The morphology of small volcanic ash particles is fundamental to our understanding of magma fragmentation,
and in transport modeling of volcanic plumes and clouds. Until recently, the analysis of 3D features in small objects
(b250 μm) was either restricted to extrapolations from 2D approaches, partial stereo-imaging, or CT
methods having limited spatial resolution and/or accessibility. In this study, an X-ray computed-tomography
technique known as SEM micro-CT, also called 3D X-ray ultramicroscopy (3D XuM), was used to investigate
the 3Dmorphology of small volcanic ash particles (125–250 μmsieve fraction), aswell as their vesicle andmicrocrystal
distribution. The samples were selected from four stratigraphically well-established tephra layers of the
MeerfelderMaar (West Eifel Volcanic Field, Germany). Resolution tests performed on a Beametr v1 pattern sample
along with Monte Carlo simulations of X-ray emission volumes indicated that a spatial resolution of 0.65 μm
was obtained for X-ray shadow projections using a standard thermionic SEM and a bulk brass target as X-ray
source. Analysis of a smaller volcanic ash particle (64–125 μm sieve fraction) showed that features with
volumes N 20 μm3 (~3.5 μmin diameter) can be successfully reconstructed and quantified. In addition, newfunctionalities
of the Blob3D softwarewere developed to allowthe particle shape factors frequently used as input parameters
in ash transport and dispersionmodels to be calculated. This study indicates that SEMmicro-CT is very
well suited to quantify the various aspects of shape in fine volcanic ash, and potentially also to investigate the 3D
morphology and internal structure of any object b 0.1 mm3.