Brittle deformation in the Earth’s crust results in fractures, which are breaks in rock layers. These fractures are structural features that record the history of forces acting on the rock body. Geologists distinguish between two primary types: joints and faults. The distinction hinges on one specific geological action.
Joints: Fractures Without Movement
A joint is defined as a break or fracture of natural origin in a rock body where there has been no discernible movement or offset parallel to the fracture plane. This lack of movement makes the joint a simple separation, often called an extensional fracture. Joints are ubiquitous, found in nearly all rock types, and typically occur in parallel groups known as joint sets. These sets are systematically spaced and oriented, reflecting the regional stress field at the time of their formation.
One common mechanism for joint creation is the cooling and contraction of igneous rock, which can lead to distinctive geometric patterns like columnar jointing in basalt. Joints also frequently form through unloading, which is the release of confining pressure as overlying rock layers are removed by erosion. As the rock expands due to the pressure release, tensile stress builds up until the rock breaks, forming fractures often parallel to the ground surface.
Faults: Fractures With Displacement
A fault is a fracture surface along which the blocks of rock on either side have experienced measurable relative movement, or displacement. This movement is the factor that differentiates a fault from a joint. The displacement can range from a few centimeters to hundreds of kilometers, and it is a result of shear stress overcoming the rock’s frictional resistance.
Faults are classified based on the direction of this relative movement. In a dip-slip fault, the movement is primarily vertical, and the two sides are defined as the hanging wall and the footwall. The hanging wall is the block resting on top of the fault plane, while the footwall is the block beneath it. If the hanging wall moves down relative to the footwall, it is a normal fault; movement upward indicates a reverse fault; and horizontal motion defines a strike-slip fault.
The Role of Stress and Strain in Formation
The fundamental reason one type of fracture forms over the other lies in the type and magnitude of the stress applied to the rock. Stress is the force applied per unit area, categorized into three main types: tensional (pulling apart), compressional (pushing together), and shear (sliding past). Both joints and faults are products of brittle deformation, meaning the rock breaks rather than bends under stress.
Joints form when the rock is subjected to tensional stress or when internal forces like contraction cause the rock to pull apart. The applied force is sufficient to break the rock’s tensile strength, but not enough to overcome the friction along the fracture surface. Consequently, the rock breaks cleanly without subsequent sliding. Joints represent failure under lower strain conditions where movement is limited to simple separation.
Faults, conversely, form under conditions of greater stress, particularly shear stress, which causes the blocks to slide past one another. The stress must build up until it exceeds both the rock’s internal strength and the frictional resistance along the fracture plane. Once this threshold is surpassed, the rock fails, and stored elastic energy is released as movement, resulting in measurable displacement.
Geological Importance and Practical Examples
The distinction between these two structures dictates their influence on the landscape and subsurface processes. Joints, despite lacking displacement, are important in the process of weathering and erosion. Their presence creates planes of weakness that allow water, ice, and plant roots to penetrate, accelerating the breakdown of rock formations. Joint networks also create pathways that govern the movement and storage of groundwater, making them controls for aquifer systems.
Faults carry greater geological significance, as they are intrinsically linked to the major forces shaping the planet. They are the surface expressions of plate tectonics, accommodating the massive movements of the Earth’s lithosphere. Sudden movement along faults is the direct cause of most earthquakes, releasing seismic energy as the blocks slip past each other. Faults also play a major role in mountain building and the distribution of mineral and petroleum resources.