DEFINITION OF BRECCIA

Definition of breccia

A coarse-grained rock with angular clasts held

together by a finer grained matrix. There are sedimentary,

igneous, and structural breccias, each of which forms in very

different ways. Sedimentary breccias are distinguished from

conglomerates that have rounded clasts set in a fine grained

matrix. The angular nature of clasts in a sedimentary breccia

indicates that the clasts have not been transported far from

their source of origin. Many breccias form along the bases of

cliffs or steep slopes in talus aprons, and some may be eroded

from fault scarps. Breccias may also form above solution cavities

such as caves that collapse in karst terrains. The clasts in

breccias can often be traced to a nearby source rock, and the

geometry of the breccia deposit can be used to help decipher

the sedimentary environment in which it formed.

Igneous breccias include both intrusion breccias and volcanic

breccias. Intrusion breccias typically form when fluid

and gas-rich magmatic rocks intrude country rocks, causing

many fragments of the country rock to break off and be

incorporated into the magmatic rock. Igneous breccias may

also form through a process called stoping, where hot magma

causes angular fragments of the country rock to shatter off

and drop into the magma chamber. Volcanic breccias form by

several processes, including the slow movement of lava flows

that may be partly molten at depth and solid at the surface.

Movement of the molten lava causes the overlying lava to

fracture and break into many small angular pieces, forming a

volcanic breccia. Aa flows that form in basalts, and are common

in Hawaii, are one example of a volcanic breccia.

Another type of volcanic breccia is known as a hyaloclastite.

These are produced by the eruption of pillow lavas underwater.

When magma inside the pillows drains out, typically

forming lava tubes of new pillows budding off older pillows,

the older pillow may collapse inward (implode), forming a

breccia made of angular fragments of pillow lava. Subaerial

volcanic eruptions can also produce a variety of breccias,

including both fragments of cooled magma and fragments

ripped off of from older volcanic rocks.

Structural or tectonic breccias are produced along fault

zones. When a fault moves, the two adjacent sides grind

against each other and often break off pieces of the wall

rocks from either side of the fault. These get concentrated in

the fault zone, or in damage zones on either side of the main

fault zone. If deformation in the fault zone is strong, the breccia

may be further broken down into a finer-grained rock

called fault gouge, consisting of sand and clay-sized angular

to rounded clasts produced by breaking and grinding of the

rocks, a process called cataclasis.

See also FAULT; IGNEOUS ROCKS; SEDIMENTARY ROCKS;

STRUCTURAL GEOLOGY.

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