Rocky secrets – fifty shades darker?

Originally posted on the EGU blog network

You might remember Charly’s “Fifity Shades of Grey” post, highlighting the colourful spectrum of rocks; fifty shades of grey they are not. On a purely aesthetic level, rocks are incredibly varied and interesting.

But the thing is, a lot of the samples us volcanology and petrology PhD students spend our time looking at are some shade of grey. The trick to staying interested in all things grey is to learn to read between the lines. Grey volcanic ash, for example, has a whole host of dark secrets to share if you know what to look for…

Here in the Wills Building, there’s a great deal to be learnt about volcanic ash from back-scattered electron (BSE) images taken using the scanning electron microscope (SEM). For example, high resolution images of ash allow calculation of the bubble size distribution, from which conclusions about eruption style and conduit processes can be drawn. At one end of the scale, there are bubble size distributions dominated by small, homogenous bubbles; such a distribution would imply rapid ascent speeds or a late stage blast, where conditions have favoured bubble nucleation (the formation of new bubbles). In contrast, more heterogeneous distributions including large bubbles – where bubbles have had time to grow and coalesce – suggest a slower passage to the surface, possibly with magma stored during its ascent. These contrasting bubble size distributions can be used to infer the plumbing systems under volcanoes, as well as giving insight into pre-eruptive processes.

Grimsvotn Ash
A back scattered electron (BSE) scanning electron microscope (SEM) image of ash grains from the 2010 eruption of Grimsvotn, Iceland. The eruption was phreatomagmatic (it involved abundant magma-water interaction). Total image width ~ 1.5mm. Credit: Emma Nicholson
Fuego Ash
A very different BSE SEM image of ash, from the 2012 eruption of Fuego, Guatemala. This eruption was purely magmatic. Total image width ~ 1mm. Credit: Emma Nicholson

Outside of the lab, there are volcanologists who map ash deposits of both historic and pre-historic eruptions in the field. Deposits of ash are measured at various locations around the volcano, and used to generate contour maps. Isopach maps display contours of equal deposit thickness, while isopleth maps show contours of equal grain size. In combination, isopach and isopleth maps can be used to estimate the size of pre-historic eruptions, their intensities, and the height of their eruption columns. For many historic eruptions these parameters can be observed directly, but mapping the distribution of ash is still of great use in furthering our knowledge of volcanic eruptions.

isopleth isopach
An isopach (deposit thickness) and isopleth (grain size) map of the C-1 layer of the Middle Toluca Pumice, produced by Nevado de Toluca volcano, Mexico, 12.1 ka ago (from Arce et al., 2005)

My research doesn’t actually involve looking at rocks at all; I am concerned with volcanic hazard and risk. However, there are so many things to be learnt from uncovering the “dark secrets” held within ash that have implications outside of physical volcanology: what kind of eruption precursors (such as seismicity and deformation) are associated with rapidly ascending magma, and how do these differ from those prior to eruptions in which magma rises more slowly? If there are discernible differences in pre-eruptive signals, how can that information be used to better eruption forecasts? Where do isopach and isopleth maps, in combination with climatic conditions, suggest ash hazards will be most severe? And what are the hazards likely to be – remobilised airborne ash, or secondary lahars?

Left hand side: Remobilised airborne ash at Mount St. Helens, USA. (Credit: Melanie Auker). Right hand side (4 photos): Garage owners fight a losing battle with secondary lahars following the 1991 eruption of Pinatubo, Philippines. (Credit: Chris Newhall).
Left hand side: Remobilised airborne ash at Mount St. Helens, USA. (Credit: Melanie Auker).
Right hand side (4 photos): Garage owners fight a losing battle with secondary lahars following the 1991 eruption of Pinatubo, Philippines. (Credit: Chris Newhall).

Whilst grey rocks might be lacking in beauty, they certainly make up for it elsewhere!

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About Melanie Auker

I’m an applied mathematician bumbling my way through a geology PhD.

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