Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the contrasts between oceanic and continental crust; the seismic evidence for the layering (changes in wave velocity at boundaries such as the Moho); the P and S wave shadow zones as evidence for a liquid outer core; the link between the core and the Earth's magnetic field.

What this dot point is asking

OCR wants you to describe the layered internal structure of the Earth (crust, mantle, outer core, inner core), to contrast oceanic and continental crust, to explain the seismic evidence for the layering (velocity changes and the Moho), to explain the P and S wave shadow zones as evidence for a liquid outer core, and to link the core to the magnetic field.

The answer

The layered structure

The Earth is divided into concentric layers of differing composition and state:

• Crust. The thin, rigid outer 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 $2900\ \mathrm{km}$; mainly peridotite; solid but able to flow slowly (it convects).
• Outer core. Liquid iron and nickel; the movement of this conducting liquid generates the Earth's magnetic field.
• Inner core. Solid iron and nickel (solid despite the high temperature because of the immense pressure).

Oceanic versus continental crust

The contrast is examinable: oceanic crust is thinner, denser, basaltic, younger and subducts; continental crust is thicker, less dense, granitic, older and too buoyant to subduct. This is why subduction consumes oceanic, not continental, crust.

Seismic evidence for the layering

We cannot drill to the core, so the structure is deduced from seismic waves, which change velocity at boundaries between layers of different density and composition. A sudden change in velocity (and the refraction it causes) marks a boundary.

The shadow zones

The decisive evidence for the core's state comes from where waves do and do not arrive:

• S wave shadow zone. S waves are not detected on the far side of the Earth from an earthquake. Because S waves cannot travel through liquid (liquids 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 zone where direct P waves do not arrive. The position of this zone helps define the size of the core.

The core and the magnetic field

The liquid outer core of molten iron is an electrical conductor; its convective motion, driven by heat and the Earth's rotation, generates the geomagnetic field (a geodynamo). This links directly to palaeomagnetism, since rocks record this field when they form.

Examples in context

Example 1. The geodynamo and palaeomagnetism. Because the liquid outer core generates the magnetic field, and that field is recorded by cooling basalts, Earth's deep structure links directly to the palaeomagnetic evidence for sea-floor spreading.

Example 2. Refraction at the Moho. Seismic refraction surveys exploit the velocity jump at the Moho to map the thickness of the crust, a technique used both in research and in resource exploration.

Try this

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

• Cue. Any two of: oceanic crust is thinner, denser, basaltic, younger and subducts; continental crust is thicker, less dense, granitic, older and does not subduct.

Q2. Explain what the S wave shadow zone shows about the outer core. [2 marks]

• Cue. S waves cannot pass through liquid, so their absence on the far side of the Earth shows the outer core is liquid.

Q3. State what the Moho is and how it is detected. [2 marks]

• Cue. The crust-mantle boundary; it is detected by an abrupt increase in seismic wave velocity as waves enter the denser mantle.