Introduction

At atmospheric pressure and room temperature most rocks are brittle; that is, they behave nearly elastically until they fail by fracture. Cracks or fractures in rock along which there has been little or no relative displacement are known as joints. They occur on all scales in both sedimentary and igneous rocks. Joints are commonly found in sets defining parallel or intersecting patterns of failure related to local stress orientations. The breakdown of surface rocks by erosion and weathering is often controlled by systems of joints along which the rocks are particularly weak and susceptible to disintegration and removal. These processes in turn enhance the visibility of the jointing. Igneous rocks often develop joints as a result of the thermal stresses associated with cooling and contraction. Columnar jointing in basaltic lava flows (Figure 7–1) and parallel jointing in granitic rocks (Figure 7–2) are examples.

Faults are fractures along which there has been relative displacement. Faults also occur on all scales; examples of faults have already been given in Figures 1–58 and 4–34b and another example is given in Figure 7–3. The mechanical aspects of faulting are discussed in the next chapter.

Although fracture is important in shallow crustal rock at low temperatures and pressures, there are many circumstances in which rock behaves as a ductile material. In determining the transition from brittle to ductile behavior, pressure, temperature, and strain rate are important. If the confining pressure of rock is near the brittle strength of the rock, a transition from brittle to ductile behavior will occur.