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Conglomerate
What is Conglomerate?
Conglomerate is a clastic sedimentary rock that contains large (greater than two millimeters in diameter) rounded clasts. The space between the clasts is generally filled with smaller particles and/or a chemical cement that binds the rock together.
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Conglomerate: The specimen shown is about two inches (five centimeters) across. It is made up of chert and limestone clasts bound in a matrix of sand and clay.
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What is the Composition of Conglomerate?
Conglomerate can have a variety of compositions. As a clastic sedimentary rock it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz or they can be sedimentary, metamorphic or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud and chemical cement.
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Conglomerate Close-Up: A detailed view of conglomerate showing the pebble-size clasts with sand and smaller size particles filling the spaces between them. The largest pebbles in this view are about ten millimeters across. Image by the United States Geological Survey.
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How Does Conglomerate Form?
Conglomerate forms where a sediment of rounded clasts at least two millimeters in diameter accumulates. It takes a strong water current to transport particles this large. So the environment of deposition might be along a swiftly flowing stream or a beach with strong waves. There must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveal that they were tumbled by running water or moving waves.
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Conglomerate-Forming Environment: A beach where strong waves have deposited rounded, cobble-size rocks. If buried and lithified these materials might be transformed into a conglomerate. Image copyright by iStockPhoto and Jason van der Valk.
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Conglomerates often begin by being deposited as a sediment consisting mainly of small clasts as shown in the photo below. The finer size sand and clay which fill the spaces between the larger clasts is often deposited later on top of the large clasts and then sifts down between them to fill the interstitial spaces. .
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Conglomerate-Size Sediment Clasts: Pebble-size clasts of many compositions deposited together on a beach. Quartz, sandstone and limestone clasts are all easily recognizable. Largest clast is about two inches (five centimeters) across. Image copyright by iStockPhoto and Ivan Ivanov.
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What is Conglomerate Used For?
Conglomerate has very few commercial uses. Its inability to break cleanly makes it a poor candidate for dimension stone and its variable composition makes it a rock of unreliable physical strength and durability. Conglomerate can be crushed to make a fine aggregate that can be used where a low-performance material is suitable. Many conglomerates are colorful and attractive rocks that are rarely used as an ornamental stone for interior use.
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Jasper Conglomerate: A conglomerate composed mainly of chert pebbles, some of which are red in color and known as "jasper". This specimen is about three inches (7.5 centimeters) across.
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Analysis of conglomerate can sometimes be used as a prospecting tool. For example: most diamond deposits are hosted in kimberlite. If a conglomerate contains clasts of kimberlite then the source of that kimberlite must be somewhere upstream.
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The rock photos above were taken by Ann Bryant and are copyright by Geology.com. |
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"A Grain of Sand": A gallery of sand grains from around the world as photographed through a microscope by Dr. Gary Greenberg. |
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Igneous Rocks: Photos and facts about intrusive and extrusive igneous rocks. |
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