Amy Ryan – University of British Columbia, Vancouver (Canada)
Have you ever looked at your coffee mug and wondered: “How was this made?”
In most cases dishware was initially a loose mineral/oxide powder. It was transformed into the nonporous composite you trust to hold your coffee thanks to “hot pressing” – the powder was molded and compressed, heated and left at these conditions for a few hours.
Ceramic manufacturing temperatures are often high enough to cause some of the components within the initial powder to melt. However, melting is not required – hot pressing temperatures can be 50-75% the melting temperature. The reduced temperature lengthens the time needed to create the composite, but the result is the same: a powder goes in and a solid comes out.
This process operates at moderate temperatures and pressures because atomic diffusion between adjacent crystalline particles causes particles to coalesce, ultimately forming a coherent solid. Geologists will recognize this mechanism as analogous to diffusion creep. In the ceramics literature, it is called “solid-state sintering”.
Given that solid-state sintering occurs where crystalline powders are held at elevated pressures and temperatures for extended times, I’ve been working during my PhD to answer following questions: Does solid-state sintering occur within volcanic environments? If so, does it happen on timescales relevant to volcanic processes?
The volcanic shear zone exhumed during the 2004-2008 eruptions at Mount St. Helens (Washington, USA; MSH) (Figure 1) gave the first indication that this mechanism operates in volcanic conduits. The shear zone formed at depth (~ 1 km) when the friction between the ascending crystal-rich lava and conduit wall rocks caused the lava to break apart, producing volcanic fault gouge.
Figure 1: One of the lava spines extruded at Mount St. Helens during the 2004-2008 eruptions. A shear zone that developed at depth mantles the lava spine.
Given this, one would expect the extruded shear zone to comprise unconsolidated gouge. Instead the deposit exposed at the surface is variably densified, and includes low-porosity gouge-derived cataclasites next to unconsolidated gouge (Figure 2).
Figure 2: Shear zone rocks from Mount St. Helens. These gouge-derived materials have identical compositions and but are variably densified as they underwent less (left) or more (right) sintering during their time in the conduit.
Based on surface observations, the estimated ascent times for the fault gouge, from its source to the surface, were months to years. I hypothesized that during this time much of the unconsolidated gouge was sintered to produce the dense rocks erupted at surface. This work was the first to suggest that solid-state sintering occurs within the volcanic conduit.
Next, I wanted to know just how quickly sintering operates. To do this I commissioned my own “hot pressing” experiments that mimic volcanic environments – steel canisters filled with MSH gouge were heated to moderate temperatures (still below the melting temperature), compressed and left at P-T for hours to days. To my surprise, all experimental products are solid composites,even after only a few hours of hot pressing (Figure 3)!
Figure 3: Experimental starting material (left) and the products of hot pressing – competent solid cores (center) that show microscopic evidence of sintering (right) such as the formation of patches of coalesced particles and “necks” of crystalline materials that now join grains.
This work is ongoing, but our first results indicate composite porosity and permeability decrease and material competence increases in a predictable manner with increasing temperature, pressure and time.
Based on these results, solid-state sintering appears to occur in volcanic environments on short timescales (hours to days). At active volcanoes this means sintering may cause outgassing pathways (e.g. intra-conduit shear zones, tuffisite veins) to seal, leading to the development of gas overpressures that can trigger explosive activity. In other volcanic settings (e.g., near intrusions, geothermal fields), solid-state sintering can suppress fluid flow in the subsurface and lead fractured and brecciated materials to progressively recover their competence and strength.
Overall, my work challenges the assumption that fractured and unconsolidated crystalline materials will remain that way indefinitely. These materials, much like the fine mineral powder that became your coffee mug, can readily be transformed.
She is an experimental volcanologist and conducts high temperature experiments that explore how the physical and transport properties of volcanic materials (e.g., melt foams, crystal powders, vesicular lavas) change as a result of deformation.
More of her work, including collaborative projects with other volcanologists and rock mechanists in France, Italy, the US and the UK, is available here.
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