Notes · GeologyUK · A-Levels

Geology of the lithosphere

This optional advanced theme looks in depth at the outer shell of the Earth and the engine that drives it. It covers the Earth's internal heat and how it is lost, the mechanisms that drive plate motion, the creation, ageing and recycling of oceanic lithosphere, and the processes that build and preserve the continental crust, developing quantitatively the plate-tectonic ideas introduced earlier in the course.

4 sections~15 min reading time3 competenciesLevel Advanced 4

T·131313 / 13

Exam profile

AO1 · Describe the Earth's heat budget, plate-driving forces and the life cycle of oceanic lithosphereAO2 · Apply the geothermal gradient and ocean-floor subsidence relationships quantitativelyAO3 · Analyse and evaluate the mechanisms that drive and recycle the lithosphere

Operators:describeexplaincalculateinterpretanalyseevaluate

basic level

This is an optional advanced theme; a foundation grasp is the Earth's internal heat, the plate-driving forces and the creation and destruction of ocean floor.

higher level

The full theme requires you to work quantitatively with the geothermal gradient and ocean-floor subsidence and to evaluate the driving mechanisms.

Depth

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Contents · 4 sections

Geology of the lithosphere

The Earth's heat and heat loss#

AdvancedWJEC/Eduqas A level Geology (Component 3, optional theme) — Geology of the lithosphere

The near-surface geothermal gradient

Fig. 1The near-surface geothermal gradient (here 25 degrees C per km from a surface temperature of 15 degrees): temperature rises steeply through the conductive crust.

Everything the lithosphere does is ultimately powered by the Earth's internal heat, which comes from two sources: the primordial heat left over from the planet's formation and from the sinking of iron to the core, and the radiogenic heat continually produced by the decay of long-lived radioactive isotopes (chiefly uranium, thorium and potassium) in the mantle and crust. This heat flows outward toward the cooler surface, and the way it is lost drives mantle convection, plate tectonics, volcanism and metamorphism, so understanding the heat budget is the foundation of the whole theme.

Close to the surface the temperature rises with depth along the geothermal gradient, which in the upper continental crust averages roughly 25 to 30 degrees Celsius per kilometre. This gradient is steep near the surface, where heat is carried by conduction through the rigid lithosphere, but it flattens dramatically deeper down: if the near-surface gradient continued unchanged the mantle would be completely molten, which it is not, so the gradient must decrease with depth. The geothermal gradient is measured directly in deep boreholes and mines and is fundamental to geothermal energy and to metamorphism.

The reason the gradient flattens is a change in how heat is transferred. Through the cool, rigid lithosphere heat moves by conduction, a slow process across a large temperature difference, giving the steep near-surface gradient. Below the lithosphere, in the hot, ductile asthenosphere and mantle, heat is carried far more efficiently by convection — the physical movement of hot material upward and cool material downward — which keeps the deep temperature gradient gentle. The boundary between conductive lithosphere and convecting asthenosphere is, in effect, a thermal boundary layer.

This distinction has a profound consequence: mantle convection, driven by the outward flow of internal heat, is the engine of plate tectonics. Hot mantle rises, spreads beneath the lithosphere and sinks again as it cools, and the plates are the cool, rigid top of this convecting system, created where hot mantle rises at ridges and destroyed where cold lithosphere sinks at trenches. The heat budget therefore links the deep Earth to the surface: the loss of internal heat by convection is what moves the continents and recycles the ocean floor, the subjects of the following sections.

T = T_0 + g \times z

Temperature and depth (geothermal gradient)

The temperature T at depth z equals the surface temperature T0 plus the geothermal gradient g (about 25-30 degrees C per km in the crust) times the depth; the gradient is steep near the surface (conduction) and flattens at depth (convection).

Worked example

Temperature at depth

The surface temperature is 15 degrees C and the geothermal gradient in the crust is 25 degrees C per km. Calculate the temperature at a depth of 4 km, and comment on what happens to the gradient at greater depth.

State the relationship
Temperature at depth = surface temperature + gradient x depth.
Substitute
T = 15 + 25 x 4.
$T = 15 + 25 \times 4 = 115\ ^{\circ}\text{C}$
Comment
At greater depth the gradient flattens: if 25 degrees C per km continued, the temperature at 100 km would be far above the melting point of the mantle, which it is not, because convection carries heat efficiently below the lithosphere.

Result: The temperature at 4 km is 115 degrees C; the gradient must flatten at depth, where convection replaces conduction, or the mantle would melt.

Typical mistakes

Extrapolating the near-surface gradient to great depth; it flattens because convection takes over from conduction, or the mantle would be molten.
Forgetting radiogenic heat; much of the Earth's present heat comes from radioactive decay, not only from its formation.

Active revision

The geothermal gradient near the surface is 30 degrees C per km and the surface temperature is 10 degrees C. Calculate the temperature at 3 km depth and explain why this gradient cannot continue to the base of the mantle.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — Earth's heat and geothermal energy (British Geological Survey)

Plate-driving mechanisms#

AdvancedWJEC/Eduqas A level Geology (Component 3, optional theme) — Geology of the lithosphere

The forces that drive a plate

Fig. 2Plate-driving forces: ridge push slides the plate off the elevated ridge, slab pull drags it down at the trench, and mantle convection underlies both; slab pull dominates.

The plates move because the outward flow of the Earth's internal heat sets the mantle convecting, but the forces that actually move each plate are more specific than 'convection' alone, and the theme requires them to be distinguished. Three forces are recognised: the drag exerted on the base of a plate by the convecting mantle beneath it, ridge push acting at constructive margins, and slab pull acting at destructive margins. The modern view, supported by the observation that plates with long subducting edges move fastest, is that slab pull is the dominant force.

Ridge push is a gravitational force. At a mid-ocean ridge the lithosphere is young, hot, thin and buoyant, so it stands high; as it moves away from the ridge it cools, thickens and becomes denser, so the elevated ridge effectively slides the plate away downhill under gravity, pushing it from behind. Ridge push is therefore not the magma at the ridge shoving the plates apart, but the gravitational consequence of the ridge's height, a subtle point often misunderstood.

Slab pull is the negative buoyancy of a subducting slab. Old oceanic lithosphere, having cooled for tens of millions of years, is colder and denser than the hot asthenosphere beneath it, so once it begins to sink at a trench its own weight pulls it down into the mantle, dragging the rest of the plate behind it. Because the slab keeps cooling and remains dense as it descends, slab pull is a powerful and self-sustaining force, which is why it is considered the main driver, and why plates anchored by long subducting slabs move fastest.

In reality all three forces act together and interact with the convecting mantle, and their balance varies from plate to plate: a plate with little subducting edge and a long ridge is pushed, one anchored by a long trench is pulled, and basal drag may either help or resist depending on the mantle flow beneath. Evaluating the relative importance of ridge push, slab pull and mantle drag for a given plate, and appreciating that they are all expressions of the same underlying loss of internal heat, is the analytical heart of this section.

Worked example

Weighing the driving forces

Plate A has a long subducting edge at a deep trench; plate B has a long mid-ocean ridge but no subducting edge. Predict which moves faster and explain in terms of the driving forces.

Analyse plate A
Plate A is anchored by a long, cold, dense subducting slab, so it experiences strong slab pull, the dominant driving force.
Analyse plate B
Plate B is driven mainly by ridge push, a weaker gravitational force, and lacks the powerful slab pull.
Conclude
Plate A moves faster, because slab pull is stronger than ridge push; this matches the observation that plates with long trenches move fastest.

Result: Plate A (with the subducting slab) moves faster, because slab pull is the dominant force and exceeds the ridge push driving plate B.

Typical mistakes

Describing ridge push as magma pushing the plates apart; it is gravitational sliding off the elevated, buoyant ridge.
Treating 'mantle convection' as a separate alternative to ridge push and slab pull; the latter are the specific forces through which convection and cooling move the plates.

Active revision

Explain why a plate with a long subducting edge tends to move faster than one with none, referring to slab pull and ridge push.

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)

Formation and recycling of oceanic lithosphere#

AdvancedWJEC/Eduqas A level Geology (Component 3, optional theme) — Geology of the lithosphere

Ocean-floor depth against age

Fig. 3Ocean-floor depth increases with the square root of crustal age as the lithosphere cools and sinks (illustrative constants): from about 2500 m at the ridge to about 6000 m at 100 Ma.

Oceanic lithosphere has a life cycle: it is born at mid-ocean ridges, ages and cools as it moves away, and is destroyed at subduction zones, so no ocean floor is very old. At a ridge, hot mantle wells up and partially melts as the pressure falls (decompression melting), and the basaltic magma produced builds new oceanic crust — pillow lavas at the surface, sheeted dykes beneath, and gabbro at depth, over mantle peridotite. This layered structure, occasionally thrust up onto land as an ophiolite, is the direct evidence for how ocean floor is made.

As the new lithosphere moves away from the ridge it cools, and cooling has two measurable consequences. It thickens, because the base of the lithosphere is a cooling front that penetrates deeper with time; and it becomes denser and therefore sinks, so the ocean floor deepens away from the ridge. The relationship is quantitative and famous: for young ocean floor the depth below the ridge crest increases in proportion to the square root of the age of the crust, a signature of simple thermal cooling that is beautifully confirmed by the shape of the ocean basins.

Because the lithosphere keeps cooling and densifying, it eventually becomes denser than the asthenosphere beneath it and is ready to sink; at a subduction zone the old, cold, dense slab plunges back into the mantle, driven by its own negative buoyancy (slab pull), and is recycled. This is why the ocean floor is nowhere older than about 200 million years — a tiny fraction of Earth history — even though the continents preserve rocks billions of years old: the oceans are continually made and destroyed, while the buoyant continents ride above and survive.

This creation-and-recycling of oceanic lithosphere is the clearest expression of the whole plate-tectonic system, tying together the ridge magmatism, the cooling and subsidence of the ocean floor, and the slab pull that recycles it. It also explains the fundamental asymmetry of the Earth's surface between the young, dense, ephemeral ocean floor and the old, light, persistent continents, and it provides one of the neatest quantitative tests in geology — the square-root-of-age depth relationship — for exactly the kind of AO2 application this theme rewards.

d = d_0 + k\sqrt{\text{age}}

Ocean-floor depth and age (thermal subsidence)

For young ocean floor the depth d below sea level increases with the square root of the crust's age as it cools and sinks; here d0 (about 2500 m) is the ridge-crest depth and k (about 350 m per square-root-million-years) is an empirical constant.

Worked example

Ocean-floor depth from age

Using the relationship depth = 2500 + 350 x square root of the age (depth in metres, age in millions of years), calculate the depth of ocean floor that is 25 million years old, and compare it with the ridge crest.

Take the square root of the age
The square root of 25 is 5.
Substitute
Depth = 2500 + 350 x 5.
$d = 2500 + 350 \times \sqrt{25} = 2500 + 350 \times 5$
Evaluate
Depth = 2500 + 1750 = 4250 m.
$d = 2500 + 1750 = 4250\ \text{m}$
Compare
The ridge crest is at about 2500 m, so in 25 million years the cooling, sinking lithosphere has deepened by about 1750 m.

Result: The 25-million-year-old ocean floor lies at about 4250 m, some 1750 m deeper than the 2500 m ridge crest, through thermal subsidence.

Typical mistakes

Thinking ocean floor deepens in direct proportion to age; the depth increases with the square root of the age (thermal cooling).
Forgetting why no ocean floor is old; it is continually recycled by subduction, unlike the persistent continents.

Active revision

Explain why the ocean floor is deepest far from a mid-ocean ridge and why no oceanic crust is older than about 200 million years.

Active recall

Recall the key points — then reveal.

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

Continental-crust processes#

AdvancedWJEC/Eduqas A level Geology (Component 3, optional theme) — Geology of the lithosphere

Why continents persist and oceans recycle

Fig. 4The asymmetry of the crust: dense oceanic crust is recycled by subduction, while buoyant, silica-rich continental crust, made at subduction zones, resists subduction and persists.

The continental crust is fundamentally different from the oceanic crust, and its processes explain why continents persist while oceans come and go. Continental crust is thick (typically 30 to 70 km), old (preserving rocks up to about 4 billion years old), and, crucially, of lower density than oceanic crust because it is broadly granitic (silica- and aluminium-rich) rather than basaltic. This low density makes the continents buoyant, so that, like a cork that cannot be pushed under water, they float on the mantle and resist subduction, which is why they survive while the denser ocean floor is recycled.

New continental crust is generated chiefly at subduction zones. Where oceanic lithosphere descends, water driven off the slab lowers the melting point of the overlying mantle, generating magma that rises and, through repeated melting and crystallisation (differentiation following Bowen's reaction series), becomes progressively more silica-rich and less dense. This buoyant, silica-rich magma is added to the overriding plate as volcanic arcs and granite batholiths, so subduction zones are factories of new continental crust, and continents grow outward by the accretion of these arcs and of scraped-up sediments and terranes.

The continental crust is also stabilised and preserved by its own buoyancy and by the way it thickens. During continental collision the crust is shortened and thickened into mountain belts, and the low-density crustal root that forms floats high (isostasy), so mountains are supported from below like an iceberg. Over long times, erosion of the mountains and the slow adjustment of this root return the crust toward a stable thickness, but the ancient, buoyant cores of the continents (the cratons and shields) are so stable that they have survived largely intact for billions of years.

Taken together, these processes explain the grand asymmetry of the Earth: a two-way traffic in which oceanic crust is endlessly created and destroyed while continental crust, once made buoyant by differentiation at subduction zones, accumulates and is preserved. The continents are thus the long-term memory of the planet, recording its whole history, while the oceans record only the most recent chapter. Understanding why this is so — the interplay of density, subduction-zone magmatism, differentiation and isostasy — is a fitting synthesis of the lithosphere theme and of the plate-tectonic thread that runs through the entire course.

Worked example

Explaining the persistence of continents

Explain, using density and subduction-zone processes, why continental crust survives for billions of years while oceanic crust is continually recycled.

Density of the crusts
Oceanic crust is basaltic and dense, so as it cools it becomes denser than the mantle and can subduct; continental crust is granitic and less dense, so it is buoyant.
Making continental crust
At subduction zones, melting and differentiation produce silica-rich, low-density magma that is added to the continents, keeping them buoyant.
Consequence
Because the buoyant continental crust resists subduction, it is not recycled and accumulates, preserving rocks billions of years old, while the dense ocean floor is recycled within about 200 million years.

Result: Continents persist because their low-density, silica-rich crust (made and differentiated at subduction zones) is buoyant and resists subduction, unlike the dense, recyclable ocean floor.

Typical mistakes

Thinking continents float because they are thick; it is their lower density (granitic composition), not just thickness, that makes them buoyant.
Assuming continental crust subducts like oceanic crust; it is too buoyant, so it thickens and persists instead.

Active revision

Explain why the oldest continental rocks are billions of years old whereas the oldest ocean floor is only about 200 million years old.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — the crust and continents (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Geology of the lithosphere

4 sections~15 min reading time3 competenciesLevel Advanced 4

Geology of the lithosphere

Key points

Temperature at depth

Key points

Weighing the driving forces

Key points

Ocean-floor depth from age

Key points

Explaining the persistence of continents

Sources

Geology of the lithosphere

Key points

Temperature at depth

Key points

Weighing the driving forces

Key points

Ocean-floor depth from age

Key points

Explaining the persistence of continents

Sources