Doc Brown's Chemistry - Earth Science Revision Notes

7. The Structure of the Earth - three layers

Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

based on a GCSE Geological & Earth Science TASK SHEET * Earth Science Homepage * 5 multi-word fill GCSE worksheets + answers * GCSE Earth Science Quiz: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz

ANSWERS-NOTES 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. 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. It is much cooler, harder, brittle and less dense than the other layers. The crust is divided into sections or 'plates' which 'float' and move on the mantle. 2/3rds of the surface 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.

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. The core consists of an outer liquid layer and a solid inner layer. The heat is generated by radioactive decay of longer lived isotopes and is transferred into the mantle. It is this heat that drives the convection currents in the mantle, which ultimately moves the tectonic plates of the crust. 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.

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). This ratio can be measured in an analytical instrument called a mass spectrometer. The ratio of potassium-40 / Argon-40 is measured. 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. Its half-life is too small at only 5700 years and is not very long in terms of geological time.

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