Structure of the Earth

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Structure of the Earth

The earth is an oblate spheroid. It is almost spherical, flattened a little at the poles with a slight bulge at the centre ( equator ). The structure of earth’s interior is layered. It is composed of three major layers : the crust, the mantle and the core. The crust forms only 0.5 per cent of the volume of the earth, the mantle forms 16 per cent and the core has its share of 83 per cent. Earth being a spherical body, has its centre at 6,400 km. viz., the mean radius of the earth.

  1. The outer shell of the Earth is called the CRUST ( breadcrumbs )
  2. The next layer is called the MANTLE ( sausagemeat )
  3. The next layer is the liquid OUTER CORE ( egg white )
  4. The middle bit is called the solid INNER CORE ( egg yolk ).

Density of the Earth’s Atmosphere :

  • The overall density of the Earth is much higher than the density of the rocks we find in the crust. This tells us that the inside must be made of something much denser than rock.
  • Meteorites ( created at the same time as the Earth, 4.6 billion years ago ) have been analysed. The commonest type is called a chondrite and they contain iron, silicon, magnesium and oxygen ( Others contain iron and nickel ). A meteorite has roughly the same density as the whole earth. A meteorite minus its iron has a density roughly the same as Mantle rock ( e.g. the mineral called olivine ).
  • Iron and Nickel are both dense and magnetic.
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  • Scientists can follow the path of seismic waves from earthquakes as they travel through the Earth. The inner core of the Earth appears to be solid whilst the outer core is liquid ( waves do not travel through liquids ). The mantle is mainly solid as it is under extreme pressure. We know that the mantle rocks are under extreme pressure, diamond is made from carbon deposits and is created in rocks that come from depths of 150 – 300 kilometres that have been squeezed under massive pressures.
  • The Earth is sphere with a diameter of about 12,700 Kilometres. As we go deeper and deeper into the earth the temperature and pressure rises. The core temperature is believed to be an incredible 5000 – 6000°c.
  • The crust is very thin ( average 20Km ). This does not sound very thin but if you were to imagine the Earth as a football, the crust would be about ½ millimetre thick. The thinnest parts are under the oceans ( OCEANIC CRUST ) and go to a depth of roughly 10 kilometres. The thickest parts are the continents ( CONTINENTAL CRUST ) which extend down to 35 kilometres on average. The continental crust in the Himalayas is some 75 kilometres deep.
  • The mantle is the layer beneath the crust which extends about half way to the centre. It’s made of solid rock and behaves like an extremely viscous liquid. The convection of heat from the centre of the Earth is what ultimately drives the movement of the tectonic plates and cause mountains to rise. Click here for more details.
  • The outer core is the layer beneath the mantle. It is made of liquid iron and nickel. Complex convection currents give rise to a dynamo effect which is responsible for the Earth’s magnetic field.
  • The inner core is the bit in the middle. It is made of solid iron and nickel. Temperatures in the core are thought to be in the region of 5000 – 6000°c and it’s solid due to the massive pressure.

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Inner Core :
Depth of 5,150 – 6,370 kilometres. The inner core is made of solid iron and nickel and is unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure – freezing which occurs to most liquids under extreme pressure.

Outer Core : Depth of 2,890 – 5,150 kilometres. The outer core is a hot, electrically conducting liquid ( mainly Iron and Nickel ). This conductive layer combines with Earth’s rotation to create a dynamo effect that maintains a system of electrical currents creating the Earth’s magnetic field. It is also responsible for the subtle jerking of Earth’s rotation. This layer is not as dense as pure molten iron, which indicates the presence of lighter elements. Scientists suspect that about 10% of the layer is composed of sulphur and oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron.

D’ layer : Depth of 2,700 – 2,890 kilometres. This layer is 200 to 300 kilometres thick. Although it is often identified as part of the lower mantle, seismic evidence suggests the D” layer might differ chemically from the lower mantle lying above it. Scientists think that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density.

Lower Mantle : Depth of 650 – 2,890 kilometres. The lower mantle is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminium. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites.

Transition Region :
Depth of 400 – 650 kilometres. The transition region or mesosphere ( for middle mantle ), sometimes called the fertile layer and is the source of basaltic magmas. It also contains calcium, aluminium, and garnet, which is a complex aluminium – bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma.

Upper Mantle : Depth of 10 – 400 kilometres. Solid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions. Olivine ( Mg, Fe ) 2SiO4 and pyroxene ( Mg, Fe ) SiO3 have been found. These and other minerals are crystalline at high temperatures. Part of the upper mantle called the asthenosphere might be partially molten.

Oceanic Crust : Depth of 0 – 10 kilometres. The majority of the Earth’s crust was made through volcanic activity. The oceanic ridge system, a 40,000 kilometre network of volcanoes, generates new oceanic crust at the rate of 17 km 3 per year, covering the ocean floor with an igneous rock called basalt. Hawaii and Iceland are two examples of the accumulation of basalt islands.

Continental Crust : Depth of 0 – 75 kilometres. This is the outer part of the Earth composed essentially of crystalline rocks. These are low – density buoyant minerals dominated mostly by quartz ( SiO2 ) and feldspars ( metal poor silicates ). The crust is the surface of the Earth. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere ( the rocky or strong layer ).

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