The main aim of our study on Ascension is to understand how frequently the volcano erupted (and what type of eruption it was), to constrain how the style and timing of eruption is linked to subsurface processes traced in crystal and glass chemistry, and finally how these all relate to past and potential future eruptions (and their associated hazards) on Ascension.
Volcanological Context of the Project:
“The ultimate goal of volcanological research is to anticipate better the timing and impact of volcanic eruptions on civilization. Understanding the time-size distribution of eruptions from an individual volcanic system provides the most important first step in determining both long and short-term volcanic hazard. However this is limited by a lack of process-based understanding of these patterns. By taking advantage of recent advances in techniques applied to volcanic time series and to understanding and describing magmatic processes it should now be possible compare both magmatic and eruptive time-scales for a single system. This will enable us to evaluate the role of magmatic processes in governing eruptive behaviour. The study volcanic system needs to be relatively isolated, present a range of well-exposed eruptive styles and compositions and be situated such that even a minor eruption has the potential to cause significant local or international disruption. Ascension Island, a British Overseas Territory represents such a system.”
Excerpt from Leverhulme Grant Trust Application
In order to address our aims, we have three key hypotheses which we will test during this project:
- There is a link between magma composition, storage conditions, and eruption frequency, style and impact and this link can be quantified through geological time.
- There is a link between magma recharge and eruption timing, style and impact and this link can be quantified through geological time.
- These combined data provide sufficient detail to be able to anticipate surface patterns of unrest associated with pre-eruptive magma assembly (short-term planning) and to evaluate most likely eruptive behaviour within a quantitative hazard assessment framework (long-term planning).
In order to address these hypotheses we will use a variety of methods, both in the field, and in the lab:
- Stratigraphic correlation: By making measurements of pyroclastic units thickness, pumice type and size, lithic type and size we hope to be able to correlate these units across areas of the island. This in turn will allow for potential vent location (where the magma reached the Earth’s surface), plume height and estimated eruptive volume and intensity.
- Lava flow measurements: This will be undertaken both in the field and from remote-sensing. Measurements of flow thickness, surface extent, periodicity and inferred vent locations will be used to build up an understanding of the frequency and size of effusive eruptions on Ascension. This will be used to inform hazard assessment side of the project.
- 40Ar/39Ar dating: The 40Ar/39Ar dating dating method has its foundation in the K/Ar technique. Both approaches are based on the natural decay of an unstable radioactive isotope of potassium, 40K, contained in rocks and minerals, to an isotope of argon, radiogenic 40Ar, sometimes written as 40Ar*. The half-life of 40K is 1250 million years, which means that it takes 1250 million years for half of the 40K atoms present in the rock or mineral to decay to 40Ar*. This means that the amount of 40Ar* compared to the amount of 40K is directly related to the time since the 40K was trapped inside the rock, which in the simplest case of igneous rocks, e.g. an unaltered lava flow, represents the time since eruption and cooling. Therefore, the eruption age can be calculated by measuring the ratio of 40Ar* accumulated to the amount of 40K remaining. In the 40Ar/39Ar dating technique, the sample is first irradiated in a nuclear reactor to transform some atoms of another naturally occurring isotope of potassium, 39K, into 39Ar (sometimes written as 39ArK) , via interaction with fast neutrons. The age can be determined by measuring the 40Ar*/39ArK, which is proportional to 40Ar*/40K. This is because the amount of 39ArK is dependent upon the amount of 39K present in the rock or mineral and the 39K/40K ratio is essentially constant in nature. The K/Ar method involves splitting the sample into two and making separate K and Ar measurements. In the 40Ar/39Ar technique, K and Ar are effectively measured simultaneously on the same aliquot of samples, providing the advantage of enabling small amounts of sample to be analysed and providing greater internal precision. These analyses have been carried out by Katie Preece alongside Ben Cohen.
- Cosmogenic 3He dating: Atoms of 3He are produced at the Earth’s surface when they are hit by cosmogenic rays (from the Sun) which interact with normal 4He atoms. The number of 3He atoms (compared to 4He) is therefore proportional to the length of time the rock (in this case, lava surface) has been exposed at the Earth’s surface. As we specifically sampled lavas which have no evidence for erosion, or having been buried at any point in their history, this length of time is directly related to the eruption age of the lava flow. We will use this cosmogenic dating to date the very youngest lava flows on Ascension. In order to measure the concentration of 3He and 4He in the lavas, we will pick crystals of either olivine or pyroxene, and then crush them up to release the gas, whilst simultaneously measuring each isotope’s abundance in the crystals. This work is being undertaken by Katie Preece and Fin Stuart at SUERC.
- Whole-rock chemistry: The chemical composition of lavas and pyroclastic deposits on can be used to determine the different processes which have formed the magma below the Earth’s surface, and what has subsequently happened to it on its way from where it was generated to eruption (up to tens of kilometres!)
- Crystal compositions: Crystals can preserve evidence for changing conditions within the magma chamber(s) similar to tree rings recording different stages of growth of a tree. By extracting crystals from the lava and pumice samples we can measure chemical changes both within single crystals and between different eruptions to infer changes in pressure, temperature and composition between and within single magma batches.
- Diffusion modelling: Zones within different crystals record changes in growth conditions in the magma as described above. Diffusion modelling of elements across and within these zones can allow timescales (relative to eruption) for when these changes occurred to be inferred. For example, we hope to be able to identify if certain processes such as magma mixing happen immediately prior to explosive eruptions, and this could be used in the future to monitor potential activity on Ascension. This work is being carried out by Katy Chamberlain.