Folding and faulting in the Earth's crust
The topographic map illustrated in Figure 1 suggests that the Earth's surface has been deformed. This deformation is the result of forces that are strong enough to move ocean sediments to an elevation many thousands meters above sea level. This displacement of rock can be caused by tectonic plate movement and subduction, volcanic activity, and intrusive igneous activity.
Deformation of rock involves changes in the shape and/or volume of these substances. Changes in shape and volume occur when stress and strain causes rock to buckle and fracture or crumple into folds. A fold can be defined as a bend in rock that is the response to compressional forces. Folds are most visible in rocks that contain layering. For plastic deformation of rock to occur a number of conditions must be met, including:
- The rock material must have the ability to deform under pressure and heat.
- The higher the temperature of the rock the more plastic it becomes.
- Pressure must not exceed the internal strength of the rock. If it does, fracturing occurs.
- Deformation must be applied slowly.
A number of different folds have been recognized and classified by geologists. The simplest type of fold is called a monocline (Figure 2). This fold involves a slight bend in otherwise parallel layers of rock.
Figure 2: Monocline fold.
|Figure 3: Anticline fold.|
A syncline is a fold where the rock layers are warped downward (Figure 4 and 5). Both anticlines and synclines are the result of compressional stress.
|Figure 4: Syncline fold.|
More complex fold types can develop in situations where lateral pressures become greater. The greater pressure results in anticlines and synclines that are inclined and asymmetrical (Figure 6).
A recumbent fold develops if the center of the fold moves from being once vertical to a horizontal position (Figure 7). Recumbent folds are commonly found in the core of mountain ranges and indicate that compression and/or shear forces were stronger in one direction. Extreme stress and pressure can sometimes cause the rocks to shear along a plane of weakness creating a fault. We call the combination of a fault and a fold in a rock an overthrust fault.
Faults form in rocks when the stresses overcome the internal strength of the rock resulting in a fracture. A fault can be defined as the displacement of once connected blocks of rock along a fault plane. This can occur in any direction with the blocks moving away from each other. Faults occur from both tensional and compressional forces. Figure 8 shows the location of some of the major faults located on the Earth.
There are several different kinds of faults. These faults are named according to the type of stress that acts on the rock and by the nature of the movement of the rock blocks either side of the fault plane. Normal faults occur when tensional forces act in opposite directions and cause one slab of the rock to be displaced up and the other slab down (Figure 9).
Reverse faults develop when compressional forces exist (Figure 10). Compression causes one block to be pushed up and over the other block.
A graben fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys (Figure 11).
A horst fault is the development of two reverse faults causing a block of rock to be pushed up (Figure 12).
The final major type of fault is the strike-slip or transform fault. These faults are vertical in nature and are produced where the stresses are exerted parallel to each other (Figure 13). A well-known example of this type of fault is the San Andreas Fault in California.
Folds and faults have an economic importance. Anticlines and horsts are good sites for oil accumulation forming oil reservoirs whereas synclines and grabens are suitable for water accumulation forming aquifers or groundwater basins.
Faults represent a weak zone so they should be avoided or put in mind in any civil constructions. Also, faults as a weak zone are suitable for upward leakage of either lava forming sills or dykes or groundwater forming springs.
Folding and faulting reflect the effect of the internal energy of the earth. Consequently, the criteria of folding and faulting represent a kind of eath's ability for stability.