Scaling effects on vesicle shape, size and heterogeneity of lavas from Mount Etna

Abstract

The rheology of basaltic lava flows depends on several factors including the vesicle size and shape distributions. We analysed vesicles in lavas from Mount Etna by sawing, painting and digitizing the collected samples. We find statistical properties which are common from one sample to another and which are independent of size for different types of lava including pahoehoe, aa and massive. For example, lava vesicularity shows scale invariant behaviour from ≈ 0.10 to ≈ 4.00 mm implying a simple relationship between vesicularity and the resolution at which it is estimated. In order to deduce the volume distribution from the observed area distribution, we develop transformation rules which apply to vesicles of arbitrary shape. On the 22 out of 25 samples, we find that the vesicle number-size density is scale invariant over the same ranges ( n( V) α V − B−1 ) with a power law distribution of exponent B ≈ 1. When averaging over all the samples, the results yield a somewhat more precise estimate B ≈ 0.85. For small vesicle sizes (typically less than ≈ 0.25 mm 2), another power law with an exponent B ≈ 0 is found in nearly all samples. Hence, the observed similar scaling behaviours found in the samples reveal the existence of a common vesicle pattern which may be related to vesicle growth mechanisms in very different looking samples. Moreover, even for identical volcanological/geological conditions—when B ≤ 1—the vesicularity will vary significantly from one sample to another depending on the presence or absence of a few very large vesicles, implying significant spatial rheological variations of the lava flows.