Abstract

We have developed a novel method of measuring bubble size distributions from their remains expressed on the surfaces of volcanic ash particles. The morphology of the ash fragments retains a record of bubble size at the time of fragmentation in the curvature of the convex surfaces on the ash fragments. This curvature can be measured using stereo scanning electron microscopy (SSEM), and morphology can be represented using a digital elevation model (DEM) of ash particle surfaces. Due to the vagaries of the sensitivity of SSEM imagery to surface roughness, a three-point fit technique produces more robust results for curvature than a least-squares approach for curve-fitting of ellipsoids of revolution to ash surfaces. The method allows measurement of vesicles within a size range from one to over a million cubic microns. The inferred bubble size distributions so obtained can potentially provide valuable insights regarding magma dynamics and vesiculation that lead to the explosive eruptions that produce ash. An error analysis of the methodology indicates reasonably accurate reconstruction of bubble geometries and bubble size distributions (BSD). Accuracy is constrained primarily by the size of ash particles themselves since the mode of the BSD should be at least a standard deviation smaller than the dominant dimensions of the particles for robust results.

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