Breccia Rock Geology and Uses

How It Forms

Like other clastic sedimentary rocks, breccia forms when other rock is subjected to weathering. The clasts are angular and irregular, indicating the particles forming the rock didn't travel far from their source. Other material fills in the spaces between the clasts, binding them into a rock. One way to categorize breccia is by its method of formation. For example:

• Some breccia forms as material that accumulates at the base of a steep slope or cliff.
• Cataclastic breccia is formed when fragments fall from a fault.
• Volcanic breccia, pyroclastic, or igneous breccia is formed from the compaction of lava chunks with ash.
• Collapse breccia is sedimentary breccia formed from the collapse of a cavern.
• Impact breccia is formed from a meteor impact breaking rock at the impact site.
• Hydrothermal breccia is formed when fluid fractures a rock.

The spaces between the clasts fill with silt (iron oxide), carbonate (e.g., calcite), or silica, eventually acting as the cement that binds the particles.

Sometimes, the deposition of clast and matrix material occurs at about the same time. Another class of breccia consists of rock in which the clasts and matrix are unrelated. For example, the collapse of a limestone cavern would produce both clasts and matrix material at once, while a mudslide over a fault would coat old clastic material with young matrix.

Another way to classify breccia is by the distribution of clasts and matrix. In matrix-supported breccia, clasts don't touch each other and matrix completely surrounds them. In clast-supported breccia, the matrix fills the void between touching (or nearly continuous) clasts.

What Is Breccia?

Breccia usually refers to rock of sedimentary origin, although it may also form from igneous or metamorphic rocks. A mixture of different rocks and minerals may combine. Thus, breccia composition and properties are highly variable. Usually, clasts consist of a hard, durable rock that can survive some degree of weathering. Sometimes, breccia is named to reference its composition. For example, there is sandstone breccia, basalt breccia, and chert breccia. Monomict breccia is breccia containing clasts of a single rock type. Polymict breccia or petromict breccia is breccia containing clasts of different rocks.

Properties

The identifying feature of breccia is that it consists of visible angular clasts cemented together with another mineral. The clasts should be easily visible to the naked eye. Otherwise, the properties of the rock are highly variable. It can occur in any color, and may be either hard or soft. The rock may be rough to the touch because of the angular clasts. Whether it polishes to a smooth surface depends on the similarity of clast and matrix composition.

Uses

Because of its variable composition, breccia has an interesting appearance. The rock is mainly used to make sculptures, gems, and architectural elements. The Minoan palace of Knossos on Crete, constructed around 1800 B.C., includes columns made of breccia. The ancient Egyptians used breccia to make statues. The Romans regarded breccia as a precious stone and used it to construct public buildings, columns, and walls. The Pantheon in Rome features columns made of pavonazzetto, a type of breccia with a pattern resembling peacock feathers. In modern culture, breccia is used for decorative elements, jewelry, and sometimes as a fill material for roads.

Breccia vs Conglomerate

Breccia and congomerate are similar to each other. Both are clastic sedimentary rocks containing clasts larger than two millimeters in diameter. The difference is that the clasts in breccia are angular, while those in conglomerate are rounded. This indicates the clasts in conglomerate traveled a greater distance from their source or experienced more weathering before becoming embedded in matrix than the clasts in breccia.

Key Points

• Breccia is a clastic sedimentary rock. The clasts are irregularly shaped particles greater than two millimeters in diameter. The cement binding the clasts is a matrix made of smaller particles.
• Breccia and conglomerate rock are similar. The clasts in breccia are angular, while the clasts in conglomerate rock are rounded.
• Breccia comes in many colors and compositions.
• Breccia is mainly used to make decorative architectural elements. It may be polished to make decorative features or gemstones. It can be used as a road base or fill.

Sources

• Jébrak, Michel. "Hydrothermal breccias in vein-type ore deposits: A review of mechanisms, morphology and size distribution." Ore Geology Reviews, Volume 12, Issue 3, ScienceDirect, December 1997.
• Mitcham, Thomas W. "Origin of breccia pipes." Economic Geology, Volume 69, Number 3, GeoScienceWorld, May 1, 1974.
• Sibson, Richard H. "Earthquake rupturing as a mineralizing agent in hydrothermal systems." Geology, ResearchGate, January 1987.