Volcanic bombs—their form and physical structure: comparison with cast iron cannon balls

Abstract

Volcanic bombs are ejected clots of hot viscous magma of smallest dimension about 2½ in. (50mm) dia. After cooling down, their internal structure reflects the residual stresses within their hard “spherical” shell. This we suppose is the same, at least in kind, as that calculated for cast iron spherical cannon balls which is described in Ref. [I] [ Johnson and Chandrasekar, J. Mater. Processing Technol. , in press (1992) ]. A tensile, and therefore “spongy”, central region was predicted and if it should be relieved by virtue of an imperfection then a simple void would be able to be developed. In the case of cannon balls, off-centre voids will not allow it to be shot “true”. It is alleged that all cast iron balls are so flawed, but as far as the authors know no substantial evidence of such has been presented. Verificatory physical evidence by way of similarity with what is known and seen from the terminal state of cooled magma bombs—deriving from the study of vulcanology—is remarkable. A short catalogue of what can be found out about them is given.