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The Geosphere
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3.2.
Sedimentary rocks
Before starting with sedimentary rocks, clarify the definition of
sediments
,
sedimentary basins
and
diagenesis
.
Ask students
How do sediments transform into sedimentary
rocks?
Explain the origin of sediments from the erosion of rocks
by geological agents, and the posterior transportation and
sedimentation. The illustration on page 56 will visually help
students to understand this process. Once they understand the
formation of sedimentary rocks, you can carry on explaining the
processes that could happen in the diagenesis: the compaction
and the cementation.
Reading comprehension:
THE ROCK OF AGES
This text is about how slow the geological processes happen. Stu-
dents will realise how much implicated are human beings in these
processes since millions of years while the sand transformation in
rock is still happening.
3.2.1.
Formation of sedimentary rocks
Ask students to read section 3.2.1 and then close their books.
Read out the process for either compaction or cementation and
ask them to stand up if you defined one or the other.
Students could now do questions 21 and 22. For question 21, ask
students to work on this problem in pairs. For question 22, they
should re-read section 3.2.1 first and then answer the question.
3.2.2.
The classification of sedimentary rocks
Tell students to read the text and to look at the different rocks.
Ask them:
What is the difference between detrital and non-detrital
sedimentary rocks?
When talking about detrital sedimentary rocks,
highlight the peculiarities of coal and petroleum as combustible
fossils.
These sedimentary rocks originate from the remains of living
things that have not decomposed. They can burn and produce
energy.
Coal could be from different types according to their calorific value.
From the least to themost calorific, they are classified in peat, lignite,
coal and anthracite. The calorific value depends on the quantity of
coal: more coal, more powerful.
Answer key
Analyse
21.
2.5 cm of sediments are accumulated every 10 years in
a sedimentary basin. After the process of compaction,
will the volume of sediment be thicker or thinner?
Explain your answer.
The thickness will be 0.25 cm every 10 years.
22.
Explain why water is important in the formation of
sedimentary rocks.
Because sediments, before transforming into sedimentary
rocks, must lose water by compaction or cementation.
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3. Thegeosphere
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3.2.
Sedimentary rocks
Rocks found on the Earth’s surface are changed by the action of the wind, sea,
rivers, rain or ice. The elements wear down rocks in a process of erosion. The rock
fragments are deposited in the lowest areas of the Earth’s surface.
3.2.1.
Formation of sedimentary rocks
Sediments deposited in sedimentary basins (1) often follow two essential
processes for the formation of sedimentary rocks:
❚
Compaction
(2) is when sediments lose volume. Due to the weight
of the sediments on top, water is lost and the sediments become
compacted.
❚
Cementation
(3) takes place after the water is lost and the salts form
crystals. The crystals act like cement and stick the sediments together
to form sedimentary rocks.
3.2.2.
Classification of sedimentary rocks
These rocks are classified by looking at the origin of the sediments that compose
them. Sedimentary rocks can be detrital or non-detrital.
❚
Detrital sedimentary rocks
are made up of other rocks. Depending on the
size of the fragments or grains that compose them, we can distinguish various
types.
❚
Non-detrital sedimentary rocks
are not made up of fragments of other rocks.
Theyaremadeupof sediments from skeletonsofmarine lifeormineral salts.They
can also come from the remains of living things that have not decomposed.
❚
Sediments
are fragments of rock and organic material that have been
transported and deposited by water and wind.
❚
The places where sediments are deposited are called
sedimentary basins
.
The process in which sediments transform into sedimentary rocks is
called
diagenesis
or
lithification
.
Conglomerates
contain
grains that are more
than 2 mm big, called
clasts. Clasts are joined
together by smaller
grains.
Sandstone
contains
grains smaller than
2 mm that are easily
visible. It feels rough to
the touch.
Clay
is made up of very
small grains that can
only be observed with
a magnifying glass or a
microscope. It is soft to
the touch.
Limestone
can contain
fossil remains or be composed
of mineral salts. It produces
effervescence in contact
with acids.
Gypsum
is a rock composed
of the mineral of the same
name, gypsum. It originates
from the salts of water
evaporation in shallow
2
lakes
and seas.
Coal
forms from the
accumulation and
decomposition of plant
remains over millions
of years.
Petroleum
also results
from the transformation
of organic remains. It is
considered a rock even
though it is not in a solid, but
liquid state.
Sedimentarybasins: theoriginof sedimentary rocks
Formationof sedimentary rock
Coal and petroleum originate from the remains of living things that have not
decomposed. They can burn and produce energy. This is why they are called
combustiblefossils
.Coaloriginatesfromplantremainsthatcouldnotdecompose
completely because they were buried in sediments and sand. Petroleum is the
result of the transformation of remains of marine microorganisms buried in the
ocean floor.
Analyse
21.
2.5 cm of sediments are accumulated every 10 years in a
sedimentary basin. After the process of compaction, will
the volume of sediment be thicker or thinner? Explain your
answer.
22.
Explainwhywater is important intheformationofsedimentary
rocks.
Analyse
23.
Canyouexplainwhy limestone,whichcontains remainsof living things,
cannot be burned like coal or petroleum?
2
shallow:
not deep
