Conglomerate Rock: Geology, Composition, Uses

Conglomerate or Nagelfluh, Isartal in Wallgau, Werdenfels, Upper Bavaria, Bavaria, Germany
Conglomerate or Nagelfluh, Isartal in Wallgau, Werdenfels, Upper Bavaria, Bavaria, Germany. Martin Siepmann / Getty Images

In geology, conglomerate refers to a coarse-grained sedimentary rock that resembles concrete. Conglomerate is considered a clastic rock because it contains an abundance of gravel-sized (greater than 2 mm diameter) pebbles called clasts. Sand, silt, or clay sediment, called matrix, fills the spaces between the clasts and cements them together

Conglomerate is relatively uncommon. In fact, geologists estimate only about one percent of all sedimentary rock is conglomerate.

How Conglomerate Forms

Over time, pebbles on a beach may form conglomerate rock.
Over time, pebbles on a beach may form conglomerate rock. Howard Pugh (Marais) / Getty Images

Conglomerate rock forms when gravel or even boulders are transported far enough from their original source to become rounded, or are subjected to wave action. Calcite, silica, or iron oxide fills in the spaces between the pebbles, cementing them together. Sometimes all the clasts in conglomerate are the same size, but usually there are smaller pebbles filling in part of the spaces between larger clasts.

Regions likely to produce conglomerate include beaches, riverbeds, and glaciers.

Classifying Conglomerates

The following characteristics are used to classify and categorize conglomerate rock:

  • The composition of the clasts. If all the clasts are the same type of rock or mineral), the rock is categorized as monomictic conglomerate. If the clasts are made up of two or more rocks or minerals, the rock is a polymictic conglomerate.
  • The size of the clasts. Rock comprised of large clasts is cobble conglomerate. If the clasts are pebble-sized, the rock is called pebble conglomerate. If the clasts are small granules, the rock is called granule conglomerate.
  • The amount and chemical composition of the matrix. If the clasts do not touch each other (lots of matrix), the rock is paraconglomerate. Rock in which the clasts touch each other is called orthoconglomerate.
  • The environment that deposited the material. Conglomerates may form from glacial, alluvial, fluvial, deepwater marine, or shallow marine environments.

Properties and Uses

The key characteristic of conglomerate is the presence of readily visible, rounded clasts bound within a matrix. The clasts tend to feel smooth to the touch, although the matrix can be either rough or smooth. The hardness and color of the rock is highly variable.

When the matrix is soft, conglomerate may be crushed for use as a fill material in the construction and transportation industries. Hard conglomerate may be cut and polished to make dimension stone for interesting-looking walls and floors.

Where to Find Conglomerate Rock

Santa Maria de Montserrat Abbey, Barcelona, Spain was built from conglomerate rock.
Santa Maria de Montserrat Abbey, Barcelona, Spain was built from conglomerate rock. Paul Biris / Getty Images

Conglomerate rock is found in areas where water once flowed or where glaciers were found, such as Death Valley National Park, the cliffs along the east coast of Scotland, the dome-shaped hills of Kata Tjuta in Australia, the underlying anthracite of the coal fields of Pennsylvania, and the base of the Sangre de Cristo mountains of Colorado. Sometimes the rock is strong enough to be used for construction. For example, The Santa Maria de Montserrat Abbey was constructed using conglomerate from Montserrat, near Barcelona, Spain.

Conglomerate Rock on Mars

Conglomerate rock on Mars (left) compared with conglomerate on Earth (right).
Conglomerate rock on Mars (left) compared with conglomerate on Earth (right). NASA Mars Curiosity Rover

Earth isn't the only place to find conglomerate rock. In 2012, NASA's Mars Curiosity Rover captured photographs of conglomerate rock and sandstone on the Martian surface. The presence of conglomerate is compelling evidence that Mars once had flowing water: the pebbles in the rock are rounded, indicating they were transported along a current and rubbed against one another. (Wind isn't strong enough to move pebbles this large.)

Conglomerate vs. Breccia

Conglomerate has rounded clasts, while breccia contains angular clasts.
Conglomerate has rounded clasts, while breccia contains angular clasts. Scientifica / Getty Images

Conglomerate and breccia are two closely related sedimentary rocks, but they differ significantly in the shape of their clasts. The clasts in conglomerate are rounded or at least partially rounded, whereas the clasts in breccia have sharp corners. Sometimes sedimentary rock contains a mixture of round and angular clasts. This type of rock may be called breccio-conglomerate.

Conglomerate Rock Key Takeaways

  • ​Conglomerate is a sedimentary rock that looks like concrete. It consists of large, rounded pebbles (clasts) cemented by a matrix made of calcite, iron oxide, or silica.
  • Conglomerate rock occurs where gravel can become rounded by traveling distances or being subjected to tumbling. Beaches, riverbeds, and glaciers can produce conglomerate.
  • The properties of conglomerate rock depend on its composition. It can be found in any color and may be either hard or soft.
  • Conglomerate can be used as a fill material for roads and construction. Hard rock may be cut and polished to make dimension stone.

 

Sources

  • Boggs, S. (2006) Principles of Sedimentology and Stratigraphy., 2nd ed. Printice Hall, New York. 662 pp. ISBN 0-13-154728-3.
  • Friedman, G.M. (2003) Classification of sediments and sedimentary rocks. In Gerard V. Middleton, ed., pp. 127-135, Encyclopedia of Sediments & Sedimentary Rocks, Encyclopedia of Earth Science Series. Kluwer Academic Publishers, Boston, Massachusetts. 821 pp. ISBN 978-1-4020-0872-6.
  • Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4.
  • Tucker, M.E. (2003) Sedimentary Rocks in the Field, 3rd ed. John Wiley & Sons Ltd,West Sussex, England. 234 pp. ISBN 0-470-85123-6.