MODULE 2832 (B) THE ROCK CYCLE – PROCESSES AND PRODUCTS
REVISION NOTES
2.1
The Rock Cycle
Candidates should be able to:
(a)
Describe the rock cycle and define the processes which operate within it: (i) at
the surface – weathering, erosion, transport, deposition and extrusion; (b) below the
surface – burial, diagenesis, recrystallisation, metamorphism, partial melting, magma
accumulation, crystallisation, intrusion and uplift.
You must be able to draw a fully labelled/annotated diagram showing the processes of the
rock cycle.
 Weathering is the “in situ” breakdown of rocks at the Earth’s surface, doesn’t involve
transport.
 Erosion is the gradual wearing away of material at the Earth’s surface as it undergoes
transport.
 Transport involves sediment being moved from one place to another. The agents of
transport are wind, water, ice and gravity.
 Deposition occurs due to physical (commonly loss of energy) or chemical changes and
results in material being laid down on the Earth’s surface. One sedimentary bed is the
result of one period of deposition.
 Burial occurs as sediment is covered over as more layers of sediment accumulate on top
(the overburden).
 Lithification - the process of converting loose sediment into a sedimentary rock.
 Diagenesis - all the physical (mainly compaction) and chemical (mainly cementation)
processes that act on sediment during lithification.
 Extrusion occurs when magma is erupted as lavas/pyroclastics onto the Earth’s surface.
This forms a volcanic/extrusive rock.
 Intrusion occurs when magma cools and crystallises inside the Earth’s crust. This forms a
plutonic (deep-seated) or hypabyssal (shallow level/near surface) intrusive rock.
 Partial Melting is the process by which magma forms in the Earth’s mantle – different
minerals have different melting points so only part of the rocky/silicate mantle melts to
form magma.
 Magma accumulation is the process by which the magma collects within an underground
magma chamber.
 Crystallisation occurs when molten magma cools and solidifies into individual
crystals/minerals.
 Metamorphism is the process by which rocks in the Earth’s crust are changed by the
effects of heat and pressure. The process is isochemical and involves solid state
recrystallisation.
 Recrystallisation occurs during metamorphism – it is a solid-state process by which
existing minerals are changed/recrystallised into new crystalline metamorphic minerals.
 Uplift is the process by which deeply buried rocks are returned to the Earth’s surface by
tectonic forces.
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The rates of geological processes are variable. Some are very slow and take tens of
thousands to millions of years to complete, e.g. crystallisation of an igneous intrusion,
metamorphism. Others may be very rapid, e.g. a volcanic eruption, earthquake, flash
flood.
As the Earth is 4.5 billion years old, some rocks have been round the rock cycle several
times.
The Principle of Uniformitarianism states “The present is the key to the past” and our
assumption is that the same geological processes operated in the past, at the same rates,
as they do today.
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(b)
Describe the broad classification of rocks into igneous, sedimentary and
metamorphic classes and their relationship to temperatures and pressures in the rock
cycle.
Rocks are classified according to the way they form, i.e. a genetic classification. There are 3
rock groups:
 Sedimentary rocks - These are rocks formed at the Earth’s surface. Rocks at the Earth’s
surface are broken down by weathering and erosion. The broken down material is
transported. When there is a physical/chemical change (e.g. a reduction in energy) the
particles of rock are deposited. The particles are then buried and undergo lithification and
diagenesis to make a clastic sedimentary rock, e.g. sandstone. Form at low temperatures
and pressures.
 Igneous rocks - Form from cooling and crystallisation of molten magma. There are two
main types of igneous rocks: Intrusive (hypabyssal or plutonic) igneous rocks - these have
a medium-coarse crystal size because the magma cooled slowly inside the Earth’s crust,
e.g. granite; and Extrusive (volcanic) igneous rocks - these have a fine crystal size
because the magma reached the Earth’s surface and cooled quickly, e.g. basalt. Form at
high temperatures and pressures.
 Metamorphic rocks - These are formed when rocks are subjected to heat and pressure
(metamorphism) in the Earth’s crust. This causes their structure to change as minerals in
the rock recrystallise in the solid state to form new minerals. There are two main types of
metamorphism - Contact and Regional. Regional metamorphic rocks contain a foliation
due to directed pressure, e.g. slate. Form at temperatures > 200°C and < 800°C and low
to high pressures.
(c)
Distinguish between the broad groups of igneous, sedimentary and
metamorphic rocks using characteristic features including mineral composition and
textures as seen in simplified drawings / photographs taken from thin sections
(microscope work is not expected).
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Igneous: Crystalline; Equigranular, porphyritic, vesicular, amygdaloidal, flow banded,
glassy textures; Hard silicate minerals - quartz + feldspars + mafic minerals; May show
cross-cutting relationships if intrusive; May have chilled margins and contain xenoliths;
May be fragmental if pyroclastic.
Sedimentary: Grains (except evaporites); Bedding; Sedimentary structures – cross
bedding, ripple marks, desiccation cracks, graded bedding, sole structures; May contain
fossils; Matrix or cement; Quartz + mica + calcite + clay minerals + evaporites (some are
soft).
Metamorphic: Crystalline; Porphyroblastic, granoblastic textures; Types of foliation – slaty
cleavage, schistosity, gneissose banding; Mainly hard silicate minerals - quartz + mica +
garnet + Al2SiO5 polymorphs + calcite; May contain relict structures and deformed fossils;
May form a metamorphic aureole/baked margin around an igneous intrusion.
Describe the division of the geological column into eras and systems.
The Geological Column is the time framework used by Geologists. Geological time is
divided into Eras and Periods/Systems.
Eras are the major subdivisions. The four Eras are: the Precambrian 4600-590 Ma;
Palaeozoic (old life) 590-235 Ma; Mesozoic (middle life) 235-65 Ma; and Cenozoic (new
life) 65 Ma - present.
The Eras are subdivided into Periods and the rocks laid down in a particular Period are
known as a System.
The Periods/Systems are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous,
Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary/Recent.
Angular unconformity – a plane surface representing a break in time when no deposition
occurred and erosion may have occurred. The rocks below may be much older than the
rocks above the unconformity. Beds below and above the unconformity may be very
different (different environments of deposition) and will dip at different angles.
Sequence of formation: 1. Older sediments deposited horizontally; 2. Rocks uplifted,
folded or tilted; 3. Erosion occurs → unconformity; 4. Younger sediments deposited.
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ERA
Cenozoic
Mesozoic
Palaeozoic
PERIOD/SYSTEM
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Precambian
2.2
TIME (Ma)
2.5-0
65-2.5
135-65
195-135
235-195
280-235
370-280
415-370
445-415
515-445
590-515
4600-590
Sedimentary Processes and Products
Candidates should be able to:
(a)
Describe and explain the weathering processes producing soluble products
and insoluble residues by chemical (hydrolysis, hydration, oxidation, carbonation),
mechanical (exfoliation and frost shattering) and biological processes.
Weathering is a form of low temperature differentiation producing soluble products (ions
removed in solution) and insoluble residues (clay minerals or scree).
Mechanical/Physical weathering (freeze-thaw, insolation, stress release, salt crystallisation)
- results in changes in grain size, produces angular fragments, composition is unchanged.
 Freeze-thaw – Water enters cracks, freezes and expands by ~9%, exerts pressure on
rock → eventual failure. Requires daily temperatures to fluctuate above and below 0ºC.
Produces angular fragments called scree.
 Insolation – Alternate heating and cooling, causes rock to expand and contract, causes
stress → failure. Common in deserts.
 Stress release (exfoliation) – Overlying rocks removed by weathering, rocks “spring up”
as stress is released, causes joints/fractures (unloading joints) to open up parallel to
surface and fragments to break off. “Onion skin weathering” – common in granite.
 Salt crystallisation – Salt water enters cracks, water evaporates, salt crystals grow
exerting pressure on rock.
Chemical weathering (carbonation, oxidation, hydrolysis, hydration) – results in changes in
composition of the rock, soluble ions are removed in solution leaving an insoluble residue of
clay minerals.
 Carbonation (solution) – CO2 dissolved in rainwater forms a weak acid, which slowly
dissolves susceptible rocks, e.g. limestone.
 Oxidation – Reaction with oxygen in air, iron-bearing minerals oxidise → red colour.
Indicates weathering in terrestrial (land), arid environment.
 Hydrolysis – Reaction with water, e.g. feldspar → clay minerals (very slow!!).
 Hydration – Addition of water, e.g. anhydrite CaSO4 → gypsum CaSO4.2H2O.
Biological weathering - Roots in cracks physically prise the rock apart; humic acid produced
by plants attacks rocks chemically.
(b)
Explain the influence of gravity, wind, ice, the sea and rivers on the parameters
of grain size, shape, roundness and degree of sorting and methods of transport
(solution, suspension, saltation and traction).
Grain size - measured on the Wentworth Scale – the average diameter of the grains in mm.
This scale is also used to name clastic sedimentary rocks. As the range of grain sizes found
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in nature is large a logarithmic scale is more practical than a linear scale. However, as the
Wentworth scale is rather cumbersome it can be converted into the phi scale.
The Wentworth scale:
Diameter
(mm)
Phi Ø
>2
2
-1
1
0
0.5
1
0.25
2
0.125
3
0.0625
(1/16)
> 0.0625
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Sediment Name
Rock Name
Boulders, Cobbles, Pebbles,
Gravel
Conglomerate (rounded grains)
Breccia (angular grains)
Sand – very coarse
Sand – coarse
Sand – medium
Sandstone - grain size is
between 2 – 1/16 mm.
Sand – fine
Sand – very fine
Silt, Clay
Siltstone, Mudstone,
Shale, Clay
Roundness – the degree to which the sharp corners have been knocked off, based on the
curvature of the corners. Visual estimation - compared to a chart - Very angular; angular; subangular; sub-rounded; rounded; well rounded.
Sorting – a measure of the range of grain sizes in a sediment, the degree to which the grains
are the same size. Either a visual estimation or a mathematical calculation. Very well sorted;
well sorted; moderately sorted; poorly sorted; very poorly sorted.
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During erosion and transport the rock particles are subjected to abrasion and attrition.
This results in the particles becoming smaller, rounder and better sorted. In addition,
softer minerals are gradually destroyed.
Gravity and ice – very little influence on sorting, grain size, etc., so usually texturally and
compositionally immature, poorly sorted, angular fragments.
Wind and water (high energy) – more efficient at rounding and sorting fragments, so
usually mature depending on distance from source.
You must be able to plot histograms and cumulative frequency curves, calculate the
coefficient of sorting, and be able to plot rose diagrams.
Solution – soluble ions are dissolved in water, load in solution cannot be seen.
Suspension – fine, clay-sized particles are carried in the water column, gives water a
murky appearance.
Saltation – sand and gravel sized particles are carried along the bottom in a hopping,
bouncing motion.
Traction – coarse boulders sliding and rolling along the bottom, they are too heavy to be
lifted by the water.
Make sure you can label areas of erosion, transport and deposition and interpret a graph
of particle size against velocity of flow.
Clay sized particles require high velocities for erosion because they are flat and platy in
shape and cohesive.
Clay sized particles require very low velocities/still water for deposition because they are
small, light and bouyant.
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(c)
Describe the deposition in hot desert environments of wadi conglomerates,
dune sandstones and evaporites in playa lakes.
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Wadi conglomerates – a dry valley liable to flash flooding during infrequent heavy
rainstorms. No infiltration or interception of water → runs rapidly over ground. Produces
an immature, poorly sorted, chaotic mixture of gravels/boulders → breccia/conglomerate.
Dune sandstones – result of wind transport – very high energy. There are 4 main types of
dunes: barchan (crescent); longitudinal, transverse, stellate. Produces a texturally and
compositionally mature desert/aeolian sandstone. Medium sand grains (0.5mm), well
rounded, high sphericity, well sorted – “millet seed sand”. Frosted grains, 100% quartz,
iron oxide coating. Contains large scale/dune cross-bedding (> 1m scale) formed by
sediment moving up the windward slope (stoss) by saltation, collecting at the top in the
lee and cascading/avalanching down the lee slope. Lee slope (max. 35º) is steeper than
stoss slope. As migration occurs successive lee slopes are preserved → foreset beds.
Playa lake mudstone and evaporites – temporary (ephemeral) lakes formed after rain in
the desert. As the lake evaporates → fine-grained mud/clay is deposited, this dries out →
mud cracks, followed by crystallisation/precipitation of a sequence of evaporite salts. As
the water evaporates, the salts become more concentrated. A predictable sequence of
evaporites is produced on the basis of solubility – least soluble first, most soluble last.
Calcite → (dolomite) → gypsum → (anhydrite) → halite → K-Mg salts.
(d)
Describe the deposition in deltaic environments of delta top (topsets) to form
coal, sandstones of the delta slope (foresets) and shales to form offshore deposition
(bottomsets). Understand deltaic deposition in cyclothems.
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A delta forms where a river meets the sea and deposits its load. Heavier, coarser material
(sand and gravel) is deposited first, closest to the river mouth. Finer sediment is carried
further out to sea, before settling from suspension to form the prodelta. As the coarse
material accumulates it becomes unstable and slumps down → sloping delta front/slope.
Meanwhile, the delta builds outwards and upwards (progrades). As the delta progrades,
the delta front sediments cover the prodelta, producing a coarsening upwards sequence.
If the delta top emerges from the sea it may be colonised by plants (swampy
environment) → coal.
This produces a distinctive sedimentary sequence: limestone (shallow marine) → shale
(prodelta) - bottomsets → sandstone (delta front) - foresets → seat earth (palaeo-soil) →
coal (vegetation colonising delta top) - topsets.
Deltaic sedimentation occurs as repeated cycles due to: subsidence of the delta
(local/isostatic change); or rises in sea level (global/eustatic change).
Each cycle is called a cyclothem.
(e)
Describe the deposition of clastic material formed in sediment-rich shallow
seas, to form conglomerates, sandstones and mudstones.
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Clastic material is derived from weathering and erosion of land. Main input is from rivers.
Sediment becomes finer grained as distance from land increases. Pebbles and sand –
deposited in high energy areas – beach/near shore; silts and muds – deposited in lower
energy areas – offshore on continental shelf.
The material is transported by wave and tidal currents.
Sedimentary structures include: ripple marks, small scale cross-bedding, burrows.
(f)
Describe the deposition of limestones formed in shallow seas to form oolitic
and bioclastic limestones.
Covers a wide range of environments – reefs, sand shoals/carbonate sand banks, lagoons,
deep shelf. Biological and biochemical processes are largely responsible for the formation
and deposition of limestones. They usually form in tropical seas within 30° of the Equator –
requirements include warm (>25°C), shallow (<200m, but most form within 10m of surface),
marine conditions, with normal salinity and a lack of clastic terrestrial sediment input. A wide
range of limestones are formed.
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Oolitic limestones are characteristic of higher energy environments such as carbonate
sand banks/barrier beaches and often have a carbonate cement. Ooliths/oolites form by
evaporation of lime-saturated seawater and precipitation of concentric layers CaCO 3 onto
a nucleus (sand grain, shell fragment, etc.), they become perfectly spherical as they roll
backwards and forwards in the wave zone.
Fossiliferous limestones (biogenic and bioclastic) form in reef environments – usually
formed of coral and algae, but with other carbonate debris bound into the structure. Reefs
are wave-resistant frameworks and form in conditions of high energy where the water is
well-oxygenated. They are subject to erosion by storms – which may deposit “wash-over”
debris in the sheltered lagoons to the landward side of the reef.
(g)
Describe the characteristic features of the primary sedimentary structures:
cross bedding, ripple marks, graded bedding, desiccation cracks. Describe the
environments in which they form and the processes of formation. Describe the uses of
these sedimentary structures as way up and palaeo-environmental indicators.
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These are primary features formed during the deposition of sediment.
They are very useful as they provide information about: 1. Way up – are the beds the right
way up or inverted (up side down)? Beds may be inverted by folding. 2. Palaeo-current
direction – some structures can be used to determine the wind/current direction at the
time of formation. 3. Palaeo-environment – the environment in which the structure formed,
e.g. a desert.
Cross bedding - Forms in high energy environments (wind or water). Requires flow in one
direction (unidirectional). Sediment moves up the windward slope (stoss) by saltation, collects
at the top in the lee and eventually cascades/avalanches down the lee slope. Lee slope (max
35) is steeper than stoss slope. As migration occurs all that is preserved are the successive
lee slopes → dipping foreset beds.
 Way up – foreset beds are concave upwards, older beds are truncated (cut off) at top.
 Palaeo-current/wind direction – foreset beds dip in current direction. (Could measure and
plot on rose diagram).
 Palaeo-environment – given by scale. Large scale (dune) cross bedding >1metre = wind
blown (aeolian), desert. Medium scale = beaches, deltas, shallow seas. Small scale =
rivers (fluvial).
Ripple Marks - Formed in a similar way to cross bedding. There are two types:
Asymmetrical – formed by currents moving in one direction, i.e. unidirectional → rivers,
deserts.
Symmetrical – formed by currents moving to and fro, i.e. bi-directional → wave action,
beaches.
 Way up – ripple crests point upwards.
 Palaeo-current/wind direction – asymmetrical ripple marks are steeper on down current
side; symmetrical ripple marks can only give trend at 90 to ripple crests.
 Palaeo-environment – given by shape (asymmetrical versus symmetrical).
Desiccation (mud) Cracks - Formed when wet fine-grained sediment (clay) dries out.
Requires high rates of evaporation. As the sediment dries it shrinks and polygonal shaped
cracks form. These are preserved when infilled by later, coarser sediment.
 Way up – cracks point downwards.
 Palaeo-environment – high rates of evaporation, arid, desert – playa (temporary) lake.
Graded bedding - Form when sediment settles out of water under the influence of gravity.
The coarser (heavier) grains sink to the bottom first and the bed gets progressively finer
grained upwards. Found in muddy sandstones (greywackes).
 Way up – bed gets finer upwards.
 Palaeo-environment – associated with turbidity currents – form when the current runs out
of energy.
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(h)
Define lithification. Describe and explain the diagenetic processes of
compaction and cementation.
Occurs during burial of sediment as the depth of overburden increases. Near surface process,
source of energy is geothermal energy, upper limit is 200C, above this burial metamorphism
occurs.
 Lithification – process of converting loose sediment into a sedimentary rock.
 Diagenesis – all the physical and chemical processes that operate during lithification.
 Compaction (main physical process) – grains move closer together due to weight of
overburden, porosity is reduced. Mudstones undergo more compaction than sandstones
as mudstones are made of flat, platy minerals (clay minerals, mica) that can align under
pressure, whereas sandstones are made of round quartz grains that merely rotate under
pressure.
 Cementation (main chemical process) – a crystalline cement (commonly quartz or calcite)
is precipitated in the pore space from circulating pore fluids, porosity is reduced.
 Other processes – (pressure) solution; recrystallisation; replacement; growth of new
“authigenic” minerals.
2.3
Igneous Processes and Products
Candidates should be able to:
(a)
Describe the intrusion of concordant and discordant bodies as both major and
minor intrusions. Recognise and describe the characteristics of sills, dykes,
transgressive sills and batholiths.
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Major (plutonic) intrusions – Large batholiths with surface area >100 km2, emplaced at
depth – magma rises as low density diapirs pushing the country rock aside. Produces
coarse crystal-grain size. Commonly acid-intermediate composition.
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Minor (hypabyssal) intrusions – Dykes – discordant, cross cut bedding. Sills – concordant,
parallel to bedding. Both are intruded at shallow level/near surface depths. Produces
medium crystal-grain size. Commonly basic composition.
Transgressive sills are mainly concordant but in places step up/cross cut and change
level in the country rock.
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Country rocks - rocks (often sedimentary) surrounding the igneous intrusion. These are
older than the igneous intrusion.
Contact - the junction between the intrusion and the country rock.
Xenoliths (foreign rocks) – fragments of the surrounding country rock caught up and
trapped in the magma – these are older than the igneous intrusion.
(b)
Explain the origin and nature of chilled and baked margins and metamorphic
aureoles.
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Chilled margins of finer crystals develop around the edge of igneous intrusions because
the outer part of the intrusion cools faster than the centre. Most likely to represent the
original composition of the magma.
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Metamorphic aureole - zone of altered/recrystallised/metamorphosed rocks around a
large igneous intrusion. Major igneous intrusions (batholiths) contact metamorphose the
surrounding country rocks. The size of the aureole depends mainly on the volume of
magma emplaced, the composition of the magma and the surrounding country rock.
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Baked margins - minor igneous intrusions (dykes and sills) are not large enough to
produce metamorphic aureoles, instead a thin baked margin is formed in the surrounding
country rock. Produces fine-grained, recrystallised, hard rock.
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(c)
Distinguish between intrusive and extrusive igneous rocks. Explain the
differences between rocks formed by these methods - sill and lava flow.
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Sill (intruded - must be younger than overlying rock) - medium-grained; two baked
margins top and bottom; xenoliths may occur at top and bottom; top surface is regular;
veins and stringers of sill may occur in overlying rock; may transgress from one bed to
another; may contain cumulate layering.
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Lava flow (extruded - must be older than overlying rock) – fine-grained/glassy; one baked
margin at bottom only; xenoliths will be found at bottom only; may contain vesicles, flow
banding; could be pillow lavas; may have an irregular, scoracious, reddened weathered
top surface; fragments of eroded lava may occur in overlying rock; underlying rocks
represent palaeo-land surface – may be eroded, weathered surface; may have associated
pyroclastics.
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Both may show columnar jointing and could have porphyritic textures.
(d)
Describe how crystal grain size is related to rates of cooling of volcanic,
hypabyssal and plutonic igneous rocks. Know how vesicular and porphyritic textures
are formed.
Rapid cooling at (or close to) surface (due to large temperature difference between magma
and surroundings) produces fine crystals – little time for crystals to grow.
Slow cooling at depth (due to insulation) produces coarse crystals – atoms and ions have
more time to diffuse through magma and attach themselves to growing crystals.
Crystal size indicates rate and depth of cooling:
 Volcanic rocks = <1mm – fine-grained - rapid cooling – extrusive.
 Hypabyssal rocks = 1-5mm – medium-grained - shallow-level, near surface – intrusive.
 Plutonic rocks = 5-30mm – coarse-grained - slow cooling - deep-seated – intrusive.
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Equigranular – mosaic of equidimensional crystals – formed by cooling at a constant rate.
Found in volcanic, hypabyssal & plutonic rocks.
Porphyritic – Coarse crystals (phenocrysts) set in a groundmass of finer crystals – formed
by cooling at two different rates/depths. First, slow cooling at depth forms large/coarse
crystals, then the magma rises to a higher level/the surface and more rapid cooling occurs
producing the finer grained groundmass. Found in volcanic, hypabyssal & plutonic rocks.
Vesicular – when lava is erupted onto the surface vesicles form as bubbles of gas escape
from the magma, indicates rapid cooling under low pressures. Vesicles may become
oval/elliptical as the lava flows over the ground. Only occurs in volcanic rocks.
Amygdaloidal – vesicles are infilled by later, secondary crystals/minerals (e.g. calcite).
Only occurs in volcanic rocks.
Flow banding – As lava flows along the surface, the crystals become aligned in the
direction of flow. Only occurs in volcanic rocks.
Glassy – Lava cooled so rapidly that there was insufficient time for crystals to grow →
volcanic glass (obsidian). Only occurs in volcanic rocks, particularly those erupted under
water.
(e)
Describe volcanic processes - basic magma at constructive margins and
hotspots, intermediate and acid magma at destructive plate margins. Explain why
volcanoes form at plate margins.
Igneous rocks/magmas are subdivided into acid, intermediate, basic and ultrabasic - based
on silica content. The % silica, mineralogy and colour of the rocks are all related to
composition. The crystal size is independent and is related to rate/depth of cooling. All
igneous minerals are silicate minerals - therefore, the % silica is not the same as the %
quartz.
 Acid rocks – silica content >66%; rich in orthoclase (K-feldspar), contains quartz and
plagioclase (Na-feldspar), less than 30% mafic minerals (usually mica); therefore lightcoloured.
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Intermediate rocks – silica content 66-52%; 2/3 plagioclase:1/3 orthoclase feldspar,
contains ~10% quartz, 30-60% mafic minerals (usually amphibole); therefore mediumcoloured.
Basic rocks – silica content 52-45%; 50% plagioclase (Ca-feldspar), 50%+ mafic minerals
(usually pyroxene); therefore dark coloured.
Ultrabasic rocks – silica content <45%; 100% mafic minerals (olivine and pyroxene);
therefore very dark coloured. Rarely occur as volcanic rocks.
Magma viscosity – the extent to which the magma flows is controlled by silica content.
Acid magmas (high silica content) have a high viscosity, don’t flow; Basic magmas (low
silica content) have a low viscosity and are fluid.
The type of volcano (shape/form), style of eruption and volcanic products are all related to
the magma type and viscosity. This is in turn related to the plate tectonic setting.
Basic magmas form by partial melting of the ultrabasic mantle at constructive margins and
hot spots. Fluid – volatiles/gases can escape → gently effusive eruptions of lava.
Basic volcanoes - gently effusive, fissure and shield volcanoes, main products are lavas
and gases, occur at constructive margins and hotspots.
Acid and intermediate magmas form by partial melting of the subducted slab at
destructive margins, magma picks up silica as it rises through the crust. Viscous –
volatiles/gases can’t escape → highly explosive eruptions of pyroclastics and gases.
Acid - intermediate volcanoes - highly explosive, central vent volcanoes, main products
are pyroclastics and gases, occur at destructive plate margins.
(f)
Describe the products of volcanoes: gases, pyroclasts and lavas of basic,
intermediate and acid composition. Explain the distribution of volcanic products
around volcanoes.
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Magma – molten rock below the earth’s surface – a molten mixture of silicate minerals,
water and other volatiles.
Volcanoes produce three main products: gases (gas), lavas (liquid) and pyroclastics
(solid).
Lava – molten magma erupted onto the Earth’s surface from a volcano. Most lavas are
basic in composition (fluid magma). Submarine lavas take the form of pillow lavas, e.g. at
mid ocean ridges. Subaerial lavas take the form of pahoehoe (ropy lava) or aa (blocky
lava), e.g. Hawaii. Basic lava flows may contain vesicles and flow banding. Acid lavas
(high viscosity) are rare and tend to form acid domes, e.g. Mt. Pelée. Acid lavas may be
glassy and contain flow banding.
Gases – Main gases are steam (50-80%), CO2, N, Sulphurous gases – SO2 and H2S. E.g.
Lake Nyos, Cameroon, erupted 100 million m 3 of CO2 gas in 1986, killed ~2000 people.
Viscous, acid magmas tend to trap gases that cannot escape from the magma → violent,
highly explosive eruptions.
Pyroclastics – solid fragmental material ejected from a volcano. Mainly acid composition.
Classified by size – ash < 4mm; lapilli 4-32 mm; blocks and bombs >32 mm. When
lithified – ash and lapilli → tuff; blocks and bombs → agglomerate.
Isopachytes – lines on a map joining points of equal thickness of pyroclastic material.
Thickness decreases away from the vent. Particle size decreases away from the vent.
Isopachytes may have an elliptical shape - wind blowing the material or a lateral eruption.
Other products – pyroclastic flows (nuée ardentes), lahars.
(g)
Describe the characteristics of: submarine; fissure, shield and strato-volcanoes
and explain the type of volcanic activity at each. Know about caldera formation,
pyroclastic flows and geysers.
Volcano – a hole (vent) or crack (fissure) in the ground through which volcanic products
escape. The vent or fissure is connected to an underground magma chamber via a conduit or
pipe. A vent may contain a crater and there may be parasitic vents/cones on the flanks of the
volcano.
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
Submarine volcanoes – commonly fissures at the mid ocean ridges, tend to erupt gently
effusive, basic pillow lavas. May get Surtseyan (phreatomagmatic) eruptions if rising
magma comes into contact with sea water → superheated to steam.
 Fissure volcanoes – cracks; gently effusive Icelandic style eruptions; basic pillow lavas;
mainly occur at mid ocean ridges/constructive margins, e.g. Iceland, Mid Atlantic Ridge.
 Shield volcanoes – central vent volcanoes; gently sloping/low angle sides with a very
large base; gently effusive Hawaiian style eruptions; basic lavas (pahoehoe and aa if
subaerial); mainly occur at hot spots, e.g. Hawaii (Mauna Loa).
 Strato-volcanoes/Composite cone volcanoes - central vent volcanoes; classic steep-sided
cone shape, more explosive eruptions ranging from Strombolian → Vulcanian →
Vesuvian → Krakatoan/Plinian (exceptionally violent); intermediate to acid composition;
typically alternating lava flows and pyroclastics build up the cone; mainly occur at
destructive margins, e.g. Mt. Vesuvius, Popacatapetl.
 Acid domes – steep-side dome composed of acid lava; liable to very violent eruption, e.g.
Mt. Pelée.
 Pyroclastic flows (nuée ardentes) – “glowing ash cloud” – a hot (~600°C), turbulent cloud
of ash and gas that flows down the flanks of a volcano at great speed (>100 km/h);
exceptionally dangerous Peléan style eruption; produces a characteristic rock called a
welded tuff/ignimbrite; a feature of acid-intermediate volcanic eruptions, e.g. Mt. Pelée,
Mt. St. Helens, Mt. Pinatubo, Mt. Vesuvius (Pompeii was destroyed by one!).
 Lahar – volcanic mud flow, e.g. Nevado del Ruiz (Columbia), Mt. Pinatubo.
 Caldera formation – top of the volcano is blown off in an exceptionally violent, cataclysmic
eruption leaving a very large crater (> 10 km diameter); as the magma chamber beneath
the volcano empties further subsidence takes place (caldera collapse), e.g. Krakatoa,
Yellowstone.
Minor activity:
 Fumarole – vent from which volcanic gases escape.
 Geyser – a thermal spring that produces intermittent eruptions of hot water, formed when
groundwater is heated by volcanic activity.
(h)
Appreciate the social and economic effects of volcanic activity and the danger
to life and property of different types of volcanic activity including climatic change.
Describe the advantages which volcanic activity can bring. Describe methods for the
prediction of volcanic activity: historic pattern of activity, changes in ground level,
changes in gas composition and precursor earthquake tremors. Describe the methods
of risk analysis: extent and path of lava flows, blast damage, ash falls, pyroclastic
flows and lahars using hazard maps.






Primary volcanic hazards – pyroclastics, gases, lava flows, pyroclastic flows.
Secondary volcanic hazards – lahars, landslides, tsunamis, fires.
Social impacts – concerned with human beings and their relationships – injury, death,
bereavement, stress, homelessness, lack of fresh water, famine, disease, collapse of
whole communities.
Economic impacts – concerned with financial/monetary matters – mainly losses building/property damage, loss of production, destruction of crops/farmland, disruption to
communications/infrastructure, job losses, insurance claims; some gains – construction
opportunities.
Long term impacts – Climatic change – major volcanic eruptions have been implicated in
natural climatic change – volcanic debris/dust is thrown into the upper atmosphere and
prevents solar radiation reaching the Earth, also SO2 gas aerosols produce a white
coating on particles in the atmosphere (reflect solar radiation) → Global cooling of 1-2°C
from average.
Long term advantages of volcanic activity – produces fertile soils (ash rapidly breaks
down releasing P and K); exploitation of geothermal energy; formation of mineral deposits
(e.g. gold, silver, copper, sulphur); tourism.
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Prediction
A successful prediction must pinpoint – spatial occurrence (where), timing (when) and
magnitude (how big). Based on precursor phenomenon (small events that occur before the
main event). Prediction of volcanic eruptions has been relatively successful.
Methods of prediction:
 Past eruptive history and periodicity of individual volcanoes.
 Ground swelling/inflation as magma chamber fills – measured with tilt meters.
 Harmonic tremors – increase in seismic activity (earthquakes) prior to eruption, due
pressure of rising magma.
 Changes in composition of fumarolic gases (HCl, SO 2, S and Cl).
 Rise in ground and groundwater temperatures.
 Changes in Earth’s magnetic field strength.
 Changes in seismic wave behaviour – can be used to detect areas of liquid (i.e. rising
magma).
 Remote sensing techniques – infra red imagery from satellites can detect changes in heat
flow, SAR (synthetic aperture radar) can detect ground swelling.
Risk Analysis
 Can be carried out by hazard risk mapping – delineating areas at risk from the likely paths
of lava flows, blast damage, ash falls, pyroclastic flows and lahars.
 Tephrochronology can be used – Tephra (pyroclastic) deposits can be mapped out and
dated to indicate areas at risk.
 However, hazard maps can be misleading, e.g. Mt. St. Helens, 1980 – eruption was
predicted, but the lateral blast was not!
2.4
Metamorphic Processes and Products
Candidates should be able to:
(a)
Explain thermal (contact), regional and burial metamorphism in relation to
varying temperatures and pressures.


Metamorphism = process by which rocks are changed by heat and pressure. The rock
undergoes solid-state recrystallisation without melting. This is a very slow process.
Metamorphism is isochemical – nothing is added or taken away, the minerals simply
recrystallise into new metamorphic minerals. If the rock is subjected to directed pressure
a preferred orientation of the minerals develops at 90º to the pressure – a foliation.
Limits of metamorphism – 200-800ºC. Below 200ºC = diagenesis. Above 800ºC = partial
melting (anatexis).
Types of Metamorphism:
 Thermal/Contact metamorphism – Caused by heat from a cooling igneous intrusion. Local
scale – produces a metamorphic aureole. High T (200-800ºC), low P (2-3kbars). No
directed pressure, heat only → random texture, unfoliated rocks.
 Regional Metamorphism – Associated with mountain building (orogenesis) at destructive
plate margins/continental collision zones. Large scale – 100s of km in extent. Low to high
T, low to high P. Directed pressure/compressive stress → foliated rocks.
 Burial Metamorphism – In sedimentary basins – an extension of diagenesis. Low T (but >
200ºC), medium-high P.
 Dynamic/Dislocation Metamorphism – Occurs along faults – rocks are crushed by
movement/pressure → mylonite. Very high P.

Factors Controlling Metamorphism – Temperature, Pressure (load, directed/compressive,
pore fluid), Parent Rock, Time.
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(b)
Describe the relation of parent rock composition and the environment of
metamorphism to the mineralogy of the resulting metamorphic rock: (i) mica and clay
minerals in slate from shale; (ii) mica and garnet in schist from slate; (iii) quartz,
feldspar and mafic minerals in gneiss from schist; (iv) calcite in marble from limestone;
(v) quartz in quartzite (metaquartzite) from sandstone.
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Parent rock – original rock prior to metamorphism.
Metamorphic grade – intensity/degree of metamorphism the rock has suffered. Low –
medium – high grade. Can be determined by the texture (crystal size and type of foliation)
and mineralogy of the metamorphic rock.
Index mineral – a metamorphic mineral that is stable over a particular temperature and
pressure range – indicates the metamorphic grade, e.g. mica, garnet, Al2SiO5 polymorphs
(andalusite, kyanite and sillimanite). The metamorphic grade can be mapped using index
minerals. Always mapped by first appearance of the mineral (may persist at higher T & P).
Isograds – lines on a map joining points of equal metamorphic grade.
Regional Metamorphism – Large-scale metamorphism (> 100 km 2). Result of the high
pressures and temperatures generated at destructive plate margins/continental collision
zones → mountain building (orogenesis). Directed pressure/compressive stress → foliated
rocks.
Regional Metamorphic Textures
 Types of foliation – slaty cleavage; schistosity; gneissose banding.
 Porphyroblastic texture (large new minerals that grow during metamorphism) may also be
found in any of the above.
Regional Metamorphism of Shale
Rich in clay minerals (Al-rich),  resultant rocks also Al-rich.
 Low grade = slate → fine grained (<1mm), crystalline, slaty cleavage (microscopic
alignment), dark coloured, harder than shale. Mineralogy – typically mica, clay minerals
and chlorite.
 Medium grade = schist → medium grained (1-5mm), schistosity, variable colour.
Mineralogy – typically mica and garnet (may be porphyroblastic).
 High grade = gneiss → coarse grained (>5mm), gneissose banding (mm to cm scale
segregation of light and dark coloured minerals). Mineralogy – typically quartz and
feldspar in light bands; mafic minerals (biotite, etc.) in dark bands.
The Barrovian Zones
George Barrow, 1912 – mapped a sequence of regionally metamorphosed shales in the SE
Scottish Highlands. He was able to map out zones of metamorphism using index minerals
and isograds. The metamorphism occurred during closure of the proto-Atlantic Ocean
(Iapetus).
Low grade
Medium grade
High grade
Texture
Slate
Schist
Gneiss
fine-grained,
medium-grained
coarse-grained,
slaty cleavage
schistosity
gneissose banding
Index
chlorite
biotite
garnet
staurolite
kyanite
sillimanite
mineral
Regional metamorphism of sandstones and limestones
Produces the same products as contact metamorphism, i.e. metaquartzite and marble. The
rocks are monominerallic and made of equidimensional minerals that cannot align under
pressure. In the case of hard, round quartz grains – these merely rotate under directed
pressure.
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(c)
Describe thermal metamorphism by heat from an igneous intrusion to form
different grades of unfoliated rocks within a metamorphic aureole. Understand the
factors controlling the width of contact aureoles and metamorphic grade.
Thermal/Contact metamorphism – Result of heat from cooling igneous intrusion. Heat only,
lack of directed pressure → unfoliated rocks with random texture.
 Minor intrusions – dykes and sills – insufficient heat to greatly affect country rock →
baked margin.
 Major intrusions – batholiths – retain heat for 10,000s - millions of years → metamorphic
aureole in country rock. The closer to the intrusion the higher the temperature – may
result in a zonation of metamorphic rocks.
 Metamorphic aureole = zone of altered/ recrystallised/ metamorphosed rocks around a
large igneous intrusion, up to 10 km wide. Temperature decreases with distance from
contact.
 Factors controlling the width of the metamorphic aureole – Volume of magma/intrusion;
Composition of country rocks; Temperature of magma; Composition of magma – acid
magmas contain more volatiles; Depth of intrusion – temperature difference.
Contact Metamorphic Textures:
 Sugary/Granoblastic texture – randomly oriented mosaic of equidimensional crystals.
 Porphyroblastic texture – large new minerals that grow during metamorphism.
Products of Contact Metamorphism:
Limestone (calcite) → Marble. Calcite grains/fossil fragments recrystallise to calcite crystals.
Fossils are destroyed. Produces a randomly oriented mosaic of equidimensional interlocking
calcite crystals → sugary, granoblastic texture.
Sandstone (quartz) → Metaquartzite. Quartz grains recrystallise to quartz crystals.
Sedimentary structures are destroyed. Produces a randomly oriented mosaic of
equidimensional interlocking quartz crystals → sugary, granoblastic texture.
Shale → Spotted slate, Andalusite slate, Hornfels. Shale is rich in clay minerals (Al-rich) →
new Al-rich minerals form. As shale contains a mixture of minerals several new products are
possible depending on distance from contact (temperature).
 Spotted slate (low grade) – furthest away – rock only partially recrystallises, relict textures
(sedimentary structures, fossils) are present. Spots of biotite/graphite form from organic
matter in shale.
 Andalusite slate (medium grade) – higher temperatures, spots converted to new minerals
→ randomly oriented andalusite (Al2SiO5) porphyroblasts form.
 Hornfels (high grade) – closest to contact. Shale is completely recrystallised to form hard,
medium- to fine-grained, crystalline rock with equidimensional crystals – granoblastic
texture. Minerals include sillimanite, quartz, feldspar and pyroxene.
© D. Armstrong, 2004
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