SEDIMENTARY ROCKS
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Sedimentary rock covers most of the Earth’s crust surface
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Sedimentary rocks form through the compacting and cementing of layers
of sediment
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Sedimentary rocks are classified by three basic formation processes
Clastic Rocks
 These rocks are formed from fragments of other rock
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The fragments come from the weathering of igneous, metamorphic, and
sedimentary rocks
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The formation of clastic rock is as follows:
1. A river moves sediment into a lake (fragments are smooth and
rounded from river/stream)
2. Particles are sorted by size as the river slows and flows into an
ocean or lake. The first to be deposited are the largest gravels,
followed by sands, and then silts and clays.
3. Over time, the sediments are buried and compacted. The particles
may become cemented together.
4. Conglomerates form from the larger particles, sandstones from
sand, and shale from silt and clay.
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Loose sediments become clastic rock when particles of sediments become
cemented by silica, calcite, iron oxide, or clay minerals.
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When minerals fill the spaces between sediment particles, they bind the
fragments together in a process called cementation
Chemical Rock
 The water in seas, lakes, swamps, and underground reservoirs often
contain dissolved minerals
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Chemical sediments form when these minerals precipitate
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Precipitation can occur through evaporation or through chemical action
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Common chemical sedimentary rocks are rock salt, rock gypsum, and some
limestones
Organic Rock
 These rocks form from sediments consisting of the remains of plants and
animals
 The most common organic sedimentary rocks are coal and limestone
 These limestones form through the following process:
1. Water dissolves calcite out of rocks on land and carriers it in the
form of calcium ions to a lake or ocean.
2. Certain organisms use the ions to produce calcium carbonate shells
(clam, corals).
3. When these organisms die their calcium-rich remains pile up on the
ocean floor.
4. The fragments of shells are cemented into limestone.