358 TECTONICS/Fractures (Including Joints) Figure 11 Polygonal arrays of tensile fractures cause by the desiccation of a silt layer Figure 12 Polygonal arrays of tensile fractures cause by the cooling of a lava flow (Giant’s Causeway Northern Ireland) Figure 10 (A) A regular array of tensile fractures exposed on a bedding plane in Carboniferous sandstone, Millook, Cornwall, England (B) Less well organized tensile fractures cutting Devon ian sandstones, St Anne’s Head, South Wales (C) Carboniferous sandstone cut by randomly oriented tension fractures In both these examples, the fractures are organized into polygonal arrays showing that the tensile stresses generated were the same in all directions Fracture Sets oriented fractures are to be expected in rocks, and field observations support this idea, Figure 10 As noted above, the problem of forming tensile fractures in the compressive stress field that generally characterizes the Earth’s crust can be solved by appealing to high fluid pressures However, tensile failure can occur in rocks without the aid of a high internal fluid pressure, for example, during the contraction of a layer as a result of desiccation of a sediment (Figure 11) or the cooling of an igneous body (Figure 12) Generally, the state of stress in the Earth’s crust is not hydrostatic Consequently a single episode of deformation is likely to generate a set of fractures with the same orientation However, most rocks experience several different stress regimes during their history with the result that several fracture sets are frequently found superimposed on each other to produce a fracture network (Figure 13) The interaction of late fractures with early fractures is illustrated in Figure 14 The effect of early fractures on later ones is to arrest their propagation and to modify their orientation It can be seen from Figures 14A and B that one