A geological fracture is any break in a rock mass within the Earth’s crust. These fractures are fundamental features in rock mechanics, recording the history of stress and strain experienced by the planet’s outer layers. Geologists classify these breaks based on the movement that occurs along the fracture surface. The two most common types are joints and faults, and their difference lies entirely in the presence or absence of rock mass shifting.
Understanding Joints
A joint is a fracture in a rock where there has been no measurable movement or slip parallel to the fracture plane. These breaks form when the rock material is pulled apart, a process known as tensional stress. When the pulling force exceeds the rock’s tensile strength, the rock fails in a brittle manner, creating the joint.
Joints also form due to processes that cause a rock mass to expand or contract. For example, the removal of overlying rock through erosion reduces pressure, allowing the rock to expand and crack in a process called unloading. Similarly, as magma cools and solidifies into igneous rock, the contraction in volume often creates distinctive, often hexagonal, columnar joints.
These fractures rarely appear as isolated features; they typically occur in systematic groups known as joint sets. A joint set is a family of parallel and approximately equally spaced joints. When two or more joint sets intersect, they form a joint system, which can cleave a large rock body into a series of smaller, geometrically shaped blocks.
Understanding Faults
A fault is a fracture surface along which measurable displacement of the rock masses has occurred. This slip can range from millimeters to hundreds of kilometers over geological time, marking significant crustal deformation. Faults form when the rock is subjected to shear or compressional stresses, which push rock masses past one another or forcefully together.
Geologists classify faults based on the direction of this relative movement. Dip-slip faults involve primarily vertical movement, categorized by the relationship between the two blocks of rock: the footwall (beneath the fault plane) and the hanging wall (above the fault plane).
In a normal fault, the hanging wall moves down relative to the footwall, caused by tensional stress pulling the crust apart. Conversely, a reverse fault occurs when the hanging wall moves up relative to the footwall, resulting from compressional stress that pushes the crust together. The third main category is a strike-slip fault, where movement is horizontal along the fault plane, caused by shear stress as rock masses slide past each other.
The Critical Distinction in Formation and Movement
The fundamental difference between a joint and a fault is the presence of measurable displacement along the fracture plane. A joint is an extension fracture where rock surfaces move apart perpendicular to the break, but do not slip past one another. A fault, by contrast, is a shear fracture where the rock masses have moved parallel to the fracture surface.
This difference in movement directly reflects the type of stress that caused the fracture. Joints primarily form under simple tensional stress, such as that caused by cooling or the expansion of rock upon the removal of overburden. Faults, however, require compressional or shear stress, which are associated with tectonic plate interactions.
In terms of their geological consequences, joints and faults play distinct roles. Joints significantly increase a rock body’s surface area, making it susceptible to weathering and erosion by providing pathways for water and air. Faults are directly associated with the release of accumulated strain energy, making them the source of most naturally occurring earthquakes and major agents of large-scale crustal shaping.