VHP Photo Glossary: Transition of pahoehoe to `a`a lava
Photograph by C. Heliker on 21 May 1998
`A`a flows pour down a steep slope on the southeast flank of Kilauea Volcano, Hawai`i, following a brief pause in the eruption of Pu`u `O`o vent. The flows were fed from a skylight near the top of the Pulama Pali, a 300-m tall steep fault scarp. Lava that previously had spilled from skylights at the top and bottom of the pali, where the slopes are gentle, however, typically formed pahoehoe flows. But if the pahoehoe moved down the pali, the pahoehoe transformed into `a`a almost immediately. Bottom photo (width of image is about 2 m) is a close-up of the lower right-hand corner of the upper image.
Pahoehoe to `a`a transitionNearly all lava in Hawai`i erupts as pahoehoe, and some changes to `a`a during flowage and cooling. The critical factor that determines whether pahoehoe or `a`a forms is the relationship between the viscosity of the lava (resistance to flow) and the amount of internal disturbance or shear that occurs as lava spreads from a vent downslope.
Generally, if pahoehoe viscosity is high (relatively cool pasty lava), a low rate of shear or disturbance may cause `a`a to form. For example, a slow-moving pahoehoe flow that has cooled sufficiently may transform into `a`a as the flow continues to advance. If pahoehoe viscosity is low (relatively hot fluid lava), a high rate of shear or disturbance is required for transition to `a`a. For example, when a fluid pahoehoe flow pours over a steep slope, it increases its speed and creates conditions favorable for `a`a to form.
Any factor that increases the viscosity of pahoehoe, such as cooling, loss of gas, or crystallization will favor the transition of pahoehoe to `a`a. Pahoehoe can change to `a`a, but the reverse never occurs.
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