Back when I was teaching natural disasters, one of my favorite labs involved teaching the students about viscosity. They had to take bottles of unknown liquids — dish soap, maple syrup, honey, etc. — and see how bubbles flowed through the liquid at different temperatures. We would have a set in a container full of hot water and a set in a container full of icy water. It was a little messy and a lot of fun and everyone got into it, but a question I always got was “what does this have to do with natural disasters?”

It turns out that the answer is “even more than I realized”. Scientists studying the 2018 eruption of Kīlauea were surprised to find that the flow of the volcano slowed over time. And I don’t mean the eruption flow; I mean the movement of the already erupting lava. It came out of the rift at one speed and as it kept going, it slowed down. A volcanologist from Hiroshima University in Japan, Atsuko Namiki, was on the Big Island at the time, and she wondered if bubbles in the lava could be the cause of the decreasing velocity.

So not unlike the experiments I ran in my lab class, her team used corn syrup, a common analog for lava in the lab, to test out how bubbles affect the viscosity of the liquid. They created three different liquids with differing concentrations of bubbles: pure corn syrup with no bubbles, bubbly corn syrup, and bubbly corn syrup containing suspended particles. They poured the mixtures onto a meter-long plastic plank that was set at a small angle, and they used cameras to record the flows.

Per the article: Pure corn syrup containing no bubbles moved the fastest, with bubbly corn syrup flowing slightly less rapidly. Bubbly corn syrup suffused with particle matter moved more slowly and split into channels that flowed at different speeds—liquid in the middle section moved faster, whereas the liquid on the flow’s flanks moved more slowly. The experiment also revealed a gravitational separation occurring, with bubbles floating to the top as the fluid moved—a process that creates a fragile gaseous shell called pahoehoe in real flows. As bubbles rose to the top, the flow’s more concentrated liquid touched the base of the slope, where it accelerated the flow’s overall speed.

So not only are the flows affected by the particle content, they’re affected by the gaseous content, and combining the two states of material in one liquid makes for a range of results that definitely impact predictions for how quickly an eruption may impact a community. With Kilauea erupting again, this work is even more important than ever.

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Eos article