Doc Brown's Chemistry - Earth Science & Geology Revision Notes
for KS4 Science, GCSE, IGCSE & O
INDEX OF EARTH SCIENCE PAGES
GCSE/IGCSE level chemistry revision
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5. SEDIMENTARY ROCKS - examples of types - formation and
A sedimentary rock is defined and what sedimentary
rock formations look like. The formation of the sedimentary rocks sandstone,
limestone, chalk, shale, mudstone, coal, salt deposits. The formation of fossils
is described and why they mainly occur in sedimentary rocks and their
usefulness to geological scientists studying the age of rocks and to biologists
and zoologists studying the evolution of plant and animal species.
Sedimentary Rocks slowest
to form, and weather the fastest!
rock bed is formed from plant/animal remains or weathered and eroded
particles from pre-existing rocks. The material is deposited in layers that will
eventually form the sedimentary rock. These may be transported, usually by water
(or wind in the case of
sand) and deposited to form sediments. These become buried under
later forming sediments and water or by major tectonic activity, and then become
subjected to compression as enormous pressures are created deep in the
crust from the weight of rocks or sediments above them. Over millions of
from the fragments of eroded pre-existing rocks, water is squeezed out and the particles cement
together with the help of dissolved salts and silica crystallising out. Other changes come
about depending on the type of material from which the sedimentary rock is
5(b) Types of sedimentary rock
- Shale and Mudstone is formed from relatively fine
grained weathered rock material transported into seas and lakes before
settling out as clay or mud sediment. It then becomes compressed under the
weight of water and other sediments and the water is squeezed out and the
particles cement together. These rocks are clearly layered and crumble
easily. Shale can contain significant amounts of oil-like organic
Above is the pebble beach at Charmouth
in southern England and the
Jurassic fossil bearing shale cliffs in the distance (180-195 million years
Fossils found include ammonites, nautilus, belemnites, crinoids, shells
and bones etc. Shale cliffs are structurally weak and collapses of the
cliff faces are regular, but they often reveal lots of good fossil
Limestone (above) is formed from the deposition of hard
mineral remains of sea creatures and chemically is mainly calcium
carbonate CaCO3. This sedimentary rock mineral contains the
'shelly' remains of marine organisms, including coral, that once lived
in warm shallow fertile seas. Limestone is grey-white in colour and contains fossils and sand grains. The
'shelly' remains, including coral, get buried and compressed and cemented
together by the weight of water and other sediments. Limestone tends to form
beneath warm shallow seas rich in plant and animal life.
- The picture above
shows Gordale Scar in the Yorkshire Dales in the Pennines of the North of
England. The scar was gouged out of the easily eroded limestone rock.
Because limestone has not been subjected to high temperatures it often
contains many good fossils. Limestone is relatively easily weathered and
dissolves in acid rain so after a few thousand years deep valleys are
formed like the one illustrated in the photograph. Medieval limestone
buildings have suffered grievously at the hands of acid rain from the
fossil fuel combustion smoke of the industrial revolution.
- Chalk cliffs (above) are formed from the
mineral remains of tiny marine organisms in the sea and is chemically relatively pure calcium
carbonate and it contains microscopic fossils readily seen under a
microscope. The picture above shows the chalk White Cliffs of Dover. Like
limestone, chalk is quite easily eroded over thousands of years by wind
rain, and waves if the cliffs are by the sea.
- Sandstone is formed from weathered particles of
igneous rock and these particles mainly consist of colourless silica
(silicon dioxide, SiO2).
- The rock particles are laid down in
lakes, estuaries or seas from water transportation or wind blown to form
- The layers of sand get buried and compressed and the particles
get cemented together by other minerals including iron oxides which give
sandstone its distinctive orange or red colour.
- Coal is formed from the decayed (without
oxygen) remains of plant materials e.g. giant ferns and trees from hot swampy
- The organic materials are buried, heated and compressed and form clear
sedimentary layers often showing well preserved fossils of leaves or tree
- You can think of coal as fossilised layers
of plant material with high percentage of carbon content formed under high
temperature and high pressure conditions.
- The deeper and older the layer, the more carbonised is the coal
(anthracite is almost completely the element carbon).
- Salt deposits
: These are formed from the
evaporation of ancient seas or lakes leaving huge salt deposits which become buried and compressed
underground by later sedimentation above them e.g.
- Rock salt is mainly sodium chloride
and occurs in large deposits a few hundred metres underground in the UK
(e.g. Cheshire and Teesside Co. Durham) and other countries like Germany.
- It can be
mined as a solid or extracted as a concentrated solution.
- It is used for
food preparation, de-icing roads or to make chlorine etc. via the
process of electrolysis.
- See the
Group 7 Halogens and Use of Salt
- Potash contains potassium chloride, sodium
chloride and magnesium sulphate and used in fertiliser manufacture.
5(c) Since limestone is
mainly calcium carbonate CaCO3, and a simple test is to add acid
- should giving fizzing of a colourless gas that turns limewater 'milky'
carbon dioxide CO2 is formed. Heating limestone to a high
temperature in a limekiln produces calcium oxide (quicklime, a strong
alkali). Lime is used in agriculture to treat fields which are too acidic
for healthy crop growth. Limestone is used as building stone and in the
manufacture of glass and concrete.
Any rocks which are
not eroded away, are eventually returned to the mantle when plates descend in
tectonic activity - see later.
5(e) A potted history of
are formed by
plants and animals becoming trapped in deposits or sediments and then buried
by other layers and compressed as the sedimentary rock forms.
- This is why the vast majority of fossils are
found in sedimentary rock layers.
- Fossils are the remains of or 'imprints' of
dead organisms (plants or animals).
- In most cases
the original organic material is replaced by other minerals but this leaves
the trace and structure of the original plant or animal.
- Fossils can give information on the age of
the sedimentary rock they were formed in and the ambient conditions under
which the rock was formed e.g. was it formed under water with 'tell-tail'
fossil fish, shells.
- Even ripples in the water from rivers or
seas can be detected if the sand becomes covered with other material and
then the layers harden.
- The sediment that forms rocks like sandstone
will be formed from wind-blown sand grains (forming surface layers
initially, like great sand dunes) or water-borne sand grains (underwater
deposition) will form sandstone underwater.
- By examining the shapes of sand grains in the
sandstone you can determine whether it was formed under water or out of
- In undisturbed
sedimentary layers the lower the layer the older the layer, so the
geological sequence of formation can be worked out.
- Why are fossils so useful as well as
interesting to scientific studies?
- Fossils are useful to geological scientists
studying the age of rocks and to biologists and zoologists studying the
evolution of plant and animal species.
- Whole sequences of fossils can tell you
about the evolution of the same species, new species, extinctions etc.
- Fossils allow us to date the age of the
from the species present and also the sort of 'environment' present at
the time of fossil formation e.g. the climate and the nature of the land. The
older the fossil, the older the rock!
- Note: Fossil dating is NOT
absolute and accurate dating can only be obtained from
- The fossil record provides powerful
evidence for species evolution as the development of individual species can be
followed and their divergence into other later species.
- Fossils 'emerge' when the sedimentary rocks
in which they lie in are eroded away. The original harder parts of the organism tend
to be better preserved e.g. shell, bone, coral or bark etc. They
then require careful extraction from the surrounding rock or mud material.
5(f) Why are there no fossils in igneous rocks?
- You would not expect fossils in igneous
because they are formed from molten mixed up magma.
- Any organism, plant or animal falling into
magma, would be totally destroyed.
- Even if a
pre-existing sedimentary rock had fossils in it, they would be destroyed if the rock was
re-melted e.g. in a subduction zone - see plate tectonics later.
- Fossils are rare in metamorphic rock
their trace can sometimes be preserved in e.g. slate, despite the effects of
heat and pressure involved in their formation (see 6.).
- It is not impossible
for the 'traces' of fossils in sedimentary rock to be preserved through the
re-crystallisation process in metamorphic rock formation.
- However the fossils are likely to be distorted
or destroyed by the heat and pressure factors involved in metamorphic rock
- Why are fossils so useful as well as
interesting to scientific studies?
- The formation of fossils is described and why
they mainly occur in sedimentary rocks and useful to geological
scientists studying the age of rocks and to biologists and zoologists
studying the evolution of plant and animal species.
5(g) At the surface of the Earth younger sedimentary rocks
usually lie on the top of older rocks. All sorts of
features found in sedimentary rock formations allow scientists to work out their
origin and what has happened to them over long time periods of time see Fig
9.2/Fig 7. e.g.
- order of layers
- the deeper the
layer, the earlier the sedimentary rock was formed
- discontinuous deposition where
different layers of different rocks are successively laid down at different
- a more recent (younger) rock layer might cut across an
- ripple marks can show the layer was
formed from a sea-bed or river bank from waves or currents.
- tilting of rock formations can show very large scale
movement and the angle can be followed over a large distance to show
the relationship between distant rock formations.
- folding shows the compression of
layers due to plate movement, a curve down is called a
syncline, a curve in
an arc upwards is called an anticline.
and fault lines provide evidence of
- inverted layers (turned upside down!)
provide evidence of massive plate movement and give geologists much food
for thought on deducing the 'event sequence'!
- rock layers can be buried by these
massive upheavals as well as burial by subsequent sedimentary rock
- Points 5. to 9. are evidence for the crust being
unstable and subjected to tremendous forces (see Fig 9.1/Fig 2).
Fig 9.2/Fig 7 (above) and Fig 9.1/Fig 2