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.
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.
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….
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.
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!