Eduqas A-level Geology · F4

Earth structure and global tectonics

Exam-style questions with answers for F4 Earth structure and global tectonics 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 F4, Key Idea 1 · Earth structure
Name the layers of the Earth from the surface to the centre. [2 marks]
Answer:
Crust and mantle (1); liquid outer core and solid inner core (1).
Question 2 · Spec F4, Key Idea 1 · Crust
State two differences between continental and oceanic crust. [2 marks]
Answer:
Any two from: continental crust is thicker (35–70 km v 6–10 km) (1); continental crust is felsic (granitic), oceanic is mafic (basaltic) (1); continental crust is less dense (2.7 v 3.0 g/cm³) (1); continental crust is much older (1).
Question 3 · Spec F4, Key Idea 1 · Crust
Explain why the oldest rocks are found on continents, not the ocean floor. [2 marks]
Answer:
Oceanic crust is destroyed by subduction, so none is older than about 200 Ma (1); continental crust is too buoyant to subduct, so it survives (1).
Question 4 · Spec F4, Key Idea 1 · Mantle
Name the rock that is thought to make up the upper mantle and state its main minerals. [2 marks]
Answer:
Peridotite (1); mostly olivine with some pyroxene (1).
Question 5 · Spec F4, Key Idea 1 · Mantle
Explain why mantle xenoliths are useful but limited evidence. [2 marks]
Answer:
They are real samples of deep material brought to the surface (1); but they come from only a few places and depths and may be altered during their ascent (1).
Question 6 · Spec F4, Key Idea 1 · Core
Explain how iron meteorites provide evidence for the composition of the core. [2 marks]
Answer:
They are thought to be fragments of the cores of differentiated planetary bodies (1); their iron-nickel composition supports an iron-rich core (1).
Question 7 · Spec F4, Key Idea 1 · Density
The Earth's mean density is 5.5 g/cm³ and crustal rocks are about 2.8 g/cm³. Explain what this shows. [2 marks]
Answer:
The interior must contain material much denser than the crust (1); suggesting a dense iron-rich core (1).
Question 8 · Spec F4, Key Idea 1 · Seismic waves
Distinguish between P waves and S waves. [2 marks]
Answer:
P waves are compressional, with particle motion parallel to the direction of travel, and travel through solids and liquids (1); S waves are shear waves, with motion perpendicular to travel, and only travel through solids (1).
Question 9 · Spec F4, Key Idea 1 · Seismic waves
Explain how S waves show that the outer core is liquid. [2 marks]
Answer:
S waves cannot travel through liquids (1); and they are not transmitted through the outer core, producing an S-wave shadow zone beyond 103° (1).
Question 10 · Spec F4, Key Idea 1 · Seismic waves
Explain why seismic waves follow curved paths through the mantle. [2 marks]
Answer:
Seismic velocity increases with depth as density and rigidity change (1); so the waves are continuously refracted (1).
Question 11 · Spec F4, Key Idea 1 · Shadow zone
Explain how the P-wave shadow zone forms. [2 marks]
Answer:
P waves are refracted sharply at the core-mantle boundary because velocity drops (1); so no direct P waves arrive between about 103° and 142° from the epicentre (1).
Question 12 · Spec F4, Key Idea 1 · Seismic discontinuities
State what a sudden change in seismic velocity can show. [2 marks]
Answer:
A change in composition (1); or a change in physical state or mineral structure (1).
Question 13 · Spec F4, Key Idea 1 · Seismic waves
Distinguish between surface waves and body waves. [2 marks]
Answer:
Surface waves travel along the Earth's surface and cause most damage (1); body waves (P and S) travel through the interior (1).
Question 14 · Spec F4, Key Idea 1 · Locating earthquakes
Explain why the time between P and S wave arrivals increases with distance. [2 marks]
Answer:
P waves travel faster than S waves (1); so the gap between them grows the further they travel (1).
Question 15 · Spec F4, Key Idea 1 · Calculations
P waves travel at 6 km/s. Calculate the time to travel 120 km. [2 marks]
Answer:
Time = distance ÷ speed = 120 ÷ 6 (1); = 20 s (1).
Question 16 · Spec F4, Key Idea 1 · Locating earthquakes
Explain why at least three seismic stations are needed to locate an epicentre. [2 marks]
Answer:
Each station gives only the distance to the epicentre, a circle on a map (1); three circles intersect at a single point (1).
Question 17 · Spec F4, Key Idea 1 · Moho
State what the Mohorovičić discontinuity separates and how it is recognised. [2 marks]
Answer:
It separates the crust from the mantle (1); seismic wave velocities increase sharply across it (1).
Question 18 · Spec F4, Key Idea 1 · Moho
Explain how ocean drilling can test interpretations of the Moho. [2 marks]
Answer:
Drilling recovers actual rock samples from depth (1); which can be compared with the layers inferred from seismic data (1).
Question 19 · Spec F4, Key Idea 1 · Gravity
Explain why a dense buried rock body produces a positive gravity anomaly. [2 marks]
Answer:
It has extra mass compared with the surrounding rock (1); which increases the local gravitational pull (1).
Question 20 · Spec F4, Key Idea 1 · Magnetic field
Explain how the Earth's magnetic field supports a liquid outer core. [2 marks]
Answer:
The field is generated by a geodynamo (1); which needs convection currents in a molten, electrically conducting iron outer core (1).
Question 21 · Spec F4, Key Idea 1 · Density
A basalt sample has a mass of 87 g and a volume of 30 cm³. Calculate its density. [2 marks]
Answer:
Density = 87 ÷ 30 (1); = 2.9 g/cm³ (1).
Question 22 · Spec F4, Key Idea 1 · Evidence
Explain why comparing granite and basalt densities alone cannot prove the Earth's internal structure. [2 marks]
Answer:
They are only surface samples (1); deep structure needs other evidence, such as seismic waves, gravity and meteorites (1).
Question 23 · Spec F4, Key Idea 1 · Lithosphere
Define the lithosphere. [2 marks]
Answer:
The crust and the uppermost mantle (1); which form a rigid outer layer broken into tectonic plates (1).
Question 24 · Spec F4, Key Idea 1 · Asthenosphere
Explain why it is wrong to describe the asthenosphere as liquid. [2 marks]
Answer:
It is mainly solid mantle that behaves in a ductile, plastic way (1); with only a small amount (1–5%) of partial melt (1).
Question 25 · Spec F4, Key Idea 1 · Low velocity zone
Explain how the low velocity zone provides evidence for the asthenosphere. [2 marks]
Answer:
Seismic waves slow down between about 100 and 250 km depth (1); which suggests hot, weaker rock with a small melt fraction (1).
Question 26 · Spec F4, Key Idea 2 · Plate boundaries
Describe the features of a divergent oceanic plate boundary. [3 marks]
Answer:
A mid-ocean ridge with a central rift valley (1); shallow-focus earthquakes (1); basaltic volcanism forming new oceanic crust (1).
Question 27 · Spec F4, Key Idea 2 · Subduction
Explain how earthquake depths provide evidence for subduction. [2 marks]
Answer:
Earthquake foci get deeper away from the trench, down to about 700 km (1); this inclined Benioff zone traces the descending slab (1).
Question 28 · Spec F4, Key Idea 2 · Conservative boundaries
Explain why conservative boundaries have earthquakes but no volcanoes. [2 marks]
Answer:
Plates slide past each other, so friction builds stress that is released as earthquakes (1); no crust is created or destroyed, so there is no melting (1).
Question 29 · Spec F4, Key Idea 2 · Plate boundaries
A trench lies next to a volcanic arc with an inclined zone of deep earthquakes. Identify the boundary type. [1 mark]
Answer:
A destructive (convergent) boundary with subduction.
Question 30 · Spec F4, Key Idea 2 · Earthquake distribution
Explain why plotting earthquake epicentres is useful for studying plate boundaries. [2 marks]
Answer:
Most earthquakes occur in narrow belts along plate boundaries (1); so their distribution outlines the edges of the plates (1).
Question 31 · Spec F4, Key Idea 2 · Data analysis
Explain why focal depth and magnitude should be treated as separate variables. [2 marks]
Answer:
Depth describes where the rupture occurs, while magnitude describes the energy released (1); any relationship between them must be tested from data (1).
Question 32 · Spec F4, Key Idea 2 · Data analysis
Explain what a positive correlation between two variables shows. [2 marks]
Answer:
As one variable increases, the other also increases (1); but it does not prove that one causes the other (1).
Question 33 · Spec F4, Key Idea 2 · Continental drift
Explain why Wegener's theory of continental drift was not accepted at first. [2 marks]
Answer:
He had evidence such as matching coastlines, rocks and fossils (1); but could not explain a mechanism for moving the continents (1).
Question 34 · Spec F4, Key Idea 2 · Sea-floor spreading
Explain how the age of the ocean floor supports sea-floor spreading. [2 marks]
Answer:
Oceanic crust is youngest at the ridge and gets older away from it (1); symmetrically on both sides, showing new crust forms at ridges and moves outwards (1).
Question 35 · Spec F4, Key Idea 2 · GPS
Explain how GPS provides direct evidence for plate movement. [2 marks]
Answer:
Precise positions of stations are measured repeatedly over years (1); showing changes in distance between stations on different plates (1).
Question 36 · Spec F4, Key Idea 2 · Calculations
Two GPS stations move apart by 12 cm in four years. Calculate the rate in mm/year. [2 marks]
Answer:
12 cm = 120 mm (1); 120 ÷ 4 = 30 mm/year (1).
Question 37 · Spec F4, Key Idea 2 · Plate mechanisms
Explain slab pull. [2 marks]
Answer:
Cold, dense oceanic lithosphere sinks into the mantle at a subduction zone (1); pulling the rest of the plate behind it (1).
Question 38 · Spec F4, Key Idea 2 · Plate mechanisms
Explain ridge push. [2 marks]
Answer:
New lithosphere at the ridge is hot and elevated (1); it slides down and away from the ridge under gravity as it cools and thickens (1).
Question 39 · Spec F4, Key Idea 2 · Plate mechanisms
Explain why mantle convection alone is no longer thought to explain plate movement. [2 marks]
Answer:
Plates are not simply carried on convection cells like a conveyor belt (1); slab pull and ridge push are now thought to be major driving forces, interacting with mantle flow (1).
Question 40 · Spec F4, Key Idea 2 · Seismic tomography
Explain how seismic tomography helps our understanding of plate tectonics. [2 marks]
Answer:
It maps variations in seismic velocity in 3D inside the Earth (1); showing cold, fast subducted slabs and hot, slow areas such as plumes (1).
Question 41 · Spec F4, Key Idea 2 · Mantle plumes
State two pieces of evidence for a mantle plume beneath a volcanic chain. [2 marks]
Answer:
Any two from: volcanoes get progressively older away from the active hotspot (1); high heat flow (1); slow seismic velocities beneath it (1); volcanism away from plate boundaries (1).
Question 42 · Spec F4, Key Idea 2 · Palaeomagnetism
Explain how igneous rocks record the Earth's magnetic field. [2 marks]
Answer:
Magnetic minerals such as magnetite align with the Earth's field (1); and this is locked in as the rock cools below the Curie temperature (1).
Question 43 · Spec F4, Key Idea 2 · Magnetic stripes
Explain how magnetic stripes provide evidence for sea-floor spreading. [3 marks]
Answer:
Basalt erupting at the ridge records the Earth's field (1); the field reverses periodically, giving bands of normal and reversed polarity (1); the stripes are symmetrical on either side of the ridge (1).
Question 44 · Spec F4, Key Idea 2 · Calculations
A magnetic stripe 50 km from a ridge is 2.5 Ma old. Calculate the half-spreading rate in cm/year. [2 marks]
Answer:
50 km ÷ 2.5 Ma = 20 km/Ma (1); = 2 cm/year (1).
Question 45 · Spec F4, Key Idea 2 · Palaeomagnetism
State what magnetic inclination in an ancient rock can be used to find. [1 mark]
Answer:
The palaeolatitude at which the rock formed.
Question 46 · Spec F4, Key Idea 2 · Igneous activity
Explain why basaltic magma forms at mid-ocean ridges. [2 marks]
Answer:
Mantle rises as plates move apart and pressure falls (1); causing partial melting of peridotite (decompression melting) (1).
Question 47 · Spec F4, Key Idea 2 · Igneous activity
Explain how magma forms at a subduction zone. [2 marks]
Answer:
Water released from the subducting slab rises into the mantle wedge above (1); lowering its melting point so it partially melts (1).
Question 48 · Spec F4, Key Idea 2 · Metamorphism
Explain why regional metamorphism occurs at convergent plate margins. [2 marks]
Answer:
Crust is thickened and buried, increasing pressure and temperature (1); and directed stress deforms and recrystallises rocks over a large area (1).
Question 49 · Spec F4, Key Idea 2 · Sedimentation
Explain how plate movements affect the formation of sedimentary rocks. [2 marks]
Answer:
Uplift creates mountains that are eroded, supplying sediment (1); subsidence creates basins where sediment accumulates (1).
Question 50 · Spec F4, Key Idea 2 · Evidence
Explain why a tectonic interpretation is stronger when several types of evidence agree. [2 marks]
Answer:
Each method measures different properties and has different limitations (1); so independent agreement makes the conclusion more reliable (1).

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