Joints and Veins - Global Change

2917-CH07.pdf 11/20/03 5:11 PM Page 154The origin of sheeting joints is a bit problematic. Atfirst glance, you might not expect joints to form parallelto the ground surface, because they are tensile fractures,and near the ground surface there is a compressiveload perpendicular to the ground surface due to theweight of the overlying rock, and the lack of high fluidpressure. It appears that sheeting joints form wherehorizontal stress is significantly greater than the verticalload (Figure 7.15a), allowing joints to propagateparallel to the ground surface. In this regard, formationof sheeting joints resembles cracks formed by longitudinalsplitting in laboratory specimens.The stresses causing sheeting joints may, in part,be tectonic in origin, but they may also be residualstresses. A residual stress remains in a rock even if itis no longer loaded externally (e.g., in an unconfinedblock of rock sitting on a table). Residual stressesdevelop in a number of ways. Imagine a layer of drysand that gets deeply buried. Because of the weight ofthe overburden, the sand grains squeeze together andstrain elastically. If, at a later time, groundwater fillsthe pores between the strained grains, unstrainedcement may precipitate and lock the grains together.As a consequence, the elastic strain in the grains getslocked into the resulting sandstone. When unroofinglater exposes the sandstone, the grains and the cementexpand by different amounts, and as a consequencestress develops in the sandstone. In the case of pluton,residual stresses develop because its thermal properties(e.g., coefficient of thermal expansion) differfrom those of the surrounding wall rock, and because,during cooling, the pluton cools by a greater amountthan the wall rock. The pluton and the wall rock tendto undergo different elastic strains as a result of thermalchanges during cooling and later unroofing (Figure7.15b and c). Because the pluton is welded to thesurrounding country rock, the differential strain createsan elastic stress in the rock. For example, if thepluton shrinks more than the wall rock, tensilestresses develop perpendicular to the wall. At depth,compressive stress due to the overburden countersthese tensile stresses, but near the surface, residualtensile stress perpendicular to the walls of the plutonmay exceed the weight of the overburden and producesheeting joints parallel to the wall of the pluton.We earlier noted that sheeting joints tend to paralleltopography. This relationship either reflects topographiccontrol of the geometry of joints (because thevertical load is perpendicular to the ground surface), orjoint control of the shape of the land surface (becauserocks spall off the mountainside at the joint surface).Geologists are not sure which phenomenon is moreimportant.7.5.3 Natural Hydraulic FracturingAs we saw in Chapter 3, the three principal stresses atdepth in most of the continental lithosphere are compressive.Yet joints form in these regions, and thesejoints may be decorated with plumose structure indicatingthat they were driven by tensile stress. Howcan joints form if all three principal stresses are compressive?As we described in Chapter 6, the solutionto this paradox comes from considering the effect ofpore pressure on fracturing. In simple terms, theincrease in pore pressure in a preexisting crackpushes outward and causes a tensile stress to developat the crack tip that eventually exceeds the magnitudeof the least principal compressive stress. If the porepressure is sufficiently large, a tensile stress thatexceeds the magnitude of σ 3 develops at the tip thecrack, even if the remote principal stresses are allcompressive (Figure 7.16a), and the crack propagates,a process called hydraulic fracturing. Oilwell engineers commonly use hydraulic fracturing tocreate fractures and enhance permeability in the rocksurrounding an oil well. They create hydraulic fracturesby increasing the fluid pressure in a sealed segmentof the well until the wall rock breaks. Buthydraulic fracturing also occurs in nature, due to thefluid pressure of water, oil, and gas in rock, and it isthis natural hydraulic fracturing that causes somejoints to form.If you think hard about the explanation of hydraulicfracturing that we just provided, you may wonderwhether the process implies that the pore pressure inthe crack becomes greater than pore pressure in thepores of the surrounding rock. It doesn’t! Pore pressurein the crack can be the same as in the pores of the surroundingrock during natural hydraulic fracturing.Thus, we need to look a little more closely at the problemto understand why pore pressure can cause jointpropagation.Imagine that a cemented sandstone contains fluidfilledpores and fluid-filled cracks (Figure 7.16b).Let’s focus our attention on the crack and its walls.Because the pores and the crack are connected, thefluid pressure in the pores and the crack are the same.Fluid pressure within the crack is pushing outwards,creating an opening stress, but at the same time, thefluid pressure in the pores, as well as the stress in therock, is pushing inwards, creating a closing stress. Aslong as the closing stress exceeds the opening stress,the crack does not propagate. If the fluid pressureincreases, the opening stress increases at the samerate as the increase in fluid pressure, but the closingstress increases at a slower rate. Eventually, the open-154 JOINTS AND VEINS