Notes · GeologyUK · A-Levels

Earth structure and global tectonics

This chapter builds the modern picture of the Earth's interior and how its outer shell moves. It covers the layered internal structure and the seismic evidence for it, the use of seismic waves to locate earthquakes, the theory of plate tectonics and the forces that drive it, and the three types of plate boundary with their characteristic landforms and hazards.

4 sections~15 min reading time3 competenciesLevel Standard 3 · Advanced 1

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Exam profile

AO1 · Describe the layered structure of the Earth, the evidence for it, and the theory of plate tectonicsAO2 · Apply seismic data to locate an epicentre and magnetic-stripe data to calculate plate ratesAO3 · Analyse and evaluate the evidence for plate tectonics and the features of each boundary

Operators:describeexplaincalculateinterpretanalyseevaluate

basic level

AS-Level expects you to describe the Earth's layers, the types of plate boundary, and the main evidence for plate tectonics.

higher level

The full A-Level requires you to interpret seismic evidence, calculate epicentral distance and plate rates, and evaluate the plate-driving mechanisms.

Depth

Text

Contents · 4 sections

Earth structure and global tectonics

The internal structure of the Earth and its evidence#

StandardWJEC/Eduqas A level Geology (Component 2) — Earth structure and global tectonics

The layered Earth (mechanical layers)

Fig. 1The Earth's mechanical layering: a solid inner core, liquid outer core, mantle, weak asthenosphere and rigid lithosphere (the plates).

The Earth is divided into concentric layers that can be described in two ways: by composition and by mechanical behaviour. By composition there are three layers — a thin outer crust of light silicates, a thick mantle of dense iron- and magnesium-rich silicates, and a metallic iron-nickel core. By mechanical behaviour the outermost part is divided differently: the rigid lithosphere (the crust plus the topmost, brittle part of the mantle) rides on the weak, partly molten, slowly flowing asthenosphere beneath it. It is the lithosphere, broken into plates, that moves in plate tectonics, so the mechanical layering is the one that matters for tectonics.

The core itself is in two parts: a liquid outer core and a solid inner core. The outer core is liquid because, although it is very hot, the pressure there is not high enough to keep the iron solid; the inner core, under still greater pressure, is solid despite being hotter. Movement in the liquid outer core generates the Earth's magnetic field by a dynamo action, a fact that becomes important as evidence for seafloor spreading. The boundaries between the layers — the Mohorovicic discontinuity (Moho) between crust and mantle, and the core-mantle boundary — are sharp changes in composition and physical state.

We cannot sample the deep Earth directly, so almost all of this knowledge comes from seismic waves — the vibrations from earthquakes that travel through the Earth. Two body-wave types are used: P (primary) waves, which are fast compressional waves that travel through solids and liquids, and S (secondary) waves, which are slower shear waves that travel only through solids. Because their speeds depend on the density and rigidity of the material, the waves refract (bend) and reflect at boundaries, and by mapping how they arrive around the globe geophysicists reconstruct the layers they passed through.

The single most important piece of evidence is the S-wave shadow zone. Because S waves cannot travel through a liquid, they fail to arrive on the far side of the Earth beyond about 103 degrees from an earthquake, creating a broad zone where no S waves are recorded; this proves that the outer core is liquid. P waves, which can pass through liquid, are also refracted sharply at the core-mantle boundary to produce a narrower P-wave shadow zone. The sudden increases in wave speed at the Moho and the changes at the core boundaries similarly reveal the depths and nature of the layers. Reading the interior from the pattern of wave arrivals is a triumph of indirect evidence.

Seismic waves and the S-wave shadow zone

Fig. 2S waves cannot cross the liquid outer core, so they fail to arrive beyond about 103 degrees, creating the S-wave shadow zone — the key evidence that the outer core is liquid.

Worked example

Interpreting seismic evidence

At a seismograph 130 degrees around the globe from an earthquake, P waves arrive but no S waves are recorded. Explain what this shows about the Earth's interior.

Identify the observation
The station lies within the S-wave shadow zone (beyond about 103 degrees), where S waves fail to arrive.
Apply the wave properties
S waves are shear waves and cannot pass through a liquid, whereas P waves (compressional) can pass through both solids and liquids.
Conclude
The absence of S waves but arrival of P waves shows the waves crossed a liquid layer — the liquid outer core — which blocks the S waves.

Result: The station is in the S-wave shadow zone; the missing S waves show the outer core is liquid, while the P waves confirm it is still transmitting.

Typical mistakes

Confusing the compositional layers (crust, mantle, core) with the mechanical layers (lithosphere, asthenosphere); the lithosphere includes the crust plus the topmost mantle.
Saying the inner core is liquid because it is hottest; it is solid because the pressure there is high enough to keep the iron solid despite the heat.

Active revision

Explain how the behaviour of P and S waves shows that the outer core is liquid but the mantle is solid.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — earthquakes and the Earth's interior (British Geological Survey)

Seismic waves, epicentres and travel time#

StandardWJEC/Eduqas A level Geology (Component 1) — Geological investigations; (Component 2) — Earth structure and global tectonics

Travel-time graph for P and S waves

Fig. 3A (constant-velocity) travel-time graph: the S-wave line is steeper, so the S-minus-P gap widens with distance — reading the gap gives the epicentral distance.

An earthquake originates at a point at depth called the focus (or hypocentre); the point on the surface directly above it is the epicentre. From the focus, P and S body waves spread out through the Earth and surface waves travel along the ground. Because P waves travel faster than S waves, they arrive first at any seismograph, and the time gap between the P and S arrivals grows steadily with distance from the earthquake — just as the gap between a lightning flash and its thunder grows with distance. This S-minus-P time gap is the key to locating an earthquake.

For a single station, the S-P time interval gives the distance to the epicentre. If the P- and S-wave speeds are known, the distance follows from the fact that the extra time the slower S wave takes is the distance multiplied by the difference in the reciprocals of the two speeds. A travel-time graph, which plots the arrival times of P and S waves against distance, is the practical tool: the observer reads off the distance at which the P-S separation matches the observed S-P interval. The result is a distance, not a direction — the epicentre lies somewhere on a circle of that radius around the station.

To fix the epicentre, the distances from at least three stations are combined by triangulation. A circle of the appropriate radius is drawn around each of three stations, and the single point where all three circles intersect is the epicentre. Three stations are the minimum because two circles generally intersect at two points and a third is needed to resolve the ambiguity. This method, applied automatically to thousands of stations, is how seismologists pinpoint earthquakes within minutes, and it is a standard quantitative task in the examination.

The same travel-time information also reveals the structure of the Earth, because the waves' speeds change with depth. Sudden increases in P-wave speed mark boundaries such as the Moho, where the waves refract; delays and shadow zones mark the low-velocity zones and the liquid core. So seismic waves serve a double purpose: locating earthquakes at the surface and, through the way their travel times vary, revealing the layers they have passed through — the evidence used in the previous section.

\Delta t = d\left(\frac{1}{V_S} - \frac{1}{V_P}\right)

S-P time and distance

The S-minus-P time gap (delta t) equals the epicentral distance d multiplied by the difference in the reciprocals of the S- and P-wave speeds; rearranged, d = delta t divided by that difference.

Worked example

Finding the epicentral distance

A seismograph records the S wave arriving 50 s after the P wave. Using a P-wave speed of 8.0 km/s and an S-wave speed of 4.0 km/s, calculate the distance to the epicentre.

Find the reciprocal-speed difference
1/V_S minus 1/V_P = 1/4.0 minus 1/8.0 = 0.250 minus 0.125 = 0.125 s/km.
Rearrange and substitute
Distance d = delta t divided by that difference = 50 / 0.125.
$d = \frac{\Delta t}{\frac{1}{V_S} - \frac{1}{V_P}} = \frac{50}{0.125} = 400\ \text{km}$
Interpret
The epicentre is 400 km from the station, but somewhere on a circle of that radius; two further stations are needed to fix its position.

Result: The epicentre is 400 km from the station; triangulation from three stations is needed to locate it exactly.

Typical mistakes

Confusing the focus (the point at depth where the earthquake starts) with the epicentre (the point on the surface above it).
Thinking one station gives the epicentre; a single station gives only a distance (a circle), so three are needed to triangulate.

Active revision

A station records an S-P time interval of 30 s. Taking the P-wave speed as 8 km/s and the S-wave speed as 4 km/s, calculate the distance to the epicentre and explain what else is needed to locate it.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — how we locate earthquakes (British Geological Survey)

Plate tectonic theory and the driving mechanism#

StandardWJEC/Eduqas A level Geology (Component 2) — Earth structure and global tectonics

The plate-driving engine

Fig. 4The forces that move plates: mantle convection coupled with ridge push and slab pull, of which slab pull is thought to be the strongest.

Plate tectonics is the theory that the rigid lithosphere is broken into a mosaic of plates that move slowly over the weak asthenosphere, and that most of the Earth's earthquakes, volcanoes and mountain belts are concentrated at the boundaries between them. The theory grew from Alfred Wegener's earlier idea of continental drift, which he supported with the jigsaw fit of the continents, matching fossils and rock types across oceans, and matching ancient climate evidence (such as glacial deposits now near the Equator). Wegener's idea was resisted because he could suggest no adequate mechanism to move continents through solid ocean floor.

The mechanism was supplied by the discovery of seafloor spreading in the 1960s. Mapping showed a global system of mid-ocean ridges from which new oceanic crust is created, and the decisive evidence was palaeomagnetism: as new basalt crystallises at a ridge it records the Earth's magnetic field, which reverses from time to time, so the ocean floor carries a symmetrical pattern of magnetic stripes of alternating polarity on either side of the ridge (the Vine-Matthews-Morley hypothesis). The stripes are like a tape recording of spreading, and their symmetry and matching to the dated reversal timescale proved that the ocean floor grows and moves.

The plates are driven by forces linked to mantle convection and to the plates themselves. Ridge push arises because the hot, raised ridge slides the lithosphere away downhill under gravity; slab pull arises because a cold, dense, old plate subducting at a trench sinks and drags the rest of the plate after it; and both are coupled to slow convection in the mantle driven by the Earth's internal heat. Current understanding is that slab pull is the strongest of these forces, which is why plates with long subducting edges move fastest. Mantle convection, ridge push and slab pull together form the plate-driving engine.

The theory is powerful because it unifies a huge range of observations: the distribution of earthquakes and volcanoes in narrow belts, the youth of the ocean floor (nowhere older than about 200 million years, because it is continually recycled), the growth of mountain belts, and the drift of the continents through time. It also connects directly to the rest of the course, since plate tectonics drives the metamorphism, igneous activity, mountain-building and geohazards met elsewhere. Evaluating the evidence for the theory, and the relative importance of the driving forces, is a common AO3 demand.

Worked example

Calculating the rate of plate movement

Oceanic crust at a point 400 km from a mid-ocean ridge is found from its magnetic stripe to be 8 million years old. Calculate the average rate of spreading (the half-spreading rate) in mm per year.

State the relationship
Rate = distance / time, using the distance from the ridge and the age of the crust there.
$\text{rate} = \frac{\text{distance}}{\text{time}}$
Convert units
400 km = 400 000 000 mm and 8 million years = 8 000 000 years.
Calculate
Rate = 400 000 000 mm / 8 000 000 yr.
$\text{rate} = \frac{400\,000\,000\ \text{mm}}{8\,000\,000\ \text{yr}} = 50\ \text{mm/yr}$
Interpret
The half-spreading rate is 50 mm/yr (5 cm/yr), so the full ridge spreads at about 100 mm/yr — a typical fast-spreading rate.

Result: The half-spreading rate is 50 mm/yr (5 cm/yr), giving a full spreading rate of about 100 mm/yr.

Typical mistakes

Saying Wegener proposed plate tectonics; he proposed continental drift but had no mechanism — seafloor spreading supplied it decades later.
Treating mantle convection as the only driving force; ridge push and, especially, slab pull are now regarded as major contributors.

Active revision

Describe the pattern of magnetic stripes either side of a mid-ocean ridge and explain how it demonstrates seafloor spreading.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — plate tectonics (British Geological Survey)

Plate boundaries and their features#

AdvancedWJEC/Eduqas A level Geology (Component 2) — Earth structure and global tectonics

An ocean-continent subduction zone

Fig. 5A destructive (convergent) ocean-continent boundary: the dense oceanic plate subducts, forming a trench, explosive volcanoes and a Benioff zone of earthquakes deepening away from the trench.

There are three kinds of plate boundary, defined by the relative motion of the plates. At divergent (constructive) boundaries the plates move apart and new lithosphere is created; at convergent (destructive) boundaries they move together and lithosphere is consumed or crumpled; and at transform (conservative) boundaries they slide past one another and lithosphere is neither created nor destroyed. Each produces a characteristic set of landforms, earthquakes and igneous activity, and recognising a boundary from its features is a core skill.

Divergent boundaries are marked by mid-ocean ridges, where basaltic magma rises to form new ocean floor, with shallow earthquakes and gentle (effusive) volcanism; on land, divergence creates rift valleys such as the East African Rift. Convergent boundaries come in three forms. Where ocean meets continent, the denser oceanic plate subducts beneath the continent, forming a deep ocean trench, explosive andesitic volcanoes and a fold mountain belt (as along the Andes), with earthquakes deepening away from the trench. Where ocean meets ocean, one subducts to form a volcanic island arc. Where continent meets continent, neither subducts easily, so the crust crumples into high fold mountains (as in the Himalayas) with great earthquakes but little volcanism.

The depth pattern of earthquakes is itself diagnostic of a subduction zone. Earthquakes occur along the subducting slab in a plane, the Benioff zone, that dips away from the trench, so foci are shallow near the trench and progressively deeper beneath the overriding plate. This inclined zone of seismicity traces the descending slab and is strong evidence that a slab of cold lithosphere really is plunging into the mantle, tying the boundary landforms to the slab-pull mechanism of the previous section.

Transform boundaries, such as the San Andreas Fault, produce no volcanoes and no new crust, but they generate large, shallow, often destructive earthquakes as the plates lock and then slip. Reading a plate boundary therefore means combining several clues: the landforms (ridge, trench, mountains, rift), the type and violence of any volcanism (linked, through magma composition, to whether subduction is melting wet oceanic crust), and the depth and distribution of earthquakes. The systematic matching of features to boundary type recurs in the geohazards chapter.

Worked example

Identifying a boundary from its features

A region has high fold mountains, very large shallow earthquakes, but almost no volcanoes. Deduce the plate-boundary type and explain the evidence.

Use the mountains
High fold mountains indicate crustal shortening and crumpling, characteristic of a convergent boundary.
Use the lack of volcanoes
The near-absence of volcanoes rules out subduction of oceanic crust (which would melt and feed volcanoes); it points to two continents colliding, neither able to subduct.
Use the earthquakes
Large shallow earthquakes fit the crumpling of thick continental crust rather than a deep subducting slab.
Conclude
The boundary is a continent-continent collision (as in the Himalayas).

Result: A continent-continent collision boundary: fold mountains and great shallow earthquakes, but little volcanism because neither continent subducts.

Typical mistakes

Assuming all boundaries have volcanoes; transform boundaries and continent-continent collisions produce earthquakes but little or no volcanism.
Saying continental crust subducts; it is too buoyant, so continent-continent collision crumples the crust into mountains instead.

Active revision

At a boundary there is a deep ocean trench, a chain of explosive andesitic volcanoes, and earthquakes that get deeper inland. Identify the boundary type and explain each feature.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — plate boundaries (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Earth structure and global tectonics

4 sections~15 min reading time3 competenciesLevel Standard 3 · Advanced 1

Earth structure and global tectonics

Key points

Interpreting seismic evidence

Key points

Finding the epicentral distance

Key points

Calculating the rate of plate movement

Key points

Identifying a boundary from its features

Sources

Earth structure and global tectonics

Key points

Interpreting seismic evidence

Key points

Finding the epicentral distance

Key points

Calculating the rate of plate movement

Key points

Identifying a boundary from its features

Sources