Breccia is a type of rock characterized by its distinctive texture, consisting of large, sharp-edged fragments, known as clasts, embedded within a finer-grained material called the matrix. These clasts must be larger than two millimeters in diameter. The angular shape of the fragments is the defining feature that distinguishes breccia from conglomerate, which contains clasts that have been rounded and smoothed by significant transport. The angularity of breccia fragments indicates they have been broken and deposited very close to their source, recording intense, localized mechanical breakage. The environment where this fragmentation and subsequent cementation occurs determines the type of breccia.
Breccia Formed by Tectonic Stress
Tectonic movements deep within the Earth’s crust create fault breccia, also known as cataclastic breccia. This breccia forms within fault zones where immense shear stress and friction cause the surrounding rock to shatter and grind. The process, called cataclasis, mechanically breaks down the rock into a chaotic mixture of angular fragments.
These rocks commonly form in the brittle regime of the upper to mid-crust. Fluid pressure plays a significant role, as hot water can infiltrate the fractured rock, reducing effective confining pressure and promoting hydraulic fracturing. This process aids in the initial fragmentation and later cementation of the fragments.
Breccia Formed by Surface Processes
Breccias can also form through processes at or near the Earth’s surface, driven by gravity, weathering, and dissolution. One common example is talus breccia, which forms at the base of steep cliffs or slopes. Mechanical weathering breaks rock fragments off the cliff face, which fall and accumulate as angular scree that is then cemented together.
Another type is collapse breccia, typically found in karst environments where soluble rock, like limestone or evaporites, has been dissolved by groundwater. The removal of this underlying material causes overlying layers to fall into the resulting void, shattering into angular fragments. Rapid mass movements, such as debris flows, can also transport and deposit angular fragments over short distances, creating debris flow breccia. These surface-level formations are collectively called sedimentary breccias.
Breccia Formed by Magmatic Activity
Volcanic and intrusive igneous settings create a distinct category of breccias under conditions of high heat and explosive force. Pyroclastic breccia is created during violent volcanic eruptions that explosively shatter the magma and surrounding rock. The resulting angular fragments of ash, rock, and lava (tephra) are ejected and cemented together upon deposition.
A different process, called autobrecciation, forms flow breccias when the outer crust of a moving lava flow solidifies. The continued movement of the molten interior breaks this brittle surface into angular blocks, which are then incorporated back into the flow. Intrusive breccias form beneath the surface when magma forcefully injects itself into the surrounding host rock, shattering it and incorporating the fragments into the intrusion or volcanic vent.
Breccia Formed by Extraterrestrial Impact
The most violent formation environment for breccia is a hypervelocity impact by a meteorite or asteroid. The immense energy released instantly shatters the target rock and can even melt some material, creating impact breccia. A specific variety is suevite, a polymict breccia characterized by a matrix containing fragments of melted rock and glass shards.
Suevite forms from the excavation and collapse of the crater, incorporating shocked and fragmented material. The presence of planar deformation features (PDFs) in mineral grains is a definitive indicator of an impact origin. These micro-structures are caused by extreme, sudden shock pressures and distinguish impact breccias from terrestrial breccias formed by volcanic or tectonic forces.