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3. The geosphere
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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.
They are made up of sediments from skeletons of marine life or mineral salts. They
can also come from the remains of living things that have not decomposed.
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.
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
.Coal originates fromplant remains that couldnotdecompose
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
23.
Can you explain why limestone, which contains remains of living things,
cannot be burned like coal or petroleum?
2
shallow
:
not deep
