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Earth Science 3.3
Sedimentary Rocks
Sedimentary Rocks
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Sedimentary Rocks begin to form
when existing rocks are broken
down into sediments.
These sediments, which consists
of mainly of weathered rock
debris, are often transported to
other places by wind or water, ice
or gravity.
When these sediments are
dropped, they eventually become
compacted and harden to form
sedimentary rock.
Formation of Sedimentary Rocks
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Sedimentary rocks form when
solids settle out of a fluid water
or air.
Some sediments are dropped by
moving water, some when winds
blow them from one location to
another.
The sandstone arch at right has
been eroded by wind over
centuries.
Weathering
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Recall that weathering is any
process that breaks rocks into
sediments.
Weathering is often the first step
in the formation of sedimentary
rocks.
Chemical weathering takes place
when minerals in rocks change into
new substances.
Weathering
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Weathering also takes place
when physical forces; wind and
water, breaks rocks into smaller
pieces.
Weathered sediments don’t
usually stay in one place.
Instead wind, water, ice or
gravity carries away the
fragments to new locations.
Erosion
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Erosion involves weathering and the
removal of rock.
When an agent of weathering; water
wind, ice or gravity, looses energy and
slows down it’s movement, it drops the
sediments.
This process, the dropping of the
sediments, is called deposition.
The Mississippi Delta was formed by
deposition as the Mississippi River slows
down as it arrives at the ocean.
Deposition
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Deposition is the process by which an
agent of erosion loses energy and drops
the sediment it is carrying.
When this happens, sediments are
deposited according to size and weight.
The larger sediments, such as rounded
pebbles, are deposited first.
Deposition
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Smaller sediments, like grains of sand,
are carried farther and deposited last.
Some sediments are so small they are
carried great distances before being
deposited.
The Mississippi Delta was formed from
all the materials transported by the
waters of the river carrying sediments
downstream.
Compaction and Cementation
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After sediments are deposited,
they often become lithified, or
turned to rock.
Two processes both contribute
to this process of layers of
sediment hardening (lithifying)
into rock:
○ compaction
○ cementation
Compaction
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Compaction, is a process that
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Compaction is caused by the
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squeezes or compacts sediment.
weight of sediments.
During compaction , much of the
water in the sediment is driven
out.
Cementation
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Cementation takes place when
dissolved minerals are deposited
in the tiny spaces among the
sediments.
Rocks formed by cementation can
have mixed appearances.
Cementation
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The conglomerate at right is
formed when larger rocks are
cemented together by finer
material.
The sandstone at right has a
smooth fine grain as it is the
result of layers of fine sanding
compacting over centuries.
Classification of Sedimentary Rock
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Just like igneous rocks, sedimentary
rocks can be classified into two main
groups depending on their formation
 Clastic Sedimentary
 Chemical Sedimentary
Clastic Sedimentary Rocks
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The most common materials found in clastic
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Clastic sedimentary rocks can be grouped
sedimentary rocks are clay and quartz.
“according to the size of the sediments in the
rocks”.
 When rounded, gravel sized or larger
rocks make up the rock, the rock is said
to be a conglomerate.
 When particles are angular the rock is
called breccia, and if the rock is fine
sand-sized grains it is called
sandstone.
Chemical and Biochemical Sedimentary Rock
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Chemical sedimentary rocks form
when substances dissolved in water
precipitate, or separate, from water
solutions.
This precipitation usually happens
when water evaporates or boils
away, leaving a solid product.
Examples of this type of rock
includes
 some limestones,
 rock salt,
 chert,
 flint,
 and rock gypsum
Biochemical Sediments
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About 90 % of limestones are formed
from biochemical sediments
Such sediments are the shells and
skeletal remains of organisms that
settle to the ocean floor.
In coquina, you can see the shells
cemented together.
Another biochemical rock is chalk, the
material we use to write on blackboards.
To the right you see the famous White
Cliffs of Dover, comprised of mainly
white chalk.
Features of Sedimentary Rocks
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Many unique features of
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Each layer records a period of
sedimentary deposit.
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sedimentary rocks are clues to
 how,
 when,
 and where the rocks formed.
The newest layers are on the bottom
and the oldest are near the top.
Features of Sedimentary Rocks
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Ripple marks or mud cracks found
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Fossils are sometimes found in
in a layer indicate that water
existed in this area at a certain
time period; that it may have been
an ocean or lake at one time past.
these layers and the age of the
fossil can be determined in part by
the period of formation of the rock
layer the fossil is found within.
Summery
In summary, our Key Points are:
○ Sedimentary rocks are rocks that
form because of 4 major processes.
 Weathering produces particles
called sediments.
 Wind, water, ice and gravity
erode and deposit these
sediments.
 Over time, the sediments are
compacted and cemented to form
rocks.
 Sedimentary Rocks can be
classified according to how they
form.