Fractal structure of heterogeneous ejecta from the Me-akan volcano, eastern Hokkaido, Japan: implications for mixing mechanism in a volcanic conduit

Abstract

During the somma-building activity (about 13 ka) of the Me-akan volcano, eastern Hokkaido, simultaneous eruptions of basaltic andesite, andesite, and dacite magmas yielded a variety of heterogeneous juvenile-ejecta. These heterogeneous ejecta, ranging from mafic scoria to dacite pumice, can be divided into three types based on the morphological structure of the pumicescoria boundary: type-1 is characterized by a spot pattern, type-2 by a banded pattern containing streaky, dense banded and vortical structures, and type-3 by irregular lumps with fingering patterns. These structures and the eruptive sequence suggest that the basaltic andesite magma was injected into or drawn up with the dacite magma within the volcanic conduit beneath the Nakamachineshiri crater.

A fractal analysis of boundary patterns of the heterogeneous ejecta was performed by a ☐-counting method over a length scale of 0.2 mm to 17 mm in order to characterize the mixing mechanism in the conduit. The morphological patterns of heterogeneous ejecta have a self-similar structure with fractal dimensions of D = 0.9 to 1.2 for type-1, D = 1.2 to 1.6 for type-2, and of D = 1.2 to 1.4 for type-3. The mean value of D = 1.37 ± 0.11 for types-2 and -3 ejecta is almost the same as the value measured for fluid turbulence structures such as clouds or jets, thus we infer that formation of banded and irregular-lump structures was probably produced by turbulent entrainment of dacite magma into basaltic andesite magma in the conduit. The lower fractal dimension of type-1 (D = 1.11 ± 0.08) is due to weakly turbulent or nearly laminar processes. Homogeneous andesite magma is inferred to result from complete, vigorous turbulent mixing.