Notes · GeologyUK · A-Levels

Rock forming processes

This chapter treats the three families of rock-forming process in the depth the A-Level demands. It covers igneous crystallisation and the classification of igneous rocks by silica content and grain size, the lithification and sedimentary structures that record depositional conditions, the interpretation of sedimentary environments from that evidence, and the regional and contact metamorphism that recrystallise rock in the solid state.

4 sections~15 min reading time3 competenciesLevel Standard 1 · Advanced 3

T·0333 / 13

Exam profile

AO1 · Describe igneous, sedimentary and metamorphic processes and their productsAO2 · Classify rocks and apply structures and minerals to deduce conditions of formationAO3 · Interpret sedimentary structures and metamorphic assemblages to reconstruct environments and conditions

Operators:describeexplainclassifyinterpretdeduceanalyse

basic level

AS-Level expects you to describe how igneous, sedimentary and metamorphic rocks form and to classify common examples.

higher level

The full A-Level requires you to apply Bowen's reaction series, read sedimentary structures to reconstruct environments, and relate metamorphic grade and index minerals to conditions.

Depth

Text

Contents · 4 sections

Rock forming processes

Igneous processes and classification#

AdvancedWJEC/Eduqas A level Geology (Component 2) — Rock forming processes

Bowen's reaction series

Fig. 1Bowen's reaction series: the order in which minerals crystallise as a magma cools, with the discontinuous (ferromagnesian) and continuous (plagioclase) branches converging on quartz.

Igneous rocks crystallise from magma (molten rock below the surface) or lava (molten rock erupted at the surface). Two variables control the rock that results: the composition of the magma, which ranges from silica-rich (acidic, over about 66% silica) through intermediate and basic to silica-poor (ultrabasic, under about 45% silica), and the rate of cooling, which controls crystal size. Slow cooling at depth grows large, interlocking crystals (a coarse, phaneritic texture, as in granite); rapid cooling at or near the surface grows small crystals (a fine, aphanitic texture, as in basalt); and very rapid quenching gives a glass such as obsidian.

As a magma cools it does not crystallise all at once; its minerals crystallise in a definite order set out by Bowen's reaction series. There are two branches. The discontinuous branch runs olivine, pyroxene, amphibole, biotite mica, each mineral reacting with the remaining melt to form the next at lower temperature; the continuous branch runs the plagioclase feldspars from calcium-rich (high temperature) to sodium-rich (lower temperature). The two branches converge at potassium feldspar, then muscovite mica, and finally quartz, the last to crystallise. The high-temperature minerals (olivine, calcium plagioclase) are the dense, dark, silica-poor minerals of basic rocks; the low-temperature minerals (quartz, potassium feldspar) are the light, silica-rich minerals of acidic rocks.

Bowen's series explains far more than the order of crystallisation. It explains why certain minerals occur together (a rock rich in olivine and calcium plagioclase, but never in olivine and quartz, because these crystallise at opposite ends of the series); it underlies fractional crystallisation, in which early-formed crystals are separated from the melt so that the residual magma becomes progressively more silica-rich, one way a single parent magma can produce a range of rocks; and it predicts the order of weathering, because the last minerals to crystallise (quartz) are the most stable at the surface and the first (olivine) the least stable. It is one of the most connective ideas in the subject.

Igneous rocks are classified on a simple grid of composition against grain size, and being able to name a rock from these two axes is a core skill. By grain size a rock is coarse (intrusive, slow cooling), medium (a minor intrusion such as a dyke or sill) or fine (extrusive, fast cooling); by composition it is acidic, intermediate, basic or ultrabasic. Thus a coarse acidic rock is a granite and its fine equivalent a rhyolite; a coarse basic rock is a gabbro and its fine equivalent a basalt; the intermediate pair is diorite and andesite. Acidic rocks are pale (much quartz and feldspar) and basic rocks dark (much ferromagnesian mineral), so colour is a quick guide to composition.

Classification of igneous rocks

Fig. 2Igneous rocks classified by grain size (cooling rate) and composition (silica content): granite and rhyolite are acidic, gabbro and basalt basic.

Worked example

Classifying an igneous rock

A rock is coarse-grained, pale, and made largely of quartz and potassium feldspar with a little mica. Classify it by grain size and composition, and name it.

Grain size
Coarse, interlocking crystals indicate slow cooling at depth, so the rock is intrusive.
Composition
Abundant quartz and potassium feldspar (both silica-rich, low-temperature minerals of Bowen's series) and a pale colour indicate an acidic composition.
Name it
A coarse, acidic igneous rock of quartz and feldspar is a granite.

Result: The rock is a granite: coarse (intrusive) and acidic (quartz plus potassium feldspar).

Typical mistakes

Saying quartz crystallises first; quartz is the last mineral to crystallise, at the lowest temperature.
Confusing grain size (cooling rate) with composition; a coarse rock can be acidic (granite) or basic (gabbro).

Active revision

A dark, fine-grained volcanic rock is rich in pyroxene and calcium plagioclase with no quartz. Classify it, name it, and use Bowen's series to explain the mineral assemblage.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — igneous rocks (British Geological Survey)

Sedimentary processes and structures#

StandardWJEC/Eduqas A level Geology (Component 2) — Rock forming processes

Cross-bedding and graded bedding

Fig. 3Cross-bedding (inclined laminae recording current direction) and graded bedding (grain size fining upward as a current wanes) are diagnostic sedimentary structures and way-up indicators.

Sedimentary rocks form at the surface by three routes. Clastic rocks are made of transported fragments (clasts) lithified together, and are named by grain size — conglomerate and breccia (gravel), sandstone (sand) and mudstone or shale (mud). Chemical rocks precipitate from solution, such as rock salt and other evaporites and some limestones. Biogenic (biochemical) rocks are built from the remains of organisms, such as most limestones (from shell and coral debris) and coal (from plant matter). Recognising which route a sedimentary rock took is the first step in reading its history.

Sedimentary rocks are unique in carrying sedimentary structures — features formed during or soon after deposition that record the conditions at the time. Bedding, the layering produced by successive deposition, is the most fundamental, and the principle of original horizontality holds that beds are laid down roughly horizontally, so tilted beds have since been deformed. Cross-bedding, in which inclined laminae lie within a bed, forms where migrating ripples or dunes build forward, and the direction of the inclined laminae records the direction of the current or wind. Graded bedding, in which grain size decreases upward through a bed, forms as a waning current drops its coarsest load first.

Other structures are equally diagnostic. Ripple marks on a bedding surface record currents or waves: asymmetrical ripples indicate a one-way current (a river), symmetrical ripples a to-and-fro wave motion (a shore). Desiccation (mud) cracks, polygonal cracks in fine sediment, indicate drying out and therefore periodic exposure to the air, as on a mudflat or floodplain. Way-up (younging) structures such as graded bedding, cross-bedding and desiccation cracks let a geologist tell which way was originally up, essential for reading a sequence that has been tilted or overturned.

The value of these structures is that they are the direct, physical fingerprints of the environment of deposition, and unlike fossils they are present even in barren rocks. A geologist reading a sandstone with large-scale cross-bedding, well-rounded and well-sorted grains, and no fossils will infer a desert dune field; one reading a mudstone with symmetrical ripples, desiccation cracks and shallow-water fossils will infer an intertidal flat. The systematic use of structures to deduce environment is developed in the next section and is a favourite of the examiners.

Worked example

Interpreting a graded bed

A marine sequence contains repeated beds each of which grades from coarse sand at the base to mud at the top. Explain how each bed formed and what the repetition suggests.

Read the grading
Each bed fines upward, from coarse sand to mud, which is graded bedding formed as a sediment-laden current slowed and dropped first its coarsest and then its finest load.
Identify the mechanism
Such graded beds form from turbidity currents (submarine sediment flows) surging down a slope and then waning, each surge depositing one graded bed (a turbidite).
Interpret the repetition
The repetition of graded beds indicates many successive turbidity currents, typical of deposition at the foot of a continental slope.

Result: Each fining-upward bed is a turbidite deposited by a waning turbidity current; the repetition records many such flows on a submarine slope.

Typical mistakes

Confusing cross-bedding (inclined laminae within a bed) with the boundaries between beds; the cross-laminae are internal to a single bed.
Reading a graded bed the wrong way up; grain size decreases upward, so the coarse base is the older, lower part.

Active revision

A bed shows grain size decreasing from coarse gravel at the base to fine sand at the top. Name the structure, state which way is up, and explain how it formed.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — sedimentary rocks (British Geological Survey)

Interpreting sedimentary environments#

AdvancedWJEC/Eduqas A level Geology (Component 2) — Rock forming processes

Reading environment from evidence

Fig. 4Environmental interpretation combines independent lines of evidence — grain characteristics, structures, fossils and rock type — into a facies interpretation.

A sedimentary environment is the setting in which sediment accumulates — a desert, river, delta, beach, shallow sea, reef or deep ocean — and each leaves a characteristic combination of rock type, grain characteristics, sedimentary structures and fossils. The task of environmental interpretation is to read these clues together, because no single clue is conclusive, and to reason from the present (uniformitarianism): the features made in modern deserts or reefs are the key to recognising ancient ones. The set of features that characterises an environment is its facies.

Grain characteristics narrow the environment down. Well-sorted, well-rounded sand indicates prolonged reworking by a consistent medium such as wind or waves (a desert or beach); poorly sorted, angular sediment indicates rapid deposition with little transport (a scree, or a glacial till). Grain size indicates energy: gravels and coarse sands need high energy (rivers, beaches, shallow agitated water), muds settle only in low energy (lagoons, deep sea). Composition helps too: a mature quartz sand has survived long weathering and transport, whereas an immature sand still rich in feldspar has not.

Structures and fossils then pin the environment down. Large-scale cross-bedding with frosted, well-rounded grains and no marine fossils points to a desert dune field; symmetrical ripples and desiccation cracks point to an intertidal flat; a limestone full of corals and shells points to a warm, clear, shallow sea. Fossils are especially powerful because many organisms are restricted to particular environments — corals to warm shallow seas, certain plants to swamps — so a rock's fossils constrain not only its age but its environment and even its former climate and water depth.

The most secure interpretations combine several independent lines of evidence and are honest about their limits. A single feature can be ambiguous — cross-bedding forms under both water and wind — so a good answer weighs the whole assemblage: the rock type, the sorting and rounding, the structures, the fossils and the way the beds change upward. Being able to marshal this evidence into a justified conclusion, and to acknowledge where the evidence is equivocal, is exactly the AO3 skill the specification rewards, and it recurs in the map and stratigraphy chapters.

Worked example

Deducing a desert environment

A red sandstone is very well sorted and well rounded, shows very large-scale cross-bedding, has frosted grain surfaces and contains no marine fossils. Deduce the environment and give the evidence for each conclusion.

Sorting and rounding
Very good sorting and rounding indicate prolonged reworking by a consistent medium; with the frosted grains this points to wind transport.
Structures and colour
Very large-scale cross-bedding is typical of migrating sand dunes, and the red colour indicates oxidation of iron in a well-aerated, dry setting.
Fossils
The absence of marine fossils is consistent with a subaerial, arid environment rather than a sea.
Conclude
Together the evidence indicates a hot desert dune field (an aeolian environment).

Result: The rock formed in a hot desert dune field: wind-blown (frosted, well-rounded, well-sorted), dune cross-bedded, oxidised red and lacking marine fossils.

Typical mistakes

Relying on one feature; cross-bedding alone cannot distinguish a desert from a river, so the whole assemblage must be weighed.
Ignoring fossils when interpreting environment; fossils often constrain water depth, salinity and climate as well as age.

Active revision

A well-sorted, well-rounded sandstone shows large-scale cross-bedding and contains no marine fossils. Interpret its environment of deposition and justify your answer.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — depositional environments (British Geological Survey)

Metamorphic processes, grade and zones#

AdvancedWJEC/Eduqas A level Geology (Component 2) — Rock forming processes

Progressive metamorphism of a mudstone

Fig. 5With increasing grade under regional metamorphism, a mudstone passes from slate to phyllite to schist to gneiss, the foliation coarsening throughout.

Metamorphism is the transformation of a pre-existing rock (the protolith) in the solid state by heat, pressure and chemically active fluids, without melting. New minerals grow that are stable under the new conditions, and the texture is reorganised, but the rock never becomes a liquid — a key distinction from igneous processes. The intensity of metamorphism is its grade, from low grade (relatively low temperature and pressure) to high grade (high temperature and pressure), and the grade is read from the minerals and textures produced.

There are two main settings. Regional metamorphism affects large areas during mountain-building, under high temperature and, crucially, directed (differential) pressure, which aligns platy and elongate minerals to produce foliation. As grade increases, a mudstone passes through a classic sequence: slate (low grade, fine, with a slaty cleavage), phyllite, schist (medium grade, visible aligned micas, schistosity), and gneiss (high grade, coarse, segregated into light and dark bands). Contact (thermal) metamorphism, by contrast, occurs in the aureole around an igneous intrusion, driven by heat alone without directed pressure, and so produces non-foliated rocks such as hornfels, and marble from limestone.

Certain index minerals appear at particular grades and so act as natural thermometers and barometers of metamorphism. In metamorphosed mudstones, chlorite indicates low grade, then biotite, garnet, staurolite, kyanite and finally sillimanite at the highest grade; mapping where each first appears defines metamorphic zones and isograds across a region. The appearance of a given index mineral therefore lets a geologist assign a grade and reconstruct the temperatures and pressures a region experienced, linking the rocks to the tectonic events that produced them.

Protolith and product are systematically related, and predicting one from the other is a standard skill. Mudstone gives slate, phyllite, schist and gneiss with increasing grade; a pure limestone recrystallises to marble; a quartz sandstone recrystallises to the tough, sugary quartzite; and basalt gives greenschist and then amphibolite. Because the process is solid-state recrystallisation, the chemistry is largely inherited from the protolith while the minerals and texture record the conditions, so a metamorphic rock is a record of both what it was made of and what it went through.

A contact metamorphic aureole

Fig. 6Contact metamorphism: heat from an intrusion bakes the surrounding rock into a metamorphic aureole, the grade decreasing outward away from the contact.

Worked example

Predicting metamorphic products

A mudstone is caught up in a mountain-building event and metamorphosed to increasing grade. State the sequence of rocks produced, name a low-grade and a high-grade index mineral, and explain why the rocks are foliated.

The rock sequence
With rising grade the mudstone passes through slate, phyllite, schist and finally gneiss.
Index minerals
Chlorite indicates low grade; garnet or sillimanite indicates higher grade, acting as natural thermometers.
Explain the foliation
Regional metamorphism applies directed pressure, which aligns platy micas perpendicular to the maximum stress, producing the foliation (slaty cleavage, then schistosity, then gneissic banding).

Result: Mudstone gives slate, phyllite, schist then gneiss; chlorite (low) to sillimanite (high) index the grade, and the directed pressure of regional metamorphism produces the foliation.

Typical mistakes

Saying metamorphic rocks melt; metamorphism is a solid-state process — melting would produce magma and an igneous rock.
Assigning foliation to contact metamorphism; foliation requires the directed pressure of regional metamorphism, not the heat-only setting of an aureole.

Active revision

Predict and name the metamorphic products of (a) a limestone in a contact aureole and (b) a mudstone at increasing grade in a mountain belt, explaining the difference in texture.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — metamorphic rocks (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

British Geological Survey — igneous rocks

Rock forming processes

4 sections~15 min reading time3 competenciesLevel Standard 1 · Advanced 3

Rock forming processes

Key points

Classifying an igneous rock

Key points

Interpreting a graded bed

Key points

Deducing a desert environment

Key points

Predicting metamorphic products

Sources

Rock forming processes

Key points

Classifying an igneous rock

Key points

Interpreting a graded bed

Key points

Deducing a desert environment

Key points

Predicting metamorphic products

Sources