Conglomerate: Definition, Uses & Facts

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  • 0:02 Conglomerate
  • 1:16 Formation
  • 2:23 Uses
  • 3:05 Interesting Facts
  • 4:08 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about conglomerate, a sedimentary rock formed from large, rounded rock fragments in a matrix of fine grained sediments. While not as widely useful as other rocks, conglomerate does have some applications in the construction industry.

Definition of a Conglomerate

So, what is a conglomerate? No, it's not a multinational collection of businesses, but rather an internationally distributed type of rock. Conglomerate is a type of sedimentary rock that is composed of large, rounded rock fragments contained within a smaller grained matrix of sediments. The large fragments, known as clasts, can come in a range of sizes, but all must be greater than two milimeters in diameter in order for the rock to be classified as a conglomerate.

They look very similar in appearance and structure to another type of sedimentary rock known as a breccia, but the key difference between the two is that conglomerates contain rounded rock fragments, while breccias contain angular and jagged rock fragments. This makes telling the difference between the two in samples quite easy. If you notice in the picture here that conglomerates look very similar to older concrete aggregate driveways and sidewalks, that would be because, structurally, they are similar in that both contain larger rock fragments (the aggregate in the concrete and the rounded pebbles in conglomerate) set within a fine grained matrix of sedimentary particles that act as the glue that holds the rock—and the driveway—together. As we will see later, conglomerate actually plays a role in some concrete materials.

Formation of a Conglomerate

Conglomerate can form all over the world, as long as the geologic setting is correct. So, what's required to turn random pebbles and small sand or silt grains into a conglomerate? It has to take place in an aquatic environment, where a combination of strong water flow and a variety of sediment and clast sizes are available. The strong current allows for both the rounding of the larger clasts, typically at the base of a river or along coastline, and also for the breakdown of the smaller clasts (the sand, silt and clay-sized particles that form the matrix). The presence of water is also extremely helpful because it helps turn the smaller particles into a concrete-like slurry, which can then later solidify into the solid, rocky matrix that is tough enough to hold the larger clasts together.

Because conglomerates can form in a wide range of locations—provided strong water currents are available—the composition of the rock clasts can be highly variable, ranging from glacial quartz pebbles to shale to large chunks of minerals. The fine-grained matrix is most frequently composed of tiny bits of silica-rich sand, calcite, or occasionally metallic minerals.

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