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The STRUCTURE of the EARTH

 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 & earthquake waves ... 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?

doc b's Earth Science NotestopFig 9. The structure of the Earth

The Earth is almost spherical and is composed of three principal layers.

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

X is the crust: is the relatively thin and cool outer solid layer of the Earth. The thickness of this upper part ranges from 6km to 100km surrounded by the atmosphere of air W. It is much cooler, harder, brittle and less dense than the other layers of the Earth. The crust is divided into sections or 'tectonic plates' which 'float' and move on the very hot mantle at the rate of a few cm per year relative to each other (for more details see Tectonic plates). This plate movement means that most parts of the Earth's crust are in very different locations from millions of years ago.  It should also be noted that 2/3rds of the surface of the Earth is water.

The lithosphere consists of the crust and the almost solid upper part of the mantle next to the crust.

Y is the mantle: is very hot rock material, it is almost solid, quite rigid, but the 'thick plastic' rock can be deformed and moves very slowly due to huge convection currents driven by heat from radioactive decay in the metallic core. It is these convection currents which move the 'plates'. The mantle's 'thickness' is 3000 km (about halfway to the centre of the Earth) and its temperature is usually over 1000oC. The deeper you go, the hotter the mantle gets and the rock gets less rigid i.e. flows more easily.

Chemically, the mantle 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.

topSome 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 and bone.
      • For more details see Radioisotopes and dating rocks and archaeological finds

top

APPENDIX - How to we get our knowledge of the cross-section of the Earth?

  • How far can we drill into the Earth's crust and mantle?

  • At the most, we can only drill down into the Earth's crust to a depth of 12km, which is not even through to the mantle.

  • Therefore, we must find other scientific methods to investigate the inner structure of the Earth since we can't even penetrate the crust to get to the mantle!

  • The monitoring and recording of earthquake waves has proved the most fruitful direct scientific probing of the Earth's structure and these seismic shock waves have given us most of the detailed scientific knowledge we have of the total structure of the Earth.

  • Powerful earthquake waves always emanate from the central point of an earthquake, called the epicentre.

  • I hope the diagram and accompanying notes explain the basic ideas how violent earthquakes help us to understand the structure of the Earth.

  • Fig 13. Earthquake Waves, S waves & P waves

  • Fig 13. S and P earthquake waves, shows the paths of two types of earthquake waves (P, blue on diagram) and S, green on diagram) as they travel through the Earth from an earthquake.

    • Earthquake waves shadow zones

    • The diagram shows the zones on the Earth's surface where the S and P earthquake waves can both be detected by a seismometer (purple zone), AND, just as importantly, where the S and P waves cannot be detected (no S waves can reach the black and blue zones and no P waves can reach the black zone).

    • The reason for these shadow zones is explained below (see also the 'physics' section at the end).

  • Around the world are seismographic stations fitted with seismometers that can detect the P and s earthquake waves and portrayed as a seismogram (seismographic trace).

  • P waves can travel right through the Earth, passing through the mantle and core in the process.

  • The wave paths are curved because the waves are gradually refracted by the gradual change in density as you go deeper into the Earth.

  • The refraction and absorption effects causes the formation of zones where one or more of the earthquake waves cannot be detected.

  • The S waves travel through the mantle showing that it is almost solid and certainly not liquid.

  • However, the S waves are absorbed by the outer core and are not detected on the other side of the Earth from where the earthquake occurred, creating the S wave shadow zone (which overlaps with a smaller P wave shadow zones).

    • The absorption of S waves by the outer core proves that this layer is liquid (mainly metal). see physics section at the end.

  • As well as travelling through the crust and mantle, P waves can also travel right through the core layers and be detected on the other side of the Earth.

  • However because of the refracted curved paths of the P waves, there are two smaller zones where P waves are not detected.

  • By collating lots of data from different seismometers around the world on the Earth's crust, it is possible to work out from the speed and path patterns of the P and S wave detected the structure of the Earth, that we cannot in any way investigate directly.

    • The earthquake wave data is put into a mathematical computer model, from which you can get the thickness of the crust and the rocky plastic mantle, the thickness of outer liquid metal core and the radius of the solid inner core of metal.

    • For example, the P waves speed up in the inner core suggesting that not only is the density greater, but that the inner core is also solid.

  • Physics notes and my Fig 13. diagram!

    • P waves, the primary waves, are longitudinal waves and can pass through liquids and solids (blue paths on the diagram).

      • The oscillation/vibration of longitudinal waves is in the direction of wave motion, remember the 'push & pull' of the 'slinky spring' in your physics lessons.

      • This compression and decompression of the material helps retain the energy of the wave and so can pass through liquids or solids (rather like sound can pass through any material).

      • The energy from P waves is much more gradually absorbed by the medium than in the case of S waves.

    • S waves, the secondary waves, are transverse waves and can only travel through solids (green paths on the diagram).

      • In transverse waves, the oscillation/vibration is a right angles to the direction of wave motion and in liquids or 'plastic' material, the energy of the wave is quite quickly dispersed and the amplitude drops to zero, i.e. the wave is no longer existing.

      • This is why you get the shadow zone beyond the liquid outer core below the mantle.

    • In the diagram Fig 13., you should notice all the paths of the earthquake waves are curved.

    • The curvature is due to the waves being continually refracted as the density of the mantle and core changes.

      • This isn't the sharp change in direction you see when light rays are refracted as they pass through an abrupt interface boundary like air/water or air glass.

    • The refraction is more prominent at the mantle - outer core boundary and the outer core - inner core boundary where the density changes the most abruptly.

    • Technically, as with light, when the waves pass into a more dense material they bends towards the normal and when the waves pass into a less dense medium they bend away from the normal.

      • If you look carefully, I think I've just most of the deviations right on the diagram, but the curves are a bit exaggerated!

    • -


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


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