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Lava microtextures reflect sub-volcanic geometry

Analysis of microtextures in lava samples can be used to calculate key eruption characteristics, such as magma chamber depth and discharge rate

Microscopic imaging of erupted lavas reveals such rocks comprise a mix of glass (rapidly quenched molten rock), bubbles (originally filled with gases), and crystals. The arrangement of these components is described as the rock’s ‘texture’ and is highly variable between different eruptions and volcanoes. Lavas from explosive eruptions, where lava is rapidly erupted to the surface, are dominated by bubbles; in contrast, slowly extruded samples have time to crystallise tiny minerals (microphenocrysts) and degas dissolved volatiles.

MSH lava texture

Back scattered electron (BSE) image of an erupted lava from Mt St Helens showing distribution of vesicles (voids which were originally filled with gas – now black), glass (light grey) and crystals (geometric shapes in glass). Image width is 50 µm. Photo credit: Kathy Cashman

Melnik and co-workers used crystal size distributions (CSD), a way of analysing crystal size and ’roundness’, as a method of quantifying volcanic texture. The study determined the relationship between the volume fraction of crystals in a sample, and parameters such as the cross-sectional area of the volcanic conduit and crystal nucleation rate. CSD can therefore be used as a measure of how magma travels through the sub-volcanic system at different depths and to track the ascent of molten rock to the surface.

The model is applied to a lava sample from the 1980 eruption of Mt St Helens volcano, USA. High resolution imaging allows the study of small (<10 µm) crystals and extends the depth range of the calculations to ~8 km, which agree with other estimates from seismic imaging and petrologic studies. Not only does the new model of Melnik et al. pave the way for more advanced understanding of how lava ascend through volcanic conduits, it also allows estimation of discharge rate and conduit geometry for prehistoric or unmonitored eruptions.

Melnik, OE, Blundy, JD, Rust, AC & Muir, DD (2011) ‘Subvolcanic plumbing systems imaged through crystal size distributions’ Geology, vol 39(4), pp. 403 – 406. http://dx.doi.org/10.1130/g31691.1

SCIENTIFIC ABSTRACT

The configuration of subvolcanic magma storage regions exercises a fundamental control on eruptive style and hazard. Such regions can be imaged remotely, using seismic, geodetic, or magnetotelluric methods, although these are far from routine and rarely unambiguous. The textures of erupted volcanic rocks, as quantified through crystal size distributions (CSD), provide space- and time-integrated information on the subvolcanic plumbing systems, although these data cannot be used readily for reconstruction of key parameters such as conduit geometry or magma chamber depth. Here we develop a numerical approach to interpretation of CSD in products of steady eruptions, based on crystallization kinetics and hydrodynamic flow simulation, to image subvolcanic plumbing systems. The method requires knowledge of magma properties, crystal growth kinetics (measured experimentally), and discharge rate (measured observationally). The method is applicable to steady-state eruptive regimes. Distributions of pressure, temperature, crystal content, and conduit cross-section area with depth are obtained from a CSD from a sample erupted from Mount St. Helens volcano, USA. Values of average conduit diameter (∼30 m) and magma chamber depth (∼14 km below the summit) are in good agreement with independent estimates.

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