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

for KS4 Science, GCSE, IGCSE & O Level Courses

7. The Structure of the Earth - three layers

The structure of the Earth is described in terms of the three layers - core (inner and outer), mantle and the crust and comments on the dating of rocks.

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

7. The Structure of the Earth - A sort of egg?

7(a-b) The three layered structure of the Earth.

X is the crust: is the relatively thin and cool outer layer of the Earth. The thickness ranges from 6 to 40km surrounded by the atmosphere of air W. It is much cooler, harder, brittle and less dense than the other layers. The crust is divided into sections or tectonic 'plates' which 'float' and move on the mantle. It should also be noted that ²/₃rds of the surface of the Earth is water.

Y is the mantle: is very hot rock material, it is almost solid but the 'plastic' rock can move very slowly as huge convection currents driven by the heat from radioactive decay in the core. It is these convection currents which move the 'plates'. The mantle's 'thickness' is 3000 km and its temperature is usually over 1000^oC. It consists mainly of non-metallic silicates with some metal ions. Magma is heated molten rock, from the more 'runny' mantle material and comes up to the surface in volcanic activity or igneous intrusions. The mantle has a higher density and a different chemical composition compared to the crust. It is relatively cold and rigid just below the crust, but lower down it is much hotter and non-rigid and so is able to flow. Heat is generated by radioactive decay of longer lived isotopes and it is this heat that drives the convection currents in the mantle, which ultimately moves the tectonic plates of the crust which 'float' above the mantle.

Technical note: Most of the heat (~90%) generated in the Earth's interior is fuelled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 present in the mantle. These isotopes generate heat as they lose excess energy when changing to more stable atoms (nuclides).

Z is the core: is composed mainly of iron, nickel and other metals. Its diameter is about half that of the Earth (3500 km radius) and its is very hot and dense. It is believed that the core consists of an outer liquid layer (outer core) and a solid inner layer (inner core). The mainly iron core generates a magnetic field through and around the Earth.

Some general points:

The overall density of the Earth is much greater than the average density of the rock of the crust. This is evidence that the inner layers of the Earth are made of different more denser materials from that of the crust e.g. the metallic core.

The lithosphere is the rigid, relatively cool crust, and the outer or upper part of the mantle. It is split into sections called plates - the base of tectonic theory.

7(c) The age when rocks where formed in or on the crust can be estimated in various ways ..

• Fossils: As plants and animals evolve, species die out and new ones emerge. The sequence and type of fossils can be worked out and the timescale estimated. Therefore the fossils present in a layer can be used to estimate the age of the sedimentary rocks. This dating method is not absolute like radioisotope studies of igneous rocks but its the most useful for sedimentary rocks.
• Radioactive isotope dating: This is a more accurate method for dating very ancient igneous rocks. As certain isotopes, with VERY long half-lives, decay to form more stable atoms, there is a change in the isotope ratio of less stable / more stable. This ratio gets smaller, and by knowing the rate of change from the half-life of the more unstable atom, the age at which the magma cooled to give igneous rock can be estimated.
  • For example: potassium-40 decays to Argon-40 with a half-life of 1300 million years (1.3 x 10⁹y).
    • The potassium-40/argon-40  ratio can be measured in an analytical instrument called a mass spectrometer.
    • If 50% of the potassium-40 remains, the rock is 1.3 x 10⁹y old
      • if 25% is left the age is 2.6 x 10⁹ y old
        • if 12.5% is left the age is 3.9 x 10⁹ years etc.
  • Age of the Earth: Using this method it is estimated to be 4.5 x 10⁹ years.
  • The radioisotope carbon-14, ¹⁴C, is of new use for dating rocks.
    • Carbon-14's half-life is far too small at only 5700 years to be of any use for the geological dating of rocks.
    • 5700 years is not very long in terms of geological time because most rocks are at least hundreds of thousands, or millions of years old, from their date of formation.
      • However, carbon-14 is very useful to archaeologists for dating artefacts of organic origin like wood.
    • For more details see Radioisotopes and dating rocks and archaeological

• NOTES Cue, QUIZ and WORKSHEET QUESTION LINKS

• A structured question covering much of the GCSE Earth Science

• Earth Science GCSE study revision notes index (responses to the above Questions)

• foundation-easier multiple choice quiz on Earth Science

• higher-harder multiple choice quiz on Earth Science

•  Five Earth Science word-fill worksheets Q1 * Q2 * Q3 * Q4 * Q5

• Answers to the five GCSE Earth Science worksheets listed above

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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