Breccia is a rock formed from angular gravel and boulder-sized clasts cemented together in a matrix. The angular nature of the clasts indicates that they have not been transported very far from their source. There are several modes of formation for breccia. Some represent consolidated material accumulated on steep hill slopes or at the foot of cliffs. Cataclastic breccias are produced by the fragmentation of rocks during faulting. Volcanic breccias (agglomerates) comprise blocks of lava in an ash matrix and are the product of an explosive eruption. Hydrothermal breccias are formed when hydrothermal fluid fractures a rock mass. Impact breccias are formed when a meteor impacts the Earth's surface, fracturing rock at the site of the impact. The cement that binds the clasts in a breccia is generally one of either calcite, silica or iron oxide. The matrix can consist solely of the cementing material, but may also contain sand and / or silt sized clasts cemented together among the coarser clasts. Breccia can be further divided according to:
Class - breccia can be divided into two broad classes:
Clast supported - where the clasts touch each other and the matrix fills the voids; and
Matrix supported - where the clasts are not in contact and the matrix surrounds each clast;
Clast size - fine (2 - 6mm), medium (6 - 20mm), coarse (20 - 60mm), very coarse (> 60mm);
Sorting - a breccia comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted;
Lithology - a breccia where the clasts represent more than one rock type is termed polymictic (or petromictic), while one where the clasts are of a single rock type are monomictic (or oligomictic).
Other specimens - Click the thumbnails to enlarge
Grain size - > 2mm; clasts easily visible to the naked eye, should be identifiable.
Hardness - variable, soft to hard, dependent on clast composition and strength of cement.
Colour - variable, dependent on clast and matrix composition.
Clasts - variable, but generally harder rock types and / or minerals dominate.
Other features - rough to touch due to angular clasts.
Uses - as dimension stone for decoration of walls and floors; if hard can be used as aggregate, fill etc. in the construction and roading industries.
New Zealand occurrences - sporadic occurrence throughout the country; volcanic breccia can be seen exposed along the north Manukau Heads.