Download 8.5 Earth Science

8.5 Earth science
8.5.1 Igneous rocks
Igneous comes from the Latin word igneous word meaning fire. These rocks have
crystallised from molten lava or magma. Igneous rocks are made of interlocking
crystals of different chemical composition.
The size of crystals.
Practical- Salol can be used as “artificial” magma or lava. A test tube of salol can
be easily melted with a water bath. Once taken out it will cool and crystallises at
41.5 ºC.
Use a dropper and put a small amount of molten salol on to a warm slide. Use a hand
lens and watch as it crystallises.
Repeat this with some new molten salol and put a small amount on to a cold slide.
Use a hand lens to compare the size of crystals.
Q1. Which slide contained the bigger crystals?
_____________________________________________________________
Q2. Which slide contained the smaller crystals?
_______________________________________________________
Q3. What affects the size of the crystals that are formed?
_______________________________________________________
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Igneous rocks that cool quickly have small crystals. These are likely to form from
volcanic eruptions into the air or volcanic eruptions in the sea.
A common example of an igneous rock
with small crystals is basalt. This is
said to be an example of a rock formed
extrusively.
If magma gets trapped underground it
will cool very slowly. This will form an
igneous rock with large crystals. An
example of a rock with large crystals is
granite.
Magma can be forced into rocks and this forms sills or dykes. A dyke cuts across
all the rocks, where as a sill exploits a plane of weakness and usually runs
horizontally. In the North-East, the great Whin sill can be seen in many places
such as Teesdale.
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8.5.2 Weathering
Weathering is the breaking down of a rock into smaller pieces. There are three
main types of weathering:
Physical weathering
Freeze–thaw weathering
This happens in areas where day
temperatures are above 0 °C but night
time temperatures drop below 0 °C.
Ice takes up 9% more volume than the
equivalent amount of water. Therefore as
water freezes it expands. Natural cracks
and fissures allow water to seep in and if
it freezes it expands can make these
gaps larger. This water can melt and
refreeze many times over a winter and shatter rocks into smaller pieces.
Onion skin weathering
This occurs in areas with high day time temperatures and low night time
temperatures, typically deserts. The sun’s energy leads to the rock expanding but
rapid cooling at night leads to a contraction. As this is repeated many times the
outer layer can peel off just like in an onion!
Biological weathering
This is where living organisms break down
rocks into smaller pieces. Trees and their
roots are probably the most common example.
Tree roots are incredibly strong and have
been known to damage house foundations. In
nature, they can force rocks apart. Animals
can also weather rocks, with rabbits
burrowing being a good example.
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Chemical weathering.
Rainwater is naturally acidic and will slowly react with carbonate rocks. Limestone
is calcium carbonate and over time is weathered by rain water.
A reaction that can be simplified as:
Calcium carbonate + sulphuric acid
water
calcium sulphate + carbon dioxide +
This process will weather the rock and can often form spectacular landscapes:
This process will also weather limestone and marble statues and buildings:
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8.5.3 Sedimentary rocks
Sedimentary rocks usually form from weathered pieces of rock that have been
transported. This can be done by rivers, glaciers, wind or the sea. When sediments
have stopped being transported they are deposited. Over time they are compacted
and sometimes cemented together. It can take millions of years to turn sediments
into a hard rock.
The type of sedimentary rock depends on the sediments it is made of. A rock made
of sand grains will form a sandstone, clay sediments form mudstones and gravel will
form conglomerates.
Mudstone
Sandstone
Conglomerate
Sometimes sedimentary rocks contain fossils. These are the remains of living
creatures that have been washed in to the rocks and then compacted. Fossils are
important as they can date the rock and can tell you about the environment that
the rock formed.
An Ammonite found in rocks
near Whitby. This creature
lived 150 million years ago in
the Jurassic age.
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Common fossils
Link the fossil pictures with the matching names.
Ammonite
Bivalve
Graptolite
Echinoderm
Trilobite
Mammoth tooth
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8.5.4 Metamorphic rocks
These are igneous or sedimentary rocks that have been changed (but not melted)
by heat and/or pressure. High temperatures and/or pressure cause minerals in the
rock to recrystallise.
Practical: 6 spatulas of clay, 2 spatulas of sand, 2 spatulas of talc, ½ spatula of
sodium chloride and ½ spatula of alum are placed into a vessel.
These are mixed and a few drops of water are added. Keep mixing until a dough
consistency is achieved. This is then moulded into a ball shape. This ball can then
be placed into a crucible and heated with a roaring flame for approximately 5
minutes. A metamorphic rock should be the outcome!
Equipment: clay, talc, alum (hydrated aluminium potassium sulphate), sodium
chloride, sand. Bunsen burners, crucibles, pipe clay triangles, bench mats,
spatulas.
The type of metamorphic rock depends on the starting rock:
Starting rock
Mudstone
Sandstone
Limestone
Basalt
Granite
Resulting metamorphic rock
Slate
Quartzite
Marble
Schist
Gneiss
Metamorphic rocks can form near lava
flows or igneous intrusions. The intense
heat changes the rock that is in contact
with the lava flow. This type of
metamorphism
is
called
contact
metamorphism.
Metamorphic rocks can also form where
there are rocks that are under intense
pressure. This usually happens near
plate boundaries where plates are being forced together. This type of
metamorphism is called regional metamorphism.
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TOPIC SUMMARY (rocks)
____________ rocks are formed as molten __________ or ____________
crystallises. If cooling is rapid, the size of the crystals will be __________ than
if cooling takes place over a long period of time. ____________ is a typical
igneous rock.
Rock formations are broken down by the process of __________; there are three
main types - ______________, ______________ and chemical.
Physical weathering is associated with fluctuations in ____________ . If water
collects in cracks in rocks, it will _________ if the temperature drops below 0 °C
and as it ________ it will cause the gaps to become bigger. This is known as
_____________ weathering.
If a rock is heated by the sun during the day, it will _________but as it rapidly
cools at night it will _________. This causes tiny cracks to form in the outer
layer, and after prolonged action of this kind the layer will start to peel away – this
is known as ___________ _________ weathering.
Biological weathering is where rocks are broken down by the action of living
organisms such as _________ or animal __________.
Rainwater is slightly _________, and this will attack limestone and other
_________ rocks, including statues and buildings of which they are made. This is
the most common example of ____________ weathering.
Sedimentary rocks are the result of weathering processes on other rocks which
have been ____________, deposited and then ____________ over millions of
years. These rocks often contain ____________, which are able to tell us about
the age of the rock and the environment in which it was formed. ____________ is
a typical sedimentary rock.
Metamorphic rocks are the result of extreme conditions of ________ and/or
___________ on existing igneous or sedimentary rocks. ______________
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metamorphism occurs when these rocks are changed by the intense heat of lava
flows or bodies of magma; ___________ metamorphism is the result of intense
pressure, often close to plate boundaries and other tectonic activity.
____________ is a typical metamorphic rock.
trees
biological
regional
slate
acidic
freeze
contact
heat
smaller
weathering
compacted
magma
contract
fossils
onion skin
pressure
carbonate
freeze-thaw
igneous
chemical
expand
physical
sandstone
temperature
lava
transported
granite
burrows
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8.5.5 Calcium Carbonate Chemistry
Calcium carbonate (CaCO3) is an important raw material that occurs naturally as
the sedimentary rocks limestone and chalk, and as the metamorphic rock, marble.
These rocks are obtained by quarrying.
Malham cove (limestone)
Dover cliffs (chalk)
Marble quarry
Calcium carbonate (i.e. limestone or marble) undergoes some interesting
chemical reactions.
1. Thermally decomposing it produces calcium oxide (CaO).
2. Adding water to calcium oxide produces calcium hydroxide, Ca(OH)2.
3. Treating calcium hydroxide with carbon dioxide regenerates calcium
carbonate.
Thermal Decomposition
When some substances are heated, they decompose (break down) into simpler
substances. This process is called THERMAL DECOMPOSITION. Calcium
carbonate undergoes thermal decomposition when heated in a kiln to about 1100 oC,
forming calcium oxide (quicklime) and carbon dioxide.
CaCO3 (s)
→ CaO (s) +
CO2 (g)
Other metal carbonates decompose in a similar way. The lower the metal is in the
reactivity series, the lower the temperature at which this decomposition takes
place.
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Hydration (reaction with water)
Calcium oxide, CaO (sometimes called quicklime) reacts with water in a very
exothermic reaction, forming calcium hydroxide, Ca(OH)2 (sometimes called slaked
lime).
CaO (s) + H2O
(l)
→ Ca(OH)2 (aq)
Quicklime
Slaked lime
Limewater reaction with carbon dioxide
Calcium hydroxide solution is known as limewater. When carbon dioxide is bubbled
through calcium carbonate is regenerated. As calcium carbonate is insoluble in
water, when it is made in water it appears as a white precipitate.
Ca(OH)2 (aq) +
CO2(g)
→
CaCO3(s) +
H2O(l)
Because calcium carbonate is regenerated in the third reaction we call this series
of reactions the limestone cycle.
A summary of the limestone cycle
+ CO2
Calcium hydroxide
(slaked lime)
Ca(OH)2
Calcium carbonate
(limestone)
CaCO3
+ H2 O
You need to learn this cycle.
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Heat
Calcium oxide
(quicklime)
CaO
The word and symbol equations that describe these reactions are as follows:
1. Word:
Symbol:
Calcium carbonate
CaCO3
2. Word: Calcium oxide
Symbol:
CaO
→
→
+ water
+ H2O
Calcium oxide + carbon dioxide
CaO
+
CO2
→
→
Calcium hydroxide
Ca(OH)2
3. Word: Calcium hydroxide + carbon dioxide → Calcium carbonate + water
Symbol:
Ca(OH)2
+
CO2
→
CaCO3
+ H2 O
You will have investigated these reactions of calcium carbonate in an experiment
looking at the effect of heat on a marble chip.
9.5.6 Uses of calcium carbonate & calcium hydroxide
1. Building material
Limestone is used as a building material. A form of limestone, called Portland stone,
was used in the construction of St Paul’s Cathedral in London. Limestone is a hard
rock, but it weathers badly in polluted urban environments, because it dissolves
readily in acids. Marble is used in sculpture and as a building material.
St. Paul’s Cathedral
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marble bust
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2. Removal of acidity
Powdered limestone is used to neutralise acidity in lakes and in light, sandy soils.
Slaked lime (calcium hydroxide) is used to neutralise acidity in heavy soils and in
drinking water.
3. Cement manufacture
Cement is made by roasting
 powdered limestone with
 powdered clay in a rotary kiln.
The hard, stone-like building material, concrete is made by mixing:
 cement
 water
 sand
 crushed rock
The soft mixture undergoes a slow chemical reaction and eventually sets hard.
4. Glass manufacture
Glass is made by heating a mixture of:
 limestone
 sand
 sodium carbonate, Na2CO3
5. Iron production
Limestone is added to the Blast Furnace, in the production of iron, to remove acidic
impurities present in iron ore.
Firstly, the heat in the Blast Furnace causes CaCO3 to thermally decompose to CaO
CaCO3 → CaO + CO2
Then, CaO reacts with the silicon dioxide (SiO2) impurity in the iron ore to form
calcium silicate (CaSiO3), which forms a molten crust on top of the iron, called slag.
CaO + SiO2 → CaSiO3
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

NEUTRALISATION


GLASS
heavy soils
drinking water
SLAKED
LIME
lakes
light soils
add water
QUICKLIME
heat + sand &
sodium carbonate
heat
LIMESTONE
CHALK
MARBLE
CALCIUM
CARBONATE
heat + powdered clay
BUILDING
removal of impurities
CEMENT
IRON & STEEL
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CONCRETE
mix with water,
sand, crushed
rock
Topic Summary (calcium carbonate)
Calcium carbonate (formula = _______ ) is found in the sedimentary rocks
_____________________ and __________________ and also in the metamorphic
rock ___________________.
It undergoes several reactions that allow it to provide us with useful products for the
______________________ industry.
When _________________ strongly it breaks downs to calcium __________________
and ___________________________ gas. Another name for calcium oxide is
______________.
This is an example of _____________ ______________________, Other metal
carbonates can also undergo this type of process. The more reactive the metal in the
metal carbonate the ____________ the temperature needed to cause decomposition.
The calcium oxide can then react with ____________ to form
_____________________ hydroxide ( formula _______ ). This is also known as
______________ __________. When it is dissolved in water it is used in labs as
_____________________ to test for ________.
When carbon dioxide is bubbled into the solution it turns cloudy (ie makes a
_________ precipitate. This solid is actually _______________ ______________
which is insoluble.
Because we have regenerated limestone after these 3 steps, they are known as the
________________ ____________.
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Limestone is used as building stone, in farming to ______________ soil acidity, in the
Blast Furnace to form the waste substance __________ and in __________
manufacture by heating with sand and sodium carbonate. It is also the basis of the ____
_____ industry.
quicklime
thermal decomposition
carbon dioxide
marble
calcium
Ca(OH)2
heated
limestone
CaCO3
building
glass
calcium carbonate
neutralise
TOPIC 8.5 EARTH SCIENCE
oxide
cement
chalk
water
slaked lime
Limestone Cycle
16
white
limewater
CO2
slag
higher