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.

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.

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

My name is Annabelle and I am a master’s by research student at Durham University studying an unusual glassy pyroclastic rock (some say they resemble chocolate brownies/cookies, however I can assure you, they are not). To geologists they immediately stand out as they are super weird and don’t resemble what usually is erupted out of a volcano! I was eager to undertake this project due to the unique morphology and intricate detail of the samples, as unlocking the secrets of their formation will help us to understand vent processes in fissure eruptions – one of the most common types of volcanic eruptions.

Recent fieldwork to Ascension Island uncovered these mysterious glassy pyroclasts which were co-erupted with scoria during a small-volume basaltic eruption. They are comprised of dense glass bombs that vary in size from milimeter spherical droplets, to blobs 10’s of centimeters across. They occur in low abundances in scoria-dominated ramparts (elongate walls that are found adjacent to fissure).

The unusual shapes and features of these pyroclasts pose numerous questions about their formation and origin.

Some key questions are;

1. Are they a product of stagnant lava that has drained back into the vent?
2. How was it entrained into the lava fountain and erupted alongside normal scoria?
3. Were they erupted at a normal temperature or superheated somehow?

I am fortunate to be investigating these hypotheses under the supervision of a large group; Rich Brown, Kate Dobson, Fabian Wadsworth and Katy Chamberlain. Rich (my lead supervisor) and I are carrying out a comprehensive 2D and 3D textural and physical analysis of the samples to gain a better understanding of their origin. I have also been working with Fabian using numerical techniques to constrain an approximate temperature of the glassy drops when they were erupted. Determining whether they were erupted unusually hot may be a reason why they look so weird.

Additionally, I have been working with Kate on a machine which fires X-rays onto either small samples or cores that have been drilled out of the pyroclasts. This creates 2D pictures and 3D renders of the samples to gain a better understanding of the textures and interactions between the glass and scoria. In the coming months I will be working alongside Katy to investigate the geochemistry of the erupting magma, this will be completed by using an electron microprobe for major element chemistry. Preliminary qualitative chemical analysis work indicates that the glass is a more evolved composition that the co-erupted scoria.

I am very excited to gain more insight into the formation of these extremely weird rocks over the course of my masters and I hope to be back and able to explain further about their origin so make sure to keep an eye out for updates!

Updates from our 2018 field season.

Way back in June our team of researchers (and toddling assistants) from BGS, Durham, UEA and Granada landed on Ascension eager to get back to/see for the first time the spectacular geology of Ascension. The 6 week field season had been carefully planned to cram in as much work as possible; collecting rocks (lots of them), digging pits, making maps, and working with the Ascension Island Government and other stakeholders on Ascension. Six weeks soon felt like hardly any time at all – lucky for us a 1 week delay on the flight home meant we could cram in some extra work (with the much appreciated help of some RAF employees…).

Follow our new blog series for highlights from our 2018 field season along with key updates on our ongoing projects….

Part 1: Pumice Pilfering

Bridie kicked off her PhD investigating the underlying processes that lead to variability in eruptive style on Ascension, by looking in detail at the pumice fall and felsic lava flow deposits of Ascension (pumices associated with high explosivity eruptions and lavas with more gentle ones). Although she made detailed observations and collected samples from all around the island one of the most exciting deposits she worked on was located near to NE-Bay.

One of the key aims for Bridie’s project was to find lavas and pumices that can be linked to the same magmatic plumbing system. It is important to be able to link the different products in this way because we want to see how a volcanic and magmatic processes evolve over time for a single volcano. If we can understand what causes a change in eruptive behaviour we may be better able to monitor for signs that the volcano is about to change the way it is erupting. This is key for volcanic hazard assessment because the dangers posed by a thick, slow moving lava flow are very different to those posed by a hot, fast moving pyroclastic flow.

Exposed in the side of Echo Canyon is a sequence of rocks consisting of a thick pumice fall at the base, thinly bedded pumices at the top, followed by a stripy looking, brecciated (broken into lots of sharp angular fragments) “pink” lava flow and finally topped by a sprinkling of pumice. This sequence not only provided some of the best photos from the field season (see above) it could also be the key to understanding why the volcanoes on Ascension change their eruptive style over time.

From the relationships between the units in the field Rich had suspicions that the sequence in Echo Canyon could indeed be linked to a single magmatic plumbing system (huzzah!). We couldn’t rely of field relationships alone and so we set out to hunt for some additional evidence that could link all the units together…..Luckily, we found it in the form of some VERY special crystals!

The images below show the HUGE (by a geologist’s standard) crystals we found in all the pumices and lavas of the Echo Canyon sequence.

These crystals are the mineral feldspar – which is very common in the Ascension Island volcanic rocks. However, these crystals are special as they are unusually big (up to 4mm), are fairly “stubby” and often show the same inter-grown structure (see photo A above). We haven’t seen crystals like this elsewhere on Ascension and so it is highly likely that the magmas that carried them to the surface came from the same magmatic plumbing system.

Safe to say Bridie was pretty excited when they found those crystals as they linked all the units together. Jane and Bridie set about sampling all up through the sequence in order to collect as much information as possible about how this eruption/series of eruptions progressed. They collected several bags of pumice from the units exposed in the canyon – so much in fact that both of them had to empty their field packs to fit the pumice inside (top tip: bin bags are very useful for carrying field gear that would otherwise have been sacrificed for the good of geological research!).

Although pumice is very full of air bubbles and therefore not very heavy, it does take up quite a lot of room….

Back in the UK, the next step for Bridie’s project is to take the pumice she collected from this sequence and measure the pieces to find out the typical vesicularity of pumice from each part of the deposit (aka what percentage of the pumice is actually empty space). By doing this she can identify a few representative clasts (out of the hundreds that she has) to take thin slices of to analyse in more detail.

Bubbles/vesicles are key in eruptions that produce pumice as their shapes, sizes and abundance tell us about the processes occurring in the magma as it approaches the surface and can ultimately offer insight into what happens in the magma right before it explodes causing the potentially devastating pyroclastic flows you have likely seen on the news.

Her very high-tech equipment (consisting of a mutilated coat hanger and a piece of bamboo) enables Bridie to start making these measurements – the first step in her PhD analysis.

 

Watch this space for updates on Bridie’s progress and more from the 2018 field season!

In just a few days myself (Bridie) and a team of geologists from UEA, Durham and BGS (Jane, Rich and Charlotte) will land on Ascension for the first (and rather long) field season of my PhD.

With a grand total of ~36 days on Island we will have the chance to look in detail at several aspects of Ascension’s eruptive history, with each member of the team focussing on a slightly different aspect.

I will be visiting some of the outcrops already identified by Katie Preece and Katy Chamberlain during their 2014 and 2015 field seasons to get some really detailed sampling done. One such locality is a pumice fall deposit that shows a transition from pumice to scoria accompanied by a compositional transition from trachyte to trachy-basaltic andesite  (find the link to Katy’s paper here). Other sites of interest include a pumice-scoria breccia from the central felsic complex (see geology of ascension map) and various lava domes and flows located in the central and eastern parts of the island. E.g. White Horse and the Devils Cauldron trachyte lava flow.

My research focuses on trying to understand what causes the volcanic activity on Ascension to change from effusive (lava flows and domes) to explosive (pumice and ash falls, pyroclastic density currents etc) and vice versa. To do this I will be sampling the larger pumice fall deposits in detail so that I can carry out in-depth studies on their vesicles (bubbles) and crystals to understand more about the processes leading to their formation. I will also target lava flows and domes that I can tie to explosive eruptions in the same area (e.g. that can be traced back to the same volcanic vent, show a similar chemical signature or erupted around the same time in that region). By sampling both styles of eruption I will be able to compare the processes acting on the magma as it evolves and moves towards the surface.

My project links quite closely with that of Jane Scarrow a fairly new member of the Ascension team who focuses on finding zircon crystals in igneous rocks so that she can date them and provide timescales for magmatic processes occurring deeper in the volcanic plumbing system. As Jane focuses on the deeper processes and I on the shallow ones, combining our research should provide some interesting insights into magmatic evolution and volcanism on Ascension.

Getting my hands on lots of amazing samples is the first step of my PhD research, the overall aim of which is to to improve our understanding of the volcanic system on Ascension in order to better mitigate and prepare for potential volcanic hazards in the future.

Other objectives of this field season include:

• Completion of the Ascension geological map (Charlotte Vye-Brown, BGS, and Rich Brown – Durham)
• Ongoing interaction and communication with Ascension Island Government regarding volcanic hazards (Charlotte Vye-Brown)
• Mapping of lava flows in the North of the island (Charlotte Vye-Brown)
• Collection of plutonic rocks to use for zircon dating (Jane Scarrow – UEA)

We are all very excited to get our feet on the ground and start hunting for rocks that will help us to unlock more of Ascension’s volcanic secrets!

Watch this space for updates from our field season!