Open Site - Science: Geology: Rocks and Minerals: Sedimentary

Top: Science: Geology: Rocks and Minerals: Sedimentary

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Overview

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface.

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Clastic

Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their grain size.

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Chemical

Chemical sedimentary rocks are formed by chemical precipitation. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals are redeposited when the water evaporates away or when the water becomes over- saturated.

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Biologic

Biologic sedimentary rocks form from once-living organisms. They may form from accumulated carbon-rich plant material or from deposits of animal shells.

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Rocks and Minerals

Name Description

Limestone Formed from compacted sea shells (microscopic animals falling to the sea floor). Formula is CaCO3. Can form by chemical deposition - stalagmite.

Sandstone Highly compressed particles of sand. Usually soft. From beach. Larger particles.

Silt stone Highly compressed particles of sand. Usually soft. From river. Smaller particles.

Mudstone Highly compressed particles of sand. Usually soft. From swamp. Very small particles.

Greywache Very old. Very hard - almost metamorphic.

Conglomerate Formed from different rock types cemented together. May have number of colours but commonly brown or grey. Often crumbly depending on condition.