Note vehicles for scale. Photograph by R.W. Decker, U.S. Geological Survey, July 2, 1983. Since the mid-1800s, geologists have tried to explain the causes of pahoehoe and aa lava. In the last few decades, with careful observations of numerous lava flows, geologists have reached a better understanding of the transition of pahoehoe to aa. One important influence is the viscosity of the lava.
Viscosity is the resistance of a fluid to flow.
For example, molasses has a higher viscosity that water. The following paragraphs outline some of the more important observations on the transition of pahoehoe to aa. Pahoehoe lava crossing Chain of Craters Road. Photograph by R.B. Moore, U.S. Geological Survey, November 16, 1979. Pahoehoe may change to aa, but never the reverse.