Eduqas A-level Geology · G1

Rock forming processes

50 exam-style questions with answers for G1 Rock forming processes 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. Not official Eduqas material.

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Question 1 · Spec G1, Key Idea 1 · Partial melting
Explain why rocks partially melt rather than melting completely. [2 marks]
Answer:
Rocks contain minerals with different melting temperatures (1); so over a range of temperatures some minerals melt while others remain solid (1).
Question 2 · Spec G1, Key Idea 1 · Partial melting
Explain how a fall in pressure causes mantle rock to melt. [2 marks]
Answer:
As mantle rises, pressure decreases and the solidus temperature falls (1); the rock crosses the solidus and partially melts without gaining heat (1).
Question 3 · Spec G1, Key Idea 1 · Partial melting
Explain how adding water affects the melting of rocks. [2 marks]
Answer:
Water lowers the solidus (melting temperature) (1); so partial melting begins at a lower temperature than in dry rock (1).
Question 4 · Spec G1, Key Idea 1 · Partial melting
A geotherm crosses the wet solidus of the lower crust at 28 km. State what this predicts. [1 mark]
Answer:
Partial melting begins at about 28 km depth (not complete melting).
Question 5 · Spec G1, Key Idea 1 · Magma types
Explain why melting of mantle and continental crust produces different magmas. [2 marks]
Answer:
The source rocks have different mineral and chemical compositions (1); mantle peridotite gives mafic (basaltic) magma, while continental crust gives felsic (granitic) magma (1).
Question 6 · Spec G1, Key Idea 1 · Bowen's reaction series
Describe the discontinuous branch of Bowen's reaction series. [2 marks]
Answer:
Ferromagnesian minerals crystallise in sequence as magma cools (1); olivine → pyroxene → amphibole → biotite, each reacting with the melt to form the next (1).
Question 7 · Spec G1, Key Idea 1 · Bowen's reaction series
Explain why plagioclase forms a continuous series. [2 marks]
Answer:
Plagioclase is a solid solution (1); its composition changes gradually from calcium-rich to sodium-rich as the magma cools (1).
Question 8 · Spec G1, Key Idea 1 · Fractional crystallisation
Explain how removing early-formed mafic crystals changes the remaining magma. [2 marks]
Answer:
Iron, magnesium and calcium are removed in the early crystals (1); so the remaining melt becomes richer in silica, sodium and potassium (more felsic) (1).
Question 9 · Spec G1, Key Idea 1 · Fractional crystallisation
Explain how cumulate layers form by gravity settling. [2 marks]
Answer:
Early-formed crystals such as olivine are denser than the magma (1); so they sink and accumulate on the floor of the magma chamber (1).
Question 10 · Spec G1, Key Idea 1 · Fractional crystallisation
Explain why a cumulate rock has a different composition from its parent magma. [1 mark]
Answer:
It is made of accumulated crystals of selected minerals, not the whole original liquid.
Question 11 · Spec G1, Key Idea 1 · Assimilation
Distinguish between a xenolith and assimilation. [2 marks]
Answer:
A xenolith is a fragment of country rock enclosed in the igneous rock (1); assimilation is when country rock melts or dissolves and becomes part of the magma (1).
Question 12 · Spec G1, Key Idea 1 · Magma mixing
State what an enclave of different igneous rock suggests about the magma. [1 mark]
Answer:
That two magmas of different composition or temperature mixed incompletely.
Question 13 · Spec G1, Key Idea 1 · Viscosity
Explain why magmas with more silica are more viscous. [2 marks]
Answer:
More silica forms more linked chains of silicon-oxygen tetrahedra (polymerisation) (1); which makes the magma resist flow (1).
Question 14 · Spec G1, Key Idea 1 · Ionic substitution
State the two properties of ions that control substitution in a crystal lattice. [2 marks]
Answer:
Ionic radius (size) (1); and charge (1).
Question 15 · Spec G1, Key Idea 1 · Solid solution
State what the olivine solid solution series shows. [2 marks]
Answer:
Mg²⁺ and Fe²⁺ can substitute for each other because they have similar size and the same charge (1); giving a range of compositions between forsterite and fayalite (1).
Question 16 · Spec G1, Key Idea 1 · Solid solution
Explain why substitution in plagioclase involves aluminium and silicon as well as sodium and calcium. [2 marks]
Answer:
Ca²⁺ and Na⁺ have different charges (1); so Al³⁺ and Si⁴⁺ are also exchanged to keep the crystal electrically neutral (1).
Question 17 · Spec G1, Key Idea 1 · Phase diagrams
State what a point between the liquidus and solidus on a phase diagram represents. [1 mark]
Answer:
A mixture of crystals and melt at that temperature and composition.
Question 18 · Spec G1, Key Idea 1 · Phase diagrams
Name the line crossed when a cooling liquid first begins to crystallise. [1 mark]
Answer:
The liquidus.
Question 19 · Spec G1, Key Idea 1 · Phase diagrams
Explain why the crystals and melt in a two-phase field have different compositions. [2 marks]
Answer:
The crystals and liquid are in equilibrium but have different compositions (1); read from where the temperature line meets the solidus and liquidus (1).
Question 20 · Spec G1, Key Idea 1 · Magma chambers
Explain why the shape of magma chambers beneath mid-ocean ridges is debated. [2 marks]
Answer:
Seismic surveys and drilling give only indirect and incomplete evidence (1); so different models of chamber shape and size can fit the data (1).
Question 21 · Spec G1, Key Idea 1 · Decay calculations
Using N/N₀ = e^(−λt), calculate t when N/N₀ = 0.5 and λ = 1.0 × 10⁻⁹ per year. [2 marks]
Answer:
t = −ln(0.5) ÷ λ = 0.693 ÷ 1.0 × 10⁻⁹ (1); = 6.93 × 10⁸ years (693 Ma) (1).
Question 22 · Spec G1, Key Idea 1 · Decay calculations
Explain why logarithmic scales are useful for plotting decay data. [2 marks]
Answer:
They display values ranging over several orders of magnitude (1); and turn an exponential decay curve into a straight line (1).
Question 23 · Spec G1, Key Idea 1 · Uncertainty
An age is 120 ± 3 Ma. Calculate the percentage uncertainty. [2 marks]
Answer:
3 ÷ 120 × 100 (1); = 2.5% (1).
Question 24 · Spec G1, Key Idea 1 · Radiometric dating
Explain why an isotopic age may need checking for later disturbance. [2 marks]
Answer:
Later heating, alteration or fluids can cause loss or gain of isotopes (1); so the age may record a later event rather than crystallisation (1).
Question 25 · Spec G1, Key Idea 2 · Metamorphic reactions
Explain how metamorphism forms new minerals without changing the bulk composition. [2 marks]
Answer:
New temperature and pressure conditions make different minerals stable (1); so elements already in the rock recombine in solid-state reactions (1).
Question 26 · Spec G1, Key Idea 2 · Parent rock
Explain why limestone and mudstone form different minerals under the same metamorphic conditions. [2 marks]
Answer:
They have different chemical compositions (1); so different elements are available to form new minerals (e.g. calcite in marble, micas in schist) (1).
Question 27 · Spec G1, Key Idea 2 · Index minerals
State what an index mineral shows. [2 marks]
Answer:
The minimum metamorphic grade reached (1); so zones of index minerals can be mapped to show the pressure and temperature conditions (1).
Question 28 · Spec G1, Key Idea 2 · Contact metamorphism
Explain why metamorphic grade increases towards an igneous intrusion. [2 marks]
Answer:
Heat is conducted from the intrusion into the country rock (1); so temperatures are highest near the contact and decrease with distance (1).
Question 29 · Spec G1, Key Idea 2 · Aureole investigation
Describe how to model the effect of distance from a heat source in a metamorphic aureole. [2 marks]
Answer:
Measure temperature at set distances from the heat source at regular time intervals (1); keeping starting conditions the same and repeating the readings (1).
Question 30 · Spec G1, Key Idea 2 · Al₂SiO₅ polymorphs
Explain why andalusite, kyanite and sillimanite are useful indicators of metamorphic conditions. [2 marks]
Answer:
They are polymorphs with the same composition (Al₂SiO₅) but different structures (1); each is stable over a different range of pressure and temperature (1).
Question 31 · Spec G1, Key Idea 2 · Al₂SiO₅ polymorphs
Which Al₂SiO₅ polymorph indicates high pressure? [1 mark]
Answer:
Kyanite.
Question 32 · Spec G1, Key Idea 2 · Al₂SiO₅ polymorphs
Which Al₂SiO₅ polymorph indicates high temperature? [1 mark]
Answer:
Sillimanite.
Question 33 · Spec G1, Key Idea 2 · P-T paths
Explain why a single peak temperature cannot describe a rock's metamorphic history. [2 marks]
Answer:
Rocks follow a pressure-temperature-time path during burial, heating and uplift (1); different minerals and textures may record different stages of the path (1).
Question 34 · Spec G1, Key Idea 2 · Metamorphic textures
Describe a granoblastic texture. [2 marks]
Answer:
A mosaic of interlocking, equal-sized crystals (1); with no preferred orientation, as in marble or metaquartzite (1).
Question 35 · Spec G1, Key Idea 2 · Metamorphic textures
Distinguish between a porphyroblast and a phenocryst. [2 marks]
Answer:
A porphyroblast grows in the solid state during metamorphism, such as garnet in schist (1); a phenocryst crystallises from magma in an igneous rock (1).
Question 36 · Spec G1, Key Idea 2 · Dynamic metamorphism
Explain how mylonite forms. [2 marks]
Answer:
Intense ductile shearing in a fault zone (1); reduces grain size and recrystallises the rock into a fine-grained, strongly foliated fabric (1).
Question 37 · Spec G1, Key Idea 2 · Dynamic metamorphism
Explain why fault zones are sites of dynamic metamorphism. [2 marks]
Answer:
Strain is concentrated in the fault zone (1); causing crushing, shearing and recrystallisation of the rocks (1).
Question 38 · Spec G1, Key Idea 2 · Metamorphic grade
Explain why grade should be inferred from mineral assemblages, not grain size alone. [2 marks]
Answer:
Grain size also depends on the parent rock, time, fluids and deformation (1); mineral assemblages are more directly linked to pressure and temperature (1).
Question 39 · Spec G1, Key Idea 3 · Hjulström curve
Explain why clay needs a higher velocity to erode than sand. [2 marks]
Answer:
Clay particles are cohesive and stick together (1); and form a smooth bed, so more energy is needed to lift them (1).
Question 40 · Spec G1, Key Idea 3 · Hjulström curve
Sand settles below 0.10 m/s and is eroded above 0.40 m/s. Predict what happens to moving sand at 0.20 m/s. [1 mark]
Answer:
It stays in transport, but sand on the bed would not be eroded.
Question 41 · Spec G1, Key Idea 3 · Turbidity currents
Describe how a turbidity current transports sediment. [2 marks]
Answer:
A mixture of sediment and water is denser than the surrounding water (1); so it flows rapidly down submarine slopes under gravity (1).
Question 42 · Spec G1, Key Idea 3 · Turbidity currents
Explain how a turbidity current forms graded bedding. [2 marks]
Answer:
As the current slows, it loses energy (1); so coarse grains settle first, followed by finer grains (1).
Question 43 · Spec G1, Key Idea 3 · Modelling
State one limitation of a laboratory model of a turbidity current. [1 mark]
Answer:
It cannot reproduce the scale, grain sizes, flow depth and turbulence of natural flows.
Question 44 · Spec G1, Key Idea 3 · Data analysis
Deposit thickness is 12 mm at 2 minutes and 28 mm at 6 minutes. Calculate the rate of deposition. [2 marks]
Answer:
(28 − 12) ÷ (6 − 2) (1); = 4 mm per minute (1).
Question 45 · Spec G1, Key Idea 3 · Walther's Law
State Walther's Law. [2 marks]
Answer:
Facies that occur in a conformable vertical sequence (1); were deposited in laterally adjacent environments (1).
Question 46 · Spec G1, Key Idea 3 · Transgression
Define marine transgression. [1 mark]
Answer:
When the sea advances over the land, moving the shoreline landward.
Question 47 · Spec G1, Key Idea 3 · Regression
Explain how a regression produces a shallowing-upward sequence. [2 marks]
Answer:
The shoreline moves seaward, due to falling sea level or high sediment supply (1); so shallow-water facies are deposited on top of deeper-water facies (1).
Question 48 · Spec G1, Key Idea 3 · Diachronous beds
Explain why the base of a beach sandstone may be different ages in different places. [2 marks]
Answer:
The beach environment migrates as the shoreline moves (1); so it reaches each place at a different time, making the deposit diachronous (1).
Question 49 · Spec G1, Key Idea 3 · Correlation
Explain why similar lithology alone does not prove two beds are the same age. [2 marks]
Answer:
Similar environments can occur at different times, and facies boundaries can be diachronous (1); fossils or dated horizons are needed for reliable correlation (1).
Question 50 · Spec G1, Key Idea 3 · Walther's Law
Explain why Walther's Law cannot be applied across an unconformity. [2 marks]
Answer:
The unconformity represents missing time and an erosional break (1); so the facies above and below were not necessarily laterally adjacent (1).

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