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Doc Brown's Chemistry - Earth Science & Geology Revision Notes

for KS4 Science, GCSE, IGCSE & O Level Courses

5. Sedimentary Rocks

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.


GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets


5. Sedimentary Rocks slowest to form, and weather the fastest!

doc b's Earth Science Notesdoc b's Earth Science Notes5(a) A sedimentary rock bed is formed from plant/animal remains or weathered and eroded particles from pre-existing rocks. These may be transported, usually by water (and 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. For those from 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 formed.

 top5(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 material.
  • topAbove is the pebble beach at Charmouth in southern England and the Jurassic fossil bearing shale cliffs in the distance (180-195 million years old). Fossils found include ammonites, nautilus, belemnites, crinoids, shells and bones etc.
  • View of Gordale Scar from above, not surprisingly, a major tourist attraction of the Yorkshire Dales.
  • topLimestone (above) is formed from the deposition of hard mineral remains of sea creatures and chemically is mainly calcium carbonate CaCO3. It 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.
  • 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
  • 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 sand dunes. 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 forests. The organic materials are buried, compressed and form clear sedimentary layers often showing well preserved fossils of leaves or tree trunks. 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 and become buried and compressed underground e.g.
    • Rock salt is mainly sodium chloride. 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.
    • 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' i.e. 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.

top5(d) Any rocks which are not eroded away, are eventually returned to the mantle when plates descend in tectonic activity - see later. 

doc b's Earth Science Notes 5(e) A potted history of fossils ... doc b's Earth Science Notes

  • Fossils 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 water.
  • 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 rocks 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 radioisotope studies.
  • 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.

top5(f)  Why are there no fossils in igneous rocks?

  • You would not expect fossils in igneous rocks because they are formed from molten mixed up magma. Even if a 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 but 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. However the fossils are likely to be distorted or destroyed by the heat and pressure factors involved in metamorphic rock formation.
  • 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 e.g.

  1. order of layers - the deeper the layer, the earlier the sedimentary rock was formed
  2. discontinuous deposition where different layers of different rocks are successively laid down at different times. (see Fig 9.2)
  3. a more recent (younger) rock layer might cut across an older layer.
  4. ripple marks can show the layer was formed from a sea-bed or river bank from waves or currents.
  5. 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.
  6. 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. (see Fig 9.2)
  7. fractures and fault lines provide evidence of earthquake activity. (see Fig 9.1)
  8. inverted layers (turned upside down!) provide evidence of massive plate movement and give geologists much food for thought on deducing the 'event sequence'!
  9. rock layers can be buried by these massive upheavals as well as burial by subsequent sedimentary rock formation.
  10. Points 5. to 9. are evidence for the crust being unstable and subjected to tremendous forces.

doc b's Earth Science Notes Fig 9.1

doc b's Earth Science Notes Fig 9.2

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GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets


Studying Revision for KS4 Earth Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Earth Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  WJEC gcse science chemistry CCEA/CEA gcse science chemistry (revise courses equal to US grade 8, grade 9 grade 10)

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