QUANTIFYING THE EFFECT OF RHEOLOGY ON LAVA-FLOW MARGINS USING FRACTALGEOMETRY

This study aims at quantifying the effect of rheology on plan-view sha pes of lava flows using fractal geometry. Plan-view shapes of lava flo ws are important because they reflect the processes governing flow emp lacement and may provide insight into lava-flow rheology and dynamics. In our earlier investigation (Bruno et al. 1992), we reported that fl ow margins of basalts are fractal, having a characteristic shape regar dless of scale. We also found we could use fractal dimension (D, a par ameter which quantifies flow-margin convolution) to distinguish betwee n the two endmember types of basalts: a'a (D: 1.05-1.09) and pahoehoe (D: 1.13-1.23). In this work, we confirm those earlier results for bas alts based on a larger database and over a wider range of scale (0.125 m-2.4 km). Additionally, we analyze ten silicic flows (SiO2: 52-74%) over a similar scale range (10 m-4.5 km). We note that silicic flows t end to exhibit scale-dependent, or non-fractal, behavior. We attribute this breakdown of fractal behavior at increased silica contents to th e suppression of small-scale features in the flow margin, due to the h igher viscosities and yield strengths of silicic flows. These results suggest we can use the fractal properties of flow margins as a remote- sensing tool to distinguish flow types. Our evaluation of the nonlinea r aspects of flow dynamics indicates a tendency toward fractal behavio r for basaltic lavas whose flow is controlled by internal fluid dynami c processes. For silicic flows, or basaltic flows whose flow is contro lled by steep slopes, our evaluation indicates non-fractal behavior, c onsistent with our observations.