Brittle Deformation: Types and Examples

Brittle deformation is a type of deformation that occurs in rocks when they are subjected to stress beyond their elastic limit. This means that the rocks break rather than bend or flow. Brittle deformation is typically associated with tectonic forces, such as those that cause earthquakes and mountain building.

Brittle deformation can occur in a variety of rock types, but it is most common in hard, brittle rocks such as sandstone, limestone, and granite. Brittle deformation can also occur in softer rocks, such as shale and mudstone, but this is less common.

There are two main types of brittle deformation: fractures and faults. Fractures are small breaks in rocks that can be caused by a variety of factors, such as impact, thermal stress, or weathering. Faults are larger breaks in rocks that are caused by tectonic forces.

Terminology of Brittle Deformation

Faults

Faults: Faults are one of the most common examples of brittle deformation. These are large-scale fractures or displacements in the Earth's crust, caused by tectonic forces such as plate movement. Faults can extend for hundreds of kilometers and can have a significant impact on the landscape. Faults can be found all over the world, and they can range in size from a few meters to hundreds of kilometers. Some famous examples of faults include the San Andreas Fault in California, the Dead Sea Fault in the Middle East, and the Alpine Fault in New Zealand.

There are different types of faults, classified based on the movement along the fault plane:

• Normal faults: Blocks of rock move downward relative to each other.
• Reverse faults: Blocks of rock move upward relative to each other.
• Strike-slip faults: Blocks of rock move horizontally past each other.

Joints: Joints are another common type of brittle deformation. Joints are fractures in rocks that do not have any displacement associated with them. Joints are often caused by cooling or drying of rocks, and they can be found in a variety of rock types.

Cleavage: Cleavage is a type of brittle deformation that occurs in metamorphic rocks. Cleavage is caused by the alignment of mineral grains during metamorphism, and it can give rocks a layered appearance. Some common examples of rocks with cleavage include slate and phyllite.

Brittle fault zone A band of finite width in which slip is distributed among many smaller discrete brittle faults, and/or in which the fault surface is bordered by pervasively fractured rock.

Brittle fault A single surface on which movement occurs specifically by brittle deformation mechanisms.

Cataclasis A deformation process that involves distributed fracturing, crushing, and frictional sliding of grains or of rock fragments.

Crack Verb: to break or snap apart. Noun: a fracture whose displacement does not involve shear displacement (i.e., a joint or microjoint).

Fault Broad sense: a surface or zone across which there has been measurable sliding parallel to the surface. Narrow sense: a brittle fault. The narrow definition emphasizes the distinctions between faults, fault zones, and shear zones.

Fracture zone A band in which there are many parallel or subparallel fractures. If the fractures are wavy, they may anastomose with one another. Note: The term has a somewhat different meaning in the context of ocean-floor tectonics.

Fracture

Fracture A general term for a surface in a material across which there has been loss of continuity and, therefore, strength. Fractures range in size from grain-scale to continent-scale.

These are small-scale breaks or cracks in rocks caused by various factors like:

• Impact: A sudden application of force, such as from a falling object or an earthquake.
• Thermal stress: Rapid changes in temperature, such as those caused by volcanic activity or weathering.
• Weathering: The gradual breakdown of rock by physical, chemical, and biological processes.

Fractures can be planar (flat) or curvilinear (curved) and can vary in size from microscopic cracks to large fissures.

Healed microcrack A microcrack that has cemented back together. Under a microscope, it is defined by a plane containing many fluid inclusions. (Fluid inclusions are tiny bubbles of gas or fluid embedded in a solid).

Joint A natural fracture which forms by tensile loading, i.e., the walls of the fracture move apart veryc slightly as the joint develops. Note: A minority of geologists argue that joints can form due to shear loading.

Microfracture A very small fracture of any type. Microfractures range in size from the dimensions of a single grain to the dimensions of a thin section.

Microjoint A microscopic joint; microjoints range in size from the dimensions of a single grain to the dimensions of a hand-specimen. Synonymous with microcrack.

Shear fracture A macroscopic fracture that grows in association with a component of shear parallel to the fracture. Shear fracturing involves coalescence of microcracks.

Shear joint A surface that originated as a joint but later became a surface of sliding. Note: A minority of geologists consider a shear joint to be a joint that initially formed in response to shear loading.

Shear rupture A shear fracture.

Shear zone A region of finite width in which ductile shear strain is significantly greater than in the surrounding rock. Movement in shear zones is a consequence of ductile deformation mechanisms (cataclasis, crystal plasticity, diffusion).

Vein A fracture filled with minerals precipitated from a water solution.

Examples of brittle deformation

Broken glass: Glass is a classic example of a brittle material that will shatter easily when dropped or struck with force.

The San Andreas Fault: Strike-slip fault. This is a major fault system in California that is responsible for many earthquakes.

The Grand Canyon: The Grand Canyon was formed by normal faulting and subsequent erosion. The layers of rock in the Grand Canyon were formed by the deposition of sediments over millions of years. These layers have been tilted and fractured by tectonic forces, creating the dramatic canyon landscape.