SCALING EFFECTS ON VESICLE SHAPE, SIZE AND HETEROGENEITY OF LAVAS FROM MOUNT-ETNA

The rheology of basaltic lava flows depends on several factors includi ng the vesicle size and shape distributions. We analysed vesicles in l avas from Mount Etna by sawing, painting and digitizing the collected samples. We find statistical properties which are common from one samp le to another and which are independent of size for different types of lava including pahoehoe, aa and massive. For example, lava vesiculari ty shows scale invariant behaviour from approximate to 0.10 to approxi mate to 4.00 mm implying a simple relationship between vesicularity an d the resolution at which it is estimated. In order to deduce the volu me distribution from the observed area distribution, we develop transf ormation rules which apply to vesicles of arbitrary shape. On the 22 o ut of 25 samples, we find that the vesicle number-size density is scal e invariant over the same ranges (n(V) proportional to V--B-1) with a power law distribution of exponent B approximate to 1. When averaging over all the samples, the results yield a somewhat more precise estima te B approximate to 0.85. For small vesicle sizes (typically less than approximate to 0.25 mm(2)), another power law with an exponent B appr oximate to 0 is found in nearly all samples. Hence, the observed simil ar scaling behaviours found in the samples reveal the existence of a c ommon vesicle pattern which may be related to vesicle growth mechanism s in very different looking samples. Moreover, even for identical volc anological/geological conditions-when B less than or equal to 1-the ve sicularity will vary significantly from one sample to another dependin g on the presence or absence of a few very large vesicles, implying si gnificant spatial rheological variations of the lava flows.