Clastic rock

Conglomerate

Breccia. Notice the angular nature of the large clasts

Marble breccia from Naples.

Clastic rocks are composed of fragments, or clasts, of pre-existing rock.

Although, in principle, almost any rock made of eroded former rock is clastic (such as sandstone or mudstone), in practice the term is used for clasts of at least gravel size upwards. That means clasts are at least visible to the naked eye, and may be huge.

There are two types of clastic rock, by shape. conglomerates have rounded clasts, and breccias have jagged clasts.

In a clastic rock, the clasts are often of a different make-up from the ground rock or matrix.

Clastic Rock Media

• 

  A thin section of a clast (sand grain), derived from a basalt scoria. Vesicles (air bubbles) can be seen throughout the clast. Plane light above, cross-polarized light below. Scale box is 0.25 mm.

• 

  Claystone from Montana

• 

  Basalt breccia, green groundmass is composed of epidote