In the summer of 2018, an eruption on the flanks of the active volcano Kīlauea in Hawaii sent lava flowing through the Puna district toward Kapoho Bay. The relentless threat from wide channels of molten rock forced about 2,000 residents to evacuate. By the end of the eruption, 24 people were injured, 716 structures were destroyed, and the flows left $800 million worth of damage in their wake.
Although it’s not uncommon for lava flows to have bubbles, samples from the 2018 Kīlauea event revealed a high percentage of gaseous bubbles by volume—more than 50% in some cores taken after the flows solidified, according to new research presented at AGU’s Fall Meeting 2020. Some of the bubbles were large, roughly a meter in diameter. The high-profile event provided an opportunity for volcanologists to observe up close how dramatic outgassing affected the way lava flows down slopes and spreads across flatter areas.
However, re-creating a stream of molten rock in the lab to capture detailed physics in action isn’t feasible—or safe. So a team of international scientists created an analogue using corn syrup, baking soda, citric acid, and an inclined slope with the aim of shedding light on how a high volume of bubbles affects the way lava flows.
Observing the Flow
Atsuko Namiki, a volcanologist from Hiroshima University in Japan, was on Hawaii when the eruption occurred. She noticed that the flow was “amazingly quick” at first, but it became much slower as it continued from the rift zone on the volcano’s slopes.
“I was really surprised at the difference,” said Namiki, who presented her team’s results at AGU’s virtual Fall Meeting 2020. “I wanted to explain the abrupt change of the flow patterns with bubbles and without bubbles.”
The researchers’ findings showed that bubbles clearly affect lava’s viscosity, or its relative thickness and fluidity. “Small changes in texture can lead to big and lasting changes in dynamics,” said Janine Birnbaum, a graduate student at the Lamont-Doherty Earth Observatory at Columbia University who worked on the study.
Corn Syrup, Just Add Bubbles
For their experiments, the team members used corn syrup modified with citric acid and baking soda. (Corn syrup is a common experimental analogue for lava. According to Birnbaum, the syrup’s viscosity can be tweaked by researchers, which makes it ideal to work with.) 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.
The researchers then poured the liquids down a meter-long plastic plank propped up at an acute angle to mimic how lava flows from a volcano. A camera tracked movement while a laser sensor monitored the bubbly flow’s thickness.
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.
The Importance of Understanding Lava
According to Pranabendu Moitra, a physical volcanologist at the University of Arizona who was not involved in the study, the research represents an effort to understand lava flow in all three of its phases: liquid, bubbly, and particle.
“This is one of the first of its kind,” Moitra said. “This has potential to be the basis for a lot of future research.”
Future lab experiments analyzing the detailed physics of lava flows could help provide more accurate predictions for communities at risk for damage from volcanic eruptions, said Birnbaum. Observations of a flow’s likely movement patterns and speeds, depending on its gas content, could aid public safety authorities in preparing more reliable evacuation notices.
Namiki hopes the simplicity of their experiments, which used common and inexpensive materials, inspires others to continue similar research. Not all results require expensive gear or trips to dangerous zones. “There are many things we can learn in our daily lives,” she said.
—Allison Gasparini (@astrogasparini), Science Writer