Cryptoachneliths: Hidden glassy ash in composite spheroidal lapilli

►Optical microscopy, SEM and EDX techniques applied to the study of lapilli size (2-64 mm) tephra from the Cabezo Segura volcano in the Calatrava volcanic province of Spain allowed the discovery of nanometer to micrometer in size, welded cryptoachneliths, not previously recognized, that form composite lapilli. Some of the processes that operate within eruptive columns from explosive events (particle collision, coalescence, etc.), whose observation is always problematic due to their opacity and inhomogeneity, might thus be inferred. ►Welding of juvenile pyroclasts inside eruptive columns is the subject of study and debate in recent bibliography. The dispersion and sedimentation of tephra, both in the proximal and distal regions of the volcano, have been modelled using sedimentation models. These simple models, although providing invaluable insights about fundamental processes, do not explicitly predict the dynamics and physical processes that may take place among the various types of particles that compose the eruptive column. The particles there can experience a variety of modifications which are poorly understood. Some of them are revealed by our study impying that the absence of detailed characterization of the lapilli-rich tephra layers may lead to wrong conclusions on the eruptive dynamics.