Breccia

A megabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed by landslides,[2] impact events,[3] or caldera collapse.

For example, scree deposited at the base of a cliff may become cemented to form a talus breccia without ever experiencing transport that might round the rock fragments.

[9] Thick sequences of sedimentary (colluvial) breccia are generally formed next to fault scarps in grabens.

Subsequent cementation of these broken fragments may occur by means of the introduction of mineral matter in groundwater.

This may include rocks plucked off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge.

[17] Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation.

[22][23] Clastic rocks are also commonly found in shallow subvolcanic intrusions such as porphyry stocks, granites and kimberlite pipes, where they are transitional with volcanic breccias.

Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150 and 350 °C, when seismic or volcanic activity causes a void to open along a fault deep underground.

[37] In the mesothermal regime, at much greater depths, fluids under lithostatic pressure can be released during seismic activity associated with mountain building.

The pressurised fluids ascend towards shallower crustal levels that are under lower hydrostatic pressure.

On their journey, high-pressure fluids crack rock by hydrofracturing, forming an angular in situ breccia.

[37] For thousands of years, the striking visual appearance of breccias has made them a popular sculptural and architectural material.

[38] Breccia was used on a limited scale by the ancient Egyptians; one of the best-known examples is the statue of the goddess Tawaret in the British Museum.