Rocky walks, public talks and a really big box….updates from the 2018 field season.

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.

photo 1 - Bridie Spy

Bridie using a laser range finder to measure the thicknesses of her favourite sequence of rocks on Ascension, looking from the Echo Canyon letterbox (which she climbed to twice because she loved this view so much).

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.

photo 2 - EC crystals

A selection of photos showing the same set of crystals are present in all the units of the Echo Canyon sequence; A: a piece of pumice from the thick pumice fall at the base of the canyon with the typical feldspar crystals – two here growing into each other. B: pumice from the layered units at the top of the canyon. C: in the pumice at the very top of the sequence. D: feldspars in a weathered lava associated with the sequence but not seen from Echo Canyon. E and F: same crystals within the more and less weathered pink lava as seen in the photo looking into the canyon.

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….

photo 3 - EC bags of pumice

Left and right: Bridie and Jane standing triumphantly at the entrance to Echo Canyon with all their belongings attached to the outsides of their packs as they are filled with pumice at the end of sampling. Centre: typical, very fresh pumice clasts from the Echo Canyon pumice fall.

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.

photo 4 - density measurements

Very technical stuff: the experimental set up to find the vesicularity of the pumice clasts from Echo Canyon. Each piece is wrapped in (science-y) cling film and weighed suspended in water to find the volume (using Archimedes principle) so that Bridie can calculate the density and therefore the vesicularity of pumice from each part of the deposit. To be statistically sound 100 pieces of pumice must be measured for each sample location….that’s around 700 pieces of pumice in total!

 

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

Ascension here we come……

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.

map ascension geo background

Geological map of Ascension Island – Adapted from Chamberlain et al., (2016)

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.

cropped-2015_11.jpg

Mafic lava flows and scoria cones extending across Ascension island

DSCF8808

Katie Preece examining lava dome at Little White Hill

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!

New type of volcanic bomb uncovered on Ascension Island

Geological mapping uncovered some unusual volcanic rocks to the west of White Horse. These dense glassy basaltic bombs are very unusual and so far are only known here on Ascension. Normally basaltic bombs are full of bubbles (trapped magmatic gas), but these are almost entirely free of bubbles. They landed hot as a sticky liquid (like tar) (top photo) and dribbled down into the gaps between the bubbly bombs beneath them (middle photo). The dribbles have beautiful droplet shapes (bottom photo). Work is underway to understand how they form.

Why are these interesting?

These new bombs can be analysed to provide information on processes that happen during volcanic eruptions. This may tell us how the magma behaved under the ground or at the surface and how changes in magma properties affected the nature of an eruption.

new volcanic bomb

Unusual glassy volcanic bombs found on Ascension

New geological map of Ascension Island being prepared

Work has started on a new geological map of Ascension Island by staff at the British Geological Survey and Durham University. Mapping is being achieved using satellite imagery and ground-truthing fieldwork. It requires the scientists to pore over every rock on the island and is providing many new insights into Ascension Island’s volcanic past. So far fieldwork has focused on Upper Valley Crater, Northeast Bay, Cricket Valley, Weatherpost, and Devil’s Inkpot. Work will continue next year around the south and west sides of Green Mountain. The map will be published digitally by the British Geological Survey.

Why do we need a geological map?

Geological maps provide information on the distribution of volcanic deposits, such as lavas, layers of ash and pumice, and on the location of the vents and craters for individual eruptions. This can help when considering the nature and location of future eruptions on the island. Geological maps also act as starting points for geological research and scientific studies by providing researchers with accurate spatial information on volcanic rocks. Geological maps are dynamic sources of information and as new research happens they can be updated and modified.

printout of satelite photo used in the field for geological mapping

Printout of satelite photo used in the field for geological mapping.

August 2017 – Goldschmidt 2017 Conference

In August 2017, Darren, Katie and Ben attended Goldschmidt 2017 in Paris. Goldschmidt is one of the foremost annual, international conferences on geochemistry and related subjects. Each year, the week-long conference brings together thousands of scientists from throughout the world to talk about subjects including the origin of the Earth and planets, the chemical processes that have shaped Earth over time, the search for new resources, and the environmental challenges facing today’s world. At the conference, Darren convened a session about timescales of crustal processes, and Katie presented a talk about the team’s research and findings entitled “1 million years of volcanism on Ascension Island: insights from stratigraphy, 40Ar/39Ar dating and petrology”. Both the session and the talk were well attended and interest in Ascension geology was strong. After the conference the team even managed to find a bit of time to do some sightseeing and sample the delights of French patisserie.

IMAGE_0__1493223406307