Dear Mike,
Temperature and gases certainly influence whether the lava becomes aa or pahoehoe. Probably the two biggest factors are viscosity and rate of shear strain. Viscosity is just how sticky something is (how much it resists flowing). An example of rate of shear strain is how quickly or slowly force is applied across a deck of cards. Factors influencing viscosity or rate of shear strain are listed below:
| viscosity | rate of shear strain |
| temperature | flow velocity and duration |
| gas content | flow dimensions |
| lava vesicularity | ground slope |
| crystallinity | channel configuration |
Peterson and Tilling (1980, p. 273) suggested two general conditions that determine whether pahoehoe or aa forms:
1. If lava slows, cools, and stops in direct response to the corresponding increase in viscosity only, it retains its pahoehoe form.
2. If lava is forced to continue flowing after a certain critical relationship between viscosity and rate of shear strain is achieved, the lava changes to aa.
Peterson and Tilling called this critical relationship the "transition threshold." They found that if the rate of shear strain is high, the transition threshold is reached at a lower viscosity than if the shear strain rate is low. The converse is also true. If the viscosity of the lava is high, a relatively low rate of shear strain may achieve the transition threshold, and the lava changes to aa.
You might enjoy Lava in Volcano Lessons.
Steve Mattox, University of North Dakota
Source of Information:
Peterson, D.W., and Tilling, R.I., 1980, Transition of basaltic lava from pahoehoe to aa, Kilauea Volcano, Hawaii: field observations and key factors: Journal of Volcanology and Geothermal Research, v. 7, p. 271-293.
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