What is the internal structure of the Earth and how do we know it?
The compositional and mechanical layering of the Earth (crust, mantle, outer and inner core; lithosphere and asthenosphere) and the seismic evidence (P and S wave behaviour, shadow zones, discontinuities) used to deduce it.
A focused answer to WJEC and Eduqas A-Level Geology F4 on Earth structure, covering the compositional layers (crust, mantle, outer and inner core) and mechanical layers (lithosphere and asthenosphere), and the seismic evidence (P and S wave behaviour, shadow zones and discontinuities) that reveals them.
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What this dot point is asking
WJEC wants you to describe the layered structure of the Earth, both by composition (crust, mantle, core) and by mechanical behaviour (lithosphere, asthenosphere), and to explain how seismic waves reveal that structure. This is the foundation of F4: you cannot understand plate tectonics without knowing what the plates are and what they move over.
The answer
The compositional layers
By chemical composition the Earth has three main shells. The crust is the thin, outermost rocky layer: oceanic crust is thin (about 7 km), dense and basaltic, while continental crust is thicker (about 30 to 70 km), less dense and granitic. The mantle is the thick (about 2900 km) silicate layer of dense, iron and magnesium rich rock (rich in olivine and pyroxene). The core is metallic (mostly iron and nickel) and is divided into a liquid outer core and a solid inner core.
The mechanical layers
Mechanically (by how the rock behaves) the outer Earth is divided differently. The lithosphere is the rigid outer shell, made of the crust plus the uppermost, rigid part of the mantle; it is brittle and forms the tectonic plates. Beneath it the asthenosphere is a weaker, partially molten, ductile layer of the upper mantle that can flow slowly. The rigid lithospheric plates move over the mobile asthenosphere, which is why this mechanical distinction is the one that matters for plate tectonics.
The seismic evidence
We cannot drill to the core, so the structure is deduced from seismic waves from earthquakes. P waves (primary, compressional) travel through solids and liquids and are the faster waves. S waves (secondary, shear) travel only through solids, because liquids have no shear strength. Two key observations follow: waves speed up or change direction sharply at boundaries called discontinuities (the Moho, and the core-mantle boundary), revealing the layers; and there are shadow zones where waves do not arrive. The S-wave shadow zone (no S waves beyond about 103 degrees from the source) shows the outer core is liquid; the P-wave shadow zone (a band where P waves are refracted away) confirms the liquid outer core surrounding a solid inner core.
Examples in context
Locating earthquakes. The different speeds of P and S waves let seismologists find an earthquake's distance from the time gap between the two arrivals, the same wave physics that reveals the deep structure. Iron in the core from density. The core's metallic, iron-nickel composition is inferred partly from the Earth's overall density being far higher than crustal rock, consistent with a dense metal centre. Mantle convection and the asthenosphere. Because the asthenosphere is weak and can flow, it is the layer in which mantle convection decouples the rigid plates from the deeper mantle, linking structure to plate motion.
Try this
Q1. Distinguish oceanic crust from continental crust by composition and thickness. [2 marks]
- Cue. Oceanic crust is basaltic, dense and thin (about 7 km); continental crust is granitic, less dense and thick (about 30 to 70 km).
Q2. Why can S waves not travel through the outer core? [1 mark]
- Cue. The outer core is liquid, and liquids have no shear strength, so they cannot transmit shear (S) waves.
Q3. State what the lithosphere is composed of. [2 marks]
- Cue. The crust plus the rigid, uppermost part of the mantle, together forming the rigid plates.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20194 marksExplain how the behaviour of P and S seismic waves provides evidence that the outer core is liquid.Show worked answer →
Begin from the wave properties, because the marks reward the link between wave behaviour and the physical state of the layer.
P waves are compressional and can travel through solids and liquids; S waves are shear waves and can travel only through solids, because liquids have no shear strength.
S waves from an earthquake are not detected on the far side of the Earth: there is an S-wave shadow zone covering the whole region beyond about 103 degrees from the source.
The simplest explanation is that S waves are stopped by a liquid layer, so the outer core must be liquid; P waves do pass through but are refracted, creating a P-wave shadow zone, consistent with a liquid outer core surrounding a solid inner core.
Markers reward the point that S waves cannot pass through liquid, the existence of the S-wave shadow zone, and the conclusion that the outer core is liquid.
WJEC Eduqas 20214 marksDistinguish between the lithosphere and the asthenosphere, and explain why the distinction matters for plate tectonics.Show worked answer →
The lithosphere is the rigid outer shell of the Earth, comprising the crust and the uppermost, rigid part of the mantle; it is brittle and behaves as a solid plate.
The asthenosphere is the weaker, partially molten and ductile layer of the upper mantle beneath the lithosphere; it can flow slowly over geological time.
The distinction is compositional in part but mainly mechanical: it matters because the rigid lithospheric plates move over the weaker, mobile asthenosphere, which provides the decoupling layer that allows plate tectonics to operate.
Markers reward defining the lithosphere as rigid crust plus upper mantle, the asthenosphere as weak and ductile, and explaining that plates slide over the asthenosphere.
Related dot points
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A focused answer to WJEC and Eduqas A-Level Geology F4 on plate tectonic theory, covering the development from continental drift, and the evidence (continental fit, matching geology and fossils, palaeoclimate, sea-floor spreading and the symmetry of palaeomagnetic stripes) that confirmed it.
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- The abundance of elements in the crust, the silicon-oxygen tetrahedron as the building block of the silicate minerals, and how the degree of tetrahedral linkage (isolated, chain, sheet and framework) controls cleavage, hardness and density.
A focused answer to WJEC and Eduqas A-Level Geology F1 on silicate minerals, covering the most abundant crustal elements, the silicon-oxygen tetrahedron, and how isolated, single-chain, sheet and framework silicate structures control cleavage, hardness and density in olivine, pyroxene, amphibole, mica, feldspar and quartz.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)