Structural geology shows that rock bodies contain countless breaks or discontinuities, which record the forces acting on the Earth’s crust. The two most common forms of these fractures are joints and faults, both representing ways rock has failed under stress. Understanding the difference between these features is necessary to interpret a region’s geological history.
Joints: Fractures Lacking Displacement
A joint is a break or crack in a rock body where there has been no visible or measurable movement parallel to the fracture surface. Geologists often describe joints as “Mode 1” fractures, meaning the movement is purely an opening or pulling apart perpendicular to the fracture face.
Joints frequently appear in the field as sub-parallel, evenly spaced sets, often creating a systematic, blocky appearance in the rock mass. These features can form under simple tensional stress, where the rock is stretched beyond its inherent strength. A common type is an unloading joint, which develops when massive amounts of overlying rock are removed by erosion, causing the rock below to expand upward and fracture as the confining pressure is released.
Another mechanism for joint formation is cooling contraction, particularly in igneous rocks like basalt. As molten rock cools and solidifies, it shrinks, creating polygonal or columnar joints that appear as systematic, vertical cracks. The presence of joints significantly influences the rock’s bulk properties, such as its permeability, making them important conduits for groundwater flow and weathering processes.
Faults: Fractures Defined by Movement
A fault is a fracture surface along which there has been significant, measurable slip or displacement of the rock masses. This displacement means one side of the break has shifted substantially with respect to the other. Displacement can range from a few centimeters to hundreds of kilometers in major crustal features.
The actual fracture surface on which the movement takes place is called the fault plane, and its existence is confirmed by observing offset layers or distinct rock units. To describe the relative movement, geologists use terms like the hanging wall and the footwall. The hanging wall is the block of rock that lies above the fault plane, while the footwall is the block below it.
In a normal fault, the hanging wall block moves downward relative to the footwall, resulting from extensional forces pulling the crust apart. Conversely, a reverse fault occurs when the hanging wall moves up relative to the footwall, typically in response to compressional forces. This sudden, rapid movement along a fault plane is the mechanism that releases stored elastic energy, causing most earthquakes.
The Underlying Causes: Differing Stress Regimes
The fundamental difference between joints and faults is rooted in the type and magnitude of the stress that causes the rock to fail. Joints form primarily in response to tensional stress, where the rock is pulled apart, allowing a crack to open perpendicular to the pulling force. This type of stress requires less overall energy to fracture the rock, resulting in a static break without significant slide.
Faults, however, require a more intense and complex stress environment involving shear or significant compression. Shear stress involves forces that are nearly parallel but acting in opposite directions, causing one part of the rock mass to slide past the other. Compressional stress pushes rock masses together, forcing one block to be shoved over the other.
Both shear and compressional regimes require the rock’s strength to be overcome to initiate and sustain movement, resulting in dynamic failure and measurable displacement. The development of a fault demands a much higher concentration of force than a joint, leading to a break that involves friction and sliding.