BrecciaBreccia, derived from the Latin meaning "broken," is a rock formed of angular fragments in a matrix which may be of a similar or a different material.
Fault breccias result from the grinding action of two fault blocks as they slide past each other. Subsequent cementation of these broken fragments may occur by means of mineral matter introduced by ground water. Talus slopes may become buried and the talus cemented in a similar manner.
Volcanic breccias result from the cementation of fragments that have been broken by volcanic action. Sometimes the surface of a lava flow will harden while the interior will remain liquid; the fracturing of the surface layer and its subsequent cementation by uncooled lava produces flow breccia.
The invasion of plutonic rocks will often shatter the invaded country rock, producing a shatter breccia. In the case of plutonic rocks partly cooled and subsequently broken by further invasions of magma, intrusive breccias are formed.