Quantifying degassing-driven crystal growth in basaltic lavas

Lead Research Organisation: Lancaster University
Department Name: Lancaster Environment Centre


A primary aim of lava flow research is the development of accurate flow models that can be used to forecast areas of inundation, and to estimate how far lavas will advance before stopping. Lava flows are complex fluids comprising mixtures of crystals, liquid and gas bubbles and, as they flow, they cool and lose volatile species (mainly water and carbon dioxide) that were initially dissolved in the melt at high pressure beneath the surface. Both cooling and degassing lead to crystallisation of the liquid melt, and thus have significant influence on flow advance. Cooling is a major driver of crystallisation, but its effects are mainly restricted to the thermal boundary layers, where it is an integral process in the formation of surface crust and lateral levées. In contrast, degassing is not restricted to boundary layers and occurs throughout flows, with the potential to affect the entire bulk rheology. Although the effects of cooling-driven crystallisation are accounted for in the current generation of lava flow models, crystal growth due to degassing has not yet been sufficiently quantified to allow its incorporation into models. In recent laboratory experiments, we have been able to simultaneously measure degassing and crystallisation for the first time, and we propose to further this research by examining the growth of crystals directly using hot stage microscopy. This will provide the data on crystal sizes, growth rates and morphologies necessary to quantify the contribution of degassing to the overall crystallisation of lavas. Ultimately, these results will allow degassing-induced crystallisation to be accounted for in numerical lava flow models.


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Description A primary aim of lava flow research is the development of accurate flow models that can be used to forecast areas of inundation, and to estimate how far lavas will advance before stopping. However, the growth of crystals due to degassing

of magma has yet to be incorporated into such models, potentially leading to significant error in the used values for effective viscosity, a parameter that represents how fluid the flows are. This project aimed to provide the key laboratory

measurements of degassing-driven crystal growth to enable the process to be represented in flow models. Our experiments allowed us to observe samples of lava through a microscope as they melted. During this process, bubbles were produced, indicating that the samples were degassing. Following this degassing, and with samples held at high temperatures, the growth of crystals was then observed. We were able to determine the crystal morphologies and measure their rates of growth from digital time-lapse images recorded through the microscope. Crystals initially grew fast, but then growth slowed down as space to grow into diminished and crystals approached their neighbours. Our measured growth rates can be used to estimate the time required for specific proportions of crystals to grow in a magma as it rises during eruption. For lavas from Etna, which degas significantly as they ascend, 20-30 % (by volume) of the magma can crystallise due to degassing. This can occur in less than 7 hours and can increase the effective viscosity of the magma by several

orders of magnitude. In lava samples from Hawaii, which contain much less water to degas, we observed fewer, slowergrowing crystals than in the Etna samples. Consequently, magma rheology at Hawaii is affected significantly less by degassing-driven crystallisation than it is at Etna.
Exploitation Route The results of the project have been discussed with the lava flow modelling group at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Catania, Sicily. The scientists at INGV, who have developed several of the leading flow models, are responsible for running the simulations used by the Italian Civil Defence during eruptive crises.
Sectors Environment

Description The results of the project are having academic impact, being cited in papers on magmatic processes, magma properties and lava flow emplacement.
First Year Of Impact 2012
Sector Environment
Impact Types Societal