Deformation and Metamorphism - GeoScience - Homegeoscience.msc.sa.edu.au/library/1-3-3 and 4...

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Student: ………………………… Date received: ……………. Handout 5 of 14 (Topic 1.3.3) Deformation and Metamorphism Overturned microfolding of marble (pale layers) and phyllite (dark layers) produced by low-temperature regional metamorphism of Cambrian strata, Second Valley, Fleurieu Peninsular (Photo: Bernd Michaelsen). The geometry of these folds is related to the major fold in the area – the Normanville anticline that outcrops along the coast.

Transcript of Deformation and Metamorphism - GeoScience - Homegeoscience.msc.sa.edu.au/library/1-3-3 and 4...

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Student: ………………………… Date received: …………….

Handout 5 of 14

(Topic 1.3.3)

Deformation and Metamorphism

Overturned microfolding of marble (pale layers) and phyllite (dark layers) produced by low-temperature regional metamorphism of Cambrian strata, Second Valley, Fleurieu Peninsular (Photo: Bernd Michaelsen). The geometry of these folds is related to the major fold in the area – the Normanville anticline that outcrops along the coast.

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Regional Processes Deformation

Key Ideas Intended Student Learning Deformation

Compressional and tensional forces acting on rocks cause joints, faults, and folds.

Contrast the conditions under which rocks are likely to break or fold. Explain the difference between a joint and a fault. Explain the difference between normal, reverse, and lateral faults. Describe forces that cause each type of faulting to occur. Explain the difference between an anticline and a syncline. Recognise, in the field, at least one of the deformation structures listed above.

The sections of the Intended Student Learning that are italicised must form part of the fieldwork or practical materials submitted for moderation. They will not be examined in the public examination

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Regional Processes Metamorphism

Metamorphism

Rocks change in the solid state to become metamorphic rocks

Define the term ‘metamorphism’. Explain how heat, pressure, fluids, and time contribute to metamorphism.

Thermal metamorphism is caused by heat from an igneous intrusion.

Explain the formation of a metamorphic aureole surrounding an igneous intrusion. Identify the following thermal metamorphic rocks, name their parent sedimentary rocks, and describe the textural and mineralogical changes that have occurred: Hornfels Quartzite. Marble

Regional metamorphism is due to directed pressure and heat.

Explain the difference between load pressure and directed pressure. Explain, with the aid of diagrams, the development of foliation by directed pressure. Explain the difference between cleavage and bedding in rocks that have been subjected to regional metamorphism. Identify the following regional metamorphic rocks: Slate Schist. Gneiss Describe the textures and mineralogies of the rocks listed above. Describe the progressive formation of these three regional metamorphic rocks from their parent sedimentary rock, shale. Explain why the changes: sandstone → quartzite limestone → marble may occur in both thermal and regional metamorphism.

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1.3 – Regional Processes 1.3.3 – Deformation BENDING OR BREAKING

Forces (or pressure) acting on rocks may cause deformation, which is likely to take one of two forms — bending or breaking.

In response to the forces acting on it, the undeformed strata shown in the adjacent diagram may either:

bend to form folds,

or break to form joints.

FACTORS AFFECTING NATURE OF DEFORMATION

1. Pressure

The pressure acting on a rock mass is likely to be a combination of load pressure, due to the weight of overlying rocks, and directed pressure caused by forces within Earth's crust. Load pressure acts equally in all directions, merely compressing the rock. By contrast, directed pressure squeezes the rock in only one direction.

Deformation of rocks is caused by directed pressure but the load pressure affects the nature of the deformation. If the load pressure is low, directed pressure is likely to cause the rocks to fracture. If the load pressure is high, the rocks are more likely to bend under the influence of directed pressure. Rocks deep within the crust are therefore likely to fold, while those closer to the surface are more likely to break.

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2. Temperature

Changes in temperature affect the way in which rocks behave when subjected to stress. At higher temperatures, folding is more likely to occur than breaking. This is another reason why folding usually occurs deep below Earth's surface, whereas breaking occurs closer to the surface. 3. Fluids

Fluids which are present in the pore spaces of rocks and joints, act as lubricants, making is easier for rock layers to slide over each other. Thus the presence of fluids makes it more likely that the rocks will bend rather than break. 4. Time

Solid, brittle materials normally bend rather than break if they are deformed very slowly. Small forces applied over very long periods of time may cause rocks to deform by a process called creep. This can be observed in old buildings, where floor boards and marble mantelpieces may sag. JOINTS & FAULTS

Joints and faults are formed at or near Earth's surface when sudden forces act (e.g. earthquakes).

When rock fractures a joint is formed.

A fault is created if the rocks on one side of the joint move relative to rocks on the other side

The terms associated with faults are shown in the adjacent diagram. Learn all these terms and their meanings (i.e. footwall, fault scarp, hanging wall)

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Normal Faults

Extensional forces (i.e. forces pulling the rocks apart) produce normal faults. Here, the hanging wall block moves down relative to the footwall block. Reverse Faults

Compressional forces cause reverse faults. The hanging wall moves up relative to the footwall.

Lateral (Strike-slip) Faults

Lateral faults are formed when two blocks slide horizontally past each other. The San Andreas Fault (discussed in more detail later in the course) is an example of a lateral fault.

FOLDS

Folds develop deep in Earth's crust over very long periods of time, usually in response to horizontal compressional forces. The world's great mountain ranges were formed due to large-scale folding of Earth’s crust.

The diagram below shows some of the terms used to describe different types of folds, and parts of folds — learn all these terms.

Visit the excellent web site on “structural geology” at:

http://earth.leeds.ac.uk/learnstructure/index.htm

and link to the sections on folding and faulting.

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(Tightly) folded rock formation at Moruya, NSW (photo D Vernon 2006; http://en.wikipedia.org/wiki/Image:Folded_Roc

k.jpg The process of forming “drag”folds (fault-propagation folds) like these are associated with faulting and can be can be viewed in cartoon movie at http://www.uib.no/people/nglhe/StructModules

Textbook/Contraction02.swf

Folded Neoproterozoic (Ediacaran) sedimentary layers, Finnmark, northern Norway. (http://www.geo.uib.no/struct/Figures.html)

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The diagram above illustrates an orogeny (caused by compressional tectonics) in eastern North America between 543 and 440 Ma. Folding and faulting are associated with regional metamorphism (Source of image: http://upload .wikimedia.org/wikipedia/en/2/20/Taconic_orogeny.png). The diagram shows how orogenic processes caused the North American continent to grow eastwards. The Australian continent has similarly grown eastward since 543 Ma (i.e. the beginning of the Phanerozoic eon).

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1.3 – Regional Processes 1.3.4 – Metamorphism DEFINITION

The term metamorphism encompasses processes by which rocks within Earth's crust are changed over a long period of time by heat, pressure and fluids.

NB The processes of weathering and sedimentary rock formation are excluded from the definition of metamorphism, since these processes occur at or near Earth’s surface, rather than within the crust.

Processes in which the rocks actually melt are defined as igneous and are not defined as metamorphic.

Metamorphic changes take place in the solid state. FACTORS AFFECTING METAMORPHISM

1. Temperature (below the melting point of rock)

Minerals in sedimentary rocks (usually quartz, calcite and clays) are stable at low temperatures. Increasing temperature causes these minerals to change to different minerals (which are stable at these higher temperatures).

Increasing temperatures also cause a change (usually a decrease) in the water content of the rock. 2. Pressure

The pressure acting on a rock mass is likely to be a combination of load pressure, due to the weight of overlying rocks, and directed pressure caused by forces within Earth's crust, such as those which cause folds and faults to form. Both types of pressure compress minerals, squeezing their atoms together to form denser minerals that are stable under higher pressures.

Pressure can also alter the texture of a rock, resulting in an increase in grain size. Directed pressure results in the formation and alignment of flat (platey) minerals such as micas (e.g. biotite, muscovite). Micas are therefore characteristic of metamorphic rocks which have been affected by directed pressure. This texture is known as foliation. Fossils, or the pebbles in a conglomerate, can also become elongated by directed pressure.

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3. Fluids

Fluids from magma bodies or from groundwater can affect the metamorphic process to:

i. increase the rate of metamorphic change.

ii. assist recrystallisation (forming the foliations usually associated with metamorphic rocks).

iii. cause reactions between the chemicals dissolved in the fluids and the minerals present, that is chemical metamorphism (= metasomatism) that increases the likelihood of new minerals forming.

4. Time

Metamorphic processes are extremely slow. They take many millions of years.

The table below summarises the factors affecting metamorphic change, giving the effect/s of each factor.

Factor Effect/s of this factor 1. Temperature Formation of new minerals.

Recrystallisation. Change (usually decrease) in water content.

2. Pressure Increase in density. Formation of platy minerals (i.e. micas). Foliation - alignment of platy minerals. Elongation of fossils & pebbles.

3. Fluids Increased rate of metamorphic changes. Assist recrystallisation. Cause chemical metamorphism to occur.

4. Time Metamorphic processes require millions of years.

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THERMAL (CONTACT) METAMORPHISM

Thermal (= contact) metamorphism is change in rocks due to heat from a cooling body of magma. Here the rocks surrounding an igneous intrusion are affected ('cooked') by the heat from the magma. The region of metamorphic rocks around the intrusion is known as a metamorphic aureole.

Rock Changes Caused by Thermal Metamorphism

For three common sedimentary rocks, the rocks changes brought about by thermal metamorphism are:

SANDSTONE → QUARTZITE

LIMESTONE → MARBLE

SHALE → HORNFELS Textural and Mineralogical Changes due to Contact Metamorphism Sandstone → Quartzite

There is no change in mineralogy. The quartz grains in the sandstone are recrystallised. There is, however, a change in the texture of the rock. Sandstone consists of quartz grains cemented together, while quartzite consists of interlocking quartz grains.

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Limestone → Marble

Again, there is no change in mineralogy. Both limestone and marble consist of calcite (calcium carbonate: CaCO3), and both rocks effervesce with acid.

There is a change in texture. Whereas fossiliferous limestone consists of cemented remains of living organisms, marble has a crystalline texture that resembles sugar (i.e. saccharoidal texture).

Shale → Hornfels

When shale (and siltstone) undergoes thermal metamorphism, new minerals are formed. Shale consists largely of clay minerals with minor quartz. Clay minerals are stable under low temperature and pressure conditions that prevail at or near Earth's surface. However, under high temperature and pressure conditions associated with metamorphism, clay minerals change to feldspars and biotite. Quartz remains unchanged, since this mineral is stable under a wide range of temperature and pressure conditions.

The change in mineralogy that occurs when shale changes to hornfels can therefore be summarised as:

SHALE → HORNFELS

Biotite, feldspars, quartz Clay minerals, quartz There is also a change in both colour and texture of the rocks. Shale is normally light coloured, and shows layering, as clay minerals are flat. Hornfels shows no alignment of grains. Its texture is described as non-aligned or non-foliated. REGIONAL METAMORPHISM

Regional metamorphism is associated with the folding of rocks in mountain building (or orogenic) activity. This occurs when two continental plates collide, as India collided with Asia forming the Himalayas.

Orogenesis = mountain building ( = metamorphism + folding + faulting)

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Over many millions of years, two continents move towards each other (e.g. India moving northwards and eventually colliding with Eurasia. Sediments weathered from the colliding continents are deposited in the long, narrow basin between them. This basin is called a geosyncline. When the continents eventually collide, sediments buckle and fold, forming a mountain range like the Alps or the Himalayas, shown in the photographs below.

Mont Blanc (left) at 4808 m Mt Blanc is the highest peak in continental Europe and consists of granite, gneiss, schist, marble and sedimentary strata. (Image source:

http://en.wikipedia.org/ wiki/Image:Montblanc_166186.jpg).

Much sedimentary strata in the European Alps were laid down during the Eocene (~ 55 Ma), about the same time as sediments were deposited at Maslin Bay. However, there

was no orogeny at Maslin Bay where the strata remains near sea-level, unmetamorphosed, unfolded and essentially horizontal.

The image above is taken from the Zugspitze (Germany’s highest mountain) across the Austrian Alps (Source: http://en.wikipedia.org/wiki/Image:Zugspitze_panorama1.jpg). The Alps are a zone of active orogenic activity caused by the Africa colliding with Europe. The heat and pressure associated with mountain building (= orogenic activity) causes rocks to be regionally metamorphosed, especially where the heat and pressure are greatest.

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Opposite: The orogeny that created the Himalayas and the Tibetan Plateau was (and still is!) caused by the collision of India with the Eurasian Plate (Source: http://en.wikipedia .org/ wiki/Image:Himalaya-formation.gif) The image below was captured by the International Space Station in 2004. It shows the Himalayan mountain belt with the Tibetan Plateau in the foreground. (Source of image: http://en.wikipedia.org/wiki/Image:Himalayas.jpg). Four of Earth’s 8000-plus metre peaks are shown in this image, including Mt Everest at 8850 m, the highest.

The diagram below shows that when a mountain range is formed by orogenic activity, regional metamorphism of ever-increasing grade is associated with increasing severity of folding of the rocks.

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Orogenic activity, and hence regional metamorphism, occurs over wide areas. For example, rocks affected by regional metamorphism may be found from Victor Harbor to Birdwood, and eastwards as far as Palmer, the eastern boundary of the Mount Lofty Ranges.

The adjacent map shows the main settled areas of South Australia including the Mount Lofty Ranges, Kangaroo Island, and the southern Flinders Ranges. Many of the rocks in this area have been affected by regional metamorphism. ROCKS FORMED BY REGIONAL METAMORPHISM

Some examples of the rock changes that occur during regional metamorphism are:

SANDSTONE → QUARTZITE

LIMESTONE → MARBLE

SHALE → SLATE → SCHIST → GNEISS

→ increasing metamorphism →

As indicated above, limestone → marble and sandstone → quartzite occur in both thermal and regional metamorphism. However, regional metamorphism of shale produces a series of metamorphic rocks. The rock that is finally formed depends on the degree of metamorphism.

The progression from shale to slate to schist to gneiss is caused by increasing degrees of regional metamorphism.

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TEXTURES OF METAMORPHIC ROCKS

The texture of igneous rocks is described in terms of grain/crystal size and arrangement.

For sedimentary rocks, the term “texture” covers three properties: grain size, grain shape and sorting of grains.

However, when describing the texture of metamorphic rocks, the alignment of grains is of particular importance. Metamorphic rocks are either non-aligned or foliated. Non-aligned Rocks

The grains of non-aligned rocks show no “preferred orientation”, or layering. Their appearance is the same in all directions (see the pictures of quartzite, marble and hornfels earlier these notes).

Non-aligned rocks are formed either: i. when there is no directed pressure acting on the rock.

(e.g. hornfels is formed by thermal metamorphism)

or

ii. when the rock contains no platy (flat) minerals.

(e.g. sandstone → quartzite and limestone → marble)

These transformations may occur in both thermal and regional metamorphism because the mineral grains in both quartzite and marble are not flat and therefore cannot align in any particular direction. This is shown in the diagram below. The adjacent diagram shows a rock (e.g. quartzite) which has a non-aligned texture, i.e. it is not foliated. Even directed pressure associated with regional metamorphism cannot produce a foliated texture in this rock because the grains are rounded rather than platy. Topics 1.3.3 & 1.3.4 Deformation & Metamorphism Page 16 of 33

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Foliated Rocks

As shown in the diagrams below, the minerals in foliated rocks are arranged in layers, so that the rocks may tend to split into thin slices (e.g. slate), or a layered pattern may be visible, as in schist or gneiss. Foliated metamorphic rocks are formed by regional metamorphism of shale (or similar rocks e.g. siltstone). Foliation is due to the effect of directed pressure on the platy minerals present in the shale. When the metamorphic change from shale to slate occurs, the mica flakes become aligned perpendicular to the directed pressure, as shown in the diagram below.

Three types of foliation are developed in rocks formed by regional metamorphism of shale, namely:

Slaty cleavage Schistosity Gneissic layering

The type of foliation depends on the amount of directed pressure acting on the shale, as shown in the diagram below.

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1. Slaty cleavage

NB: Until now you have thought of cleavage as a property of minerals. However, slaty cleavage is a diagnostic property of fine-grained, regionally metamorphosed rocks.

Slate contains layers of microscopic mica crystals. Weaknesses, or cleavage planes, exist between these layers. 2. Schistosity

Like slaty cleavage, schistosity is caused by directed pressure acting on platy minerals. To produce a schistosity, heat and directed pressure must be more intense, causing the growth of visible mica flakes. These align themselves in layers, again perpendicular to the directed pressure. 3. Gneissic layering

Increasing heat and pressure cause the minerals to flow, producing a rock with variously coloured layers called a gneiss.

(Remember, gneiss is layered or striped). The composition and structure of a gneiss are shown in the adjacent diagram.

Cleavage and Bedding

The direction of the directed pressure that caused shale to become slate or schist or gneiss generally differs from that of the original bedding planes, as shown in the diagram below.

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Both the original bedding planes and the cleavage directions formed during metamorphism are likely to be planes of weakness in the rocks.

Slate typically has at least two planes of weakness — one due to the original bedding and the other due to slaty cleavage, perpendicular to the directed pressure during orogenesis. In a schist, the sedimentary layers (bedding planes) do not usually form planes of weakness. However, they may be indicated by layers of crystals of metamorphic minerals, as shown in the adjacent diagram.

SUMMARY OF METAMORPHIC ROCKS

The table below contains the names of all the metamorphic rocks listed in the SSABSA syllabus, together with the name of the parent rock, the type of metamorphism and the mineralogy.

Metamorphic rock 'Parent' rock Types(s) of metamorphism Mineralogy

Marble Limestone Thermal or regional Calcite (CaCO3) Quartzite Sandstone Thermal or regional Quartz (SiO2) Hornfels Shale Thermal Feldspars, quartz,

biotite Slate Shale Regional Quartz, muscovite,

chlorite* Schist Shale Regional Quartz, muscovite,

biotite, garnet. Gneiss Shale Regional Quartz, orthoclase,

biotite, garnet.

* Chlorite (a green mica) is not listed in the SSABSA syllabus.

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EXERCISES DEFORMATION

1. Draw diagrams to show two of the deforming effects which forces can have on rocks.

Deforming effect 1:

Deforming effect 2:

2. Explain, with the aid of diagrams, the difference between load pressure

and directed pressure acting on rocks.

Load Pressure

Directed Pressure

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3. Use the table below to summarise the effects of four factors that affect the nature of the deformation caused by pressure acting on rocks.

Factor Effect of this factor

4. Explain, with the aid of diagrams, the difference between a joint and a

fault.

A joint

A fault

5. On the adjacent diagram of a fault label:

a. the hanging wall. b. the footwall. c. the fault scarp.

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The adjacent diagram shows a sequence of horizontal sedimentary strata containing a joint.

The directions of the forces acting on the strata are also shown. 6 a. In the adjacent space,

draw a second diagram showing the type of fault which would be produced by these forces.

b. Name the type of fault you have drawn.

7 a. Draw a diagram showing the type of fault produced by compressional forces acting on rock strata.

Include the directions of these forces on your diagram.

b. Name the type of fault you have drawn:

……………………fault

8 a. Describe the type of forces which produce a lateral fault.

b. Draw a block diagram of a lateral fault, including the direction of movement on both sides of the fault.

c. Name an example of a lateral fault.

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9. Label the adjacent diagram, which shows the different parts of a fold.

10. Name the structure shown in each of the following diagrams.

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METAMORPHISM

1. Define the term metamorphism.

2. Are weathering and the formation of sedimentary rocks examples of metamorphic change? Explain your answer.

3. In what way do igneous processes differ from metamorphism?

4. In which of the three states of matter do metamorphic changes take place?

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Factors Affecting Metamorphism 1. In the table below, list four factors which affect metamorphic change,

and describe the effect/s of each factor.

Factor Effect/s of this factor 1.

2.

3.

4.

2. Explain, with the aid of diagrams, the difference between conglomerate

and meta-conglomerate.

Conglomerate

Meta-conglomerate

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Thermal Metamorphism 1. What is thermal metamorphism?

2. What is a metamorphic aureole?

3. Some examples of igneous intrusions are shown in the diagrams below. Shade in the metamorphic aureoles in each case.

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4. What can you say about the relative amounts of heat energy produced by the batholith and the dyke shown in the diagrams above?

5. Compare the degree of metamorphic change you would expect in the rocks around the dyke with that around the batholith.

6. How would the size of the metamorphic aureole around the dyke compare with the metamorphic aureole around the batholith?

7. Collect specimens of the sedimentary rock shale and the metamorphic rock hornfels, which is formed by thermal metamorphism of shale. Use the table below to compare their properties.

Rock type Feature Shale Hornfels

Colour

Texture

'Hardness'

Density

Mineralogy

Layering present?

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8. Collect specimens of the sedimentary rock limestone and the metamorphic rock marble, which is formed by both thermal and regional metamorphism of limestone. Use the table below to compare their properties.

Rock type Feature

Limestone Marble

Colour

Texture

'Hardness'

Density

Mineralogy(try some acid on each)

9. Collect specimens of the sedimentary rock sandstone and the

metamorphic rock quartzite, which is formed by both thermal and regional metamorphism of sandstone. Use the table below to compare their properties.

Rock type Feature

Sandstone Quartzite

Colour

Texture

'Hardness'

Density

Mineralogy

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Regional Metamorphism

1. Describe, with the aid of diagrams, the processes which give rise to regional metamorphism.

Stage 1:

Stage 2:

2. What factors cause the changes in the rocks?

3. The diagram below shows part of a mountain range which has been formed by orogenic activity. Indicate on the diagram where you would expect to find various grades of metamorphism, ranging from low-grade to high-grade.

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4. The diagram below shows a fold mountain range in which different degrees of orogenic activity have occurred.

Write the names of the rock types formed by metamorphism of shale in the appropriate blocks.

5. Explain, with the aid of diagrams, the difference between non-aligned

and foliated rocks.

Non-aligned rocks:

Foliated rocks:

6. Explain, with the aid of a diagram, the cause of foliation in rocks.

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7. a. Give the words that are used to describe the textures of three rocks formed by regional metamorphism of shale.

Name of rock Word used for texture

b. Draw diagrams showing each of the textures you named in part a.

8. Explain, with the aid of a diagram, the difference between cleavage and

bedding in rocks that have been subject to regional metamorphism.

Cleavage:

Bedding:

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9. Collect a specimen of the ‘parent’ (sedimentary) rock shale, and also specimens of slate, schist and gneiss — rocks formed by increasing degrees of regional metamorphism of shale.

Record the characteristics of each rock type in the table below.

Rock type Feature

Shale Slate Schist Gneiss

Colour

Approximate grain size (mm)

Layers present?

Name of texture

Labelled sketch of rock, showing

texture.

10. The diagrams below represent photomicrographs showing some

examples of textures present in metamorphic rocks.

Foliated/non aligned

Foliated/non aligned

Foliated/non aligned

Foliated/non aligned

a. By crossing out the incorrect words, indicate which of the diagrams show a non-aligned texture and which show some form of foliation.

b Name the likely rock types, choosing from the following names: Quartzite, marble, schist, gneiss

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11. Summarise the characteristics of the different textures of metamorphic rocks by completing the table below.

Texture Description/sketch Rock examples Non-aligned

Types of foliation

1 ....................

2 ....................

3 ....................

12. Explain why the changes:

sandstone → quartzite limestone → marble

may occur in both thermal and regional metamorphism.

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