Sedimentary Rock

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Sedimentary rock covers 70% of the Earth. Erosion is constantly changing the face of the Earth. Weathering agents…wind, water, and ice…break rock into smaller pieces that flow down waterways until the settle to the bottom permanently. These sediments( pebbles, sand, clay, and gravel) pile up and for new layers. After hundred or thousands of years these rocks become pressed together to form sedimentary rock.

Sedimentary rock can form in two different ways. When layer after layer of sediment forms it puts pressure on the lower layers which then form into a solid piece of rock. The other way is called cementing. Certain minerals in the water interact to form a bond between rocks. This process is similar to making modern cement. Any animal carcasses or organisms that are caught in the layers of sediment will eventually turn into fossils. Sedimentary rock is the source of quite a few of our dinosaur findings.

There are four common types of sedimentary rock: sandstone, limestone, shale, and conglomerate. Each is formed in a different way from different materials. Sandstone is formed when grains of sand are pressed together. Sandstone may be the most common type of rock on the planet. Limestone is formed by the tiny pieces of shell that have been cemented together over the years. Conglomerate rock consists of sand and pebbles that have been cemented together. Shale forms under still waters like those found in bogs or swamps. The mud and clay at the bottom is pressed together to form it.

Sedimentary rock has the following general characteristics:

• it is classified by texture and composition
• it often contains fossils
• occasionally reacts with acid
• has layers that can be flat or curved
• it is usually composed of material that is cemented or pressed together
• a great variety of color
• particle size varies
• there are pores between pieces
• can have cross bedding, worm holes, mud cracks, and raindrop impressions

This is only meant to be a brief introduction to sedimentary rock. There are many more in depth articles and entire books that have been written on the subject. Here is a link to a very interesting introduction to rocks. Here on Universe Today there is a great article on how sedimentary rock show very old signs of life. Astronomy Cast has a good episode on the Earth’s formation.

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Sources:
http://en.wikipedia.org/wiki/Sedimentary_rock
http://geology.com/rocks/sedimentary-rocks.shtml
http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/sedimentary_rocks.html

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Tagged as: Earth science, sedimentary rock