While on a family vacation in 2018, Jenny Suckale was rambling across an old Hawaiian lava flow when an abrupt change in the jet-black rocks caught her eye. On one side was the smooth, undulating lava type called pahoehoe (pronounced pah-hoy-hoy); on the other was the sharp, jagged kind known as aa (pronounced ah-ah). Ever since that day, a deceptively simple question has nagged at Suckale, a geophysicist at Stanford University: What causes the dramatic transformation in texture, seen in lavas worldwide? 

Over the years, scientists have pointed to a slew of possible culprits: the speed of a flow, the slope of the ground it oozes over, or the amount of lava erupted. But no single factor has explained the shift. Now, by modeling the dynamics of lava flows, Suckale and her colleagues have offered up another explanation: The abrupt transition could be triggered by a chaotic churn within the flood of molten rock, the team reported last month in Geophysical Research Letters.

Understanding how pahoehoe morphs into aa is more than just scientific curiosity because the two lava types move at different speeds and pose distinct hazards. Although scientists are good at predicting where lava goes overall, how fast and far lava travels is a much trickier problem. “Being able to map out this transition between flow types would go a long way towards figuring those questions out,” says Leif Karlstrom, an earth scientist at the University of Oregon who was not part of the study team. 

For the new model, Suckale and her colleagues drew inspiration from an experiment conducted almost a century ago by geologist O. H. Emerson at the University of Hawaii at Manoa. Emerson pulverized a bit of hardened pahoehoe from a 1920 eruption of Hawaii’s Kilauea volcano and heated the powdered rock in a furnace until it was white-hot and oozed like honey. He then turned off the furnace and stirred the molten material with a metal rod.

Within minutes it had solidified into a chunky aa-like texture. Like stirring a cup of tea, mixing the lava sped up cooling—and seemingly triggered the formation of aa, says lead study author Cansu Culha, a postdoctoral fellow at the University of British Columbia.

The researchers wondered whether mixing might also spark the transition in nature, through a phenomenon called shear instability. When two layers within a flowing substance move at different speeds, the faster layer drags along the slower layer as it passes. “They just kind of rub each other the wrong way,” Suckale says. The result is that any ripples along the boundary between layers are magnified into turbulent waves.

To test the idea, the researchers crafted a fluid dynamics model to simulate the behavior of a virtual lava split into two layers: a cooler, stickier top sitting on a hotter, faster bottom. At the boundary between the two layers, they introduced small ripples, which inevitably form because of imperfections common in nature, like underlying bumps on the ground or bubbles in the lava.

The team then tested the stability of the lava under a range of speeds, viscosities, and layer thicknesses. The analysis revealed that in many cases, changes in the environment, such as steepening slopes or increasing eruption rates, amplified the ripples, which could lead to runaway mixing—and the likely formation of aa. “That can spiral out of control pretty quickly,” Suckale says.

The new model may help explain the variety of conditions previously observed to trigger the pahoehoe-to-aa transition—or why it sometimes doesn’t happen at all. “It’s a very exciting possibility,” says Arianna Soldati, a volcanologist at North Carolina State University who was not part of the study team.

But more testing is needed to show that the model reflects the reality under lava’s sizzling surface. Because lava is difficult to directly study, one possibility would be to test the model in the laboratory using an analog for molten rock like wax, Karlstrom suggests.

Still, the new research emphasizes the value of studies focused on basic mechanics. “I think this is where we gain insight into the processes that are driving these large-scale natural phenomena,” Karlstrom says.

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