|The Pahoehoe to 'A'a transition in Hawaiian Lava flows|
Kathy Cashman, University of Oregon
Jim Kauahikaua, Hawaii Volcanoes Observatory
Surface morphology (pahoehoe and `a`a) was mapped on three short-duration, high effusion rate Kilauean lava flows to determine what information about flow emplacement is preserved on the solidified flows. I utilize the distribution of surface morphology as a function of distance from the vent as an indicator of the cumulative effects of both intrinsic (i.e. composition, temperature, crystallinity) and extrinsic (i.e. topography, effusion rate, flow velocity) parameters of emplacement. A flow facies evolution common to Hawaiian `a`a flows of (1) pahoehoe sheet flows, followed by (2) `a`a filled channels within pahoehoe sheets, and lastly (3) channelized `a`a is present on each flow and results in a characteristic sigmoidal surface morphology distribution. By quantifying parameters affecting surface morphology I am able to explain similarities and differences in the surface morphology distribution for two flows with known emplacement histories (July and December 1974), and one flow that was not observed during emplacement (1823 Keaiwa). We identify a critical groundmass crystallinity at which the transition from pahoehoe to `a`a occurs in all flows supporting the idea that the development of a yield strength is responsible for the transition (Kilburn, 1981; Cashman et al., 1999). The critical crystallinity is comparable to that found in transitional morphologies of active flows and may reflect the first onset of a yield strength at high shear rates. The short flow duration allows excellent preservation of surface features. I analyze several techniques that utilize these features for assessing flow velocity, as well as develop our own. Finally, we examine the spatial and temporal development of channels, an important step in `a`a flow evolution.