Unravelling the secrets of the Sister’s Scoria – a Masters project

I was first introduced to Ascension when Katy Chamberlain presented her research on the ‘zoned fall deposit’ in one of my third-year modules at the University of Derby. Shortly after this, I made contact with Rich Brown about the MSc by Research (Volcanology) at Durham and when he proposed a project focusing on Ascension I jumped at the chance!

My research focuses on the scoria falls and lava flows from the Sister’s Volcanic Complex, with one of the final aims being to chemically and texturally (through crystal populations) correlate the individual falls with their associated flows, and the second to understand the dynamics of the eruption through textural analysis. To do this a quantitative textural analysis will be completed using optical microscopy, SEM images and X-CT (which produces a 3D image of the internal texture of the clasts) to identify mineral phases, groundmass and phenocryst textures and vesicle distribution. Chemical analysis of the tephra units and lava flows will also be undertaken, using whole rock and trace element data to look for any changes in composition as the eruption progressed. Identifying and understanding variation between the “linked” deposits will give clues as to how this system evolved as the eruptive period progressed.

 

Images showing Sister’s scoria cones, lava flows, and scoria falls.

(Photos from Rich Brown)

The first stage in the project is to sieve all the scoria units to separate them into size fractions. Once this has been done, and the two textural end members have been identified, the vesicularity will be measured (using Archimede’s principle and the method of Houghton and Wilson (1989) which has been adapted by Bridie Davies) and a representative sample will be used for further textural analysis. At the same time, more samples will be ground up and sent away for chemical analysis of the concentrations of major and trace elements present in the units.

Understanding changes in the texture of the scoria clasts (for example vesicle and crystal shape and size) is vital as it provides insight into the internal and external process of the volcanic system.

figure 3

There are obvious textural changes seen in the scoria, both from different units (B, D) and from beds within the same unit. They show a range of iridescent colours from blue (A) to green and pink (E) whilst others are dull with an ‘ashy’ coating (C). (Photos B and D from Rich Brown)

I am also learning to model, with the aim of producing a series of isopach maps that can be used to forecast the dispersal of tephra from an eruption of similar style and size to Sister’s in the future. Isopach maps can be produced using field data by mapping maximum thicknesses of distinct ash or scoria layers in the stratigraphy across an area. When complete these maps provide vital information; maximum thickness of ash, maximum extent and the dispersal axis of the volcanic plume. To model future eruptions I will work with Julia Crummy from the BGS and use the Tephra Pro model, into which data collected by Rich Brown during the 2018 field season will be fed. Prior to using Tephra2 I have created isopachs and isopleths for the Sister’s eruption through Google using only maximum thickness data, but it will be interesting to see how the modelled maps will differ when more input parameters, such as plume height, are considered. Modelling these eruptions is very important for volcanic hazard assessment due to the close proximity of Georgetown to Sister’s.

I am very lucky to be working with a great team of people; Rich Brown and Kate Dobson from Durham University, Charlotte Vye-Brown from the BGS and Katy Chamberlain from the University of Derby. I can’t wait to see how the project evolves and I hope to get some interesting results that will inform future studies and hazard assessment on Ascension.

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Scoria fall overlying a lava flow and draping over the Sister’s scoria cone. (Photo by Rich Brown)

This blog post was written by Rebecca Winstanley – A Master’s student at Durham University.

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