The Earth’s crust is constantly being shaped by immense tectonic forces. These forces, driven by the planet’s internal heat, cause rocks to change their size, shape, and orientation. Geology studies how these forces, known as stress, lead to measurable changes in the rock body, which is termed strain. A rock’s response depends on factors including its composition, temperature, and pressure. Understanding this relationship between stress and strain is fundamental to interpreting the large-scale features of our planet.
Stress and Strain: Defining the Fundamentals
In geological terms, stress is the force applied to a rock body over a specific unit of area. Tensional stress is a directed force that acts to pull a rock mass apart, stretching it in opposite directions. Tensional stress is the dominant force at divergent plate boundaries, where tectonic plates move away from one another.
When a rock is subjected to tensional stress, the resulting physical change in its shape or volume is called strain. Strain is the measurable effect of the applied stress and records the rock’s deformation. The response can be temporary, known as elastic strain, where the rock returns to its original shape once the stress is removed. If the stress exceeds the rock’s strength limit, the strain becomes permanent and irreversible, leading to lasting deformation.
The Direct Result: Extensional Strain
The result of tensional stress on a rock body is a form of deformation known as extensional strain. Extensional strain means the rock is elongated or stretched parallel to the pulling force. This stretching causes a corresponding reduction in the rock body’s dimensions perpendicular to the applied tension.
This geometric change is often referred to as “necking,” where the rock thins out as it gets longer. The total amount of extensional strain a rock can sustain depends on its composition and environmental conditions. This deformation creates space in the crust, which is accommodated by either fracturing or flowing.
Brittle Failure: The Formation of Normal Faults
When tensional stress is applied to rocks near the Earth’s surface, where temperatures and pressures are relatively low, the rock tends to fracture in a process called brittle failure. The rock breaks abruptly once the accumulated strain exceeds its strength. This fracturing leads to the formation of a specific geological structure known as a normal fault.
A normal fault is a fracture where the block of crust situated above the fault plane, called the hanging wall, moves downward relative to the block below the fault, known as the footwall. This downward movement accommodates the stretching and lengthening of the crust. The displacement along these normal faults creates a series of dropped blocks, called grabens, separated by uplifted blocks, called horsts, which are characteristic of rift valleys.
The East African Rift Valley is a large-scale extensional process, where the Arabian and Somalian plates are pulling away from the Nubian plate. The rift is marked by a system of normal faults, creating a trough that is tens of kilometers wide. The seismicity in rift zones is directly caused by this brittle failure and slip along the newly formed or reactivated normal faults. This mechanism effectively thins the continental crust.
Ductile Deformation: Stretching and Thinning
If tensional stress is applied to rocks deep within the crust, generally below 10 to 20 kilometers, the material responds differently. High temperatures and immense confining pressure prevent the rock from fracturing. Instead, the rock deforms in a ductile manner, meaning it flows without losing cohesion.
Ductile deformation allows the rock to accommodate extensional strain by stretching and thinning. The rock body is elongated parallel to the tensional stress, and internal mineral grains may become visibly stretched. Structures like boudinage, where stronger layers are pinched and segmented like a string of sausages, are a common result of this deep-seated stretching. This flow allows the lower crust and mantle lithosphere to thin beneath a developing rift zone.