What evidence shows that the outer core is liquid?

The S wave shadow zone. S waves are shear waves and cannot travel through a liquid, because a liquid has no rigidity and cannot resist shear. After an earthquake, S waves are not recorded over a wide region on the far side of the Earth, which shows they have been blocked by a liquid layer deep inside: the outer core. P waves do pass through the liquid outer core but are refracted as they enter and leave it, producing a separate ring-shaped P wave shadow zone that helps fix the size of the core. The high mean density of the Earth and the iron-meteorite analogy add that this liquid core is metallic iron and nickel.

Why was Wegener's theory of continental drift rejected at first?

Not for lack of evidence. Wegener had the jigsaw fit of the continents, matching fossils such as Mesosaurus across oceans, matching rocks and mountain belts, and palaeoclimatic evidence such as glacial deposits now in the tropics. What he lacked was a convincing mechanism: he could not explain what force could move solid continents through the solid ocean floor. Without a driver the idea was dismissed for decades. The mechanism arrived later with Hess's sea-floor spreading and mantle convection, and the decisive proof came from palaeomagnetism and the symmetrical magnetic stripes of the ocean floor.

What is the difference between magnitude and intensity?

Magnitude measures the energy released at the source of an earthquake, calculated from the amplitude of the waves on a seismogram. It is a single value for the whole earthquake and the scales are logarithmic (each whole number is about ten times the amplitude and about thirty-two times the energy). Intensity measures the effects at a particular place (the shaking felt and the damage done) on the Modified Mercalli scale, so it varies with location, being greatest near the epicentre. The Richter scale saturates for very large earthquakes, so the moment magnitude scale, based on the seismic moment, is preferred for the largest events.

What controls whether a volcano erupts gently or explosively?

The silica content of the magma, through its effect on viscosity and gas escape. Low-silica basaltic magma is runny (low viscosity), so dissolved gas escapes easily and the eruption is gentle and effusive, building broad shield volcanoes and fissures at constructive margins and hotspots. High-silica andesitic or rhyolitic magma is sticky (high viscosity), so gas is trapped and pressure builds until the magma fragments explosively, producing ash and pyroclastic flows and building steep stratovolcanoes and calderas at destructive margins. Temperature matters less: basalt is hotter but still erupts gently because it is runny.

What is isostasy and why are mountains said to have roots?

Isostasy is the gravitational balance in which the rigid lithosphere floats on the denser, plastic asthenosphere, like wood on water. Thicker or less dense crust floats higher and projects deeper, so a mountain range both stands high and has a deep crustal root, just as a taller iceberg sits higher and reaches deeper. When the load on the crust changes, the crust adjusts vertically: an ice sheet pushes it down, and when the ice melts the crust slowly rises back (isostatic rebound), which is why Scandinavia and Scotland are still uplifting since the last ice age. Erosion of a mountain is offset by uplift from below.

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Eduqas A-Level Geology Earth structure and global tectonics: the layered Earth, plate tectonics, earthquakes, volcanoes and the lithosphere

A deep-dive Eduqas A-Level Geology guide to the Earth structure and global tectonics concept. Covers the layered and mechanical Earth and the seismic evidence, the development of plate tectonics, the three kinds of plate margin, earthquakes and seismic waves, volcanic activity, and the lithosphere, isostasy and hotspots, with the calculations and exam patterns Eduqas repeats.

Generated by Claude Opus 4.819 min readEduqas-A220-Earth-Structure-TectonicsUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this concept actually demands
The layered Earth and the seismic evidence
Plate tectonics theory and evidence
Plate margins and their features
Earthquakes and seismic waves
Volcanic activity and eruption styles
The lithosphere, isostasy and hotspots
How this concept is examined
Check your knowledge

What this concept actually demands

Earth structure and global tectonics is the engine room of Eduqas A-Level Geology: it explains why volcanoes and earthquakes happen where they do, how oceans and mountains are built and destroyed, and how we know what the unreachable interior of the Earth is made of. The examiners test two linked skills: explaining how seismic and geophysical evidence builds our picture of the deep Earth and the moving plates, and applying that understanding (and a handful of calculations) to interpret margins, earthquakes, eruptions and the floating lithosphere.

This guide walks through the six clusters in a sensible build order, then sets out the exam patterns Eduqas repeats. Each cluster has a matching dot-point page; this overview ties them together and gathers the three calculations you must master.

The layered Earth and the seismic evidence

The Earth has two layer schemes. By composition: a thin crust (oceanic dense and basaltic; continental lighter and granitic), a slowly convecting peridotite mantle, a liquid iron-nickel outer core (the magnetic field source) and a solid inner core. By mechanical behaviour: a rigid lithosphere (the plates) over a weak, plastic asthenosphere. We deduce this from seismic waves: the Moho is a sharp velocity increase, the S wave shadow zone proves the outer core is liquid (S waves cannot cross liquid), and the high mean density plus the meteorite analogy show the core is metallic.

Plate tectonics theory and evidence

The theory grew from Wegener's continental drift (evidence: continental fit, matching fossils and rocks, palaeoclimate; rejected for lack of a mechanism), through Hess's sea-floor spreading (oceanic crust youngest at ridges), to the unified theory. The clinching evidence is palaeomagnetism: as basalt cools below its Curie temperature it locks in the field, and spreading lays down symmetrical magnetic stripes about a ridge, proving crust is created and carried away equally on both sides. The plates move by mantle convection, ridge push and slab pull (the strongest force).

Plate margins and their features

There are three margin types. Constructive (divergent) margins pull apart, with decompression melting making basaltic magma, shallow earthquakes and gentle volcanism at mid-ocean ridges. Destructive (convergent) margins converge: ocean-continent and ocean-ocean sub-types subduct dense oceanic crust along a Benioff zone (deep trench, shallow-to-deep earthquakes, explosive andesitic arcs fed by flux melting), while continent-continent collision thickens buoyant crust into fold mountains. Conservative (transform) margins slide past, giving powerful shallow earthquakes but no volcanism.

Earthquakes and seismic waves

An earthquake starts at the focus (the rupture point at depth), below the epicentre, by elastic rebound (strain builds across a locked fault, then it slips). P waves are fastest and cross solids and liquids; S waves cross solids only; surface waves are slowest and most damaging. Magnitude measures energy at the source (logarithmic; moment magnitude beats the saturating Richter scale for big quakes); intensity measures effects at a place (Modified Mercalli). The P-S arrival gap on a travel-time graph gives the distance to a station, and triangulation from three stations fixes the epicentre.

Volcanic activity and eruption styles

Eruption style is set by silica content, through viscosity and gas escape. Low-silica basaltic magma is runny and gas escapes, so eruptions are effusive, building shield volcanoes and fissures at constructive margins and hotspots. High-silica andesitic or rhyolitic magma is viscous and traps gas, so eruptions are explosive, producing ash and pyroclastic flows and building stratovolcanoes and calderas at destructive margins.

The lithosphere, isostasy and hotspots

A plate is a slab of lithosphere (thin, dense, young oceanic versus thick, light, ancient continental) riding on the asthenosphere. Isostasy is the floating balance: thicker or lighter crust floats higher with a deeper root, and the crust adjusts vertically (isostatic rebound) when loads such as ice sheets are added or removed, as in still-rising Scandinavia. Mantle plumes are fixed columns of hot mantle feeding hotspots and intraplate volcanism; because the plate moves over a fixed plume, hotspot tracks form chains of volcanoes that age with distance from the active hotspot (Hawaii). These ideas underpin the Component 3 geology of the lithosphere option.

How this concept is examined

A typical Eduqas profile for Earth structure and global tectonics:

Evidence and explanation questions. How the shadow zones prove a liquid outer core, how palaeomagnetism proves spreading, and how elastic rebound generates earthquakes.
Calculations. Three recur: the epicentre distance from the P-S travel-time gap, the spreading rate (distance over time, with a kilometres-to-centimetres conversion and doubling the half-rate), and the isostasy density-ratio (the fraction of a crustal block below the surrounding mantle).
Identification questions. Identifying a margin from its trench, earthquake depths and volcano chemistry, or a volcano from its shape and lava type.
Levels-of-response extended answers. Describing the processes and features of a named margin, or explaining why basaltic eruptions are gentle and rhyolitic ones explosive.

Eduqas Earth structure and global tectonics covers the layered and mechanical Earth and the seismic evidence (the Moho velocity jump and the S wave shadow zone proving a liquid outer core), the development of plate tectonics (Wegener's drift, Hess's spreading, and the clinching symmetrical magnetic stripes from palaeomagnetism, driven by mantle convection, ridge push and slab pull), the three plate margins (constructive with decompression melting, destructive with subduction along a Benioff zone and flux melting, conservative sliding past), earthquakes (focus and epicentre, elastic rebound, P, S and surface waves, magnitude versus intensity, and triangulation), volcanic activity (silica controls viscosity and gas, so basaltic eruptions are effusive and rhyolitic ones explosive), and the lithosphere, isostasy and hotspots (floating crust with roots and rebound, and fixed plumes building ageing island chains like Hawaii). Master the evidence and the three calculations.

Check your knowledge

Recall and application questions covering the whole concept. Attempt them under timed conditions, then check the solutions.

State what the S wave shadow zone shows about the outer core and why. (2 marks)
Define the lithosphere and the asthenosphere, and state why the distinction matters. (3 marks)
State two pieces of evidence Wegener used and the reason his theory was rejected. (3 marks)
Explain why the magnetic stripes are symmetrical about a mid-ocean ridge. (3 marks)
Name the three plate-margin types and give the dominant volcano style (if any) at each. (3 marks)
Explain the difference between magnitude and intensity, and why moment magnitude is preferred for large quakes. (3 marks)
A P-S gap of 50 seconds corresponds to 450 km on a travel-time graph. State the distance to the epicentre and what else is needed to locate it. (2 marks)
Explain why high-silica magmas erupt explosively while basaltic magmas erupt gently. (3 marks)
Crust of density 2.7 g per cubic centimetre floats on mantle of density 3.3 g per cubic centimetre. Calculate the fraction below the surrounding mantle, and state what happens when an overlying ice sheet melts. (3 marks)

Solutions

Q1: S waves cannot travel through a liquid (no rigidity to resist shear), so their absence over a wide region on the far side of the Earth (the S wave shadow zone) shows the outer core is liquid.
Q2: The lithosphere is the rigid outer shell (crust plus rigid uppermost mantle) that forms the brittle plates; the asthenosphere is the weak, plastic upper mantle beneath. It matters because a plate is a slab of lithosphere that moves by riding on the slowly deforming asthenosphere.
Q3: Any two of: the jigsaw fit of the continents; matching fossils such as Mesosaurus; matching rocks and mountain belts; palaeoclimatic evidence such as glacial deposits now in the tropics. It was rejected because Wegener had no convincing mechanism to move the continents.
Q4: New basalt locks in the field's polarity as it cools below its Curie temperature; as crust spreads equally outwards while the field periodically reverses, the same sequence of normal and reversed stripes is laid down on each side, giving a mirror-image pattern about the axis.
Q5: Constructive (divergent): gentle effusive basaltic volcanism. Destructive (convergent): explosive andesitic volcanism (none at completed continent-continent collision). Conservative (transform): no volcanism.
Q6: Magnitude measures the energy at the source (one value, logarithmic); intensity measures the effects felt at a place (Modified Mercalli, varying with distance). The Richter scale saturates for very large earthquakes, while moment magnitude is based on the seismic moment (fault area, slip and rigidity) and does not, so it is preferred for big quakes.
Q7: The epicentre is $450\ \mathrm{km}$ from the station (read off the graph from the $50\ \mathrm{s}$ gap). This gives only a distance, so circles of that radius are needed around two more stations; where all three intersect is the epicentre (triangulation).
Q8: High-silica magma is viscous, so dissolved gas is trapped and the pressure builds until the magma fragments explosively, producing ash and pyroclastic flows. Basaltic magma is low in silica and runny, so gas escapes easily and the eruption is gentle (effusive lava flows).
Q9: By isostasy the fraction submerged equals the density ratio: $\frac{2.7}{3.3} \approx 0.82$ , so about $82\%$ of the column lies below the surrounding mantle. When the overlying ice melts, the load is removed and the crust slowly rises back up (isostatic rebound) until equilibrium is restored.