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The Evolution of Sedimentary Rocks

Sedimentary rocks begin to form when bits of other rocks in the form of sand and even finer particles get moved along by wind and moving water. Eventually the water slows down or stops moving altogether and these bits drop to the bottom and become mud. Animals and plants in the water die, or land animals get into the water and die there, and they become part of the mud. As mud continues to form, its weight puts pressure on the mud already there. Eventually, over long periods of time, the pressure of new layers slowly turns the bottom layer to rock.
  1. Lithification

    • Lithification is the process that turns sediment, or mud, into sedimentary rock. The smallest particles in the soft sediment get pushed closer together during compaction, the crystals in it grow and the spaces between the larger particles get filled in during a process called cementation. When all of the components lock together, sedimentary rock is formed.

    Stratification

    • Wind never stops blowing and water never stops flowing, but they do change course. Sedimentary rock is always forming, but not always in the same place. If sediment stops coming down for a time and then starts again, or if a different kind starts to arrive, layers form that may be a different texture or color. These layers are called strata, and are characteristic of sedimentary rock.

    Fossilization

    • Fossilization is the preservation of the remains of dead plants and animals. Hard parts, such as bones and shells, may be preserved by petrification. Impressions of soft parts or even traces of activity, such as footprints, may be preserved by infilling with different sediment, or the entire organism may be preserved by being frozen in ice or trapped in amber. Sedimentary rock is a primary source of fossils.

    Petrification

    • Petrification is the process of replacing organic matter, such as dinosaur bones and large pieces of wood, with minerals. When small plants die and drop into sediment, they form a layer that, over millions of years, turns into coal, a form of sedimentary rock. Sediment at the bottom of a body of water is an optimal place for this process to occur because free oxygen is excluded, the temperature is cooler, retarding bacterial decay, and predators have limited access to the remains. Water also tends to contain the abundant dissolved minerals necessary for petrification to occur.


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