Download Sedimentary Rock 3

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CHAPTER 6
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Rocks
SECTION
3 Sedimentary Rock
KEY IDEAS
As you read this section, keep these questions in mind:
• How do compaction and cementation happen?
• How do chemical and organic sedimentary rocks
form?
• How does clastic sedimentary rock form?
• What are seven features that sedimentary rock can
have?
What Are Compaction and Cementation?
Recall that sedimentary rock is made up of fragments
of other rocks, minerals, and organic matter. These fragments are called sediment. Most sedimentary rock forms
when sediment is laid down, or deposited, by water, wind,
or ice. Over time, two processes can change the sediment
into sedimentary rock: compaction and cementation.
After a layer of sediment is laid down, more sediment may bury it. The weight of the top layers of sediment presses down on the bottom layers. The pressure
squeezes water and air out of the spaces between the
sediment particles. The particles get closer together. The
sediment becomes smaller, or more compact. Therefore,
this process is called compaction.
Sometimes, water moves through sediment. The water
may have minerals, such as calcite and quartz, dissolved
in it. As the water moves through the sediment, those
minerals may crystallize between the sediment particles.
The minerals “glue,” or cement, the sediment particles
together. Therefore, this process is called cementation.
Pressure squeezes water
out of the sediment.
Compaction
10-20% water
READING TOOLBOX
Summarize Cause and
Effect As you read this
section, mark examples
of cause and effect or a
chain of events. Then, make
chain-of-events charts to
summarize each example.
READING CHECK
1. Describe What happens
during compaction?
LOOKING CLOSER
2. Compare How is
compaction different from
cementation?
50-60% water
Cementation
There are empty spaces
between sediment particles.
Water moves through Minerals crystallize
the empty spaces.
between the particles
and “glue” them together.
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Sedimentary Rock continued
What Are the Different Kinds of Sedimentary
Rock?
Scientists put sedimentary rocks into three different
groups: chemical sedimentary rocks, organic sedimentary
rocks, and clastic sedimentary rocks. The rocks in the
different groups form in different ways.
CHEMICAL SEDIMENTARY ROCK
READING CHECK
3. Explain What causes
chemical sedimentary rocks
to form?
Critical Thinking
4. Apply Concepts Some
kinds of limestone form
when calcium carbonate crystallizes from shallow ocean
water. Are these limestones
chemical sedimentary rocks
or organic sedimentary rocks?
The water in natural water bodies, such as lakes and
oceans, contains many dissolved minerals. As the water
evaporates, the minerals are left behind. As the minerals
crystallize, they form a type of sedimentary rock called
chemical sedimentary rock.
ORGANIC SEDIMENTARY ROCK
Remember that organic matter is matter that comes
from living things. Some kinds of sedimentary rock are
made from organic matter. These sedimentary rocks are
called organic sedimentary rocks.
Coal is one example of an organic sedimentary
rock. Coal forms from the remains of plants that live
in swamps. When the plants die, their remains fall to
the bottom of the swamp. Over time, they are buried.
Pressure changes the plant remains into coal, which is
made up mostly of the element carbon.
Some kinds of limestone are also organic sedimentary
rocks. These kinds of limestone form from the shells of
animals that live in the oceans. The animals use the compound calcium carbonate to make their shells. When the
animals die, their shells sink to the bottom of the ocean.
Over time, the shells are pressed together. They form the
rock limestone.
Formation of Organic Sedimentary Rock
Talk About It
Describe Discuss this figure
with a partner. Take turns
describing to each other
what is happening in each
part of the figure.
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Sedimentary Rock continued
CLASTIC SEDIMENTARY ROCK
The most common kind of sedimentary rock is clastic
sedimentary rock. Clastic sedimentary rock is made
up of pieces of rocks that have been carried from their
source and deposited. The rock pieces in a clastic sedimentary rock may be pieces of igneous, metamorphic,
or other sedimentary rocks.
Scientists classify clastic sedimentary rock based on
the sizes of the rock pieces in the rock. If most of the
rock pieces are more than 2 mm in diameter, the rock is
a conglomerate. If the rock pieces have sharp angles, the
rock is a breccia. If most of the rock pieces are the size
of sand grains, the rock is a sandstone. If most of the
rock pieces are very small, the rock is a shale.
No matter what size the particles in the rock are,
all clastic sedimentary rocks form the same way. First,
water, wind, or ice deposit the sediment in layers. As the
layers build up, the sediment at the bottom is compacted.
Over time, the sediment may be cemented together to
form clastic sedimentary rock.
What Are Some Characteristics of Sediment?
The sediment in clastic sedimentary rocks can have
different characteristics. The characteristics of the sediment can tell scientists about how the sediment formed
and how it was laid down. Two important characteristics
of sediment are sorting and angularity.
READING CHECK
5. Identify Where do the
sediments that form clastic
sedimentary rock come from?
Math Skills
6. Calculate The rate at
which sediment collects,
or accumulates, is called the
sedimentation rate. Suppose
the sedimentation rate for an
area is 1.5 mm per year. How
many years will it take for
10 cm of sediment to
accumulate?
SORTING
Some sediment is made up of particles that are all
about the same size. Other sediment contains particles
of many different sizes. The range of particles sizes in a
sediment determines how well sorted the sediment is.
LOOKING CLOSER
7. Compare What is the
main difference between
well-sorted sediment and
poorly sorted sediment?
Very
Well
well
sorted
sorted
Moderately
sorted
Poorly
sorted
Very
poorly
sorted
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Sedimentary Rock continued
WHAT SORTING INDICATES
READING CHECK
8. Identify What type of
water can drop poorly sorted
sediment?
Water and wind can carry sediment over long distances. However, the speed of the water or wind affects
the size of the sediment they can carry. Fast-moving
water can carry very large sediment pieces. Slow-moving
water can carry only small sediment pieces.
When fast-moving water slows suddenly, it may drop a
lot of sediment all at once. The sediment pieces it drops
will have a range of sizes. In other words, water that
slows suddenly can drop poorly sorted sediment.
However, if the water slows gradually, it will drop the
sediment over a larger distance. In other words, water
that slows gradually can drop well-sorted sediment.
Therefore, the sorting in a sediment can tell a scientist
how the sediment was deposited.
ANGULARITY
READING CHECK
9. Explain What causes a
sediment particle to become
smoother and rounder over
time?
Some sediment pieces are smooth and rounded.
Others are rough and have sharp angles. A sediment’s
angularity describes how smooth or rough the sediment
particles are.
As wind or water carry sediment over Earth’s surface,
the sediment particles hit one another. They may also hit
other objects, such as larger rocks. When the particles
hit other objects, they can break apart. They may also
become worn down and smooth. The longer the particle
is carried along, the smoother and rounder it becomes.
Therefore, the angularity of a sediment can tell a scientist
how far the sediment has traveled.
What Structures Can Sedimentary Rock Form?
The areas where sediment is laid down are called
depositional environments. Beaches, lakes, deltas, and
oceans are examples of depositional environments.
Sediment laid down in different depositional environments will form different features. Those features include
•
•
•
•
•
stratification
cross-beds and graded beds
ripple marks
mud cracks
fossils and concretions
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Sedimentary Rock continued
STRATIFICATION
Most sediment is laid down in layers. Sedimentary
rock that forms from layers of sediment generally
also has layers. Layering in sedimentary rock is called
stratification.
READING CHECK
10. Describe What is
stratification?
CROSS-BEDS AND GRADED BEDS
The layers in a sedimentary rock are called beds. Most
beds are parallel to one another. However, in some rocks,
the beds form angles with one another. Beds that are at
different angles are called cross-beds.
This photo shows sedimentary rock with cross-beds.
Talk About It
Discuss Use a dictionary to
look up the word root strat.
With a partner, discuss why
this word root is found in the
word that describes layering
in sedimentary rocks.
Most beds are made up of sediment particles that are
all about the same size. However, some beds contain
particles of different sizes. The particles in the bed are
sorted from top to bottom. A bed made up of particles
of different sizes that are sorted from top to bottom is a
graded bed.
RIPPLE MARKS AND MUD CRACKS
When water flows over sediment, it may move some
of the sediment to form ripples. Ripples are wavy lines
in sediment. Sedimentary rocks that form from sediment
that has ripples may show ripple marks.
Sedimentary rocks may also show mud cracks. Mud
cracks form when wet mud dries. As mud dries, it shrinks
and cracks. The cracks may then fill with sediment and
form sedimentary rock.
READING CHECK
11. Explain What causes
mud to crack as it dries?
FOSSILS AND CONCRETIONS
A fossil is a sign that a living thing once existed. The
hard parts of organisms can become fossils. Footprints
and other marks can also become fossils. Most fossils are
found in sedimentary rock.
Some sedimentary rocks contain lumps of rock that
are different from the rest of the rock. These lumps are
called concretions. They form when minerals crystallize
out of water that flows through the rock.
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Section 3 Review
SECTION VOCABULARY
cementation the process in which minerals
precipitate into pore spaces between sediment
grains and bind sediments together to form
rock
chemical sedimentary rock sedimentary rock
that forms when minerals precipitate from a
solution or settle from a suspension
clastic sedimentary rock sedimentary rock that
forms when fragments of preexisting rocks are
compacted or cemented together
compaction the process in which the volume
and porosity of a sediment is decreased by the
weight of overlying sediments as a result of
burial beneath other sediments
organic sedimentary rock sedimentary rock that
forms from the remains of plants or animals
1. Classify A scientist is studying a sedimentary rock that did not form through
compaction and cementation. What kind of sedimentary rock is the scientist
studying? Explain your answer.
2. Identify List the seven features that sedimentary rocks can have.
3. Infer A clastic sedimentary rock is made up of smooth, round sediment pieces
that are all about the same size. What can you infer about how the sediment that
formed the rock was transported and deposited? Explain your answer.
4. Synthesize Concepts Which structure are you more likely to find in an organic
sedimentary rock, mud cracks or fossils? Explain your answer.
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