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SECTION TWO
Information Book
(a)
Introduction
Landscape is significantly affected by the characteristics of the rocks below the
Earth’s surface. For example, harder rocks are generally more resistant to
weathering and erosion than softer rocks.
There are three main types of rocks – igneous, sedimentary and metamorphic –
distinguished by their different properties and different methods of formation.
(b)
Igneous rocks
(i)
The experiment in Activity 1 will have demonstrated that igneous rocks
are made of crystals and are hard.
They are formed when molten magma cools down and becomes solid.
As it cools, crystals form and lock together like pieces of a jig -saw.
This strong interlocking structure makes igneous rocks difficult to break
(Fig 2.1).
(ii)
The longer the magma takes to cool the larger the crystals are able to
grow (Fig 2.1).
Fig 2.1: Views of magnified igneous rocks
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It is much warmer deep within the crust than it is at the Earth’s surface, so rocks
like granite and gabbro, which cool slowly at great depth, have large crystals.
These can be quite sharp and rock climbers soon get raw, tender fingertips on the
gabbro of Skye! Basalt, on the other hand, forms when lava cools quickly as it
flows over the Earth’s surface.
The colour of the rock depends on its composition. Granite is much lighter in
colour than basalt and gabbro, because it contains 30% quartz – a white or
colourless mineral. Basalt and gabbro have no quartz, but have more dark minerals
than granite.
(c)
Sedimentary rocks
Section One showed that rocks at the Earth’s surface are broken down by a process
called weathering. Some weathering processes break rocks into smaller fragments.
The land is also worn away or eroded by rivers, the sea, wind and ice. The
materials picked up are then carried or transported to be deposited as sediment in
another place.
Sedimentary rocks are formed in three ways.
1.
Fragments of other rocks (pebbles, sand or mud) become buried by layers of
other sediment, squashed by their weight a nd cemented together by minerals
(Fig 2.2).
Fig 2.2: Formation of fragmental sedimentary rock
new sediment
w e i g h t o f o ve r l yi n g
beds compacts
loose sediments into
solid rock
old sediment
A sedimentary rock made of pebble-sized fragments is called conglomerate.
Sandstone is made of sand-sized particles.
Mudstone and clay are made of fine particles.
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2.
Some minerals are transported in solution. These, like calcite (lime), are
deposited if the water evaporates.
A rock made of the mineral calcite is called limestone.
3.
Some sedimentary rocks are made from the remains of living things.
Coal is formed from the remains of land plants.
Chalk is made from the skeletons of tiny sea plants.
Limestone can be made from coral or shell fragments (Fig 2.3).
Fig 2.3: Limestone with fossils
The preserved remains of traces of plants and animals are called fossils. They
can include leaves, shells, worm-burrows or dinosaur footprints (Fig 2.4).
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Fig 2.4: Trace fossils
Sedimentary rocks are usually laid down in layers or beds.
Sediments deposited in water often lie in distinct layers. The layers are called
beds and the layering is called bedding.
Generally, sedimentary rocks are soft and easily eroded, because they do not
have an interlocking crystal structure (Fig 2.5).
Fig 2.5: Sandstone
Individual sand grains can be easily broken off, especially if the cement is
weak.
However, the hardness of sedimentary rocks varies. For example, chalk and
limestone resist erosion better than mudstone and clay.
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(d)
Metamorphic rocks
(i)
These rocks have been changed by heat or by heat and pressure.
When other rocks are heated, or heated and squeezed, they remain solid
but they recrystallise to form metamorphic rocks (Fig 2.6).
Fig 2.6: Showing how the minerals in a rock recrystallise during
metamorphism.
There are two main ways in which metamorphic rocks are formed:
1.
Thermal Metamorphism
The rocks are heated by an igneous intrusion. In this case, the
rocks are not squeezed so the minerals in the metamorphic rock are
not lined up and flattened.
2.
Regional Metamorphism
When mountains are pushed up a great deal of heat and pressure is
generated. Deep within the mountain range huge volumes of rock
recrystallise. The high pressure causes the mineral grains to line
up to produce a layering called foliation (Fig 2.7).
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Fig 2.7: Formation of foliation
The three main foliated rocks are slate, schist and gneiss. Slate is very finegrained. It splits easily into thin sheets. Schist is medium -grained and it
usually has flakes of mica lying in the foliation ma king it glitter (Fig 2.8).
Fig 2.8: Views of magnified metamorphic rocks
Gneiss is a coarse-grained rock where the foliation usually takes the form of
obvious banding. The differences among slate, schist and gneiss represent
differences in the strength of metamorphism. If a mudstone is weakly
metamorphosed a slate is produced. As the strength of metamorphism
increases, schist then gneiss result:
no metamorphism
mudstone
weak metamorphism
slate
strong metamorphism
schist gneiss
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Like igneous rocks, metamorphic rocks are usually hard, because they consist
of interlocking crystals.
(e)
Rocks and Relief
Relief describes height differences in an area. Since igneous and metamorphic
rocks are generally more resistant to weather ing and erosion than sedimentary
rocks, they tend to form higher ground.
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Activity Sheets
Outcomes
Intermediate 2
Knowledge and Understanding
Characteristics, formation and examples of the three main rock types:
• igneous rocks
• sedimentary rocks
• metamorphic rocks.
The differing hardness of these rock types and their effect on landscape in the
British Isles.
GMTs
Conduct experiments.
Draw annotated sketches from photographs.
Identify rock types from magnified views of rocks (Extension only).
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Two rock samples (one sedimentary and the other igneous), magnifying
glass, cloth, penknife, hammer, safety goggles.
Carry out the following tests on both rocks or VIDEO.
1.
2.
Look at the rocks through a magnifying glass.
(a)
Do the particles look like sharp-edged crystals or rounded grains?
(b)
Can you see tiny surfaces glinting in the light?
(i)
Rub the rock between your fingers. If it doesn’t crumble,
(ii)
scrape the rock firmly with a penknife. If it doesn’t crumble,
(iii) wrap it with a cloth and hit it with a hammer.
(iv) Look at the broken bits with a magnifying glass. Is the rock made from
separate grains or crystals?
(a)
Which rock is harder?
(b)
Does the harder rock consist of grains or crystals?
(c)
Does the softer rock consist of grains or crystals?
Section 2 (a): Introduction
3.
Why do different rocks have different effects on the shape of the landscape?
4.
Name the three different types of rocks.
Section 2 (b) (i): Igneous Rocks
5.
How are igneous rocks formed?
6.
Why are igneous rocks difficult to break?
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Workcard 1 and a microscope.
Do the experiment or VIDEO.
7.
(a)
Make drawings of the crystals which have been produced by rapid and
by slow cooling.
(b)
Use these words to complete the sentences:
slowly; quickly; long; grow; small; large
A rapidly cooled liquid freezes
. Its crystals are
because they have not had time to
to large sizes. A slowly cooled liquid freezes
. Its crystals are
because they have
had a
time to grow.
8.
Some igneous rocks are made up of large crystals. They are described as
coarse-grained. Rocks made up of small crystals are fine-grained.
(i)
At what rate will coarse-grained igneous rocks have cooled?
(ii)
At what rate will fine-grained igneous rocks have cooled?
(iii) At what rate will medium-grained igneous rocks have cooled?
9.
Intrusive igneous rocks form from magma which solidified before it reached
the Earth’s surface. Extrusive igneous rocks form from magma which has
run as lava onto the Earth’s surface.
(i)
Use these words to complete the sentence:
fine-; medium-; coarse-; insulated; extrusive; quickly.
Instrusive igneous rocks are
by the rocks around them.
This allows them to cool fairly slowly so they
are usually
or
grained.
igneous rocks are not insulated by other rocks. They
cool
so they are usually
grained.
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10.
Copy Fig Q10 and place the following in the correct places:
intrusive; extrusive; fine-; medium-; coarse-; magma; quickly; slowly; density.
Fig Q10
. The rock is
igneous rock cooled
grained.
Intrusive igneous rock cooled fairly
slowly. The rock is
grained.
igneous rock cooled
at depth. The rock is
grained.
The liquid rock rises because it has a
relatively low
.
Deep within the Earth, rocks partly melt
to form liquid
.
Section 2 (b) (ii)
11.
Why does gabbro have bigger crystals than basalt?
12.
Why do rock climbers in Skye get sore fingertips?
13.
Why is granite paler in colour than gabbro?
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14.
Look at Fig Q14
Draw a sketch and annotate it to show the paler Red Cuillins and the darker
Black Cuillins. Also indicate the two rock types.
Fig Q14: Cuillin Hills, Skye
Checkpoint 1
Answers
1–13
Answer
14
Section 2 (c): Sedimentary Rocks
15.
Name three sedimentary rocks formed from fragments.
16.
Explain how these rocks are changed from loose sediment into solid rock.
17.
Name a rock made from the remains of land plants.
18.
Name a rock made from fossil shells.
19.
In what type of climate will calcite be deposited from sea water to form
limestone?
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20.
What are fossils?
21.
Give two examples of trace fossils.
22.
Why are sedimentary rocks such as sandstone softer than igneous rocks?
Screwtop jar, soil, salt
Add soil to a screwtop jar until it is about one third full. Add a little salt then
pour in water until the jar is nearly full. Put the lid on tightly. Shake the jar
very hard for about a minute then lay it on the bench. When the soil has
settled draw what you see.
23.
What happens to the sizes of the particles from the bottom towards the top?
24.
Can you see some evidence of layers in the settled soil?
25.
What name is given to layering in sedimentary rocks?
Checkpoint 2
Answers
15–25
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Pieces of damp, dried and fired clay.
26.
(a)
Draw a table with three columns, headed ‘damp clay’, ‘dried clay’, ‘fired
clay’.
Use these words and phrases to complete the table. You may use any
description more than once.
hard; brittle; flexible; soft; inflexible; tough; strong; weak; plastic;
becomes plastic when water added; non-plastic; remains non-plastic
when water added
(b)
Which type of clay resembles sediment?
(c)
Which type of clay resembles sedimentary rock?
Workcard 2
Do the experiments or VIDEO.
27.
For Experiment 1 draw the layers and veins before and after squeezing.
28.
For Experiment 2 draw the positions of the rice grains before and after
squeezing.
Section 2 (d): Metamorphic rocks
29.
Refer back to 26. Which type of clay resembles a metamorphic rock?
30.
During metamorphism, what happens to the minerals in a rock?
31.
What changes rocks during thermal metamorphism?
32.
Describe what happens during regional metamorphism.
33.
In which type of metamorphism do rocks become foliated?
34.
Why is gneiss coarser-grained than schist and slate?
35.
Why are fossils only found very rarely in igneous and metamorphic rocks?
36.
Are metamorphic rocks harder or softer than sedimentary rocks? Give a
reason for your answer.
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37.
Draw a table with three columns headed ‘igneous rocks’, ‘sedimentary rocks’,
‘metamorphic rocks’. Write the names of these rocks in the correct columns:
sandstone; gabbro; slate; gneiss; chalk; basalt; granite; coal; mudstone; schist;
conglomerate; limestone.
Section 2 (e): Rocks and Relief
38.
Study Fig Q38 which shows the general distribution of the three main rock
types in Britain, and an atlas map showing relief. Which rock type is
associated with:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
the main mass of the Scottish Highlands?
the lowlands of Eastern England?
the highest ground in Southern England (Dartmoor)?
the English Lake District?
Snowdonia in North Wales?
the lowlands in the centre of Ireland?
the Wicklow Mountains south of Dublin?
Fig Q38
Checkpoint 3
Answers
26, 29–38
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Answers
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Extension exercises
E1.
Look at Fig QE1
Match the rocks below to the correct numbers
slate; sandstone; mudstone; schist; conglomerate; limestone; basalt; gabbro.
Fig QE1
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E2.
Find an atlas containing a more detailed Geology map of the UK than Fig
Q38. Compare it to a Relief map. Which rock types are associated with:
The Cotswold Hills?
The Yorkshire Wolds?
The Lowlands of East Anglia?
The Peak District, east of Manchester?
Your answer should confirm that there is great variation in the resistance of
sedimentary rocks.
Checkpoint 4
Answers
E1, E2
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Workcard 1
The effect of cooling rate on crystal size
WEAR YOUR SAFETY GOGGLES
1.
Melt some urea on two glass slides.
2.
Lay one slide on a cold surface and place a cold slide on top of the liquid
drop.
3.
Place a hot slide on top of the other slide. Lay these slides on a cloth to allow
them to cool slowly.
4.
When the crystals have formed, examine them under a microscope.
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Workcard 2
Effects of squeezing
Experiment 1
1.
Strongly squeeze some layers and veins of Plasticine.
WEAR YOUR SAFETY GOGGLES FOR ALL EXPERIMENTS
2.
Watch what happens to the layers and veins.
Experiment 2
1.
Embed some rice grains in a random pattern in one face of a block of
Plasticine.
2.
Strongly squeeze the Plasticine.
3.
Watch what happens to the arrangements of the grains.
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