Abstract: Many features of dynamic fracture caused by shock-wave action are most pronounced under spalling conditions, i.e., when an intense wave pulse is reflected from a free boundary. Therefore, such material tests are one of the main ways to study the fracture processes occurring in a solid under ultrafast dynamic stress. Nevertheless, the geometrical characteristics of the fracture zone formed under spalling conditions are relatively little studied. It was experimentally established that the qualitative appearance of such a multiple fracture region is characterized by strong instability and depends on the parameters of the applied pulse, such as the rate of increase and decrease in stress, amplitude, and duration. Based on an analysis of a number of experimental results, it will be shown below that the unstable behavior of the fracture zone formed in the spalling conditions observed in the experiments can be explained and predicted using the structural-temporal approach based on the concept of incubation fracture time.