An igneous rock contains large, well-formed crystals of plagioclase set in a fine-grained groundmass. What does this porphyritic texture show?

The magma cooled slowly at first, growing the large crystals, and then cooled quickly

The magma cooled at one constant rate throughout, so crystals grew steadily without any interruption

The rock formed from a very water-rich residual melt that crystallised entirely into giant crystals

The magma cooled slowly at first, growing the large crystals, and then cooled quickly

The rock was deformed long after solidification, so its crystals were crushed and then recrystallised

Which volcanic rock is the fine-grained equivalent of the plutonic rock diorite?

Andesite, which has intermediate silica and plagioclase with hornblende

Basalt, which has the same sodic plagioclase but contains far more pyroxene

Rhyolite, which is rich in quartz and potassium feldspar and is the acid equivalent

Trachyte, which is dominated by alkali feldspar and has little or no quartz

Andesite, which has intermediate silica and plagioclase with hornblende

A plutonic rock has by volume quartz 25 %, K-feldspar 10 %, plagioclase 40 %, biotite 15 % and hornblende 10 %. Use this rule for rocks with quartz in the 20 to 60 range after normalisation: granite if the plagioclase share P/(A + P) is below 65 %, granodiorite if it is 65 to 90 %, and tonalite if it is above 90 %. What is the rock?

Granodiorite, because P/(A + P) = 40/50 = 80 %

Granite, because quartz is above 20 % of the whole rock

Granodiorite, because P/(A + P) = 40/50 = 80 %

Tonalite, because plagioclase is the most abundant felsic mineral

Diorite, because the rock contains hornblende and biotite

Which statements about Bowen's reaction series are correct?

In the discontinuous branch olivine is followed by pyroxene, then amphibole, then biotite as the magma cools

Calcium-rich plagioclase crystallises before sodium-rich plagioclase along the continuous branch

In the discontinuous branch olivine is followed by pyroxene, then amphibole, then biotite as the magma cools

Quartz crystallises first, because silica is the most abundant oxide in magma

Calcium-rich plagioclase crystallises before sodium-rich plagioclase along the continuous branch

Minerals from the two ends of the series commonly crystallise together, as olivine does with quartz in granite

A sandstone is made of angular, poorly sorted grains with more than 25 % feldspar, and it lies close to an uplifted granitic source. Which sandstone type is it?

Arkose, because it is rich in feldspar from rapid erosion of granitic rock

Quartz arenite, because the grains are mostly silicate minerals

Greywacke, because it is poorly sorted and occurs near a mountain source

Lithic arenite, because the grains came from a granitic parent rock

Arkose, because it is rich in feldspar from rapid erosion of granitic rock

A limestone consists of about 30 % carbonate grains floating in a matrix of lime mud, so the grains do not touch one another. Which Dunham class is it?

Wackestone, because it is mud-supported with over 10 % grains

Mudstone, because the rock is dominated by the lime mud matrix

Wackestone, because it is mud-supported with over 10 % grains

Packstone, because it contains both grains and lime mud together

Grainstone, because carbonate grains are the main component here

As seawater evaporates in a closed basin, which sequence lists the minerals in the order in which they precipitate?

Calcite, then gypsum, then halite, then potash salts

Halite, then gypsum, then calcite, then potash salts

Calcite, then gypsum, then halite, then potash salts

Potash salts, then halite, then gypsum, then calcite

Gypsum, then calcite, then potash salts, then halite

In a vertical cliff face the original orientation of a sedimentary sequence is unknown. Wedge-shaped infills of desiccation cracks in a mudstone layer are seen in section, each narrowing toward the left side of the face. Which way are the younger beds?

To the right, because mud cracks taper toward older beds

To the left, because a crack narrows toward the layer that was deposited later

To the left, because mud cracks always open downward from the top surface

It cannot be told, because desiccation cracks give no way-up information

Asymmetrical ripple marks in a sandstone bed have a gentle slope facing north and a steep slope facing south. In which direction did the current flow?

Toward the south, because the steep lee face points down-current

Toward the north, because the gentle slope faces the current's destination

Toward the south, because the steep lee face points down-current

Toward the east or west, because ripples always form across the flow

It cannot be told, because only symmetrical ripples show flow direction

Which of the following can be used as way-up (younging) criteria in a tilted or overturned sedimentary sequence?

Graded bedding in which the grains become finer toward the top of the bed

Desiccation cracks that taper downward into the underlying bed

Graded bedding in which the grains become finer toward the top of the bed

Desiccation cracks that taper downward into the underlying bed

Parallel lamination of uniform thickness with no change in grain size

A massive bed of uniform grain size with no internal structure

Which sequence lists the metamorphic rocks formed from shale in order of increasing metamorphic grade?

Slate, then phyllite, then schist, then gneiss

Schist, then slate, then phyllite, then gneiss

Phyllite, then slate, then schist, then gneiss

Slate, then phyllite, then schist, then gneiss

Gneiss, then schist, then phyllite, then slate

A metabasalt contains glaucophane together with lawsonite and jadeite. Which setting does this assemblage indicate?

A subduction zone, with high pressure and low temperature, in the blueschist facies

A contact aureole around a granite pluton, with high temperature and low pressure

The base of a thick continental crust, with very high temperature and little water

A subduction zone, with high pressure and low temperature, in the blueschist facies

A mid-ocean ridge flank, with low pressure and hydrothermal circulation of seawater

Regional metamorphic rocks such as schist are foliated, while hornfels from contact metamorphism is not. Which explanation is correct?

Regional metamorphism has directed stress that aligns platy minerals; contact metamorphism is thermal and nearly stress-free

Hornfels forms at a lower temperature than schist, so its minerals have no chance to recrystallise into platy shapes

Hornfels forms only from protoliths that contain no clay minerals, while every schist comes from original shale

Hornfels forms by complete melting of the protolith, so any earlier alignment of its minerals is destroyed

Which of the following pairings of protolith and metamorphic rock are correct?

Limestone gives marble, a non-foliated rock of interlocking calcite grains

Quartz sandstone gives quartzite, a granoblastic rock of recrystallised quartz

Limestone gives marble, a non-foliated rock of interlocking calcite grains

Shale gives quartzite, a hard rock made almost entirely of quartz

Quartz sandstone gives quartzite, a granoblastic rock of recrystallised quartz

Basalt gives marble when it is heated in a contact aureole

Common Igneous, Sedimentary and Metamorphic Rocks: Composition, Structure and Formation

This chapter answers the Part A heading “Common Igneous, Sedimentary, and metamorphic rocks: composition, structure, and formation processes” of the Geology and Geophysics (GG) paper. It teaches igneous textures, forms and classification (silica-based, mineral-based and QAPF), Bowen's series and magma generation, clastic and chemical sedimentary rocks with their sedimentary structures, and metamorphic types, grade and facies in outline. The numericals are modal normalisation to QAP, phi conversions, Inman mean and sorting, and porosity from densities.

1. Igneous rocks: textures, structures and modes of occurrence

Igneous rocks form by the cooling and crystallisation of magma. The place of cooling sets the crystal size: plutonic (intrusive) rocks cool slowly at depth and are coarse; hypabyssal rocks in dykes and sills are medium-grained (dolerite); volcanic (extrusive) rocks cool fast at the surface and are fine or glassy. Texture records the cooling history, so it is the first thing to describe.

Phaneritic (crystals visible to the eye, plutonic) and aphanitic (too fine to see, volcanic); glassy (obsidian) when cooling was too fast for crystals.
Porphyritic: large phenocrysts in a finer groundmass, showing two stages of cooling, slow at depth and then fast after eruption or emplacement.
Vesicular (gas bubbles; amygdaloidal when the vesicles are filled by zeolite, calcite or silica), pyroclastic (fragments of ash and lapilli), pegmatitic (very coarse crystals from water-rich residual melt) and graphic (quartz–feldspar intergrowth).
Ophitic: plagioclase laths enclosed in larger pyroxene crystals, typical of dolerite and basalt.

Forms of occurrence: a dyke cuts across the host layers, a sill lies parallel to them, a laccolith is a lens that domes the layers above, a stock is a small pluton and a batholith a huge one (exposed area above about 100 km²). On the surface basalt gives pillow lava under water, columnar joints on cooling and flood-basalt flows such as the Deccan Traps.

2. Classification of igneous rocks: silica, minerals and QAPF

Silica-based scheme (a widely used convention; texts differ by one to three per cent): acid above 66 wt % SiO₂ (granite, rhyolite), intermediate 52 to 66 (diorite, andesite, syenite), basic 45 to 52 (gabbro, basalt), ultrabasic below 45 (peridotite, dunite). Mineral-based scheme: felsic minerals (quartz, feldspars, feldspathoids) against mafic ones (olivine, pyroxene, amphibole, biotite). The colour index M is the volume percentage of dark minerals, which separates light (leucocratic) from dark (melanocratic) rocks. Quartz and feldspathoids never occur together in equilibrium.

The QAPF (Streckeisen) scheme classifies by the modal proportions of Q (quartz), A (alkali feldspar), P (plagioclase) and F (feldspathoids), after the mafic minerals are set aside and the four are recalculated to 100 %. In a rock with quartz, Q of 20 to 60 gives the granitoid family, 5 to 20 the quartz-bearing syenites, monzonites and diorites, and below 5 the syenite–monzonite–diorite/gabbro fields. Within the granitoids, the plagioclase share P/(A + P) decides the name: granite (below about 65 %), granodiorite (65 to 90 %) and tonalite (above 90 %). Diorite has plagioclase more sodic than about An50 and gabbro more calcic; dunite is over 90 % olivine.

Plutonic rocks and their volcanic equivalents
Plutonic	Volcanic	Main minerals
Granite	Rhyolite	Quartz, K-feldspar, sodic plagioclase, biotite
Syenite	Trachyte	K-feldspar dominant, little or no quartz, hornblende
Diorite	Andesite	Intermediate plagioclase, hornblende
Gabbro	Basalt	Calcic plagioclase, pyroxene, olivine
Peridotite	Komatiite (rare)	Olivine, pyroxene

Worked normalisation: a rock has by volume quartz 20, K-feldspar 25, plagioclase 25, biotite 10 and hornblende 20 per cent (total 100). Colour index M = 10 + 20 = 30. The QAP sum is 20 + 25 + 25 = 70, so Q′ = 20/70 = 28.6 %, A′ = 35.7 % and P′ = 35.7 %. Q′ lies in 20 to 60 and P/(A + P) = 25/50 = 50 %, so the rock is a granite (monzogranite).

⚠️ Leave the mafics out first

QAPF percentages are computed from Q + A + P + F only. Dividing quartz by the whole rock (100) instead of the QAP sum (70) gives 20 % instead of 28.6 %, and can move the rock into the wrong field.

3. Formation of igneous rocks: melting, Bowen's series and differentiation

Magma forms by partial melting. Mantle peridotite melts by decompression (ridges, plumes) or by water added above a subducting slab, giving basaltic magma, the commonest magma and the source of ocean crust and flood basalts. Melting of continental crust gives granitic magma. Viscosity rises with silica content, so basalt flows far while rhyolitic magma builds domes or erupts explosively as ash and ignimbrite.

Bowen's reaction series gives the order in which minerals crystallise from a cooling basaltic magma. The discontinuous branch runs olivine → pyroxene → amphibole → biotite, each early mineral reacting with the melt to make the next. The continuous branch runs from calcium-rich to sodium-rich plagioclase by gradual change of composition. The branches meet at K-feldspar, then muscovite and quartz crystallise last. Minerals that crystallise together are neighbours on the series, so gabbro holds olivine, pyroxene and calcic plagioclase, and granite holds quartz, K-feldspar, sodic plagioclase and biotite or muscovite.

Differentiation by fractional crystallisation removes early crystals (they settle or are separated), so the residual melt becomes richer in silica and alkalis; this is how a basaltic parent can yield andesitic and rhyolitic liquids, and how olivine-rich cumulates such as dunite form at the base of a chamber. Late water-rich melts give pegmatites.

4. Sedimentary rocks: origin and classification

Sediment is made by weathering, moved by water, wind or ice, deposited, and turned to rock by lithification: compaction and cementation by calcite, silica or iron oxide. Clastic rocks are named by grain size on the Udden–Wentworth scale; chemical and biochemical rocks come from precipitation or organisms.

Udden–Wentworth grain-size classes
Class	Diameter (mm)	φ	Rock
Boulder	above 256	below −8	Conglomerate or breccia
Cobble	64 to 256	−6 to −8	Conglomerate or breccia
Pebble	4 to 64	−2 to −6	Conglomerate or breccia
Granule	2 to 4	−1 to −2	Fine conglomerate
Sand	1/16 to 2	4 to −1	Sandstone
Silt	1/256 to 1/16	8 to 4	Siltstone
Clay	below 1/256	above 8	Claystone, mudstone, shale (fissile)

Conglomerate has rounded clasts (long transport); breccia has angular clasts (short transport, near the source).
Sandstones: quartz arenite is almost pure quartz, well sorted and mature; arkose has over about 25 % feldspar, is angular and poorly sorted, from rapidly eroded granitic terrain; lithic arenite is rich in rock fragments; greywacke (wacke) has over about 15 % fine matrix and is poorly sorted, typical of turbidity-current deposits.
Limestone is calcite, from shells or direct precipitation, and effervesces in dilute HCl. Dunham's textural classes are mudstone (under 10 % grains, mud-supported), wackestone (over 10 % grains, mud-supported), packstone (grain-supported with mud), grainstone (grain-supported, no mud) and boundstone (organically bound). Dolostone is dolomite; chert is microcrystalline silica; coal is plant carbon; evaporites precipitate as seawater evaporates in the order calcite and dolomite, gypsum or anhydrite, halite, then potash salts.

5. Grain size, sorting and porosity: the numericals

The phi scale is logarithmic: φ = −log₂ d, with d in millimetres, and d = 2^(−φ). So 1 mm is 0 φ, 0.5 mm is 1 φ, 0.25 mm is 2 φ and 0.0625 mm is 4 φ, while 2 mm is −1 φ and 8 mm is −3 φ. A larger φ means a finer grain, and the sign is the usual trap. Worked example: d = 8 mm gives φ = −log₂ 8 = −3.

Grain-size statistics read percentiles φ16, φ50 and φ84 from a cumulative curve. In Inman's measures the mean is Mφ = (φ16 + φ84)/2 and the sorting is σφ = (φ84 − φ16)/2; a smaller σφ means better sorting, and 0 would be a single size. Worked example: φ16 = 0.5 and φ84 = 2.7 give Mφ = 1.6 (a medium sand of 0.33 mm) and σφ = 1.1. Folk and Ward's graphic measures use more percentiles and give slightly different numbers, so the formula must be stated.

Porosity n = volume of voids ÷ total volume. From densities, n = 1 − ρ_b/ρ_g for a dry rock of bulk density ρ_b and grain density ρ_g (about 2.65 g/cm³ for quartz). If the pores are fully water-filled, the saturated density is ρ_sat = (1 − n) ρ_g + n ρ_w, so n = (ρ_g − ρ_sat)/(ρ_g − ρ_w). Worked example: ρ_sat = 2.35, ρ_g = 2.65, ρ_w = 1.00 g/cm³ gives n = 0.30/1.65 = 0.18, or 18 %. Uniform spheres pack to 47.6 % porosity (cubic) and 26 % (rhombohedral); good sorting raises porosity, a mix of sizes and cement lowers it.

6. Sedimentary structures and way-up criteria

Bedding and lamination are the primary layering. Graded bedding (grains fining upward) marks a waning current, as in a turbidite; the Bouma sequence is massive graded sand, parallel lamination, ripple cross-lamination, upper parallel lamination and mud. Cross-bedding forms where ripples and dunes migrate; its foresets dip down-current, and sets are truncated at the top. Current ripples are asymmetrical with the steep face on the down-current side; wave ripples are symmetrical. Hummocky cross-stratification records storm waves.

Sole and top marks and what they show
Structure	Meaning	Way-up or direction clue
Mud cracks	Desiccation on exposure	Cracks taper downward, so the tip points to older beds
Graded bedding	Waning current, turbidite	Coarse base, fine top
Cross-bedding	Migrating ripples or dunes	Foresets dip down-current, truncated at set top
Flute casts	Scours filled from above	Seen on the bed sole; the deeper, rounded end faces up-current
Load casts and flames	Sand sinking into soft mud	Bulges point down, flames of mud point up

⚠️ Which end is the flute?

A flute cast is the cast of a scour hollow, so on the base of a sandstone bed its blunt, deep end points up-current and it widens and flattens down-current. Reading it the other way round reverses the palaeocurrent. For ripples, the steep lee face points the way the current went.

7. Metamorphic rocks: agents, textures, grade and facies

Metamorphism is the solid-state recrystallisation of a rock to new minerals and textures under changed temperature, pressure and fluids, below melting. Contact metamorphism bakes rock around an intrusion at low pressure and gives non-foliated hornfels. Regional metamorphism affects whole orogenic belts under directed stress and gives foliated rocks. Dynamic (cataclastic) metamorphism in fault zones gives breccia and mylonite; burial and hydrothermal metamorphism need little or no directed stress, and shock metamorphism follows meteorite impact.

Foliation comes from alignment of platy minerals: slaty cleavage (very fine, slate), phyllitic sheen, schistosity (visible micas) and gneissic banding (light and dark layers). The pelite sequence of rising grade is shale → slate → phyllite → schist → gneiss, then migmatite where partial melting begins. Non-foliated rocks are marble (from limestone), quartzite (from sandstone) and hornfels, with equant granoblastic grains. Porphyroblasts (garnet, staurolite) are large new crystals in a finer matrix.
In pelites Barrow's zones appear in order of rising grade: chlorite, biotite, garnet, staurolite, kyanite and sillimanite. Grade is the intensity of metamorphism; a facies is a set of mineral assemblages formed in a given pressure–temperature range, read from mafic rocks in the table.

Metamorphic facies of mafic rocks
Facies	Setting	Diagnostic minerals
Greenschist	Low-grade regional	Chlorite, actinolite, albite, epidote
Amphibolite	Medium-grade regional	Hornblende with plagioclase, often garnet
Granulite	High temperature, deep crust	Orthopyroxene, clinopyroxene, plagioclase; hydrous minerals scarce
Blueschist	Subduction zone: high pressure, low temperature	Glaucophane, with lawsonite or jadeite
Eclogite	Very high pressure, deep subduction	Omphacite and garnet; no plagioclase
Hornfels	Contact aureole: low pressure, high temperature	Fine granoblastic aggregates; cordierite or andalusite in pelites

Key takeaways

Igneous texture records cooling: phaneritic is plutonic, aphanitic and glassy are volcanic, porphyritic means two stages, and vesicular, pegmatitic, ophitic and pyroclastic are further textures. A dyke cuts the layers, a sill lies along them, a batholith exceeds about 100 km².
Silica classes are acid, intermediate, basic and ultrabasic; QAPF is recalculated to 100 % on Q + A + P + F alone. Granite, granodiorite and tonalite follow P/(A + P); diorite or gabbro follows the plagioclase An content. Plutonic and volcanic pairs: granite–rhyolite, diorite–andesite, gabbro–basalt.
Bowen's series: olivine to pyroxene to amphibole to biotite, and calcic to sodic plagioclase, joining at K-feldspar, muscovite and quartz. Partial melting of peridotite gives basalt and of crust gives granite; fractional crystallisation drives differentiation.
Clastic rocks follow grain size (gravel, sand 1/16 to 2 mm, silt, clay); φ = −log₂ d, so 8 mm is −3 φ. Inman: Mφ = (φ16 + φ84)/2, σφ = (φ84 − φ16)/2. Porosity n = 1 − ρ_b/ρ_g, or (ρ_g − ρ_sat)/(ρ_g − ρ_w) when saturated.
Arkose is feldspathic, greywacke matrix-rich, quartz arenite mature. Dunham's classes run mudstone, wackestone, packstone, grainstone, boundstone. Evaporites precipitate calcite, gypsum, halite, potash salts. Cross-bedding, graded bedding, mud cracks and flute casts give way-up and current directions.
Metamorphic grade rises shale, slate, phyllite, schist, gneiss, migmatite; Barrow zones run chlorite, biotite, garnet, staurolite, kyanite, sillimanite. Regional directed stress makes foliation; contact metamorphism gives hornfels. Facies: greenschist, amphibolite, granulite, blueschist, eclogite.

Practice questions (18)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

An igneous rock contains large, well-formed crystals of plagioclase set in a fine-grained groundmass. What does this porphyritic texture show?
1. The magma cooled at one constant rate throughout, so crystals grew steadily without any interruption
2. The rock formed from a very water-rich residual melt that crystallised entirely into giant crystals
3. The magma cooled slowly at first, growing the large crystals, and then cooled quickly
4. The rock was deformed long after solidification, so its crystals were crushed and then recrystallised
Show answer
Answer: C — The magma cooled slowly at first, growing the large crystals, and then cooled quickly
Phenocrysts in a finer groundmass record two stages of cooling: slow growth at depth, then rapid cooling after eruption or emplacement. A single cooling rate would give uniform grain size. Very water-rich melts give pegmatite, where all crystals are large and no fine groundmass is present, which is the tempting confusion.
Which volcanic rock is the fine-grained equivalent of the plutonic rock diorite?
1. Basalt, which has the same sodic plagioclase but contains far more pyroxene
2. Rhyolite, which is rich in quartz and potassium feldspar and is the acid equivalent
3. Trachyte, which is dominated by alkali feldspar and has little or no quartz
4. Andesite, which has intermediate silica and plagioclase with hornblende
Show answer
Answer: D — Andesite, which has intermediate silica and plagioclase with hornblende
Diorite and andesite share an intermediate composition (about 52 to 66 wt % SiO₂, plagioclase and hornblende), differing only in grain size. Basalt is the equivalent of gabbro, rhyolite of granite and trachyte of syenite.
The modal composition of a plutonic rock by volume is quartz 20 %, K-feldspar 25 %, plagioclase 25 %, biotite 10 % and hornblende 20 %. After setting aside the mafic minerals and recalculating Q + A + P to 100 %, what is the normalised quartz percentage Q′, to the nearest whole number? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 29
The mafic minerals (biotite and hornblende, 30 %) are set aside, so Q + A + P = 20 + 25 + 25 = 70. Q′ = 20 ÷ 70 × 100 = 28.6, which rounds to 29. Dividing by 100 gives 20, the usual slip, and 28.6 falls in the 20 to 60 granitoid range.
A plutonic rock has by volume quartz 25 %, K-feldspar 10 %, plagioclase 40 %, biotite 15 % and hornblende 10 %. Use this rule for rocks with quartz in the 20 to 60 range after normalisation: granite if the plagioclase share P/(A + P) is below 65 %, granodiorite if it is 65 to 90 %, and tonalite if it is above 90 %. What is the rock?
1. Granite, because quartz is above 20 % of the whole rock
2. Granodiorite, because P/(A + P) = 40/50 = 80 %
3. Tonalite, because plagioclase is the most abundant felsic mineral
4. Diorite, because the rock contains hornblende and biotite
Show answer
Answer: B — Granodiorite, because P/(A + P) = 40/50 = 80 %
Q + A + P = 25 + 10 + 40 = 75, so Q′ = 33 %, in the granitoid range. The plagioclase share is P/(A + P) = 40/(10 + 40) = 80 %, which lies between 65 and 90 %, so it is granodiorite. Plagioclase being the most abundant feldspar is not enough for tonalite, which needs more than 90 %, and diorite contains little or no quartz.
Which statements about Bowen's reaction series are correct?
1. In the discontinuous branch olivine is followed by pyroxene, then amphibole, then biotite as the magma cools
2. Quartz crystallises first, because silica is the most abundant oxide in magma
3. Calcium-rich plagioclase crystallises before sodium-rich plagioclase along the continuous branch
4. Minerals from the two ends of the series commonly crystallise together, as olivine does with quartz in granite
Show answer
Answer: A — In the discontinuous branch olivine is followed by pyroxene, then amphibole, then biotite as the magma cools; C — Calcium-rich plagioclase crystallises before sodium-rich plagioclase along the continuous branch
Olivine → pyroxene → amphibole → biotite is the discontinuous branch, and the plagioclase branch runs from calcic to sodic compositions as temperature falls. Quartz is one of the last minerals to crystallise, and minerals from opposite ends of the series, such as olivine and quartz, do not normally coexist, since the early crystal would have reacted with the melt.
A pebble has an intermediate diameter of 8 mm. Using the phi scale φ = −log₂ d with d in millimetres, what is its φ value? Give the number only, including its sign.
Numerical answer — type the value.
Show answer
Answer: -3
φ = −log₂ 8 = −3, because 8 = 2³. The negative sign shows a coarse grain: φ is negative for grains larger than 1 mm, zero at 1 mm and positive for finer sand, silt and clay. Writing +3 corresponds to a grain of 0.125 mm, the usual sign slip.
A cumulative grain-size curve for a sand gives φ16 = 0.5 and φ84 = 2.7. Using Inman's sorting σφ = (φ84 − φ16)/2, what is the sorting, in phi units, to one decimal place? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 1.1
σφ = (2.7 − 0.5)/2 = 2.2/2 = 1.1. The mean would be (0.5 + 2.7)/2 = 1.6 φ, which is the other statistic the same percentiles give; answering 1.6 or 2.2 confuses mean and sorting or omits the halving.
A sandstone with fully water-filled pores has a saturated bulk density of 2.35 g/cm³. The grain density is 2.65 g/cm³ and the pore water density is 1.00 g/cm³. What is the porosity, as a percentage, to the nearest whole number? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 18
The saturated density is ρ_sat = (1 − n) ρ_g + n ρ_w, so n = (ρ_g − ρ_sat)/(ρ_g − ρ_w) = (2.65 − 2.35)/(2.65 − 1.00) = 0.30/1.65 = 0.182, or 18 %. Using 1 − ρ_sat/ρ_g = 11 % ignores the water in the pores, which is the usual slip.
A sandstone is made of angular, poorly sorted grains with more than 25 % feldspar, and it lies close to an uplifted granitic source. Which sandstone type is it?
1. Quartz arenite, because the grains are mostly silicate minerals
2. Greywacke, because it is poorly sorted and occurs near a mountain source
3. Lithic arenite, because the grains came from a granitic parent rock
4. Arkose, because it is rich in feldspar from rapid erosion of granitic rock
Show answer
Answer: D — Arkose, because it is rich in feldspar from rapid erosion of granitic rock
Feldspar above about 25 % with angular, immature grains from granitic terrain defines arkose. Greywacke is the tempting alternative because it is also poorly sorted, but it is defined by a fine matrix of over about 15 % and lithic grains, not by feldspar. Quartz arenite is a mature, quartz-rich sandstone, and a lithic arenite is dominated by rock fragments.
A limestone consists of about 30 % carbonate grains floating in a matrix of lime mud, so the grains do not touch one another. Which Dunham class is it?
1. Mudstone, because the rock is dominated by the lime mud matrix
2. Wackestone, because it is mud-supported with over 10 % grains
3. Packstone, because it contains both grains and lime mud together
4. Grainstone, because carbonate grains are the main component here
Show answer
Answer: B — Wackestone, because it is mud-supported with over 10 % grains
Mud-supported rocks with more than 10 % grains are wackestones. Mudstone has under 10 % grains, and packstone is grain-supported, with the grains touching and mud filling the gaps, which is the tempting neighbour. A grainstone is grain-supported with no mud.
As seawater evaporates in a closed basin, which sequence lists the minerals in the order in which they precipitate?
1. Halite, then gypsum, then calcite, then potash salts
2. Calcite, then gypsum, then halite, then potash salts
3. Potash salts, then halite, then gypsum, then calcite
4. Gypsum, then calcite, then potash salts, then halite
Show answer
Answer: B — Calcite, then gypsum, then halite, then potash salts
The least soluble phases precipitate first and the most soluble last: carbonates (calcite, dolomite), then gypsum or anhydrite, then halite, and finally the potash and magnesium salts. The reverse and mixed orders put the very soluble salts too early, and halite never comes before gypsum in seawater evaporation.
In a vertical cliff face the original orientation of a sedimentary sequence is unknown. Wedge-shaped infills of desiccation cracks in a mudstone layer are seen in section, each narrowing toward the left side of the face. Which way are the younger beds?
1. To the right, because mud cracks taper toward older beds
2. To the left, because a crack narrows toward the layer that was deposited later
3. To the left, because mud cracks always open downward from the top surface
4. It cannot be told, because desiccation cracks give no way-up information
Show answer
Answer: A — To the right, because mud cracks taper toward older beds
Desiccation cracks open at the exposed top surface and taper downward, so the narrow ends point toward older beds; with the narrow ends to the left, older beds are on the left and the younger beds lie to the right. The statements that mud cracks open downward or give no way-up information are both wrong, because the V-shaped taper is a standard younging criterion.
Asymmetrical ripple marks in a sandstone bed have a gentle slope facing north and a steep slope facing south. In which direction did the current flow?
1. Toward the north, because the gentle slope faces the current's destination
2. Toward the south, because the steep lee face points down-current
3. Toward the east or west, because ripples always form across the flow
4. It cannot be told, because only symmetrical ripples show flow direction
Show answer
Answer: B — Toward the south, because the steep lee face points down-current
In a current ripple, water climbs the gentle stoss slope and sand avalanches down the steep lee face, so the steep face points down-current, here to the south. Symmetrical ripples are the ones that give no flow direction (wave oscillation), and ripple crests lie across the flow, whose direction is given by the steep face.
Which of the following can be used as way-up (younging) criteria in a tilted or overturned sedimentary sequence?
1. Graded bedding in which the grains become finer toward the top of the bed
2. Desiccation cracks that taper downward into the underlying bed
3. Parallel lamination of uniform thickness with no change in grain size
4. A massive bed of uniform grain size with no internal structure
Show answer
Answer: A — Graded bedding in which the grains become finer toward the top of the bed; B — Desiccation cracks that taper downward into the underlying bed
Normal graded bedding fines upward, and mud cracks taper downward, so both show which side is the top. Uniform parallel lamination and a massive uniform bed have no asymmetry between top and bottom, so they cannot indicate way-up.
Which sequence lists the metamorphic rocks formed from shale in order of increasing metamorphic grade?
1. Schist, then slate, then phyllite, then gneiss
2. Phyllite, then slate, then schist, then gneiss
3. Slate, then phyllite, then schist, then gneiss
4. Gneiss, then schist, then phyllite, then slate
Show answer
Answer: C — Slate, then phyllite, then schist, then gneiss
Shale becomes slate at low grade (very fine cleavage), then phyllite (silky sheen), then schist (visible mica) and finally gneiss (coarse banding) at the highest grade. A list that begins with gneiss gives decreasing grade, and the lists that put schist before slate or phyllite before slate mix up the order of the fine-grained rocks.
A metabasalt contains glaucophane together with lawsonite and jadeite. Which setting does this assemblage indicate?
1. A contact aureole around a granite pluton, with high temperature and low pressure
2. The base of a thick continental crust, with very high temperature and little water
3. A subduction zone, with high pressure and low temperature, in the blueschist facies
4. A mid-ocean ridge flank, with low pressure and hydrothermal circulation of seawater
Show answer
Answer: C — A subduction zone, with high pressure and low temperature, in the blueschist facies
Glaucophane with lawsonite and jadeite is the signature of the blueschist facies, formed where cold oceanic crust is carried to depth in a subduction zone, giving high pressure and low temperature. Granulite, not blueschist, forms in a hot deep crust, hornfels in a contact aureole, and the ridge-flank setting gives greenschist assemblages with chlorite and actinolite.
Regional metamorphic rocks such as schist are foliated, while hornfels from contact metamorphism is not. Which explanation is correct?
1. Regional metamorphism has directed stress that aligns platy minerals; contact metamorphism is thermal and nearly stress-free
2. Hornfels forms at a lower temperature than schist, so its minerals have no chance to recrystallise into platy shapes
3. Hornfels forms only from protoliths that contain no clay minerals, while every schist comes from original shale
4. Hornfels forms by complete melting of the protolith, so any earlier alignment of its minerals is destroyed
Show answer
Answer: A — Regional metamorphism has directed stress that aligns platy minerals; contact metamorphism is thermal and nearly stress-free
Foliation develops when platy minerals grow or rotate perpendicular to the directed (differential) stress of regional deformation. Contact metamorphism near an intrusion is driven by heat under near-hydrostatic stress, so the grains stay equant and randomly oriented. Hornfels forms at high temperature, not lower, and without melting; and it can form from shale as well.
Which of the following pairings of protolith and metamorphic rock are correct?
1. Limestone gives marble, a non-foliated rock of interlocking calcite grains
2. Shale gives quartzite, a hard rock made almost entirely of quartz
3. Quartz sandstone gives quartzite, a granoblastic rock of recrystallised quartz
4. Basalt gives marble when it is heated in a contact aureole
Show answer
Answer: A — Limestone gives marble, a non-foliated rock of interlocking calcite grains; C — Quartz sandstone gives quartzite, a granoblastic rock of recrystallised quartz
Limestone recrystallises to marble and quartz sandstone to quartzite, both non-foliated granoblastic rocks. Shale cannot give quartzite because it lacks the silica-rich composition; it gives slate, phyllite, schist or gneiss. Basalt gives greenschist, amphibolite or granulite, not marble, which needs calcium carbonate.