Eduqas A-level Geology · T5

Geology of the lithosphere (option)

Exam-style questions with answers for T5 Geology of the lithosphere (option) in Eduqas A-level Geology (spec A480QS). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below.

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Question 1 · Spec T5, Key Idea 1 · Lithosphere
State the isotherm that defines the base of the thermal lithosphere. [1 mark]
Answer:
About 1300 °C.
Question 2 · Spec T5, Key Idea 1 · Lithosphere
Explain why the lithosphere is rigid while the mantle below can flow. [2 marks]
Answer:
The lithosphere is cooler, so it is stronger and behaves rigidly (1); the hotter mantle below deforms slowly by creep (1).
Question 3 · Spec T5, Key Idea 1 · Asthenosphere
Explain how the asthenosphere can flow without being molten. [2 marks]
Answer:
At high temperature, solid mantle rock deforms slowly by creep (1); it contains at most a small fraction of melt (1).
Question 4 · Spec T5, Key Idea 1 · Lithosphere
Explain why the Moho is not the base of the lithosphere. [2 marks]
Answer:
The Moho separates crust from mantle by composition (1); the lithosphere also includes the uppermost rigid mantle, extending deeper (1).
Question 5 · Spec T5, Key Idea 1 · Oceanic lithosphere
Describe how oceanic lithosphere changes as it moves away from a ridge. [3 marks]
Answer:
It cools (1); thickens (1); becomes denser and subsides, so the ocean deepens (1).
Question 6 · Spec T5, Key Idea 1 · Oceanic lithosphere
Explain why oceanic lithosphere thickens with age but oceanic crust does not. [2 marks]
Answer:
The crust thickness is fixed when it forms at the ridge (1); cooling adds rigid mantle to the base of the lithosphere (1).
Question 7 · Spec T5, Key Idea 1 · Heat flow
Explain why heat flow is high at mid-ocean ridges. [2 marks]
Answer:
Hot mantle and magma are close to the surface (1); and newly formed lithosphere is cooling rapidly (1).
Question 8 · Spec T5, Key Idea 1 · Continental lithosphere
Explain why continental lithosphere varies in thickness. [2 marks]
Answer:
Old, cold cratons have thick lithosphere (1); active rifts and young orogens have thinner, hotter lithosphere (1).
Question 9 · Spec T5, Key Idea 1 · Lithosphere thickness
Using thickness (km) = 10√age (Ma), calculate the thickness of 16 Ma lithosphere. [1 mark]
Answer:
10 × √16 = 40 km.
Question 10 · Spec T5, Key Idea 1 · Lithosphere thickness
Using thickness = 10√age, state what happens to thickness when age increases from 16 to 64 Ma. [2 marks]
Answer:
It doubles from 40 km to 80 km (1); because thickness is proportional to the square root of age (1).
Question 11 · Spec T5, Key Idea 1 · Low velocity zone
State the seismic evidence for a low velocity zone. [1 mark]
Answer:
Seismic wave velocities decrease in a zone below the lithosphere, about 100–250 km deep.
Question 12 · Spec T5, Key Idea 1 · Seismic waves
Explain how a low velocity layer affects seismic waves. [2 marks]
Answer:
Waves slow down in the layer (1); so they are refracted, changing the arrivals recorded at the surface (1).
Question 13 · Spec T5, Key Idea 1 · Seismic surveys
Distinguish between seismic refraction and seismic reflection. [2 marks]
Answer:
Refraction uses waves bent along layers to find velocities and depths of layers (1); reflection uses waves bounced back from boundaries to image structures (1).
Question 14 · Spec T5, Key Idea 1 · Seismic surveys
A reflection returns after 2.0 s. The velocity is 4000 m/s. Calculate the depth. [2 marks]
Answer:
Depth = velocity × time ÷ 2 = 4000 × 2.0 ÷ 2 (1); = 4000 m (1).
Question 15 · Spec T5, Key Idea 1 · Seismic surveys
Explain why seismic layers must be checked against rock samples. [2 marks]
Answer:
Different rocks can have similar seismic velocities (1); so drilling and ophiolites are needed to identify the rocks (1).
Question 16 · Spec T5, Key Idea 2 · Magmatism
State what MORB stands for and where it forms. [2 marks]
Answer:
Mid-ocean ridge basalt (1); formed by decompression melting of mantle at spreading ridges (1).
Question 17 · Spec T5, Key Idea 2 · Large igneous provinces
Describe a large igneous province. [2 marks]
Answer:
An enormous volume of mainly basaltic magma (1); erupted over a geologically short time, e.g. the Deccan Traps (1).
Question 18 · Spec T5, Key Idea 2 · Subduction magmatism
Compare magmatism at island arcs and continental margins. [2 marks]
Answer:
Both are caused by water from the subducting slab melting the mantle wedge (1); continental margin magmas interact with thick continental crust, giving more andesite and granite (1).
Question 19 · Spec T5, Key Idea 2 · Hotspots
Explain how an age-progressive volcanic chain forms above a hotspot. [2 marks]
Answer:
The plate moves over a fixed mantle plume (1); so volcanoes are carried away and become progressively older with distance (1).
Question 20 · Spec T5, Key Idea 2 · Hotspots
Explain why a hotspot track alone does not prove a deep mantle plume. [1 mark]
Answer:
It shows surface volcanism but not the depth of the source; seismic tomography and geochemistry are needed.
Question 21 · Spec T5, Key Idea 2 · Oceanic crust
Describe seismic layers 1, 2 and 3 of oceanic crust. [3 marks]
Answer:
Layer 1: sediments (1); layer 2: pillow lavas and sheeted dykes (1); layer 3: gabbro (1).
Question 22 · Spec T5, Key Idea 2 · Ophiolites
Describe the sequence of rocks in a complete ophiolite, from bottom to top. [3 marks]
Answer:
Mantle peridotite and layered gabbro (1); sheeted dykes (1); pillow lavas and deep-sea sediments (1).
Question 23 · Spec T5, Key Idea 2 · Ophiolites
Explain the significance of a sheeted dyke complex. [2 marks]
Answer:
Each dyke was intruded into the centre of the previous one as the plates moved apart (1); showing continuous extension at a spreading ridge (1).
Question 24 · Spec T5, Key Idea 2 · Ophiolites
State what pillow lavas show in an ophiolite. [1 mark]
Answer:
Lava erupted under water, on the sea floor.
Question 25 · Spec T5, Key Idea 2 · Ophiolites
Explain why ophiolites rarely preserve a complete sequence. [2 marks]
Answer:
They are faulted and deformed when thrust onto land (1); and may be metamorphosed or eroded (1).
Question 26 · Spec T5, Key Idea 2 · Ocean drilling
Explain how ocean drilling and ophiolites complement each other. [2 marks]
Answer:
Drilling samples oceanic crust in place, but only to limited depths (1); ophiolites expose complete sections but are deformed (1).
Question 27 · Spec T5, Key Idea 2 · Wilson cycle
Describe the stages of the Wilson cycle. [3 marks]
Answer:
Continental rifting and a narrow sea (1); a widening ocean (1); subduction, ocean closure and continental collision (1).
Question 28 · Spec T5, Key Idea 2 · Continental rifting
State three features of an early continental rift. [3 marks]
Answer:
Any three from: normal faulting and rift valleys (1); crustal thinning (1); subsidence (1); volcanism (1).
Question 29 · Spec T5, Key Idea 2 · Ocean basins
State three types of evidence that an ocean basin is growing. [3 marks]
Answer:
Any three from: sea floor getting older away from the ridge (1); symmetrical magnetic stripes (1); GPS measurements of spreading (1); oceanic crustal structure (1).
Question 30 · Spec T5, Key Idea 2 · Spreading rates
Oceanic crust 125 km from a ridge is 5 Ma old. Calculate the half-spreading rate in cm/year. [2 marks]
Answer:
125 ÷ 5 = 25 km/Ma (1); = 2.5 cm/year (1).
Question 31 · Spec T5, Key Idea 2 · Spreading rates
Distinguish between half-spreading rate and full spreading rate. [2 marks]
Answer:
Half rate is the movement of one plate away from the ridge (1); full rate is the total separation of both plates (1).
Question 32 · Spec T5, Key Idea 2 · Magnetic anomalies
Explain how magnetic anomalies on the ocean floor are dated. [2 marks]
Answer:
The pattern of reversals is matched to the magnetic polarity timescale (1); which is dated by radiometric ages of lavas on land (1).
Question 33 · Spec T5, Key Idea 2 · Plate speeds
Volcanoes 300 km apart along a hotspot chain differ in age by 6 Ma. Calculate the plate speed in cm/year. [2 marks]
Answer:
300 ÷ 6 = 50 km/Ma (1); = 5 cm/year (1).
Question 34 · Spec T5, Key Idea 2 · Plate speeds
Explain why speeds from hotspot tracks may differ from relative plate speeds. [2 marks]
Answer:
Hotspot speeds are measured relative to the mantle source, which may itself move (1); relative speeds compare two plates (1).
Question 35 · Spec T5, Key Idea 3 · Subduction
Explain why old oceanic lithosphere subducts. [2 marks]
Answer:
As it cools it becomes thicker and denser (1); until it is denser than the asthenosphere below and sinks (1).
Question 36 · Spec T5, Key Idea 3 · Continents
Explain why continental crust survives longer than oceanic crust. [2 marks]
Answer:
Continental crust is less dense (1); so it is too buoyant to be subducted (1).
Question 37 · Spec T5, Key Idea 3 · Supercontinents
State three types of evidence used to reconstruct a supercontinent. [3 marks]
Answer:
Any three from: matching mountain belts (1); matching rock sequences (1); fossils (1); palaeomagnetism (1).
Question 38 · Spec T5, Key Idea 3 · Supercontinents
Explain why reconstructions of older supercontinents are more uncertain. [2 marks]
Answer:
The oceanic crust from that time has been subducted (1); and older rocks are more altered, with fewer reliable ages and palaeomagnetic data (1).
Question 39 · Spec T5, Key Idea 3 · Palaeomagnetism
Explain why palaeomagnetism cannot fix the position of a continent exactly. [1 mark]
Answer:
It gives latitude and orientation but not longitude.
Question 40 · Spec T5, Key Idea 3 · Collision
Explain how continental collision thickens the lithosphere. [2 marks]
Answer:
Crust is shortened by folding and thrusting (1); stacking rocks to form a thick crustal root (1).
Question 41 · Spec T5, Key Idea 3 · Accretionary prisms
Explain how an accretionary prism forms. [2 marks]
Answer:
Sediments on the subducting plate are scraped off at the trench (1); and stacked against the overriding plate by thrusting (1).
Question 42 · Spec T5, Key Idea 3 · Isostasy
Describe isostasy. [2 marks]
Answer:
Lithosphere floats in balance on the denser mantle (1); thick or low-density crust stands higher and has a deep root (1).
Question 43 · Spec T5, Key Idea 3 · Isostasy
Using root = elevation × crust density ÷ (mantle density − crust density), calculate the root under a 3 km mountain (2700 and 3300 kg/m³). [2 marks]
Answer:
3 × 2700 ÷ (3300 − 2700) (1); = 13.5 km (1).
Question 44 · Spec T5, Key Idea 3 · Isostasy
Explain why erosion of mountains causes uplift. [2 marks]
Answer:
Erosion removes mass from the mountains (1); so the crust rises isostatically to restore balance (1).
Question 45 · Spec T5, Key Idea 3 · Delamination
Describe lithospheric delamination. [2 marks]
Answer:
The dense lower lithosphere detaches and sinks into the mantle (1); it is replaced by hot, buoyant mantle, causing uplift and magmatism (1).
Question 46 · Spec T5, Key Idea 3 · Orogenic collapse
Explain why a thickened mountain belt can undergo extension. [2 marks]
Answer:
The thick crust has high gravitational potential energy (1); and becomes hot and weak, so it spreads out under its own weight (1).
Question 47 · Spec T5, Key Idea 3 · Fold-thrust belts
Explain how fold-thrust belts accommodate shortening. [2 marks]
Answer:
Low-angle thrusts stack sheets of rock on top of each other (1); with folding above detachment surfaces (1).
Question 48 · Spec T5, Key Idea 3 · Nappes
Define the term 'nappe'. [2 marks]
Answer:
A large sheet of rock moved a long distance (1); along a low-angle thrust or as a large recumbent fold (1).
Question 49 · Spec T5, Key Idea 3 · Extensional basins
Explain how lithospheric extension forms a sedimentary basin. [2 marks]
Answer:
Stretching thins the crust and forms normal faults (1); then cooling of the lithosphere causes further subsidence (1).
Question 50 · Spec T5, Key Idea 3 · Foreland basins
Distinguish between a foreland basin and an extensional basin. [2 marks]
Answer:
A foreland basin forms where the weight of a mountain belt bends the lithosphere down (1); an extensional basin forms by stretching and normal faulting (1).

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