Tension cracks perpendicular to the free surface
Detonation is process that lasts for certain amount of time and propagates along the explosive charge. Explosive is placed in the borehole and charge has cylindrical shape (length is much large than diameter). Explosive charge is usually initiated from one end. Figure 1 illustrates the initiation of the explosive charge from the beginning of the blasthole. First, radial tension cracks are formed and if explosive charge is at proper distance from the free surface rock wedge is separated (as explained in Part I). Secondly, tension cracks subparallel to the free surface are formed (as explained in Part II). Separation of the $wedge$ goes from the beginning to the end of the borehole and it has form of the cantilever. Gases of explosion are bending this cantilever and in zone of tension, tensile cracks are formed that are perpendicular to the free surface.
Figure 2 illustrates the initiation of explosive charge from the bottom of the blasthole. In this case formation of the radial and subparallel tension cracks goes from the bottom of the blasthole. $Wedge$ has the form of the beam fixed at both ends. When the length of the beam exceeds the critical point, bending occurs and tension cracks perpendicular to the free surface are formed in the tension one.
Radial and cracks subparallel to the free surface are formed by the pressure wave, while tension cracks that occur from the bending of beam/cantilever are caused by the pressure of the gases. Expansion of gases produces the further collision between fragments and their further fragmentation.
Figure 1 Explosive charge initiation from the beginning of the blasthole
Figure 2 Explosive charge initiation from the end of the blasthole