Journey to the centre of Ascension: What makes this volcano tick? | Bridie Davies
View of scoria cones on Ascension Island Image credit Katie Preece
Volcanic eruptions are one of the most fascinating phenomena on the planet, capturing the imaginations of some of our greatest writers and philosophers throughout ancient history, and more recently fuelling some of Hollywood’s most ridiculous action movies (Dante’s Peak anyone?). Although volcanic eruptions can indeed be beautiful and spectacular they can also be catastrophic for communities living alongside them. Lured in by fertile soils, geothermal springs and often curiosity for the unexplained spectacles they observe, the human race insists on positioning themselves right in the path of destruction. Ever increasing populations, the rise of geo-tourism and hoards of researchers pursuing answers “for science!” means these areas continue to be densely populated – increasing the potential for loss of both human life and capital.
As such, there is an increasing demand for effective volcanic hazard mitigation strategies which require a deeper understanding of the volcanic systems to which they are applied - particularly for those areas where help is not exactly close at hand. Nowhere is this phrase more applicable than on Ascension Island, situated 90 km west of the Mid-Atlantic Ridge and just below the equator, it is one of the most remote places on Earth. Flights to the island from St Helena run only once a month, and your only other alternative is a 10 day sea voyage from Cape Town – not exactly speedy in an emergency. As you can imagine, the residents of the island (which is a UK territory and home to an RAF base) would quite like to know what they can expect should Ascension’s volcanoes transition from being dormant to active.
Map showing the location of Ascension Island, the Mid-Atlantic Ridge in red and nearby St Helena and the Cape Verde Islands. Volcanic eruptions on Ascension have covered everything from runny basaltic lava flows like you might see on Hawaii, through thick sticky rhyolitic lava domes to catastrophic explosive eruptions that generate pyroclastic density currents (hot flows of ash, rock and gas that travel up to 700km/hr and reach temperatures of >1000°C) - similar to those seen at Mt St Helens during the famous 1980 eruption. It is vital that we understand what changes deep in the magmatic system under Ascension to cause this transition from effusive to explosive activity, even among rocks of the same chemical composition. This is what my research will focus on: I will use a combination of geochemical analyses (e.g. Electron microprobe, x-ray fluorescence and laser ablative inductively coupled plasma microscopy – all the science words!) and quantitative crystal analyses to identify changes in the physical and chemical conditions in the magmatic system from rocks that sample the boundary between these different eruption styles. Geochemical analyses can tell us a huge amount about the magmatic system. Analysis of tiny inclusions of melt trapped within larger crystals can tell us the H2O and CO2 content of the melt prior to eruption – this is important as high %’s of volatiles can not only alter the viscosity of the erupted magma but they can even act as the trigger for an explosive eruption. By analysing certain mineral pairs we can actually determine the temperature and pressure the mineral experienced while it was crystallising, vital because decompression of the magma chamber is a common eruption trigger as is influx of hot magma from below.
Left: Zoning in a crystal of clinopyroxene (cpx) from a backscattered electron image. Right: melt inclusion within a host crystal which was analysed using an electron microprobe credit Katy Chamberlain. Other important information can be gained from what we call “textural analysis” this includes the shapes and abundances of crystals and the occurrence of zoning (concentric rings of differing chemistry in the same crystal). These can tell us about whether the minerals were growing at their preferred temperature and pressure, what the chemistry of the surrounding melt was like, and even how long the crystals were growing for before they were erupted. Finally, the use of 3D imaging of the rock samples can actually tell us about the porosity of the mushy magma in the chamber prior to eruption, this can affect how easily the magma could have escaped up to the surface, or how easy it was for H2O and CO2 to segregate from the melt (and trigger an eruption). By combining all these analyses I can hope to get a glimpse into the magmatic system beneath Ascension prior to both gentle and explosive eruptions, giving us a clue about what changes we should look for in real time to decide what kind of eruption the island will experience.
Spectacular views on the Letterbox peninsula, looking out over the Devil’s Inkpot lava flow and Little White Hill image credit Katie Preece
In the future, Ascension could become active once again, hopefully by then we will have a better understanding of exactly what makes the volcano tick so we can avoid a catastrophe worthy of another terrible Hollywood movie……. To find out more about research into Ascension Island’s explosive past head to ascension-island-volcanology.com Or follow the research team on twitter @AscensionVolc