Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the mechanical layers (lithosphere and asthenosphere); the seismic evidence for the layering from changes in P and S wave velocity at boundaries such as the Moho; the P and S wave shadow zones as evidence for a liquid outer core; the use of meteorites and density as evidence for the composition of the core and mantle.

What this dot point is asking

Eduqas wants you to describe the layered internal structure of the Earth (crust, mantle, outer core, inner core) and the contrasting mechanical layers (the rigid lithosphere and the weak asthenosphere), to explain the seismic evidence for that layering from P and S wave velocity changes and the Moho, to explain the P and S wave shadow zones as evidence for a liquid outer core, and to use density and the meteorite analogy as evidence for the composition of the deep Earth. This is the structural foundation for the whole tectonics module.

The answer

The compositional layers

The Earth is divided into concentric layers that differ in composition and state:

• Crust. The thin, rigid outermost shell. Oceanic crust is thin (about $5$ to $10\ \mathrm{km}$), dense and basaltic; continental crust is thick (about $30$ to $70\ \mathrm{km}$), less dense and granitic.
• Mantle. Down to about $2900\ \mathrm{km}$, made largely of the dense ultramafic rock peridotite (rich in olivine and pyroxene). It is solid but able to flow very slowly over geological time, which lets it convect.
• Outer core. Liquid iron and nickel (about $2900$ to $5150\ \mathrm{km}$). The convective motion of this conducting liquid generates the Earth's magnetic field.
• Inner core. Solid iron and nickel at the centre. It is solid despite the highest temperatures because the immense pressure raises the melting point above the local temperature.

The mechanical layers (lithosphere and asthenosphere)

The compositional layers are not the same as the mechanical layers that matter for plate tectonics.

The key idea is that a tectonic plate is a slab of lithosphere, not just crust. The plates move because they sit on the slowly deforming asthenosphere.

Density and the composition of the deep Earth

The whole Earth has a mean density of about $5.5\ \mathrm{g\ cm^{-3}}$, but surface rocks are only about $2.7\ \mathrm{g\ cm^{-3}}$. The deep interior must therefore be far denser than the crust, which points to a dense metallic (iron-nickel) core. This density argument is supported by the meteorite analogy: iron meteorites are thought to be fragments of the metallic cores of shattered planetesimals, and stony meteorites resemble the silicate mantle, so meteorites give us samples of material like the deep Earth that we can never drill to.

Seismic evidence for the layering

Because we cannot drill below the crust, the structure is deduced from seismic waves, which change velocity at boundaries between layers of different density and composition, and refract there. A sudden change in velocity (and the bending it causes) marks a boundary.

The shadow zones

The decisive evidence for the state of the core comes from where waves do and do not arrive at the surface:

• S wave shadow zone. S waves are not detected over a wide region (beyond about $103$ degrees of arc) on the far side of the Earth from an earthquake. Because S waves cannot travel through a liquid (a liquid has no rigidity and cannot resist shear), this shows the outer core is liquid.
• P wave shadow zone. P waves are refracted (bent) as they enter and leave the core, producing a ring-shaped band (roughly $103$ to $143$ degrees) where direct P waves do not arrive. The position of this zone helps define the size of the core.

Examples in context

Example 1. The geodynamo links to palaeomagnetism. Because the liquid outer core generates the magnetic field, and that field is locked into cooling basalts, the deep structure connects directly to the palaeomagnetic evidence for sea-floor spreading covered in the plate tectonics statement.

Example 2. Refraction at the Moho maps crustal thickness. Seismic refraction surveys exploit the velocity jump at the Moho to measure how thick the crust is, a technique used in both research and resource exploration.

Try this

Q1. State two differences between oceanic and continental crust. [2 marks]

• Cue. Any two of: oceanic crust is thinner (about $5$ to $10\ \mathrm{km}$), denser, basaltic and younger; continental crust is thicker (about $30$ to $70\ \mathrm{km}$), less dense, granitic and older.

Q2. Explain the difference between the lithosphere and the asthenosphere. [3 marks]

• Cue. The lithosphere is the rigid outer shell (crust plus rigid uppermost mantle) that forms the brittle plates; the asthenosphere is the weak, partially mobile upper mantle beneath it that flows plastically and over which the plates move.

Q3. Explain what the S wave shadow zone shows about the outer core. [2 marks]

• Cue. S waves cannot pass through a liquid, so their absence on the far side of the Earth (the S wave shadow zone) shows the outer core is liquid.