Breccias a rock formed from angular gravel and boulder-sized clasts that are cemented together in a matrix. The angular nature of the clasts indicates that they have not been transported very far from their source and have not been water worn smooth. Volcanic breccias (agglomerates) comprise blocks of lava in an ash matrix, and are the product of an explosive eruption.

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).