In a classic paper on the transition of pahoehoe to aa, a graphical approach was used to demonstrate how viscosity and rate of shear strain influenced the type of lava that formed. Prior to presenting this approach with your students, review this activity. The graphs can be drawn on the board or projected from an overhead.
Viscosity is on the x-axis and increases to the right.
Rate of shear strain is on the y-axis and increases upward. Point E represents the viscosity and rate of shear strain at the time of eruption. Point S is the end of the flow's history, when lava is solidified and characterized by very high viscosity. Because the lava does not cross the TTZ, it remains pahoehoe. Path crosses TTZ to reenter pahoehoe field, but lava continues to behave as aa because the pahoehoe-aa transition is irreversible.