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Video:What Are Sedimentary Rocks?

with Jen D'Amore

Sedimentary Rocks are the easiest and most accessible ways to access the Earth's history. Here's an overview of the origin of clastic, organic, and chemical sedimentary rocks and the minerals that compose them.See Transcript

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Transcript:What Are Sedimentary Rocks?

Hi, I'm Jen D'Amore for About.com, and this video is all about sedimentary rock.

What is a Sedimentary Rock?

Sedimentary rock is the second great rock class. These are formed when minerals settle and, through pressure or increase in temperature, form rock. They are usually born under water and at the earth's surface. There are three types of sedimentary rocks: Clastic, organic, and chemical.

How are Clastic Sedimentary Rocks Formed?

When particles, or "clasts," of minerals, stones, shells, and other objects are moved by wind or water and find a new place to settle and eventually form into rock, they are considered to be "clastic sedimentary" rocks. Rivers carry mud and sand to the sea. As the quartz in sand and clay minerals in mud settle and get packed together, over time with heat just over boiling point, they are cemented into rock. If they cement quickly enough around a dead organism, before that organism completely decays, an imprint, or "fossil" is left in the rock.

How are Organic Sedimentary Rocks Formed?

Organic sedimentary rock is formed when the particles are moved not by water or wind, but by microscopic organisms. Plankton use dissolved calcium carbonate or silica to build their shells. The shells of expired plankton collect on the sea floor in thick layers, where the calcium carbonate turns into limestone and the silica turns to chert. Plant material can also build up into thick layers, and be compacted into peat, which eventually turns into coal. The coal we use today was formed during past ages when the conditions were favorable for enormous coal swamps.

How are Chemical Sedimentary Rocks Formed?

Chemical Sedimentary rocks are formed when minerals precipitate or evaporate from a solution. For example, as a body of water begins to dry up, the solution becomes more concentrated, and precipitation of minerals follows. Calcite comes out creating limestone or dolomites, then gypsum precipitates, forming gypsum rock, and then halite forms rock salt. Another way chemical sedimentary rock can be formed is if different fluids circulate and interact at the chemical level.

Sedimentary Rocks Help to Piece Together the History of the Earth

While all rocks can explain pieces of geologic history, the striations and fossils of sedimentary rock are the easiest and most accessible ways to access that history.

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