flow units could be interpreted to record (A) multiple discrete PDC events, or (B) the waxing and waning of a single unsteady pyroclastic density current, dependent on the exposures available.

Objectives

The main objectives of this project are to:

  • Investigate evidence for single vs multiple PDCs during major eruptions. You will undertake fieldwork, primarily consisting of stratigraphic analysis and logging techniques, to investigate and report on the record of flow-units in space and time. Potential locations where proximal and distal exposures of the same eruption can be contrasted include Las Cañadas Caldera and the equivalent Bandas del Sur region of Tenerife (e.g. Brown et al. 2003; Smith N and Kokelaar, 2013), and the Campanian Ignimbrite of Italy (e.g. Smith V et al. 2016).
  • Develop an experimental flume set-up to model how eruption fluctuations control PDC unsteadiness. You will build upon previous work (Rowley et al. 2014; Smith G. et al. 2018) to enable the examination of how changes in eruption dynamics cause pyroclastic density currents to wax and wane through time and space.
  • Conduct experiments to investigate how density current dynamics impact the depositional record of flow-units. You will experimentally model how changes such as mass flux, current steadiness, particle size and substrate affect the depositional record of flow-units. You will attempt to replicate sedimentary architecture observed in the field.

Impact

The project will improve our interpretation of the geological record of explosive volcanic events. The research will shed light on (i) how changes in eruption mass flux and steadiness impact the run-out distance of pyroclastic density currents through time and (ii) how current run-out impacts the stratigraphic record both with distance from the vent and with location around the vent. This crucial insight will improve our ability to create hazard assessments of PDC-forming eruptions. By creating better hazard maps, we can help build the resilience of local communities at risk from explosive eruptions. The work is relevant to the fields of volcanology and hazard planning, and also potentially farther afield, to studies of other types of gravity currents (e.g. submarine turbidity currents and avalanches).

Training

The project will provide training in: (i) geological fieldwork, (ii) stratigraphic analysis, (iii) the set-up of experimental flumes, (iv) the fluid dynamics of density currents, and (viii) modelling and analytical skills. You will be associated with the Catastrophic Flows Research Cluster in the Department of Geography, Geology and Environment at the University of Hull, and will work with Dr Rebecca Williams and Dr Natasha Dowey at Hull and also with Dr Pete Rowley at the University of the West of England. The University of Hull has a thriving postgraduate community and the postgraduate training programme provides a full range of courses covering research techniques, scientific methods, information technology, scientific writing and statistical analyses, which are tailored to the needs of each student. Supervision will involve regular meetings between all supervisors.

 Student profile

You should have an interest in igneous geology, volcanology, sedimentology, and geological hazards, and be enthusiastic about using a range of different techniques, including fieldwork, to better understand density current dynamics.

If you have any informal queries about this project please feel free to contact Rebecca Williams (rebecca.williams@hull.ac.uk).

 References

Auker et al. (2013) Journal of Applied Volcanology 2 2

Branney and Kokelaar (2002) Geological Society Memoir 27

Brown et al. (2003) Geological Magazine 140 3

Brown and Branney (2013) Bulletin of Volcanology 75 727

Cas et al. (2011) Bulletin of Volcanology 73 1583

Fisher (1979) Journal of Volcanology and Geothermal Research 6:3-4 (305-318)

Lube et al. (2019) Nature Geoscience 12 (381–386)

Pollock et al. (2019) Bulletin of Volcanology 81 46

Rowley et al. (2014) Bulletin of Volcanology 76 855

Smith N. (2012) Thesis, University of Liverpool (https://livrepository.liverpool.ac.uk/6253/)

Smith N. and Kokelaar (2013) Bulletin of Volcanology 75 768

Smith V. et al. (2016) Bulletin of Volcanology 78 45

Smith G. et al. (2018) Bulletin of Volcanology 80 67

Smith G. et al. (2019) EarthArXiv PrePrint (10.31223/osf.io/6c4pv)

Sparks (1976) Sedimentology 23:2 (147-188)

Sulpizio et al. (2014) Journal of Volcanology and Geothermal Research 283 (36-65)

Williams et al. (2014) Geology 42:2 (107–110)