Dr M. Branney, Dr M. Norry, Dr T. Barry

Aims of Project

How do pyroclastic density currents behave and evolve with time during an eruption?

The Green Tuff draping caldera wall successions under Scauri Village.

Low aspect-ratio ignimbrites are thought to be emplaced by particularly hazardous, radial, high-velocity pyroclastic density currents from caldera-forming eruptions. Their circular distribution has been inferred to record simultaneous flow in all directions from source, overtopping hills, rather than passively flowing down valleys.

The Green Tuff draping near-vertical caldera wall at Bagno dell'Acqua.

We have been testing the inference of simultaneous, radial (i.e. rather than sectoral) flow by mapping out the internal chemical-architecture of a zoned, low-aspect ratio ignimbrite sheet on the island of Pantelleria, Italy. This pristine, welded ignimbrite (aspect ratio ≤ 1:5,000) was deposited during a phase of the most recent (~45,000 ka) caldera-forming explosive eruption on the island. One extensive flow-unit is zoned from pantellerite to trachyte, and records that the composition of the erupting magma changed with time.

The chemical variations allow us to divide the brief history of the sustained current into successive time-periods. The compositional zones have been mapped internally through the deposit, both (1) regionally (longitudinally from source and laterally around the broadly circular sheet), and (2) around topographic barriers draped by the ignimbrite. The study takes advantage of superlative exposure and topographic control. We have reconstructed how the footprint of the sustained current shifted as the current waxed then waned, and as it encountered and then overtopped barriers.

In particular, this study aimed to use the detailed chemical stratigraphy, in combination with field relations and structural information, to test the following hypothesis:

1      Are radial ignimbrites emplaced radially, simultaneously?

2.    How do sustained density currents interact with topography?

The unique example of the Green Tuff, Pantelleria has provided insights into the behaviour of other low-aspect ratio ignimbrites and could also feed back into the study of density currents in general.

Further work

During this PhD work, a new stratigraphy of the caldera-forming, Pre-Green Tuff eruption history for the NE side of the island was developed. This work is now being continued by Nina Jordan:

http://www2.le.ac.uk/departments/geology/extranet/staff/postgraduate-students/postgrad-pages/njj5/nina-jordan

Me at Volcán Cotopaxi, Ecuador

For more information about my previous research on modelling lahars, and further details into my skills and expertise, please visit:

http://uk.linkedin.com/in/rebeccawilliams2104