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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.
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
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 2 of 33
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
Marble
Regional metamorphism is due to directed
pressure and heat.
Quartzite.
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
Gneiss
Schist.
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 3 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 4 of 33
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)
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 5 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 6 of 33
(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 (faultpropagation 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)
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 7 of 33
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).
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 8 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 9 of 33
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
1. Temperature
Effect/s of this factor
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 10 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 11 of 33
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
Clay minerals, quartz → Biotite, feldspars, 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)
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 12 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 13 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 14 of 33
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
increasing metamorphism
→
GNEISS
→
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 15 of 33
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 nonaligned 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
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 17 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 18 of 33
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
Marble
Quartzite
Hornfels
Limestone
Sandstone
Shale
Types(s) of
metamorphism
Thermal or regional
Thermal or regional
Thermal
Slate
Shale
Regional
Schist
Shale
Regional
Gneiss
Shale
Regional
Mineralogy
Calcite (CaCO3)
Quartz (SiO2)
Feldspars, quartz,
biotite
Quartz, muscovite,
chlorite*
Quartz, muscovite,
biotite, garnet.
Quartz, orthoclase,
biotite, garnet.
* Chlorite (a green mica) is not listed in the SSABSA syllabus.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 19 of 33
EXERCISES
DEFORMATION
1.
Draw diagrams to show two of the deforming effects which forces can
have on rocks.
Deforming effect 1:
2.
Deforming effect 2:
Explain, with the aid of diagrams, the difference between load pressure
and directed pressure acting on rocks.
Load Pressure
Directed Pressure
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 20 of 33
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
4.
Effect of this factor
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 21 of 33
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
7
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.
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 22 of 33
9.
Label the adjacent
diagram, which
shows the different
parts of a fold.
10. Name the structure shown in each of the following diagrams.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 23 of 33
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?
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 24 of 33
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
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 25 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 26 of 33
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.
Feature
Rock type
Shale
Hornfels
Colour
Texture
'Hardness'
Density
Mineralogy
Layering present?
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 27 of 33
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.
Feature
Rock type
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.
Feature
Rock type
Sandstone
Quartzite
Colour
Texture
'Hardness'
Density
Mineralogy
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 28 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 29 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 30 of 33
7.
a.
Give the words that are used to describe the textures of three rocks
formed by regional metamorphism of shale.
Name of rock
b.
8.
Word used for texture
Draw diagrams showing each of the textures you named in part a.
Explain, with the aid of a diagram, the difference between cleavage and
bedding in rocks that have been subject to regional metamorphism.
Cleavage:
Bedding:
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 31 of 33
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.
Feature
Rock type
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
examples of textures present in metamorphic rocks.
Foliated/non
aligned
Foliated/non
aligned
Foliated/non
aligned
showing
some
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
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 32 of 33
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.
Topics 1.3.3 & 1.3.4
Deformation & Metamorphism
Page 33 of 33