Spherulite formation in obsidian lavas in the Aeolian Islands, Italy


  • Liam Adam Bullock University of Aberdeen
  • Ralf Gertisser Keele University
  • Brian O'Driscoll University of Manchester




Spherulites, Obsidian, Aeolian Islands, Glass Transition, Lipari, Vulcano


Spherulites in obsidian lavas of Lipari and Vulcano (Italy) are characterised by spatial, textural and geochemical variations, formed by different processes. Spherulites vary in size from <1 mm to 8 mm, are spherical to elongate in shape, and show variable radial interiors. Spherulites occur individually or in
deformation bands, and some are surrounded by clear haloes and brown rims. Spherulites typically contain cristobalite (α, β) and orthoclase, titanomagnetite and rhyolitic glass, and grew over an average period of 5 days, with modification over a few hundred years at lower temperatures. Heterogeneity relates to
formation processes of spherulite ‘types’ at different stages of cooling and emplacement. Distinct populations concentrate within deformation structures,with variations in shape and internal structure. Crystal Size Distribution (CSD) plots show differing size populations and growth periods. Spherulites which formed at high temperatures show elongation, triggered further spherulite nucleation and growth. Spherulites formed at mid-glass transition temperatures are spherical, and spherulites are modified at vapour-phase temperatures.
Enhanced undercooling, deformation, and modification are therefore pivotal in the development spherulite heterogeneity in obsidian lavas.


Author Biographies

Liam Adam Bullock, University of Aberdeen

Research Fellow, Department of Geology & Petroleum Geology

Ralf Gertisser, Keele University

Senior Lecturer in Mineralogy and Petrology, Programme Director MSc Geoscience Research, School of Geography, Geology and the Environment

Brian O'Driscoll, University of Manchester

Senior Lecturer in Petrology, School of Earth and Environmental Sciences