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Sedimentary Rocks
Chapter 4 Section 4
Sedimentary Rocks
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Sediments are loose materials like rock fragments,
mineral grains, and bits of shell.
Sediments come from already-existing rocks that are
weathered and eroded.
Sedimentary rocks form when sediments are pressed
and cemented together, or when minerals form from
solutions.
Sedimentary rocks form as layers. The oldest layers are
at the bottom because they were deposited first.
-Sometimes the forces within the Earth overturn layers
of rock, and the oldest are no longer on the bottom.
Sedimentary Rocks
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Cover 75% of the surface are sedimentary
rocks!
Classifying Sedimentary Rocks.
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Sedimentary rocks are classified by their
composition and the manner in which they
are formed.
Sedimentary rocks are classified as detrital
(clastic), chemical, or organic (nonclastic).
Detrital Sedimentary Rocks
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Detrital, or clastic sedimentary rocks are made
from the broken fragments of other rocks. These
loose sediments are compacted and cemented
together to form solid rock.
The process that breaks rocks into smaller
pieces is called weathering,.
-Rocks are weathered by things such as air
water and ice.
-Sediments are classified by size. Clay is the
smallest sediment, followed by silt, then sand,
and then gravel, the largest sediment.
Detrital Sedimentary Rocks
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Erosion moves sediments to a new location where they
are deposited. Pressure from the upper layers pushes
down on the lower layers of sediment.
If the sediments are small, they can stick together to
form a solid rock. The process is called compaction.
If the sediments are large, pressure can’t make them
stick together. Cementation occurs when minerals like
quartz and calcite, are deposited by water between
pieces of sediment. The minerals act like glue and form
a detrital sedimentary rock.
Conglomerate, sandstone, siltstone, and shale are all
detrital sedimentary rocks named for the shapes and
sizes of sediments that formed them.
Examples of Detrital Sedimentary
Rocks
Compaction and Cementation
Chemical Sedimentary Rocks
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Chemical sedimentary rocks form when
dissolved minerals come out of solution.
Minerals collect when seas or lakes evaporate.
The deposits of minerals that come out of
solution form sediments and rock.
Gypsum and rock salt form when water that is
rich in dissolved salts evaporates. Halite forms
rock salt.
Chemical Sedimentary Rocks
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Calcium carbonate is carried in solution in ocean
water. When calcium carbonate comes out of
solution it forms calcite. Limestone is a chemical
sedimentary rock that is formed as crystals of
calcite grow together.
Large areas of the United States have limestone
bedrock because seas covered much of the
country throughout geologic history.
Chemical Sedimentary Rock
Example
Organic Sedimentary Rocks
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Organic sedimentary rocks are made of the
remains of once living things.
Fossil-rich limestone is made from the remains
of once living organisms.
-If a rock is made completely out of shell
fragments that you can see it is called coquina.
-Chalk is another organic sedimentary rock
made of microscope shells.
Organic Sedimentary Rocks
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Coal is an organic sedimentary rock that is
formed when pieces of dead plants are buried
under other sediments in swamps. The
sediments are compacted over millions of years
to form coal.
Chemical and organic sedimentary rocks are also
called nonclastic sedimentary rocks, because
they are not made up of broken fragments of
other rocks.
Organic Sedimentary Rock Example