Q:

What are organic sedimentary rocks?

A:

Organic sedimentary rocks are sedimentary rocks formed from the remains of organisms. The three types of organic sedimentary rock are limestone, chert and coal. Both limestone and chert are formed largely from the hard structures of organisms, while coal is formed from dead plants after a long period of exposure to the correct environmental conditions.

Bony fish, corals and certain planktonic algae form calcium-based structures that break down and settle to the ocean floor after their deaths. These build up under time and pressure, eventually solidifying into a solid mass known as limestone. Limestone is easily dissolved by acidic substances, even including rainwater, because of its calcium content. Marble is a metamorphic rock made from limestone exposed to extreme temperatures and pressures.

Another group of photosynthetic plankton, known as diatoms, as well as sponges create structures from silica. The sediments from diatoms are the source of diatomaceous earth, but when not gathered for this purpose time and pressure cause the formation of chert.

Coal is the only type of organic sedimentary rock that is not formed primarily in oceans. Instead, it form in shallow waters, particularly swamps, where plant debris is buried by sediment and exposed to various chemical processes, purifying the carbon component of the plants.

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Related Questions

  • Q:

    What are the characteristics of sedimentary rocks?

    A:

    Sedimentary rocks are characterised by their formation from the deposition and lithification of rock material. They may be formed through physical and chemical processes, such as mechanical weathering, compaction and dissolving of rock material.

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  • Q:

    Why can't most sedimentary rocks be dated radiometrically?

    A:

    According to the Smithsonian Institute, using radiodating of sedimentary rock tells the date of formation of the original igneous rock, which, through the processes of weathering and erosion, formed the layers of the sedimentary rock. Radiodating determines the maximum age of sedimentary rock. If a layer of igneous rock forms on top of the sedimentary rock, scientists determine an age bracket for the rock sample, but not an absolute age.

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    What is the single most characteristic feature of sedimentary rocks?

    A:

    Sedimentary rocks are primarily characterized by lithification. Lithification occurs when sediment has been deposited and accumulates at the earth's surface or in bodies of water to form a rock over time. Subgroups of sedimentary rocks are defined by their specific formation.

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    Why are rocks important?

    A:

    Rocks are important because geologists use evidence from them to learn about what the Earth was like in the past. They allow scientists to build a historical record of the planet to learn what events occurred before people lived.

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