Thursday, November 12, 2009

The structure of the Earth

Imagine a Scotch egg......

  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)

There are lots of clues:

  • 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.
  • Iron and Nickel are both dense and magnetic.
  • 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 (s waves do not travel through liquids). The mantle is mainly solid as it is under extreme pressure (see below). 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 (as is the scotch egg!) with a diameter of about 12,700Kilometres. 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.


This diagram shows a detailed picture of the Earth's interior. Crust is being created at the mid ocean ridges and being eaten at the subduction zones. The movement processes are driven by the convection currents created by the heat produced by natural radioactive processes deep within the Earth.

Inner core: depth of 5,150-6,370 kilometers
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. 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 aluminum. 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 aluminum-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 km3 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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