Researchers at Stanford University, US, devised a new model to find the reason behind the transition of two lava flows, pahoehoe and aa. The research team reported that the reason could be the churning within the flood of molten rock.
The Lava Flows
Lava flows are the outpourings of molten rock, known as lava, that flows on the Earth’s surface. These streams come from an erupting vent during a volcanic eruption. These flows occur in the area with active volcanoes. Lava flows can be of different sizes and shapes. They can range from small and significant to large and widespread. When these flows cool and solidify, they turn into igneous rocks. The resulting landforms are known as volcanic landscapes and lava fields. The two major types of lava flows are pahoehoe and aa.
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Pahoehoe (pronounced as pa-hoy-hoy) is a Hawaiian term used for a lava flow that looks like braided or twisted ropes. These flows are relatively thin and have smooth, ropy or billowy textures. Pahoehoe lava flows are associated with basaltic lava which has low viscosity and silica content. Also, these flows form when the lava is hot and fluid, allowing it to flow smoothly and slowly. This means the flows erupt at the temperature range between 1100 to 1200 degree Celsius. Pahoehoe flows are found in Hawaiian islands having shield volcanoes. These lava flows have created landscapes with unique formations and patterns.
Aa (pronounced as Ah-ah) is another type of Hawaiian lava flow. Like pahoehoe, it is also associated with basaltic lava. This type of lava flow has a fragmentary, jagged and rough surface. Also, these flows are formed from the more cooled and viscous lava. The temperature at which aa lava flows erupt ranges from 1000 to 1100 degrees Celsius. These flows are thicker and move faster than the pahoehoe flows. The aa lava surface is extremely difficult than the pahoehoe lava surface to walk across. Both the lava flows are significant features of volcanic landscapes.
The New Model
On her family vacation in 2018, Jenny Suckale, a geophysicist at Stanford University, found that the Hawaiian lava surface had two different types of lava flow. One side was the pahoehoe, while the other side was covered with aa lava flow. She wanted to know the cause of the transformation in the texture of these lava flows. Many scientists pointed out the possible factors, such as the amount of lava erupted, the slope of the ground, or the speed of the lava flow. But none of these factors explains the reason behind the transformation.
As both lava types possess different speeds, Suckale and her team proposed a new model to know how pahoehoe morphs into aa. This model was inspired by an experiment conducted by O. H. Emerson, a geologist at the University of Hawaii, Manoa. In this experiment, Emerson heated the powdered rock of pahoehoe lava that erupted in 1920 in the furnace until it was white-hot and flowed like honey. Then, he turned off the furnace and stirred the molten material with the metal rod. Within a few minutes, the material changed into chunky aa lava rock.
Image Credits: Pixabay
“Like stirring a cup of tea, mixing the lava sped up cooling—and seemingly triggered the formation of aa,” said Cansu Culha, a lead study author and a postdoctoral scholar at ETH Zurich, Switzerland. Researchers also thought that this mixing method might result in transition via a phenomenon called shear instability. In this phenomenon, when two layers of a fluid move at different speeds, the faster layer moves along the slower one as it passes. This rubbing of layers results in ripples (wave-like formations) that occur between the boundary of layers. These ripples turn into turbulent waves when magnified.
The research team developed a fluid dynamics model to study the two different lava layers, a cooler and stickier one on top of the hotter and faster layer. Then, they introduced small ripples between these two layers. The team tested the stability of the lava under a variety of factors, such as layer thickness, viscosities and speeds. This analysis showed that changes in the environment, such as high eruption rates or more steep slopes, can amplify the ripples resulting in the runaway mixing of layers and formation of aa lava.
The new model may help to explain the previously observed factors that trigger the transition of pahoehoe into aa. “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,” said Leif Karlstrom, an earth scientist at the University of Oregon, US, who was not part of the research team.
The detailed study has been published in the journal Geophysical Research Letters.