1 temperature, pressure and metamorphism - pearson schools

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Metamorphism is the isochemical process by which rocks are changed by either heat or pressure, or both heat and pressure. The chemical composition of the parent rock will be the same as the metamorphic rock produced.

The rock undergoes the very slow process of solid-state recrystallisation without melting. Different temperatures and pressures cause new minerals to grow in rocks that have the same composition. The minerals produced are directly related to pressure and temperature conditions. The lower temperature limit for metamorphism is between 200 and 150 oC. Below these temperatures, changes are part of diagenesis. There is no lower pressure limit. The upper temperature limit is where melting occurs. This happens at around 800 oC.

The process of metamorphism may result in:• destruction of fossils, beds and sedimentary structures• hardening of the rock• change in colour• alignment of minerals• growth of new metamorphic minerals.

2.4 1 Temperature, pressure and metamorphism

Temperature/ �C

0 2000

200

400

Pres

sure

/MPa

600

800

1000

400 600 800 1000

Igneous rocks

Low grade

Contact metamorphism

Regional metamorphism

Buria

lm

etam

orph

ism Medium grade

High grade

Sedimentary

10

20 Dept

h/km

30

Figure 1 Relationship between metamorphism, temperature and pressure

TemperatureTemperature is a key variable in metamorphism:• High temperatures occur near to igneous intrusions, where the magma heats the

surrounding rocks.• Temperature also increases with depth, due to the geothermal gradient.

As temperature increases, the rate of metamorphic reactions also increases. This is because many of the chemical reactions require heat to take place. Higher temperatures increase the rate at which ions diffuse between minerals, though it is still a slow process because the ions have to move through solid rock during metamorphism. The whole process is greatly speeded up by water, which allows the ions to diffuse more rapidly.

Key defi nition

Isochemical means that no elements are added or removed, with the exception of volatiles such as water and carbon dioxide.

Case study

Making a brick – it’s metamorphism!• Take a mass of soft, grey-coloured,

sticky, crushed clay mixed with water and a little limestone.

• Stir well.• Press into a brick shape.• Put in the furnace at 1400 oC for 2

days.• Cool slowly and you will have a hard,

red brick.

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PressurePressure steadily increases with depth and is applied to rocks in three different ways:• Pore pressure is the pressure exerted by fluids between the grains in a porous rock.

The presence of water speeds up reactions by acting as a catalyst and increasing the rate and ease of ion exchange. In an experiment where two dry solids were heated together for 2 hours at 1300 oC, only 10% reacted. When the same solids were heated in water for the same time, the reaction was completed at only 600 oC.

• Load pressure is the weight of overlying rocks and physically brings minerals into contact with each other over very long periods of time.

• Tectonic stress or pressure is caused as the rocks undergo folding or faulting and very high pressures are exerted, but usually over relatively short periods of time.

In all cases the higher the pressure, the greater the degree of metamorphism. Reactions that depend on pressure only are less common than temperature dependent reactions.

TimeTime is very important because metamorphic reactions take place very slowly. These reactions usually take millions of years to occur. Pressure and temperature conditions that produce metamorphism have to exist over long periods of time, in order for the reactions to occur.

Types of metamorphismContact metamorphismContact metamorphism occurs adjacent to igneous intrusions, which increase the temperature in the surrounding country rock. The metamorphism is important on a local scale, producing a metamorphic aureole. Temperatures are generally high but pressure is low. As pressure is not a significant factor, the minerals are not aligned in contact metamorphic rocks.

Burial metamorphismBurial metamorphism occurs in conditions of medium to high pressure and relatively low temperature. To some extent, burial metamorphism overlaps with diagenesis, and grades into regional metamorphism as temperature increases. It affects rocks deeply buried by the weight of overlying sediments. It also occurs at subduction zones where sea floor sediments and basalts are buried. Rocks buried at the deepest levels almost always contain the blue mineral glaucophane. They are called blueschists.

Regional metamorphismRegional metamorphism affects larger areas than contact metamorphism, extending over hundreds or thousands of square kilometres. It is caused by low to high temperature and low to high pressure at convergent plate margins. It can result from either subduction or continental collision. Pressure is significant and so minerals have a preferred alignment. Regionally metamorphosed rocks occur in the cores of fold mountain belts where mountain ranges have been eroded.

Questions1 Describe the temperature and pressure conditions associated with each of the three

types of metamorphism.2 Describe the effects on the surrounding rocks of an igneous intrusion that cooled in

500 years.3 Explain why it is more difficult to know the pressure at which metamorphic rocks form

than the temperature at which they form.

Module 4Metamorphic Processes and Products

Temperature, pressure and metamorphism

Key definition

Country rock is the rock into which an igneous rock has been intruded.

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All metamorphic rocks are formed from parent rocks, the original rocks that existed prior to metamorphism. The composition of these rocks affects the mineralogy of the metamorphic rocks, so they have a kind of ‘family likeness’.

Foliated rocks produced by regional metamorphismThese rocks have all been affected by pressure, to some degree, during regional metamorphism. Any platy minerals they contain take on a preferred alignment known as foliation. All the rocks described in this spread are foliated. (For an explanation of how foliated textures form, see spread 2.4.4.) The most common platy mineral is clay so the rocks described below all have shale as the parent rock.

SlateThe parent rock of slate is shale (for a photo of shale, see spread 2.3.6). Shale is composed of clay minerals and fi ne quartz particles. Because clay minerals are rich in aluminium, so are the metamorphic minerals in slate. This is mainly composed of clay minerals and mica (although chlorite and quartz may also be present). Shale is fi ne grained (grains <1 mm diameter) and shows slaty cleavage. Traces of original bedding may still be preserved as relict bedding.

2.4 2 Identifying regional metamorphic rocks

SchistThe parent rock of schist is shale. Schist is produced by higher temperatures and pressures than those producing slate. It is medium grained (1 to 5 mm) and crystalline. Although it can occur in a variety of colours, it always has a shiny appearance where the fl at surfaces of muscovite and biotite mica crystals are visible. Schist is typically composed of mica and garnet. The garnets often form large crystals called porphyroblasts. The mica crystals are all aligned at right-angles to the maximum pressure, forming the texture schistosity.

GneissThe parent rock of gneiss is shale. Gneiss is formed by the highest temperatures and pressures during regional metamorphism. It is a coarse grained (>5 mm), crystalline rock with gneissose banding. Gneiss is typically composed of quartz and feldspar in the light bands and biotite mica (and other mafi c minerals) in the dark bands.

Key defi nitions

Foliation is the texture in metamorphic rocks, formed by the preferred alignment of fl at, platy minerals.

Slaty cleavage is the texture in fi ne grained rocks formed by low grade regional metamorphism. Platy minerals recrystallise perpendicular to the direction of stress applied during metamorphism so that the rock splits into thin sheets.

Magnificationx50

Slate is a fine grained metamorphicrock that has slaty cleavage i.e. it

splits into thin sheets

Alignment ofminerals haschanged due topressure duringmetamorphism

Slate, composed of clay mineralsand platy metamorphic minerals

mica and chlorite

Shale, composed of clayminerals aligned by compaction

during diagenesis

Figure 1 Shale and slate

Key defi nitions

A porphyroblast is a large crystal that has grown during recrystallisation in a metamorphic rock and is surrounded by a fi ner grained groundmass of other crystals.

Schistosity is the texture in medium and coarse grained metamorphic rocks formed by the preferred alignment of fl at/tabular minerals. The alignment is perpendicular to the direction of stress applied during metamorphism. No traces of original bedding remain.

Gneissose banding is the segregation of light and dark coloured minerals into layers or bands at the scale of mm to cm in thickness. The light band is normally granoblastic (granular) and the dark band normally shows schistosity.

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Summary table of rocks produced by regional metamorphism only

Parent Rock Metamorphic rock

Colour Texture Mineral composition

Type of metamorphism

Shale, composed of clay minerals

Slate Grey or purple or green or black

Slaty cleavage, fine grain size (<1 mm)

Clay minerals and muscovite mica, with some chlorite and quartz

Low grade regional

Shale Schist Silvery sheen

Schistosity, medium grain size (1–5 mm)

Muscovite and biotite mica QuartzGarnetKyanite

Medium grade regional

Shale Gneiss Dark and light bands

Gneissosebanding, coarse grain size(>5 mm)

Biotite micaMafic mineralsQuartzK feldsparSillimanite

High grade regional


Identifying regional metamorphic rocks

Questions1 Which metamorphic rock would

you associate with high grade regional metamorphism?

2 With the aid of labelled diagrams, describe the differences between schist and slate.

3 With the aid of labelled diagrams, describe one similarity and one difference between schist and gneiss.

Figure 2 Schist

0 2mm

Garnetporphyroblasts

Mica crystals givethe surface a sheen

Thin sectiondrawing of schist showinggarnetporphyroblasts(black) andmicas (brown)in preferredalignment

Garnet – mica schist

Gneiss showing gneissose banding

Band rich inbiotite mica

Quartz richband

Feldspar showing twocleavage directions atright angles

Quartz

Dark colouredband

0 2mm

Biotite mica is dark andshows preferred orientation

Light colouredband

Thin section drawing of gneiss

Figure 3 GneissGneiss showing gneissose banding

Band rich inbiotite mica

Quartz richband

Feldspar showing twocleavage directions atright angles

Quartz

Dark colouredband

0 2mm

Biotite mica is dark andshows preferred orientation

Light colouredband

Thin section drawing of gneiss0 2mm


Mica crystals givethe surface a sheen

Thin sectiondrawing of schist showinggarnetporphyroblasts(black) andmicas (brown)in preferredalignment

Garnet – mica schist

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The parent rock, for all the metamorphic rocks described in spread 2.4.2, was shale but this does not mean that shale is the only parent rock. The chemical composition of the minerals that make up shale is more varied than that of limestones, which are composed of calcite (CaCO3), or of sandstones, some of which can be almost pure SiO2 in the form of quartz. There is a much wider range of metamorphic minerals that can recrystallise from clay minerals. This means that shale can be the parent of different metamorphic rocks, but limestone and sandstone cannot.

Unfoliated rocks produced by contact or regional metamorphism

2.4 3 Identifying metamorphic rocks

Key defi nition

Granoblastic describes the texture of metamorphic rocks that contain interlocking equi-dimensional, crystals.

Oolitic limestone – parent

Concentriclayers ofcalcite

CalciteFossiliferous limestone – parent

0.5mm

CalciteFossilfragments

1mm

1mm

Nucleus

1mm

Orthoquartzite – parent

Quartz grain

Quartz cement

MarbleQuartzite

Interlockingquartz crystals

Figure 1 Thin section drawings of quartzite, marble and their parent rocks

Because these rocks do not contain platy minerals such as mica they do not show foliation. They can be produced by either regional or contact metamorphism.

QuartziteThe parent rock of quartzite is orthoquartzite, sandstone composed of quartz grains held together by quartz cement. Quartz grains in the sandstone recrystallise, forming interlocking quartz crystals. The quartz crystals are equidimensional so there can be no foliation. This texture is described as granoblastic. Any sedimentary structures or fossils in the parent sandstone are destroyed. The colour of quartzite is white or grey, unless there were other minerals in the original rock. For example, if any iron oxide was in the parent rock, there will often be a pink colour.

MarbleLimestones are made essentially of one mineral, calcite, which is stable over a wide range of temperatures and pressures. As a result, metamorphism of limestone only causes the original calcite crystals to grow larger. Calcite grains and fossil fragments in the limestone parent rock recrystallise to form an interlocking mosaic of calcite crystals. The crystals are equi-dimensional, so there can be no foliation. Marble has granoblastic texture, but the crystals of calcite make it look sugary in texture. Calcite will react with dilute HCl. Fossils are destroyed during metamorphism.

Marble from pure limestone is white. Impurities in the parent limestone give some marble a range of coloured streaks:• If there are clay minerals in the limestones, then a number of green or red minerals

such as garnet may form.• If there are sand grains present, a chemical reaction between calcite and quartz

will produce wollastonite, which can be light green, pinkish, brown, red or yellow.

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Rocks produced by contact metamorphismSpotted rockBecause contact metamorphism involves only increased temperatures, it cannot produce foliation. During contact metamorphism, spots may form in the rock where the heat has only partially recrystallised the rock. A spotted rock contains the same minerals as shale or slate. If slate is the parent rock of spotted slate, it will show foliation, but this was produced due to pressure during regional metamorphism (see spread 2.4.2). The randomly orientated spots may contain biotite, andalusite and graphite, but they are usually too indistinct to be identified in hand specimen.

Summary table of unfoliated rocks

Parent rock Metamorphic rock

Colour Texture Mineral composition

Type of metamorphism

Limestone composed of calcite(CaCO3)

Marble White Granoblastic

Medium grain size (1–5 mm) grain size increases with metamorphic grade

Calcite(reacts with dilute HCl)

Contact or regional

Sandstone composed of quartz(SiO2)

Quartzite White or grey Granoblastic

Medium grain size (1–5 mm), grain size increases with metamorphic grade

Quartz Contact or regional

Slate or shale composed of some clay minerals, mica and quartz

Spotted rock Grey or purple or green or black with darker spots

Slaty cleavage if slate parent rock

Fine grain size (<1 mm)

Clay minerals and mica

Poorly formed minerals (mica, andalusite, graphite) in spots

Contact

Questions1 Explain why marble is not always

white.2 Explain why limestone and

sandstone produce the same metamorphic rocks in both contact and thermal metamorphism.

3 State two pieces of evidence indicating that quartzite is a metamorphic rock.


Identifying metamorphic rocks

Figure 3 Identifying metamorphic rocks

Medium tocoarse grained

Yes NoMinerals havepreferred alignment

Medium tocoarse grained

Slate Spotted rockGneissSchist

Mica prominent.May contain garnet

Contains darkand light bands

Produced by regional orcontact metamorphism

Produced by regionalmetamorphism

Produced by contactmetamorphism

Fine grained Fine grained

Quartzite Marble

Composedof quartz

Composedof calcite

Containsdark spots

Key

Figure 2 Thin section diagram of spotted slate showing clay minerals and mica aligned at 90º to maximum pressure. The remains of the sedimentary bedding can just be seen as relict bedding

Darkspots

�10

Alignment of micas andclay minerals

Relictbedding

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Metamorphic rocks are classifi ed mainly based on their texture. This is because grain size and orientation tell us a lot about the conditions of metamorphism.

If rocks are subjected to directed pressure, a preferred orientation of the minerals develops at 90 degrees to the pressure. If the minerals are fl at or platy, foliation is produced (see spread 2.4.2). Because it results from pressure, foliation is a characteristic of rocks formed by regional metamorphism.

Slaty cleavageRocks with slaty cleavage will split into thin sheets along the cleavage planes. It occurs in fi ne grained rocks formed by low grade regional metamorphism:• It can only form in rocks consisting of platy minerals such

as clay minerals, chlorite and micas.• At the microscopic scale, these minerals become aligned

at 90 degrees to the direction of maximum pressure during metamorphism.

• Slaty cleavage may be at any angle to bedding, but is usually parallel to axial planes of the folds.

• It cannot occur in rocks with rounded grains, such as quartz in sandstones.

Bedding and fossils may not be completely destroyed by metamorphism, leaving traces or relict structures. Fossils may be deformed due to the high levels of compressive stress. Slates are common in North Wales and the Lake District.

Schistosity Found in schists (medium grained rocks formed by regional metamorphism), schistosity results from the alignment of fl at, platy minerals, commonly muscovite mica, at 90 degrees to the direction of maximum pressure during metamorphism. Light coloured muscovite mica is concentrated into thin parallel bands, giving the rock a characteristic shiny appearance (micaceous sheen) where fl at surfaces of mica are visible.

Garnet porphyroblasts are often present and they disrupt the alignment of mica minerals.

Schists are found in the Highlands of Scotland in Dalradian rocks.

2.4 4 Metamorphic textures

Sandstone no cleavage

Shale bedsCompression

Cleavage planes

Sandstone no cleavage

Direction ofmaximum stressduring metamorphism

Direction of maximumstress duringmetamorphism

Flat minerals like micaalign so that their longaxis is at 90° to thedirection of pressure

Foliation produced by the alignment of flat minerals e.g. mica

Relict bedding is at a different anglefrom the cleavage

Slaty cleavagedeveloped at 90°to maximumstress

Slaty cleavage

Figure 1 Foliation and slaty cleavage

Key defi nitions

A relict structure is a structure such as bedding present in the parent rock, which is partially preserved in a metamorphic rock.

Dalradian is the name of a group of rocks formed in late Precambrian times, found in Scotland.

Schistositycaused byalignmentof micas


This rock shows porphyroblastic texture in a schist

Figure 2 Porphyroblastic texture in a schist

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Gneissose bandingFound in gneisses (coarse grained rocks formed by regional metamorphism), gneissose banding is formed when light (usually quartz and feldspar) and dark coloured minerals (usually biotite mica and mafic minerals) are separated into bands. The mica-rich layer is foliated and the pale layer has granoblastic texture. The bands may be contorted or folded but are roughly at 90 degrees to the maximum pressure direction (for a thin section drawing and photo showing gneissose banding, see spread 2.4.2).

Porphyroblastic textureThis texture occurs in both regional and contact metamorphic rocks. Porphyroblasts are large crystals that grow during metamorphism and are surrounded by a finer grained groundmass. Metamorphic rocks that contain these large crystals are described as porphyroblastic. Garnet porphyroblasts found in schists may contain inclusions. Pyrite porphyroblasts can develop in slate, often forming clear cubic crystals.


Metamorphic textures

Key definitions

An inclusion is a fragment of an early formed mineral enclosed by one that grew later.

Unfoliated describes the random orientation of minerals in a metamorphic rock.

Examiner tip

How to tell the difference between the igneous texture porphyritic and the metamorphic texture porphyroblastic:• porphyritic is where large crystals –

phenocrysts – form first in the magma so grow larger than the groundmass, which cools later.

• porphyroblastic is where large crystals such as garnet grow after the groundmass has developed and they may distort the groundmass crystals.

Granoblastic textureThis is an unfoliated texture and is formed by thermal metamorphism. Pressure is not a factor in the formation of a granoblastic texture. The main characteristics are randomly orientated, equidimensional crystals usually in rocks with few, and sometimes only one, mineral. Hornfels is an example of a fine grained rock with granoblastic texture. Marble and quartzite are also granoblastic. Because of their medium grain size and white colour, their texture is sometimes described as sugary.

Figure 4 Pyrite porphyroblast in slate

Figure 3 Gneissose banding

Garnet porphyroblastscommonly have curvedcracks when seen inthin section

2mm

Inclusions of early formedminerals enclosed by agarnet porphyroblast

Thin section drawing of a garnet porphyroblast

Gneiss – dark and light bands

Questions1 Name two metamorphic rocks

that are unfoliated.2 With the aid of labelled diagrams

explain how a garnet porphyroblast affects the alignment of micas in a schist.

3 Explain why slaty cleavage commonly has a different orientation from relict bedding.

Granoblastic texture

Equidimensional crystalswith no preferred alignment

Figure 5 Granoblastic texture

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Contact metamorphism occurs when the country rock is affected by heat from a large igneous intrusion. Because temperature differences between the surrounding rock and the intruded magma are greater at shallow levels in the Earth’s crust where pressure is low, contact metamorphism is described as high temperature, low pressure metamorphism. High temperature, not pressure leads to the formation of altered, recrystallised, unfoliated rocks in a zone surrounding the intrusion. This zone is the metamorphic aureole. Around a large igneous intrusion, such as a batholith, the metamorphic aureole may be up to 10 km wide. Temperature decreases with distance from the contact with the intrusion and for this reason the effects of contact metamorphism are greatest near to the contact and decrease with distance. Metamorphic grade increases in all directions towards the intrusion.

Contact metamorphism of shaleThe chemical composition of minerals in shale is varied and so a range of different metamorphic rocks is formed, depending on the temperature and therefore the distance away from the intrusion:• Close to the contact with the intrusion, temperatures are high and so high grade

metamorphism occurs. Shale is completely recrystallised to form a fi ne grained, hard, splintery, granoblastic metamorphic rock called hornfels.

• Further away from the contact, where the heat is less intense, medium grade metamorphism occurs. Clusters of a new metamorphic mineral andalusite, form porphyroblasts. This partly recrystallised rock is andalusite slate or rock.

• In the outer part of the metamorphic aureole, temperatures are lower. Some recrystallisation occurs, causing clusters of dark minerals to grow in separate spots. Iron, carbon or biotite mica will form the spots. The rock in this outer part of the metamorphic aureole is called spotted rock and is formed by low grade metamorphism.

Key defi nitions

A metamorphic aureole is a region surrounding an igneous intrusion in which the country rocks have been recrystallised and changed by heat from the intrusion.

A metamorphic grade is a measure of the intensity of metamorphism. Although increases in temperature only result in increasing grade in contact metamorphism, grade is also used to describe regional metamorphism where both temperature and pressure vary.

Factors controlling the width of metamorphic aureolesVolume of the magmaThe size of intrusions ranges from batholiths down to minor intrusions (see spread 2.2.9). Dykes and sills are not large enough and do not produce enough heat to develop a metamorphic aureole. Because the volume of magma is small it cools quickly and there is only suffi cient heat to change the rock for a few centimetres on either side. This narrow zone of bleached and hardened rock is known as a baked margin.

Larger intrusions cool slowly and heat the surrounding rocks over long periods of time (104–106 years), allowing a wide metamorphic aureole to develop.

2.4 5 Contact metamorphism 1

A metamorphic aureole showing contact metamorphism of shale Shale with spots of partial recrystallisationAndalusite porphyroblasts

Andalusitecrystal

Black spots of iron or carbon

1cmCross-section ofandalusite crystalcm

Edge ofmetamorphic

aureole

Granite

Hornfels

Andalusiteslate

Spotted rock

Shale countryrock

0 1

Figure 1 Metamorphic aureole showing contact metamorphism of shale and photo of a spotted rock and andalusite rock

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Temperature of the magmaThe volume of magma in an intrusion affects the maximum temperature reached at any point and also the time it takes for temperatures to rise in the country rocks. Metamorphism will not occur unless the temperature rises above 200 °C for an extended period of time. A small intrusion produces little metamorphic change because the rock has little time to warm up and there is not enough time for metamorphic reactions to occur before the rock cools down. With larger intrusions there is time for metamorphic reactions to take place and for new minerals and recrystallisation to occur, because temperatures remain high for much longer periods of time.

Composition of the magmaMafic magma may be intruded at a temperature of 1200 °C, whilst silicic magma may be intruded at 850 °C. Silicic magmas contain more volatiles. When they enter the country rock they speed up metamorphic reactions. This compensates for the lower temperature of the magma, because metamorphic aureoles surrounding silicic intrusions are of similar size to those around mafic ones.

Composition of the country rockRocks largely composed of one mineral, such as limestone and orthoquartzite, show much less variation than clay-rich rocks such as shale. Quartzite and marble have larger crystals the nearer they are to the igneous intrusion and are uniform. Metamorphic aureoles formed in sandstone country rocks are typically narrower than those formed in clay-rich rocks. If the country rock is permeable and contains groundwater, heat will be able to move by convection, allowing a wider aureole to develop.

Dip of the contactThe dip of the sides of the intrusion has a major effect on the width of the metamorphic aureole. A shallow angle of dip gives a wide aureole and a steep angle of dip gives a narrow aureole. If the sides of the intrusion dip at different angles, then the metamorphic aureole will be asymmetric.

5°

5° X Y

N

Key

GraniteSandstone Quartzite

Spotted rock

MarbleShale

Limestone

Conglomerate

Y

X

Gently dipping contactproduces wider aureole

Steeply dipping contactproduces narrow aureole

Metamorphic aureole

1km

Figure 3 Map of an intrusion with dipping sides


Contact metamorphism 1

Questions1 What is the term for the zone surrounding a granite batholith?2 Hornfels forms at 460 ° C. Using Figure 3, state how far away from each intrusion

hornfels will form.3 Explain the relationship between metamorphic rocks and metamorphic grade.

0

100

200

300

400

500

600

700

0 1 2 3 4 5Distance from contact/km

Granite 10km diameterGranite 5km diameterGranite 1km diameter

GabbroDioriteGranite

Decrease in temperature with distancefrom intrusions of different size

Temperature change in intrusionsof different compositions

Temperature of intrusion Basic 1200°C;Intermediate 900°C; Acid 800°C;All intrusions are 5km wide

0

200

400

600

800

1000

1200

1400

0 1 2 3Distance from contact/km

Tem

pera

ture

/�C

T

empe

ratu

re/�

C

Figure 2 Graph showing the effects of temperature and composition of magma

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The thermal gradient and index minerals in a metamorphic aureoleWhen a batholith is intruded into beds of shale, increases in metamorphic grade are marked by the appearance of an index mineral:• Index minerals are metamorphic minerals, which are stable under specifi c temperature

and pressure conditions. They indicate the metamorphic grade.• In contact metamorphism, biotite is the low grade mineral found in spotted rocks.• The Al2SiO5 polymorph andalusite indicates medium grade and is found in andalusite

rich rocks.• Sillimanite, another Al2SiO5 polymorph, indicates high grade and is found in hornfels.• Because contact metamorphism is caused by temperature only, an increase in grade

represents a thermal gradient.

Some of the minerals that crystallise at low grades are stable at higher grades, so more than one index mineral can be found in one rock.

2.4 6 Contact metamorphism 2

Figure 1 Sketch map showing index minerals and metamorphic grade

Metamorphic aureole

Increasing temperature

Granitebatholith

Unaltered country rock

Shale

Increasing metamorphicgrade

Highgrade

Lowgrade

Medium grade

Sillimaniteappears

Andalusiteappears

Biotiteappears

Triple point

7

6

5

4

Pres

sure

/kba

r

3

2

1

00 100 200 300 400

Temperature/�C500 600 700 800

KYANITE SILLIMANITE

ANDALUSITE

Metamorphic path for contact metamorphism

Temperature and pressure fields for the Al2SiO5 polymorphs

Thin sectiondrawing of andalusitecrystals

Relict bedding

Slaty cleavage

0 1

cm

Dark grey finegrained rock Crystals of andalusite

Andalusite slate

Triple point

7

6

5

4

Pres

sure

/kba

r

3

2

1

00 100 200 300 400


KYANITE SILLIMANITE

ANDALUSITE




Relict bedding

Slaty cleavage

0 1

cm


Andalusite slate

Triple point

7

6

5

4

Pres

sure

/kba

r

3

2

1

00 100 200 300 400


KYANITE SILLIMANITE

ANDALUSITE




Relict bedding

Slaty cleavage

0 1

cm


Andalusite slate

Triple point

7

6

5

4

Pres

sure

/kba

r

3

2

1

00 100 200 300 400


KYANITE SILLIMANITE

ANDALUSITE




Relict bedding

Slaty cleavage

0 1

cm


Andalusite slate

Case study

The Skiddaw granite is part of a major intrusion in the English Lake District. The intrusion is an oval dome shape measuring 10 km × 6 km, with a wide metamorphic aureole.

The zones around the granite are:• Unmetamorphosed country rock –

Skiddaw slates are fi ne grained parent rocks showing slaty cleavage, formed by regional metamorphism before the intrusion.

• Outer zone of spotted slate – where the grain size is slightly coarser than in the country rocks and small round dark spots are visible. The spots contain biotite and organic material.

• Middle zone – Andalusite slate is medium grained and generally crystalline, containing andalusite porphyroblasts.

• Close to the intrusion – the parent slate has been completely recrystallised to hornfels that can contain sillimonite. Figure 2

Andalusite and sillimanite

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Contact metamorphism 2

Key definition

A polymorph is a mineral that has the same composition but occurs in different crystal forms.

Examiner tip

Make sure that you know the products of contact metamorphism. Do not write about slate (unless it is spotted slate or andalusite slate formed when the country rock was slate), schist and gneiss, if you are answering a question on contact metamorphism.

Figure 3 Sketch map and photos showing contact metamorphism of limestone and orthoquartzite

The Al2SiO5 polymorphs in contact metamorphismThe Al2SiO5 polymorphs andalusite and sillimanite are found in contact aureoles – andalusite is the low to medium temperature, low pressure polymorph found in andalusite slate, whereas sillimanite is the high temperature polymorph found in hornfels. With increasing metamorphic grade, contact metamorphism follows a path from andalusite to sillimanite on the Al2SiO5 polymorph phase diagram. Kyanite, the high pressure, low temperature polymorph, is not found in contact metamorphic rocks due to the lack of pressure.

Formation of quartzite and marbleWhen orthoquartzite, a sandstone composed entirely of quartz, is affected by contact metamorphism, all sedimentary structures including cross bedding and graded bedding are destroyed. The quartz grains in the sandstone recrystallise to form an interlocking mosaic of crystals giving it a granoblastic texture. Near to the contact with the igneous intrusion, in the zone of high grade metamorphism, the crystals are larger than they are further away from the contact where temperatures are not as high. The resulting rock is white or pale grey in colour and known as metaquartzite.

Where limestones are affected by contact metamorphism, all sedimentary structures and fossils are destroyed. The grains and cement composed of calcite will recrystallise to form an interlocking mosaic of crystals giving it a granoblastic or sugary texture. This metamorphic rock is called marble. Crystals are larger near to the contact with the igneous intrusion and smaller further away, due to the thermal gradient. If the parent limestone is composed purely of calcite, the resulting metamorphic rock is white in colour. Impurities in the limestone may give streaks of different colours in the marble.

Questions1 Explain why andalusite is not formed by the contact metamorphism of pure limestone.2 Draw a cross-section through a metamorphic aureole and through shale country rock.

Label the rock types that would be present on your cross-section.3 Explain why there are no relict structures in quartzite.

Limestone

Quartzite

Igneousintrusion

Marble

Orthoquartzite

Coarsemetaquartzite

Coarsemarble

100m

Edge of metamorphic aureole

Marble

Quartzite

Limestone

Quartzite

Igneousintrusion

Marble

Orthoquartzite

Coarsemetaquartzite

Coarsemarble

100m


Marble

Quartzite

Limestone

Quartzite

Igneousintrusion

Marble

Orthoquartzite

Coarsemetaquartzite

Coarsemarble

100m


Marble

Quartzite

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Most regional metamorphism is accompanied by deformation, so these metamorphic rocks will have foliated textures.

Regional metamorphism and plate tectonicsRegional metamorphism results from both heat and pressure generated at convergent plate margins during subduction and continental collision.

2.4 7 Regional metamorphism

• Along subduction zones magmas are generated, rise and intrude into the crust. Temperatures are high near the surface result so the geothermal gradient may be in the range of 50 to 70 °C/km, and contact metamorphism results.

• Compression occurs at a subduction zone where the oceanic crust starts to subduct and the edge of the non-subducting plate is deformed. The geothermal gradient is normal at 25 °C/km.

• Along a subduction zone, relatively cool oceanic lithosphere is pushed down to great depths. This produces a low geothermal gradient of 10 to 15 °C/km.

The geothermal gradient and plate tectonics

Convergent plate margins with subduction zones• When oceanic and continental plates collide, high pressure is produced as the oceanic

plate is subducted.• The result is high pressure, low temperature burial metamorphism and the formation of

blueschists.• Further away from the subduction zone, magma is rising from the melting oceanic

plate and pressures are lower, so high temperature, low pressure metamorphism occurs.

• High temperatures lead to the formation of igneous intrusions and metamorphic aureoles.

Paired metamorphic belts will form at convergent margins with subduction zones. The zone closest to the trench will have high pressure due to compressive stress and low temperature as no magma is rising. The zone further away has high temperature due to rising magma and low pressure.

Convergent plate margins continental–continentalFold mountains form at these margins (see spread 1.3.7) where the Earth’s crust is deformed, thickened and there is extensive intrusive igneous activity. The Himalayan mountain range began to form about 50 Ma when India collided with Asia. The Himalayas are still growing as the plates are still moving towards each other. High temperatures and pressures acting over such long periods create broad (>100 km2) and often complex orogenic belts affected by all grades of regional metamorphism.

At the deepest part of the orogenic belt the pressures and temperature will be highest, giving high grade regional metamorphism. Away from the collision zone and higher in the crust the grade of metamorphism will be low.

Case study

Paired metamorphic belts include areas in New Zealand, Indonesia, Washington State in the United States, Chile, and the coast of South America. All these areas lie around the Pacifi c at convergent plate margins, where subduction has occurred.

Case study

The Dalradian sedimentary rocks were deposited in late Precambrian and Cambrian times in an ancient ocean called Iapetus, which existed between Scotland and England. Continental–continental plate movements caused the ocean to close and the 13 km of sediments that had been deposited in the ocean were deformed and regionally metamorphosed to form the Caledonian orogenic belt. The area of metamorphism extends both south and north of the Great Glen Fault into the Highlands of Scotland. The metamorphic zones are displaced by the fault.

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Grades of regional metamorphic rocksRegional metamorphism of shale produces the following rocks (they are all described in spreads 2.4.2 and 2.4.3):• low grade: slate• medium grade: schist• high grade: gneiss

Regional metamorphism of orthoquartzite and limestones produces the same products as contact metamorphism – quartzite and marble. Each of these rocks is composed of only one mineral, quartz and calcite, respectively. The minerals are equi-dimensional, so they cannot align under pressure.


Regional metamorphismN

Honshu

Kyushu

Sea levelTrenchVolcanoes

Sediments

Paired metamorphic belts in Japan

High pressurelow temperaturemetamorphism

High temperaturelow pressuremetamorphism

Thrusts

MohoCrust

Oceanic crust

Asthenosphere

Lithosphere plate

Partial meltingof subductingplate

Risingmagma

Moho

Low temperaturehigh pressure belt

High temperaturelow pressure belt

Shikoku

Figure 1 Paired metamorphic belts in Japan

Key definition

Migmatite is a coarse grained mixed rock with some of the characteristics of gneiss and some of the characteristics of granite, formed by partial melting of the rock during the highest grade metamorphism, at the high temperature boundary between metamorphism and igneous activity.

Figure 3 Migmatite

Figure 2 Regional metamorphic rocks and their relationship to pressure and temperature

Questions1 Describe the formation of a

paired metamorphic belt.2 Describe two general changes

that would occur in a mudstone during regional metamorphism.

3 Draw up a table with the following headings: metamorphic grade; parent rock; metamorphic rock; mineral composition; texture. Complete it using the information in this spread and spreads 2.4.2 and 2.4.3.

00

1

2

3

Pres

sure

/kba

r

4

5

6

7

8

100 200 300

Slate

Schist

Gneiss

Increasing metamorphic grade

Migmatite

400Temperature/°C

500 600 700 800

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152

Mapping the Dalradian Supergroup• In 1893, George Barrow mapped a sequence of highly deformed regionally

metamorphosed rocks in the south-eastern part of the Scottish Highlands. The metamorphism and deformation occurred during closure of the Iapetus Ocean and the Caledonian orogeny about 400 Ma ago. These Precambrian rocks are known as the Dalradian Supergroup.

• As you already know, clay-rich sedimentary rocks such as shale produce a variety of metamorphic minerals, as temperature and pressure conditions change. When Barrow mapped rocks like these, he noticed that there was a pattern to the occurrence of metamorphic minerals. He used the fi rst appearance of some of these minerals, which he termed index minerals, to draw isograds. Some of the minerals that crystallise at low grades are stable at higher grades so more than one index mineral can be found in one rock.

• He was able to map metamorphic zones using index minerals and isograds, which defi ne the boundaries of the zones. Although he did not do all the mapping personally, the system he devised was named after him and the zones are called Barrovian zones.

2.4 8 Regional metamorphic zones

Key defi nitions

An index mineral is a metamorphic mineral that is stable over a particular temperature and pressure range (e.g. mica, garnet, Al2SiO5 polymorphs). They indicate the metamorphic grade (see spread 2.4.5 for a defi nition of metamorphic grade). It is possible to map metamorphic grade using index minerals. The fi rst appearance of the index minerals is mapped as they may still remain stable at higher temperatures and pressures.

An isograd is a line on a map joining points of equal metamorphic grade. They join places where the fi rst appearance of an index mineral occurs.

A metamorphic zone is the area between two isograds. The zone is named after the lower grade isograd. All locations within a metamorphic zone experienced the same metamorphic grade.

A Barrovian zone is a metamorphic zone mapped using index minerals identifi ed by George Barrow.

C

C

B

B

G

G

G

K

K

K

S

Isograds

NorthIndex minerals

S = SillimaniteK = KyaniteG = GarnetB = BiotiteC = Chlorite

Increasing grade

0 100km

Figure 1 Index minerals, isograds and metamorphic zones

Index minerals and metamorphic zonesMetamorphic grade

low medium high

Rock type Slate Schist Gneiss

Index minerals and metamorphic zones

Chlorite Biotite Garnet Kyanite Sillimanite

The chlorite zone represents low grade (low pressure and low temperature) regional metamorphism. The rock is slate where most of the rock has recrystallised but some clay minerals may still exist.

Schists develop as a result of increasing temperatures and pressures and can be found in both the biotite and garnet zones. The grain sizes increase with metamorphic grade. Schists formed at lower temperatures and pressures are composed of quartz, muscovite mica and biotite mica. Medium grade metamorphism results from higher temperatures and pressures and many schists formed at this grade contain garnet, and less commonly, kyanite porphyroblasts.

Kyanite is typically found in gneisses and the kyanite zone represents high grade regional metamorphism. The sillimanite zone represents high grade regional metamorphism with very high temperatures and pressures. The rocks are gneisses. Estimates based on the sillimanite zone indicate a maximum temperature of about 700 oC and maximum

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153

pressure of about 7 kb. This pressure exists at a depth of about 25 km below the surface of the continental crust. It gives a geothermal gradient of about 28 oC km–1.

Quartz and plagioclase feldspar are stable throughout the whole range of grades. This makes them no use as index minerals.

Triple point

7

6

5

4

Pres

sure

/kba

r

3

2

1

00 100 200 300 400

Temperature/°C500 600 700 800

KYANITE SILLIMANITE

ANDALUSITE


Metamorphic path for regional metamorphism

Highland Boundary Fault

Great G

len Fa

ult

Moine

Thru

st?

?

?

?

?

?

?

?

?

?

Dalradian metamorphic rocksBarrovian zones

Chlorite

50kmBiotite

Garnet

? Unknown

Kyanite

Sillimanite

Figure 2 Regional metamorphic zones

The Al2SiO5 polymorphs in regional metamorphismThe Al2SiO5 polymorphs kyanite and sillimanite are found in regional metamorphic rocks. A rock formed at high pressure and low temperature may contain kyanite. A rock formed at high temperature or at high temperature and high pressure may contain sillimanite, which can be found in contact and regional metamorphism, both of which can involve high temperature. With increasing metamorphic grade, regional metamorphism follows a path from kyanite to sillimanite on the Al2SiO5 polymorph phase diagram.

Questions1 Describe the rocks found in each of the Barrovian zones.2 Explain why clay-rich parent rocks are the most useful in mapping metamorphic

zones.3 Explain the difference between a polymorph and a pseudomorph.

Figure 3 Kyanite and sillimanite


Regional metamorphic zones

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154

1 Below is a diagram of the rock cycle.

crystall-isation

crystall-isation

From mantle

Partial melting

A

MAG

MA

Rock

s

EXTR

USIV

E

Igne

ous

Met

amor

phic

rock

s

Metamorphism Metamorphism

B

Weathering

Products

Processess

Erosion

D

C

Deposition Earth’ssurface

Figure 1

(a) (i) Complete the table below using the diagram of the rock cycle.

Location Process or product

A

B

C

D

[4] (ii) Name two processes that occur after deposition to

produce rock group D. [2] (b) Explain how the crystal grain size of igneous rocks is

related to the depth at which they crystallised. [2] (c) Explain the difference between an era and a system.

Give one example of each. [2] Total 8

(OCR 2832 May 06)

2 Descriptions of three igneous rocks are given in the table below.

Description

Rock A

• Flow banded• Light grey or red or brown colour• Very fi ne crystals <1 mm

Rock B

• Conchoidal fracture• Black colour• No crystals

Rock C

• Coarse crystal grain size• Greenish black crystals of augite and

homblende• White crystals of plagioclase fedspar• White crystals of potash fedspar

(a) (i) Identify the three rocks A, B and C. [3] (ii) Describe with the aid of a sketch the term fl ow

banding. [2] (iii) Explain why igneous rock B has no crystals. [1] (iv) Defi ne the term conchoidal fracture. [1] (b) Plagioclase feldspar, augite and hornblende are all part

of Bowen’s Reaction Series. They have been entered on the reaction series diagram below.

Na rich plagioclase feldspar

Ca rich plagioclase feldsparD

Augite

Hornblende

Biotite

E

F

G

Figure 2

(i) Name the minerals D, E, F and G from Bowen’s Reaction Series. [4]

(ii) Explain the relationship of Bowen’s Reaction Series to temperature. [2]

(iii) Name the minerals that form the discontinuous part of Bowen’s Reaction Series. [1]

(OCR 2835 June 06) (c) The table below shows the chemical composition by

percentage of oxides of four igneous rocks H, J, K and L. (i) To which igneous rock groups do H, J, K and L

belong? [4] (ii) Describe the changes in the % of oxides of silicon

and sodium compared to iron and magnesium across the four rock groups. [2]

Oxide % A B C D

SiO2 46.0 73.0 60.0 43.5

Al2O3 15.0 13.0 17.0 4.0

Fe oxides 12.0 2.0 6.0 12.5

MgO 9.0 0.5 3.5 34.0

CaO 9.0 1.5 7.0 3.5

Na2O 3.5 4.0 3.5 0.5

K2O 1.5 4.0 1.5 0.3

others 4.0 2.0 1.5 1.7

Total 20(OCR 2835 June 06)

1 Below is a diagram of the rock cycle. 1 Below is a diagram of the rock cycle.

Unit 2 Examination questions

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155155

3 (a) The table below shows the results of a student’s research into the world’s top 12 most deadly volcanic eruptions.

Rank Volcano Location Year oferuption

Death Major cause ofdeath

1 Tambora Indonesia 1815 92 000 Ash fall, starvation

2 Krakatau Indonesia 1883 36 417 Ash fall, tsunami

3 Mount Pelée Martinique 1902 29 025 Pyroclastic flows

4 Ruiz Colombia 1985 25 000 Lahars

5 Unzan Japan 1792 14 300 Volcano collapse, tsunami

6 Laki Iceland 1783 9 350 Starvation

7 Kelut Indonesia 1919 5 110 Lahars

8 Galunggung Italy 1882 4 011 Lahars

9 Vesuvius Italy 1631 3 500 Lava flows, lahars

10 Vesuvius Indonesia 79 3 360 Ash falls, pyroclastic flows

11 Pandayan Indonesia 1772 2 957 Pyroclastic flows

12 Lamington Papua New Guinea

1951 2 942 Pyroclastic flows

(i) Explain why Indonesia has so many volcanic eruptions. [2]

(ii) Using the table calculate the percentage of eruptions that had starvation as a major cause of death. Show your working. [2]

(iii) Suggest reasons why the global summer of 1816 was very cold. [3]

Total 7(OCR 2832 May 07)

4 The graphic log below shows a commonly found sequence of sedimentary rocks.

f m

50cm

c

E

D

C

B

A

Clay Silt GranulesSand

Figure 3

(a) (i) Using the graphic log, name and explain the formation of the sedimentary structure shown by the change in grain sizes in bed A. [3]

(ii) Flute casts are found at the base of bed A. Draw a labelled diagram of a flute cast. Explain how a flute cast is formed. [3]

(OCR 2835 June 06)

(b) The diagram below shows thin section drawings of two metamorphic rocks and their sedimentary parent rocks.

CalciteCement

Rock L Rock M Rock N Rock O

mm

0 1 2 Quartzcement

Quartz

Figure 4

(i) Complete the sentences below by entering the correct rock letters. Rock…………is the parent of rock………….

Rock…………is the parent of rock…………. [2] (ii) Describe how rock L forms. [2] (iii) Rock N has symmetrical ripple marks on the

bedding planes. Describe the environment in which rock N was deposited. [2]

(c) i) Describe one mechanical weathering process, operating in a cold climate, that affects limestone. [2]

(ii) State the shape of the scree fragments. [1] (iii) Describe one chemical weathering process that

affects the limestone. [2] Total 17

(OCR 2832 May 07)

5 (a) Describe how the following factors control metamorphism.

(i) temperature [2] (ii) pressure [2] (b) Regional metamorphic rocks form as a result of changes

in both temperature and pressure. (i) Name the rock type that is formed as a result of the

regional metamorphism of pure limestone and pure sandstone. [2]

(ii) Explain why shales give rise to a wide variety of new metamorphic minerals when regionally metamorphosed. [2]

(iii) Define the following terms: • index mineral [1] • isograd [1]

(OCR 2835 June 06) (c) Explain why rocks formed by contact metamorphism

lack any foliation. [2] Total 13

(OCR 2832 May 06)

6 Using diagrams explain the differences between sills and lava flows. [8]

(OCR 2832 May 06)

7 Describe with the aid of diagrams, the processes of compaction and cementation. [8]

(OCR 2832 May 07)

Unit 2Rocks

Practice questions

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1 temperature, pressure and metamorphism - pearson schools

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