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Cameroon GCE A-Level Geology study materials
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Cameroon GCE A-Level Geology practice questions

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Question 1Earth Materials and Mineralogy

Which silicate mineral structure consists of independent SiO4 tetrahedra linked only by metal cations?

  1. Nesosilicate (orthosilicate) Correct answer.
  2. Phyllosilicate (sheet)
  3. Tectosilicate (framework)
  4. Inosilicate (single chain)

Explanation

In nesosilicates the SiO4 tetrahedra share no oxygen atoms with each other; they are held together only through intervening cations such as Mg, Fe or Ca. This gives high hardness and poor cleavage, as in olivine and garnet. Chain, sheet and framework structures all involve tetrahedra sharing oxygen.

Question 2Earth Materials and Mineralogy

A mineral specimen is scratched by a steel nail but not by a copper coin. Using Mohs' scale, its hardness lies between that of which pair of reference minerals?

  1. Talc and gypsum
  2. Quartz and topaz
  3. Calcite and fluorite Correct answer.
  4. Corundum and diamond

Explanation

A copper coin has a hardness of about 3 and a steel nail about 5.5, so the specimen falls between calcite (3) and fluorite (4) on Mohs' scale. The other pairs bracket hardness ranges that do not match these two common tools.

Question 3Earth Materials and Mineralogy

Which property best distinguishes galena from sphalerite in hand specimen?

  1. Galena is magnetic; sphalerite is not magnetic
  2. Galena is harder than quartz; sphalerite is softer than talc
  3. Galena is non-metallic and white; sphalerite is metallic and black
  4. Galena has perfect cubic cleavage and a bright metallic lustre; sphalerite has dodecahedral cleavage and is resinous Correct answer.

Explanation

Galena (PbS) splits into cubes, is opaque metallic grey and is very heavy, whereas sphalerite (ZnS) shows six-directional dodecahedral cleavage and a resinous to adamantine lustre. Both are sulphides, so lustre and cleavage habit are the practical field criteria.

Question 4Earth Materials and Mineralogy

Which of the following minerals is a carbonate that reacts readily with dilute hydrochloric acid at room temperature?

  1. Halite
  2. Gypsum
  3. Calcite Correct answer.
  4. Fluorite

Explanation

Calcite (CaCO3) effervesces vigorously in cold dilute HCl because the carbonate ion releases carbon dioxide. Halite, gypsum and fluorite are respectively a chloride, a sulphate and a fluoride, and none of them fizzes in dilute acid.

Question 5Earth Materials and Mineralogy

In the discontinuous branch of Bowen's reaction series, which mineral crystallises immediately after pyroxene as temperature falls?

  1. Potassium feldspar
  2. Biotite mica
  3. Olivine
  4. Amphibole (hornblende) Correct answer.

Explanation

The discontinuous series runs olivine, pyroxene, amphibole, biotite, then potassium feldspar and muscovite. Amphibole therefore follows pyroxene directly as the melt cools. Potassium feldspar lies far later in the sequence.

Question 6Earth Materials and Mineralogy

A felsic magma cools slowly at depth. Which mineral assemblage is most consistent with this setting?

  1. Olivine, pyroxene and calcium-rich plagioclase in a fine-grained rock
  2. Pyroxene, amphibole and quartz in a glassy rock
  3. Quartz, potassium feldspar and muscovite in a coarse-grained rock Correct answer.
  4. Quartz, olivine and calcite in a glassy rock

Explanation

Slow cooling at depth allows large crystals to grow, and a felsic melt is enriched in silica and potassium, favouring quartz, potassium feldspar and muscovite. This is the assemblage of a coarse-grained granitic rock. Mafic minerals such as olivine and pyroxene belong to more mafic compositions.

Question 7Earth Materials and Mineralogy

Which cleavage description correctly matches a mineral?

  1. Halite: one perfect basal cleavage
  2. Muscovite: one perfect basal cleavage giving flexible sheets Correct answer.
  3. Quartz: three cleavages at 90 degrees
  4. Garnet: perfect rhombohedral cleavage

Explanation

Muscovite has a single perfect basal cleavage, so it splits into thin, elastic, flexible sheets. Halite has three cubic cleavages, quartz shows conchoidal fracture rather than cleavage, and garnet typically has no cleavage.

Question 8Earth Materials and Mineralogy

A dark, heavy mineral grain is strongly magnetic and leaves a black streak. Which mineral is most likely?

  1. Chalcopyrite
  2. Hematite
  3. Magnetite Correct answer.
  4. Pyrite

Explanation

Magnetite (Fe3O4) is the common strongly magnetic mineral in rocks and gives a black streak. Hematite is only weakly magnetic and gives a red-brown streak, while the sulphides are not strongly attracted to a magnet.

Question 9Earth Materials and Mineralogy

Which statement correctly describes polymorphism in minerals?

  1. Two minerals with different formulas crystallise with the same structure
  2. A mineral loses water from its crystal lattice on heating
  3. A mineral's colour changes with the type of incident light
  4. Two minerals with the same chemical formula crystallise with different structures Correct answer.

Explanation

Polymorphs share a chemical composition but differ in crystal structure, for example diamond and graphite, or calcite and aragonite. The differences in structure arise from the pressure and temperature conditions of formation and give contrasting physical properties.

Question 10Earth Materials and Mineralogy

Which pair of minerals are polymorphs of the same composition?

  1. Quartz and calcite
  2. Muscovite and biotite
  3. Diamond and graphite Correct answer.
  4. Galena and sphalerite

Explanation

Diamond and graphite are both composed of carbon but adopt different crystal structures, making them polymorphs. The other pairs differ in chemical composition as well as structure.

Question 11Earth Materials and Mineralogy

In the classification of silicate minerals, what structural feature distinguishes framework silicates from amphiboles?

  1. Framework silicates are always amorphous, whereas amphiboles are crystalline
  2. Framework silicates share all four tetrahedral oxygens, whereas amphiboles share only two or three Correct answer.
  3. Framework silicates contain no silicon, whereas amphiboles do
  4. Framework silicates contain only iron, whereas amphiboles contain only aluminium

Explanation

In a framework silicate every oxygen of each SiO4 tetrahedron is shared with neighbouring tetrahedra, producing a three-dimensional network, as in quartz and feldspar. Amphiboles are double-chain silicates in which only a limited number of oxygens are shared, leaving the chains linked by cations.

Question 12Earth Materials and Mineralogy

A crystal shows two cleavage directions intersecting at about 60 and 120 degrees, and the mineral is a dark ferromagnesian silicate. Which mineral is most likely?

  1. Gypsum
  2. Potassium feldspar
  3. Calcite
  4. Amphibole (hornblende) Correct answer.

Explanation

Amphibole displays two cleavages at approximately 60 and 120 degrees, and hornblende is a common dark ferromagnesian amphibole. The oblique angle distinguishes it from pyroxene, whose cleavages meet at about 90 degrees.

Question 13Earth Materials and Mineralogy

Which mineral is the principal source of aluminium in commercial bauxite deposits?

  1. Magnetite, formed by magmatic segregation
  2. Galena, formed by hydrothermal precipitation
  3. Halite, formed by evaporation of seawater
  4. Gibbsite, formed by tropical weathering of aluminous rocks Correct answer.

Explanation

Bauxite is a residual deposit produced by intense chemical weathering in tropical climates, and its main aluminium-bearing minerals are gibbsite and related hydroxides. Galena, halite and magnetite are respectively a sulphide, a chloride and an oxide of other elements.

Question 14Earth Materials and Mineralogy

Which property of a mineral is most directly controlled by the strength and direction of its chemical bonds?

  1. Its radioactivity only
  2. Its stratigraphic age
  3. Its fossil content
  4. Its cleavage and hardness Correct answer.

Explanation

Cleavage planes follow directions of weaker bonding in the crystal structure, and hardness reflects overall bond strength. Minerals with strong bonding in all directions, such as quartz, tend to be hard and to fracture rather than cleave.

Question 15Earth Materials and Mineralogy

Which mineral is the main constituent of the rock limestone and is also a common cement in sandstones?

  1. Calcite Correct answer.
  2. Quartz
  3. Halite
  4. Gypsum

Explanation

Limestone is composed dominantly of calcite, and calcite is the common carbonate cement that binds many sandstone grains. Quartz may be a detrital grain or cement, but it does not define limestone.

Question 16Earth Materials and Mineralogy

A mineral streak is observed on an unglazed porcelain plate. Why is streak often more reliable than colour for identification?

  1. Streak measures the density of the mineral precisely
  2. Streak indicates the crystallographic system of the mineral
  3. Streak reflects the powdered colour, which is less affected by surface weathering or impurities than the colour of the hand specimen Correct answer.
  4. Streak records the temperature of crystallisation

Explanation

The colour of a hand specimen can be altered by weathering, coatings or trace impurities, whereas the powder colour is more constant for a given mineral. Streak therefore gives a repeatable diagnostic property, as with the red-brown streak of hematite.

Question 17Earth Materials and Mineralogy

Which mineral group includes talc, kaolinite and muscovite?

  1. Nesosilicates
  2. Tectosilicates
  3. Inosilicates
  4. Phyllosilicates Correct answer.

Explanation

Talc, kaolinite and muscovite are all sheet silicates in which three of the four tetrahedral oxygens are shared, forming continuous layers. The weak bonding between layers explains their perfect basal cleavage and, for talc, its very low hardness.

Question 18Earth Materials and Mineralogy

Which statement correctly distinguishes a mineral from a rock?

  1. A mineral is always a single element, whereas a rock is always a compound
  2. A mineral is a man-made product, whereas a rock is natural
  3. A mineral is a naturally occurring crystalline solid of definite composition, whereas a rock is an aggregate of one or more minerals Correct answer.
  4. A mineral is a liquid or gas, whereas a rock is a solid

Explanation

A mineral is defined as a naturally occurring, inorganic crystalline solid with a definite chemical composition and ordered atomic structure. A rock is a consolidated aggregate of minerals, which may be monomineralic such as quartzite or polymineralic such as granite.

Question 19Earth Materials and Mineralogy

A mineral has a specific gravity of about 7.5 and a metallic lustre. Which mineral is most likely?

  1. Feldspar
  2. Calcite
  3. Galena Correct answer.
  4. Quartz

Explanation

Galena has a specific gravity of about 7.5 because of its high lead content, and it has a metallic lustre. The other minerals listed are common rock-forming silicates and a carbonate with specific gravities near 2.6 to 2.7.

Question 20Earth Materials and Mineralogy

Which process produces a mineral deposit by concentrating heavy, resistant grains such as cassiterite and gold in stream beds?

  1. Contact metamorphism
  2. Hydrothermal replacement
  3. Evaporite precipitation
  4. Placer concentration Correct answer.

Explanation

Placer deposits form when flowing water winnows away light material and leaves dense, chemically resistant grains behind in stream and beach sediments. This explains the economic concentration of gold and cassiterite.

Question 21Earth Materials and Mineralogy

Which mineral is a common ferromagnesian mineral found in peridotite and is also the first to crystallise from a basaltic magma?

  1. Muscovite
  2. Quartz
  3. Olivine Correct answer.
  4. Orthoclase

Explanation

Olivine is a magnesium-iron nesosilicate that dominates peridotite and is the highest-temperature mineral in Bowen's discontinuous series. It crystallises early from basaltic magma and is therefore common in ultramafic rocks.

Question 22Earth Materials and Mineralogy

A student identifies a mineral as fluorite because it scratches glass. Which observation would most strongly refute this identification?

  1. The mineral is purple in colour
  2. The mineral has a hardness of about 7 and shows conchoidal fracture Correct answer.
  3. The mineral is found in a vein
  4. The mineral has a hardness of about 4 and shows perfect octahedral cleavage

Explanation

Fluorite has a hardness of 4, so a mineral with hardness near 7 and conchoidal fracture is far too hard and lacks the octahedral cleavage of fluorite. The most probable identity would be quartz, which scratches glass easily.

Question 23Earth Materials and Mineralogy

Which statement about the optical properties of minerals under the petrographic microscope is correct?

  1. Anisotropic minerals show no interference colours
  2. Isotropic minerals remain dark under crossed polars through a full rotation Correct answer.
  3. All minerals show pleochroism in plane-polarised light
  4. Birefringence is the same for every mineral

Explanation

Isotropic minerals such as garnet and glass have a single refractive index in all directions and therefore stay extinct under crossed polars at every rotation angle. Anisotropic minerals, by contrast, change brightness and interference colour as the stage is rotated.

Question 24Earth Materials and Mineralogy

Which mineral is the chief ore of tin and commonly occurs in high-temperature hydrothermal veins associated with granite?

  1. Bauxite
  2. Sphalerite
  3. Cassiterite Correct answer.
  4. Gypsum

Explanation

Cassiterite (SnO2) is the principal tin ore and is typically found in greisen and high-temperature vein systems around granitic intrusions. Its high density and resistance to weathering also allow it to form placer deposits.

Question 25Petrology: Igneous, Sedimentary and Metamorphic Rocks

A magma with 52 wt% SiO₂, abundant plagioclase and pyroxene, and no quartz is best classified as which rock type?

  1. Basalt Correct answer.
  2. Rhyolite
  3. Granite
  4. Peridotite

Explanation

Intermediate-to-basic silica near 52 wt% with calcic plagioclase and pyroxene and no quartz defines basalt among the common volcanic rocks. Granite and rhyolite carry much more silica and quartz, while peridotite is ultramafic and dominated by olivine.

Question 26Petrology: Igneous, Sedimentary and Metamorphic Rocks

The rate at which magma cools most strongly controls which feature of an igneous rock?

  1. Crystal grain size Correct answer.
  2. Radioactive age
  3. Silica percentage
  4. Colour index

Explanation

Cooling rate governs nucleation and growth of crystals, so slow cooling deep underground produces coarse grains while rapid cooling at the surface produces fine grains or glass. Colour index, silica content and age depend on magma composition and history rather than cooling rate alone.

Question 27Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which sedimentary structure most reliably indicates that a bed has not been overturned?

  1. Ripple marks
  2. Mud cracks
  3. Graded bedding Correct answer.
  4. Cross-bedding

Explanation

Graded bedding normally fines upward because the coarsest grains settle first, so the coarse base and fine top record the original way-up. Cross-bedding, ripples and mud cracks all have symmetry or variability that makes polarity less certain.

Question 28Petrology: Igneous, Sedimentary and Metamorphic Rocks

A clastic sedimentary rock composed mainly of rounded quartz grains between 0.5 and 2 mm across is called:

  1. Sandstone Correct answer.
  2. Conglomerate
  3. Limestone
  4. Shale

Explanation

Sand-sized grains (0.062–2 mm) dominated by quartz define sandstone, and rounding reflects transport. Shale is finer, conglomerate is coarser with rounded gravel, and limestone is carbonate rather than clastic quartz.

Question 29Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which metamorphic rock typically forms from the regional metamorphism of limestone?

  1. Quartzite
  2. Slate
  3. Marble Correct answer.
  4. Hornfels

Explanation

Marble is the metamorphic product of limestone or dolostone, recrystallising calcite without a strong foliation. Quartzite derives from quartz sandstone, slate from shale, and hornfels from contact metamorphism of mixed rocks.

Question 30Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which mineral is the most characteristic index mineral of low-grade regional metamorphism in a pelitic sequence?

  1. Garnet
  2. Chlorite Correct answer.
  3. Staurolite
  4. Sillimanite

Explanation

Chlorite appears at the lowest grade in pelitic rocks and gives the greenish colour of slate. Garnet, staurolite and sillimanite appear progressively at medium to high grades.

Question 31Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which feature is diagnostic of a pyroclastic igneous rock?

  1. Preferred mineral alignment
  2. Interlocking coarse crystals
  3. Vesicles filled with secondary minerals
  4. Fragmental texture of ash and lapilli Correct answer.

Explanation

Pyroclastic rocks are built of fragmental volcanic debris such as ash, lapilli and blocks ejected during explosive eruptions. Alignment, coarse interlocking crystals and filled vesicles describe other igneous textures.

Question 32Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which process converts loose sand into a coherent sedimentary rock?

  1. Lithification Correct answer.
  2. Assimilation
  3. Differentiation
  4. Foliation

Explanation

Lithification combines compaction and cementation to turn loose sediment into rock. Foliation is a metamorphic fabric, differentiation and assimilation are magmatic processes.

Question 33Petrology: Igneous, Sedimentary and Metamorphic Rocks

A basalt flow shows a glassy chilled margin, a fine-grained interior and sparse olivine phenocrysts. Which interpretation best explains these features?

  1. Complete melting of an olivine-rich mantle rock at the surface
  2. Two separate magmas intruded one after the other
  3. Slow cooling at depth followed by metamorphism
  4. Rapid surface cooling of a melt that already contained some olivine crystals Correct answer.

Explanation

The glassy margin records very rapid quenching at the surface, while the phenocrysts grew earlier in the magma before eruption. The fine-grained groundmass reflects fast cooling, consistent with an extrusive origin.

Question 34Petrology: Igneous, Sedimentary and Metamorphic Rocks

A plutonic rock contains roughly equal amounts of alkali feldspar and plagioclase, abundant quartz and minor biotite. Which name fits best?

  1. Gabbro
  2. Anorthosite
  3. Syenite
  4. Granite Correct answer.

Explanation

Abundant quartz with roughly equal alkali feldspar and plagioclase is the classic granite composition. Syenite is quartz-poor, gabbro is mafic with calcic plagioclase, and anorthosite is dominated by plagioclase.

Question 35Petrology: Igneous, Sedimentary and Metamorphic Rocks

A magma chamber cools slowly and crystals of olivine and pyroxene settle to the floor. How does the residual melt change?

  1. It becomes richer in silica and alkalis Correct answer.
  2. It becomes richer in calcium and magnesium
  3. It becomes more mafic and less viscous
  4. It becomes more mafic and denser

Explanation

Removing early mafic minerals by crystal settling leaves a residual melt enriched in silica, alkalis and volatiles. This is fractional crystallisation, and it drives magmas from basaltic toward granitic compositions.

Question 36Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which pair of grain-size and sorting characteristics best describes a sediment deposited by a fast mountain river close to its source?

  1. Fine grained and poorly sorted
  2. Coarse grained and poorly sorted Correct answer.
  3. Fine grained and very well sorted
  4. Coarse grained and very well sorted

Explanation

Short transport near the source leaves coarse material with a wide range of sizes, so it is poorly sorted. Sorting improves and grains become finer only after long transport.

Question 37Petrology: Igneous, Sedimentary and Metamorphic Rocks

A limestone consists largely of ooids and carbonate mud with no visible skeletal fragments. What does this indicate about its depositional setting?

  1. Cold, dark marine water at high latitude
  2. A high-energy glacial outwash plain
  3. Deep abyssal plain below the carbonate compensation depth
  4. Shallow, warm, agitated marine water Correct answer.

Explanation

Ooids form where carbonate is supersaturated and grains are rolled by currents, which requires shallow warm agitated seawater. Below the carbonate compensation depth, carbonate dissolves rather than accumulating.

Question 38Petrology: Igneous, Sedimentary and Metamorphic Rocks

A sandstone contains abundant feldspar and angular grains with little clay matrix. What is the most likely source and history?

  1. Chemical precipitation from evaporating seawater
  2. A deep marine basin fed by slow settling of fine mud
  3. A distant lowland with prolonged weathering and reworking
  4. Rapid erosion of granitic basement with short transport Correct answer.

Explanation

Angular grains and abundant feldspar mean the sediment was buried quickly before weathering could destroy the unstable minerals, typical of rapid erosion of granitic basement. Long transport would remove feldspar and round the grains.

Question 39Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which sequence correctly shows increasing metamorphic grade for a pelitic rock?

  1. Phyllite, gneiss, slate, schist
  2. Slate, phyllite, schist, gneiss Correct answer.
  3. Schist, slate, gneiss, phyllite
  4. Gneiss, schist, phyllite, slate

Explanation

Increasing grade in pelites follows slate, phyllite, schist and gneiss, with grain size and segregation of minerals increasing along the sequence. The other orders reverse or scramble this progression.

Question 40Petrology: Igneous, Sedimentary and Metamorphic Rocks

A rock shows a strong planar fabric defined by aligned micas and is intermediate in grade between slate and gneiss. Which rock is it?

  1. Schist Correct answer.
  2. Hornfels
  3. Marble
  4. Quartzite

Explanation

Schist is characterised by a well-developed schistosity formed by aligned platy minerals such as mica, at medium grade. Hornfels is non-foliated, and quartzite and marble are dominated by quartz and calcite respectively.

Question 41Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which metamorphic reaction best explains the appearance of garnet in a pelitic schist as grade increases?

  1. Melting of quartz and feldspar
  2. Breakdown of chlorite and muscovite to form garnet and biotite Correct answer.
  3. Hydration of olivine to serpentine
  4. Precipitation of calcite from pore water

Explanation

At medium grade, reactions consuming chlorite and muscovite produce garnet and biotite in pelites. Melting occurs at higher grades, and the other reactions belong to different rock types.

Question 42Petrology: Igneous, Sedimentary and Metamorphic Rocks

A granite intrudes a shale, and within a few metres of the contact the shale becomes a hard, fine-grained, non-foliated rock. What is this rock and what caused it?

  1. Slate, formed by regional compression
  2. Mylonite, formed by fault shearing
  3. Hornfels, formed by contact metamorphism Correct answer.
  4. Gneiss, formed by high-grade regional metamorphism

Explanation

Baking of shale adjacent to an intrusion produces hornfels, a fine-grained non-foliated rock formed by contact metamorphism. Regional metamorphism would produce a foliated rock over a much wider area.

Question 43Petrology: Igneous, Sedimentary and Metamorphic Rocks

A layered rock has alternating dark and light bands of different mineral composition. Which term describes this texture?

  1. Gneissose banding Correct answer.
  2. Amygdaloidal
  3. Oolitic
  4. Porphyroblastic

Explanation

Segregation of light quartz-feldspar bands from dark mafic bands at high grade produces gneissose banding. Porphyroblastic refers to large crystals in a finer matrix, while oolitic and amygdaloidal are sedimentary and volcanic textures.

Question 44Petrology: Igneous, Sedimentary and Metamorphic Rocks

A conglomerate contains abundant unstable rock fragments and feldspar with only moderate rounding. What does this imply about its depositional history?

  1. Long transport and intense chemical weathering before deposition
  2. Precipitation from a saturated brine
  3. Rapid burial close to the source area Correct answer.
  4. Deposition in a low-energy lagoon after extensive reworking

Explanation

Unstable fragments and feldspar survive only if erosion, transport and burial were rapid and weathering was limited, indicating proximity to the source. Long transport or intense weathering would destroy these components.

Question 45Petrology: Igneous, Sedimentary and Metamorphic Rocks

A basalt has a glassy margin, a fine-grained interior and rare quartz xenocrysts rimmed by reaction coronas. Which interpretation is most consistent?

  1. The rock formed by regional metamorphism of a shale
  2. The magma was completely homogeneous and cooled slowly at depth
  3. The rock is a pyroclastic deposit of pure ash
  4. The magma assimilated silica-rich wall rock before rapid surface cooling Correct answer.

Explanation

Quartz xenocrysts with reaction rims show that foreign silica-rich material was incorporated and partly resorbed by the basaltic melt. The glassy margin records rapid cooling at the surface after assimilation.

Question 46Petrology: Igneous, Sedimentary and Metamorphic Rocks

A pelitic rock contains garnet, staurolite and kyanite but no sillimanite or melt. Which metamorphic grade and facies does this assemblage indicate?

  1. Medium grade, amphibolite facies Correct answer.
  2. Very low grade, zeolite facies
  3. Low grade, greenschist facies
  4. High grade, granulite facies

Explanation

Garnet, staurolite and kyanite are typical of medium-grade amphibolite facies pelites. Sillimanite and migmatitic melt would indicate higher grade, while chlorite and muscovite dominate low-grade greenschist assemblages.

Question 47Petrology: Igneous, Sedimentary and Metamorphic Rocks

A sediment sequence shows a basal conglomerate fining upward into sandstone and then shale, resting on an eroded granite surface. What does this record?

  1. Contact metamorphism of the granite by a dyke
  2. Chemical precipitation of evaporites in a closed basin
  3. A single turbidity current in deep water
  4. A marine transgression over an uplifted and eroded basement Correct answer.

Explanation

The erosion surface, basal conglomerate and upward fining record rising sea level flooding an eroded granitic basement, a classic transgressive succession. Turbidites, contact metamorphism and evaporites do not produce this pattern.

Question 48Petrology: Igneous, Sedimentary and Metamorphic Rocks

Two magmas have the same silica content, but one is water-rich and the other is dry. How does the water-rich magma differ during cooling?

  1. It crystallises at lower temperatures and forms larger crystals Correct answer.
  2. It becomes more mafic as water is added
  3. It cannot crystallise any minerals at all
  4. It crystallises at higher temperatures and forms glass

Explanation

Dissolved water depresses the crystallisation temperature and lowers viscosity, allowing ions to diffuse and build larger crystals. A dry magma of the same composition begins to crystallise at higher temperature.

Question 49Petrology: Igneous, Sedimentary and Metamorphic Rocks

A foliated rock contains porphyroblasts of garnet that include curved trails of matrix minerals, while the surrounding matrix fabric is straight. What does this indicate?

  1. The garnet grew before the main deformation and was rotated passively
  2. The garnet is a primary igneous phenocryst unaffected by deformation
  3. The rock is a contact-metamorphosed hornfels with no deformation
  4. The garnet grew during an earlier deformation and the matrix fabric formed later Correct answer.

Explanation

Inclusion trails in a porphyroblast record the fabric present when the crystal grew, so curved internal trails with a straight external fabric mean the garnet predates the final deformation. This relationship is used to date fabric development relative to porphyroblast growth.

Question 50Petrology: Igneous, Sedimentary and Metamorphic Rocks

Which texture is most characteristic of a rapidly cooled, silica-rich volcanic rock such as obsidian?

  1. Clastic texture with rounded quartz grains
  2. Pegmatitic texture with crystals several centimetres long
  3. Glassy texture with no visible crystals Correct answer.
  4. Coarse interlocking crystals of quartz and feldspar

Explanation

Very rapid cooling prevents crystal nucleation and growth, so the lava solidifies as a glass. Obsidian is the classic example of such a glassy rock. Slow cooling instead allows visible crystals to grow, producing coarse textures such as those of granite or pegmatite.