Q:

Why are fossils only found in sedimentary rocks?

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Quick Answer

Fossils are primarily found in sedimentary rocks because these rocks form at low temperatures and pressures. Igneous rocks form at temperatures and pressures that are high enough to destroy any organic remains. On rare occasions, fossils are found in metamorphic rocks, but they are deformed by the heat and pressure.

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Why are fossils only found in sedimentary rocks?
Credit: Laurence Ireland CC-BY 2.0

Full Answer

The majority of fossils are found in layers of sedimentary rock formed by silt or mud. When an organism dies in an area where its body is protected from scavengers and the elements, its remains are often buried by sediment. Over time, as the sediment is compressed into rock, the remains of the organism decay and dissolve. Minerals seep into the bones and change their chemical makeup. In other cases, the skeleton disintegrates entirely, leaving only a cast that is filled with minerals.

Igneous rocks form from magma at temperatures that are too high for any organic remains to survive. Any organism that is caught in magma is disintegrated and cannot fossilize. Metamorphic rocks are formed from sedimentary or igneous rocks that are heated and subjected to high pressure. If these sedimentary rocks contain fossils, these may survive the metamorphic process. However, the fossils are severely deformed and distorted by the pressure and heat.

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Related Questions

  • Q:

    What are metamorphic rocks?

    A:

    Metamorphic rocks are formed when the properties of existing igneous or sedimentary rocks are altered by contact with heat or pressure. They are typically found deep underground where the conditions necessary for their formation exist.

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  • Q:

    How do fossils form?

    A:

    Fossils form when a dead animal or plant is covered by sediment. Eventually, the organic matter in the bone or plant, such as the blood vessels and tissue, slowly turns into rock, which becomes known as a fossil.

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  • Q:

    What causes metamorphic rocks to form?

    A:

    Metamorphic rocks form when igneous or sedimentary rocks are subjected to extreme heat, pressure or chemical reactions. These forces alter the composition of the rocks, leading to changes in the rocks' density, appearance and structure.

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  • Q:

    What do fossils communicate about the past?

    A:

    Fossils communicate what organisms were alive during which period of time, as well as their shape and size. Fossils are well-preserved remains of animals and organisms where the bone has been transformed into a rock-like structure. Fossils can be examined, and by using dating techniques, paleontologists can determine the time period in which the organism was alive.

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