Abstract Tiny spalls of various growth stages were produced in poly(methyl metacrylate) (PMMA) plates at temperatures ranging from −48 to 80° by the flying plate method using a foil explosion technique. Optical inspections revealed that the spalls consist of many small, disk-shaped cracks; the cracks may be morphologically classified into three types. Some speculations on the growing and healing processes of these cracks are presented. The density of crack nuclei in PMMA is estimated to be 104 to 106 cm3, depending on the specimen properties and the amplitude of the applied stress waves. Specimen temperatures and annealing were found to have no appreciable effects on the pattern of spall cracks or on the density of crack nuclei. In the light of the observed results it is suggested that the spall patterns characteristic to PMMA are, besides the properties of the polymer itself, principally due to internal nuclei, such as flaws, which are rather randomly located.