The transition from 'A'ā to Pāhoehoe crust on flows emplaced during the Pu'u 'ō'Ō-Kupaianaha eruption

The transition from pahoehoe to 'a'a along a single lava flow is widely accepted as irreversible in volcanology. However, channelized 'a'a flows have been repeatedly observed changing into pahoehoe flows during the low-effusion-rate "pahoehoe" stage of the Pu'u O'o-Kupaianaha eruption. This change most commonly occurs when flows move from steep slopes to level ground. The transition is marked by the change from 'a'a to pahoehoe crust on the solidified levees. It is here suggested that the definitions of pahoehoe and 'a'a be restricted to solidified crustal products, as those terms were originally defined. Active lava flows can be designated as either pahoehoe flows or 'a'a flows, depending on the type of crust that is being produced at the advancing front. Many difficulties arise from applying the terms pahoehoe and 'a'a to molten lava that has not taken a solid form. Hawaiian tholeiitic lava undergoes an irreversible transition from near-Newtonian to Bingham or other non-Newtonian behavior as it cools, crystallizes, and degases during transport away from the vent. Most Hawaiian pahoehoe flows are fed by near-Newtonian lava, and most Hawaiian 'a'a flows are fed by lava with Bingham or more complex rheologies. Within a restricted rheological range, however, Hawaiian lava can form either a pahoehoe or an 'a'a crust, depending on strain rate. This limited rheologic condition allows an 'a'a flow to change into a pahoehoe flow in response to a reduction in strain rate.