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10
THE CHANGING EARTH
The Earth’s surface is constantly changing. Volcanoes and earthquakes
can cause quick changes, but most of the changes to the Earth’s
surface happen slowly. Rocks on and below the surface of
the Earth are slowly and constantly being changed by natural
events. Rocks also provide a valuable record of past events.
This aquatic reptile died over 300 million
years ago. Fossils and the rocks in which
they are found provide a valuable record
of the past.
KEY
QUESTIONS
Where does soil come from?
Which rocks are formed from the remains of living things?
What do butterflies, frogs, Mr Hyde, werewolves and metamorphic
rocks have in common?
How do we know what living things that have not existed for
millions of years looked like, how they walked and what
they ate?
How can whole bodies of animals be fully preserved for millions
of years?
What can you learn from a dinosaur footprint?
What were the very first living things on planet Earth?
How old is the Earth, and how do we know this?
Explain the methods of
formation and uses of common
types of rocks.
Describe how rocks are dated.
Describe how living things
have changed over relative
geological time, using evidence
including fossils.
Explain the difference between
weathering and erosion.
Provide some theories to
explain the extinction of the
dinosaurs.
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Thinking about
The changing Earth
1. Going to extremes
(a) The weather is an important topic in our lives. We check the
weather before we go on holidays, make weekend plans,
plant crops, go sailing, and in many other situations. But what effect
does our weather have on the surface of our planet? Extreme weather
conditions and movements within the Earth cause changes.
The first column of the table below lists some weather and geological
events that shape the Earth. Copy this table into your workbook. Divide
the events among the people in your group and, working alone,
complete the other columns of the table for your chosen events (the first
one has been done for you). Come together and share these ideas to
make a table that includes the work of the whole group.
GROUP
WORK
Conditions
produced
Event
cyclone
• storms with high
winds, high
tides
Possible effect on
the Earth’s surface
• flooding
• erosion
• damage to
buildings and
animals
What can we do to
minimise this effect?
• build above flood level
• protect animals by
bringing them under
shelter
• build cycloneprotected buildings
with shutters on
windows.
tornado
sea storm
earthquake
volcanic
eruption
drought
flood
mud slide
tsunami
(b) What do you think the word ‘weathering’ means?
2. Rocks, rocks and more rocks
(a) ‘Igneous’, ‘sedimentary’ and ‘metamorphic’ are words used to describe
rock types. Each word offers a clue about how those rocks were formed.
What do you think these words mean?
(b) Each rock type has its own name as well as being classed as igneous,
sedimentary or metamorphic — similar to having a first name and a
family name. For example, granite is an igneous rock. Write down as
many names of rocks as you can. If you know whether they are
igneous, sedimentary or metamorphic, add this information.
(c) How do you think rocks are formed?
(d) Without looking them up, write
your meaning for the words
below:
• erosion
• sediment
• deposition • soil
• humus
• fossil.
3. Prehistoric fascination
Dinosaurs fascinate both
adults and children. Even
very young children know the long,
complicated names of dinosaurs as
if these words were commonplace.
(a) Make a group mind map to
show what you already know
about dinosaurs. The following
headings will help you
remember what you already
know.
GROUP
WORK
Types of dinosaur
Dinosaur food
How dinosaurs became
extinct
When dinosaurs lived
Dinosaur sizes
What dinosaurs tell us
about our past
Other information about
dinosaurs.
Use these headings to make
lists, then decide how you want
to display your information on a
mind map. Once you have
finished, share your mind map
with other groups in the class.
You can add extra information to
your mind map at this stage.
(b) This activity should help you to
see where there are gaps in
your knowledge. In your
workbook, make a list of the
areas where you need to learn
more. After you have completed
this topic, you will be able to fill
many of these gaps. Refer to
your mind map at least once a
week to either add information
or to change ideas.
THE CHANGING EARTH
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1
10.1
0.1
Weathering and erosion
Rocks on the surface of the Earth
are slowly and continuously being
changed, physically and chemically, by natural events. They are
broken down into smaller rocks in
a process called weathering.
The action of the sea breaks
up coastal rock, often leaving
spectacular features such as the
Twelve Apostles at Port Campbell National Park, Victoria.
The wind wears rock away,
especially in dry conditions
when it blasts the rock with sand
and soil it has picked up.
Acid rain can form if there is
a lot of pollution in the atmosphere. It can react with chemicals in rocks, making them crack
and crumble more easily. Water
on the ground can react with certain chemicals in rocks, soil and
decaying plants, producing acids
and bases that speed up the
weathering of rocks.
Weathering
doesn’t
just
change rocks. It changes buildings, roads, cars and statues.
The Twelve Apostles, on the coast of southern Victoria
10.1
CHEMICAL
W E AT H E R I N G
YOU WILL NEED
Wave Rock, Western Australia, a spectacular example of weathering by the wind
piece of limestone
distilled water
dilute hydrochloric acid
two dropping pipettes
• Place a drop of distilled water
on the piece of limestone.
• Place a drop of dilute
hydrochloric acid on a different
part of the piece of limestone.
1. Does the distilled water have
any observable effect on the
limestone?
2. What is the effect of the dilute
acid on the limestone?
228
CORE SCIENCE 2
Tree roots widen the cracks in rocks.
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River sediments
Carried away
Weathered rock is usually moved
from one place to another by the
wind, running water, the sea or
glaciers. This process is called
erosion. The weathered rock
moved by erosion is deposited
and settles on the land, river
beds and floors of lakes, seas and
oceans to form sediments.
Deposits of dead plants and
animal remains are also called
sediments.
Soil is formed by weathering
and erosion and deposition of
rock. Soil also contains humus
— decaying plant and animal
material that provides nutrients
for plants to grow in it.
A fast-moving river is likely to
carry with it sand, gravel and
smaller particles. As it slows
down on its path to the sea, it
loses energy and particles are
deposited, forming sediments.
The larger particles, such as
gravel and sand, settle first. By
the time the river reaches the sea,
it is usually travelling so slowly
that the very fine mud particles
begin to settle.
10.2
S E D I M E N T S A N D WAT E R
YOU WILL NEED
mixture of garden soil, gravel, sand and clay
large jar with lid
watch or clock
1. Before commencing this experiment, form your own hypothesis about
the order in which the different types of particles in the mixture will
settle. Give reasons for your hypothesis.
• Place enough of a mixture of garden soil, gravel, sand and clay in a
large jar to quarter fill it.
• Add enough water to three-quarters fill the jar and place the lid on
firmly. Shake the jar vigorously.
• Put the jar down and watch carefully as particles begin to settle.
Note the time taken for each layer of sediment to settle
completely.
2. Which type of sediment settled first?
3. Where are the other particles of sediment while the first layers are
settling?
4. Draw a labelled diagram showing clearly any layers that form. Identify
the layers if you can.
• Leave the jar for a day or two. Then compare your observations of the
jar with your diagram.
5.
6.
7.
8.
Which sediments settled after the diagram was drawn?
Why have the last sediments taken so long to settle?
Was your hypothesis supported by your observations?
What is the relationship between the size of sediment particles and the
time taken to settle?
Activities
Remember
1. What is weathering?
2. List five causes of weathering.
3. Distinguish between erosion
and weathering.
4. What is the difference between
a soil and a sediment?
5. As a flooded river slows down,
which particles are likely to settle
first — gravel, sand or fine clay?
Think
1. The Sphinx and the Great
Pyramids of Egypt have stood
for thousands of years, yet
weathering has affected them
more during the past fifty years
than in all of the time since they
were built. What has caused the
weathering process to speed up?
2. Acid rain is a serious problem
in industrial areas where there
is a lot of air pollution.
However, rain reaching the
ground after falling through
clean air is also slightly acidic.
How could this be? (Hint: Blow
through a straw into water
containing a few drops of
universal indicator.)
3. Describe some evidence of the
weathering of buildings, cars
and roads.
4. How would the sediment at the
bottom of a still lake compare
with the sediment on the banks
of a mountain stream?
5. In alpine regions, rocks can be
shattered by frozen water.
Explain how this can happen.
6. What type of sediment would
you expect to find on the bed of
the Yarra River in Melbourne, the
Hawkesbury–Nepean River or
any river in your local district?
Imagine
How would weathering and
erosion on the moon differ from
weathering and erosion on Earth?
How long would you expect a
footprint to remain on the surface
of the moon?
Investigate
Design and carry out an
investigation to show which types
of paint offer the best protection
from weathering.
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1
10.2
0.2
Sedimentary rocks
Rocks that are formed from the particles of sediments are called sedimentary rocks. Most sedimentary rocks are formed from weathered rock
which has been eroded and then deposited. Grains
of sediment are cemented together to form a solid
rock. The process is shown in the diagram below.
Sediments are laid down by
ice, wind or water, in
horizontal layers called beds
or strata.
Rocks from living
things
Limestone is a sedimentary rock that is formed
from deposits of the remains of sea organisms
such as shellfish and corals. The hard parts of
these dead animals contain calcium carbonate.
These deposits are cemented together over a
period of time in very much the same way as
sedimentary rocks form from weathered rock.
Within each bed, the
sediment grains are squashed
together so that they are in
close contact.
Water seeps in between the
grains, bringing with it many
dissolved chemicals.
When the water evaporates,
these chemicals are left behind
as crystals around the edges
of the grains. These crystals
cement the grains of sediment
together to form rock.
Many sedimentary rocks form in this way.
This limestone, rich in corals and shells, is many metres above
sea level. How did it get there?
Sandstone is formed from grains of sand that
have been cemented together over a period of
time. Mudstone and shale are formed from finer
grains of sediment deposited by calm water in the
form of mud. Siltstone has grains slightly larger
than those of mudstone. Conglomerate contains
grains of different sizes
which have been
cemented
together.
Coal is formed from the remains of dead plants
which are buried by other sediments. In dense
forests, layers of dead trees and other plants build
up on the forest floor. If these layers are covered
with water before rotting is completed, they can
become covered with other sediments. The weight
of the sediments above compacts the partially
decayed plant material. Over millions of years the
compacting increases the temperature of the sediment and squeezes out the water, forming coal.
Rocks from chemicals
Conglomerate is formed from sediments that might be deposited
by a fast-flowing or flooded river.
230
CORE SCIENCE 2
Some sedimentary rocks form when water evaporates from a substance, leaving a layer that is
compressed after being buried by other sediments. Rock salt is an example of a rock formed
in this way. It forms from residues of salt that
remain after the evaporation of water from salt
lakes or dried-up seabeds.
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Rocks in
layers
Layers of sedimentary rock are
often clearly visible in road cuttings and the faces of cliffs. The
limestone in the photograph
opposite was formed on the
ocean floor. Layers of sediments
and sedimentary rocks can be
pushed upwards by the same
forces below the Earth’s surface
that cause mountains to be
formed. Those forces can also
bend and tilt the layers.
Layers of sedimentary rock can be pushed
upwards, bent and tilted by forces
beneath the Earth’s surface.
Using
sedimentary
rocks
Sandstone and limestone are
often used as external walls of
buildings. These sedimentary
rocks are well suited to carving
into bricks of any shape. Shale
can be broken up and crushed to
make bricks.
Limestone is broken up to produce a chemical called lime (calcium hydroxide). Lime is used to
make mortar, cement and plaster
and in the treatment of sewage.
Garden lime is also made from
calcium carbonate and is used to
neutralise acid in soil.
Coal is used as a fuel. It is
burnt in electric power stations
to boil water. The steam is used
to drive the turbines that produce
electricity. In some countries coal
is burnt in home heaters.
The chalk used to write on
chalkboards is like limestone,
but it is not as hard as most
limestone. Chalk is formed
from very fine grains of
calcium carbonate that have
separated from sea water and
settled to become a white,
muddy sediment on the sea
floor. The sediment hardens
over time to form chalk. This
process takes millions of
years. The remains of shellfish
and other sea animals are
also found in the sediment
that forms chalk, but most
of these remains are
microscopic.
The white cliffs of Dover that
overlook the English Channel
are composed of chalk.
Activities
Remember
1. Explain how sedimentary
rocks are formed.
2. Compare several types of
sedimentary rocks and use
diagrams to show how they
were formed.
3. Sedimentary rocks that have
formed from the erosion of
weathered rock cannot be
identified by their colour.
What feature allows you to
identify them?
4. Explain how a rock can be
formed from the remains of
animals.
5. How is coal formed?
Think
1. Why are sedimentary rocks
found in layers? Where are
the oldest layers?
2. A road cutting reveals a
layer of sandstone beneath a
layer of mudstone. Between
them is a much thinner layer
of conglomerate.
(a) Describe possible
conditions for the
formation of each layer.
(b) Which conditions
appeared first? Explain.
(c) If the layers were flat and
horizontal, what else
could you say about this
area?
(d) Which layer was formed
most recently?
3. Why are limestone and coal
sometimes referred to as
biological rocks?
4. Limestone is mostly formed
on the ocean floor. Why is
the Nullarbor Plain riddled
with limestone caves?
5. In which type of sedimentary
rock would you be most
likely to find embedded
seashells?
Investigate
What do peat, brown coal
and black coal have in common?
How are they different from
each other?
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1
10.3
0.3
Metamorphic rocks
Igneous rocks are those that
have formed from molten rock.
Molten rock is called magma
when it is below the surface
and lava when it is above the
surface. Some igneous rocks,
like the granite shown in the
photograph, form slowly below
the Earth’s surface. The slowly
cooling magma produces large
crystals which can be easily
seen and felt.
Other igneous rocks, such as
basalt, form above the surface
when lava flowing from volcanoes cools quickly. The crystals resulting from the rapid
cooling are very small.
by about 25°C for every kilometre below the surface. This
heat and pressure can change
the composition and appearance
of the minerals in rocks.
The process of change in the
rocks is called metamorphism.
Rocks formed by these changes
are called metamorphic rocks.
The name for these changed rocks
comes from the Greek words
meta, meaning ‘after’ or ‘changed’
and morphe, meaning ‘form’.
The changes that take place
during the formation of metamorphic rocks depend on:
• the type of original rock
• the amount of heat to which
the original rock is exposed
• the amount of pressure from
the weight of the rocks above
• how quickly the changes take
place.
Metamorphic rocks that are
mainly the result of great pressure can often be identified by
bands or flat, leaf-like layers.
These bands are evident in the
sample of gneiss (pronounced
‘nice’) pictured below. Rocks
can be changed by the high
temperatures that result from
contact with hot magma, as
shown in the diagram below.
Uses of metamorphic rock
The nature of metamorphic rocks above and below the ground can
provide clues about the history of an area. Think why the presence
of quartzite or marble high in a mountain range would suggest that
the area was once below the sea.
The presence of slate might suggest that the area was once the
floor of a still lake or river mouth. The sediments were probably
buried under many other sediments, and cemented together to
form shale. The shale was transformed, or metamorphosed, into
slate as a result of new rock formed above it.
Marble is used in statues and the walls and floors of buildings
(inside and outside). It is usually highly polished.
Slate is used in buildings, particularly in roofing and floor tiles.
Granite cools from molten rock below the
Earth’s surface. Its crystals are large.
Sedimentary rocks are those
that have formed from deposits
of weathered rock or the
remains of living things.
Igneous and sedimentary
rocks deep below the Earth’s surface are buried under the huge
weight of the rocks, sediments
and soil above them. They are
also subjected to high temperatures. The temperature increases
232
CORE SCIENCE 2
metamorphic
rock
layers of
sedimentary
rock
hot
magma
Gneiss is formed mainly as a result of
great pressure on granite.
The formation of metamorphic rock by
contact with hot magma
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The rock cycle
Here are some common
examples of the formation of
metamorphic rocks:
Shale
(sedimentary)
Granite
(igneous)
Sandstone
(sedimentary)
Limestone
(sedimentary)
mainly pressure
⇒
⇒
⇒
⇒
Slate
mainly pressure
Gneiss
mainly heat
Quartzite
mainly heat
Marble
Rocks are not the only things
that undergo metamorphism.
Many animals change form so
much that their appearance
changes completely. A
caterpillar transforms into a
butterfly; a tadpole into a
frog. There are many well
known fictional examples of
human metamorphism. In
Robert Louis Stevenson’s
novel The Strange Case of
Dr Jekyll and Mr Hyde, the
mild-mannered Dr Jekyll
transforms into the ugly, evil
Mr Hyde. Can you think of
some other fictional
examples of metamorphism?
10.3
R O C K S — T H E N E W G E N E R AT I O N
YOU WILL NEED
labelled samples of granite, gneiss, limestone, marble, sandstone,
quartzite, shale and slate
hand lens
• Try to sort the rocks into pairs of ‘parent’ rock and corresponding
metamorphic rock. Use the examples on the left if you have trouble
pairing the rocks.
1. Why is the term ‘parent’ rock used?
• Examine each pair of rocks with a hand lens.
• Copy and complete the table below by noting the similarities and
differences between the ‘parent’ and metamorphic rock of each pair.
2. Use the last column of your table to suggest whether the main cause of
metamorphism was heat or pressure.
Comparing ‘parent’ and metamorphic rocks
‘Parent’
rock
Metamorphic
rock
Similarities
Differences
Main cause of
metamorphism
shale
gneiss
sandstone
marble
Activities
Remember
1. Rocks are classified into three
groups. Metamorphic rocks
make up one of these. What
are the names of the two
groups of rock from which
metamorphic rocks are formed?
2. What can cause rocks to
change form and become
metamorphic rocks?
3. How does gneiss differ from
granite?
4. Explain how granite is
transformed into gneiss.
5. How is sandstone transformed
into quartzite?
Think
Metamorphism is a change in form.
It occurs in animals as well as rocks.
Rock types
1. If an igneous or sedimentary
rock gets so hot that it melts
completely, it does not become a
metamorphic rock. Explain why.
2. Why is limestone referred to as
the ‘parent’ rock of marble?
3. Metamorphic rocks are
generally formed deep below
the surface of the Earth.
However, they are often found
above the ground — even high
in mountain ranges. How can
this be so?
4. Explain why slate is commonly
used as floor tiles.
Investigate
Find out more about the uses of
marble and slate. Where are they
obtained? How are they used?
Why are they expensive?
Create
Devise a ‘buildings trail’ in your
city or town to locate buildings
made of different kinds of rock.
Draw a map to show the location
of the buildings and the type of
rock used in constructing and
decorating them.
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1
10.4
0.4
What’s in a rock?
If only rocks could talk! They
would have so much to say.
They would tell us about the
Earth’s history — about prehistoric creatures whose fossils lie
within them, about explosive
volcanoes, earthquakes,
about flooded rivers
that washed them
away and about
what it is like
inside the
Earth.
Cool shapes
The atoms which join together to
form minerals make up regular
geometric shapes in particles
called crystals. The way in
which crystals grow when a mineral is formed depends on the
speed of the cooling process of
the molten material from which
they form and how much space
is available. The quartz crystals
shown below have cooled
slowly and have had a lot of time
and space to grow. Quartz, one
of the most common minerals,
consists of hexagonal-shaped
crystals of silicon dioxide (SiO2).
If only rocks could talk!
Rocks are made up of substances called minerals. Any
naturally occurring solid substance with a definite chemical
composition is called a mineral.
Elements found naturally in
their uncombined form are also
minerals. These elements, called
native elements, include diamonds (pure carbon) and gold.
The chemical compounds and
native elements that make up
the Earth’s crust are minerals.
Most minerals in rocks are compounds of the elements oxygen
and silicon together with one or
more of the metal elements. The
colours, shapes and textures of
the minerals in rocks tell us
what they are made of, how
they were formed and also
provide clues about the past.
234
CORE SCIENCE 2
Quartz is one of the most common
minerals in the Earth’s crust.
Fast or slow?
The effect of cooling rate on
crystal size can be seen by comparing the crystals of two
common igneous rocks. Basalt
forms when volcanic lava cools
quickly on the ground or under
the ocean. There is not enough
time for large crystals to form.
Granite is formed when molten
rock cools more gradually under
the ground. The large crystals of
quartz, mica and feldspar of
which granite is made up can
be easily identified.
Identifying
minerals
Although colour might seem to
be the quickest way to identify
a mineral, it is not reliable.
Many different minerals have
similar colours. Some minerals,
even though they have the same
chemical composition, can have
different colours. Quartz, for
example, can be colourless like
glass, or may be pink, violet,
brown, black, yellow, white or
green. Other properties can be
used to identify minerals.
The lustre of a mineral
describes the way that light is
reflected from it. Minerals could
be described, for example, as
dull, pearly, waxy, silky, metallic,
glassy or brilliant.
The streak is the powdery
mark left by a mineral when it is
scraped across a hard surface like
an unglazed white ceramic tile.
The hardness of a mineral
can be determined by trying to
scratch one mineral with
another. The harder mineral
leaves a scratch on the softer
mineral. Friedrich Mohs’ scale of
hardness is a numbered list of
ten minerals ranked in order of
hardness. The higher numbers
correspond to harder minerals.
The hardness of a mineral is
determined by comparing it with
the minerals in Mohs’ scale. For
example, a mineral that can be
scratched by quartz but not by
orthoclase has a hardness
between 6 and 7.
The diagram at the top of the
opposite page shows that if the
minerals in Mohs’ scale are not
available, some more common
materials can be used to determine the hardness of a mineral.
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Moh’s scale
softest
Mohs’ scale of hardness
talc
1
gypsum
2
calcite
3
fluorite
4
apatite
5
orthoclase
6
quartz
7
topaz
8
corundum
9
diamond
10
Common materials
soft grey lead pencil point
fingernail
copper coin
iron nail
sandpaper
hardest
W H I C H M I N E R A L I S I T?
YOU WILL NEED
mineral kit
common materials to substitute for
unavailable Mohs’ scale minerals
magnifying glass
white ceramic tile
• Construct a table like the one
shown below and use it to
record your observations as you
work through the following steps
for each mineral.
• Describe the colour and lustre of
the mineral.
• Use the magnifying glass to look
closely at the mineral and
describe the shape and size of
its crystals.
• Scrape the mineral across the
unglazed side of a white
ceramic tile. Record the colour
of the streak.
• Use the Mohs’ scale minerals or
the common materials to
estimate the hardness of the
mineral by trying to scratch it.
An approximate range is
sufficiently accurate, for
example, 5–6.
1. Other than those already
described, what additional
properties of minerals could be
used to identify them?
2. If two unlabelled mineral
samples have the same colour
and lustre, can you be sure
that they are the same mineral?
Explain how you would find
out.
Properties of some minerals
Mineral
Colour
Lustre
Remember
1. What is a mineral?
2. What is a native element? List
two examples.
3. Which minerals are present in
granite?
4. List at least five properties that
you could observe to help you
identify an unknown mineral.
5. What is the approximate
hardness on Mohs’ scale (to
the nearest whole number) of
a mineral that can be
scratched by sandpaper but
not by an iron nail?
Think
A scale for testing the hardness of minerals
10.4
Activities
Crystal
shape
and size
Streak
Hardness
1. Explain the difference between
a rock and a mineral.
2. What does the size of the
crystals in a rock tell you
about the way the rock was
formed?
3. You have a sample of each of
two minerals but no other
equipment to test them for
hardness. How could you tell
which mineral was harder?
4. A mineral can be scratched by
a copper coin but not by a
fingernail. You know that the
mineral is one of quartz,
fluorite or calcite. Which
mineral is it?
5. Is table salt a mineral? Think
carefully about your answer
and suggest reasons for and
against classifying it as a
mineral.
Imagine
If only rocks could talk! Imagine
that you are a rock lying by the
side of a newly made roadside
cutting. Imagine that you were
magically given the ability to
walk and talk. Write a speech
that you could deliver to a class
of year 9 students about your life
— from your formation to the
present time.
Create
Find out how crystals can be
artificially grown and then grow
a crystal garden.
THE CHANGING EARTH
235
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10.5 CORE SCIENCE 2 Page 236 Tuesday, June 15, 2004 5:41 PM
1
10.5
0.5
Looking for clues
Over very long periods of time,
rivers change their course. Mountains form where seas once
existed. The climate changes. As
these changes take place, different layers of sediment can be
deposited at the same location.
Some layers will be thicker than
others.
Sedimentary
rocks,
formed by the different layers of
sediments, provide many clues
about the order in which events
took place. The older layers will
almost always be below the
younger layers. The layers of sedimentary rock in the photograph
forming a background to these
pages provide a record of the sediments deposited in the Hamersley
Ranges in Western Australia over
hundreds of millions of years.
From time to time there might
be sudden changes caused by
erupting volcanoes or earthquakes. Basalt capping at Mount
Wilson and bits of pumice on
Bondi Beach provide evidence of
volcanic activity nearby. The
Sydney Basin and Blue Mountains
are made of sandstone. How did
this sandstone, which formed
10.5
a small seashell
a small box (shoebox or milk carton)
fine sand
plaster of Paris
• Half-fill a box with fine, damp
sand.
• Make a clear imprint of a small
seashell in the sand.
• Mix some plaster of Paris and
pour it carefully into the imprint.
• Once the plaster has set, remove
the plaster cast carefully from
the sand.
CORE SCIENCE 2
Life in the past
The relative age of a rock
simply indicates whether it was
formed before or after another
rock. Fossils provide many
clues about the relative age of
rocks. By comparing fossils
found in rocks in different areas,
it is possible to compare the
ages of rocks throughout the
world. Fossils also provide a
way of finding out how living
things have changed over time.
Scientists who study fossils are
called palaeontologists.
A fossil is evidence of life in the
past. Evidence of the very oldest
living things is buried within the
deepest and oldest layers of rock.
M AKI NG A F O S S I L
YOU WILL NEED
236
beneath the sea, get raised up as
much as 1000 metres? Slow
movements caused by the forces
beneath the surface have tilted,
curved and pushed up the layers.
Weathering and erosion have
also exposed some layers that
were below the ground millions of
years ago. This explains why sea
shells are sometimes found many
metres above sea level.
You have two records of the
seashell — the mould or imprint
in the sand and the plaster cast.
1. Which parts of animals are most
likely to be preserved as casts?
2. Is the fossil of a fern leaf more
likely to be found as a cast or a
mould? Why?
3. Dinosaur fossils are found in
casts and moulds. What
evidence of dinosaurs is likely to
be found as a mould?
CAUTION: Do not put plaster
of Paris down the sink.
How fossils
form
The remains of most animals
and plants decay or are eaten
by other organisms, leaving no
trace behind. However, if the
remains are buried in sediments
before they disappear they can
be preserved, or fossilised. Fossils can take several forms.
• The hard parts of plants and
animals are more likely to be
preserved than the softer
parts. Wood, shells, bones and
teeth can be replaced or chemically changed by minerals dissolved in the water that seeps
into them. Fossils formed in
this way are the same shape
as the original remains but are
made of different chemicals;
petrified wood is an example.
Animal bones and shells can
be preserved in sediments or
rock for many years without
changing. The types of bones,
shells and other remains found
in the layers of sedimentary
rock provide clues about the
environment, behaviour and
diets of ancient animals.
• Sometimes, fossils of whole
organisms, including the soft
parts, are preserved. Such
fossils are rare and valuable.
Insects that became trapped
in the sap (called amber) of
ancient trees have sometimes
been wholly preserved. Similarly, if the remains of animals
or plants are frozen and
buried in ice they can be fully
preserved. Whole bodies of
ancient woolly mammoths
(including skin, hair and
internal organs) have been
found trapped in the ice of
Siberia and Alaska. These
Text Area: 280 mm Deep x 215 mm Wide
remains provide clues to the
way that living things have
changed since ancient times.
Whole bodies and preserved
skulls of animals can even
reveal evidence of their last
meal before death.
• The remains of animals or
plants sometimes leave an
impression, or imprint, in
hardened sediments or newly
formed rock. It is also possible
for remains trapped in rock to
be broken down by minerals
in water, leaving a mould in
the shape of the organism.
• Some fossils, called trace
fossils, only provide signs of
the presence of animals or
plants. For example, footprints
preserved in rock can provide
clues about ancient animals,
This insect was trapped in the sap of a
tree millions of years ago.
An ancient woolly mammoth. Whole
bodies of these ancient animals have been
discovered in the ice of Siberia
and Alaska.
including dinosaurs, and how
they lived. By studying the
shape, size and depth of footprints, hypotheses can be
made about the size and
weight of extinct animals as
well as how they walked or
ran. Plant, leaf and root
imprints, and feather impressions are other examples of
trace fossils.
The imprint of the leaf of an ancient fern
left in mudstone is a trace fossil.
Activities
Remember
1. A road cutting reveals the layers of rock shown in
the diagram. Which of the rocks in the cutting is:
(a) the oldest rock?
(b) the youngest rock?
(c) evidence of volcanic activity?
Shale
Sandstone
3. How does the relative age of a rock differ from its
actual age in years?
4. What does a palaeontologist study?
5. What clues about life in the past do fossils
provide?
6. Explain the circumstances that might lead to a
whole living thing being preserved as a fossil.
7. What are trace fossils and how are they useful?
8. Distinguish between a cast and a mould.
Think
Basalt
Limestone
Mudstone
Layers of rock exposed by a road cutting
2. Why are some layers of sedimentary rock tilted,
even though the sediments that formed them were
laid in horizontal beds?
1. In which rocks, shown in the diagram, would you
be most likely to find the fossil of:
(a) a seashell?
(b) the leaf of a fern usually found in swamps?
2. Why are some layers in the diagram thicker than
others?
3. Why are the hard parts of plants and animals
more likely to be preserved than the softer parts?
4. Is an ancient Egyptian mummy found in a newly
discovered tomb a fossil? Explain your answer.
5. Normally, older layers of rock will be found below
younger layers. Sometimes, however, younger
layers are found beneath older layers. How could
this happen?
THE CHANGING EARTH
237
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10.5 CORE SCIENCE 2 Page 236 Tuesday, June 15, 2004 5:41 PM
1
10.5
0.5
Looking for clues
Over very long periods of time,
rivers change their course. Mountains form where seas once
existed. The climate changes. As
these changes take place, different layers of sediment can be
deposited at the same location.
Some layers will be thicker than
others.
Sedimentary
rocks,
formed by the different layers of
sediments, provide many clues
about the order in which events
took place. The older layers will
almost always be below the
younger layers. The layers of sedimentary rock in the photograph
forming a background to these
pages provide a record of the sediments deposited in the Hamersley
Ranges in Western Australia over
hundreds of millions of years.
From time to time there might
be sudden changes caused by
erupting volcanoes or earthquakes. Basalt capping at Mount
Wilson and bits of pumice on
Bondi Beach provide evidence of
volcanic activity nearby. The
Sydney Basin and Blue Mountains
are made of sandstone. How did
this sandstone, which formed
10.5
a small seashell
a small box (shoebox or milk carton)
fine sand
plaster of Paris
• Half-fill a box with fine, damp
sand.
• Make a clear imprint of a small
seashell in the sand.
• Mix some plaster of Paris and
pour it carefully into the imprint.
• Once the plaster has set, remove
the plaster cast carefully from
the sand.
CORE SCIENCE 2
Life in the past
The relative age of a rock
simply indicates whether it was
formed before or after another
rock. Fossils provide many
clues about the relative age of
rocks. By comparing fossils
found in rocks in different areas,
it is possible to compare the
ages of rocks throughout the
world. Fossils also provide a
way of finding out how living
things have changed over time.
Scientists who study fossils are
called palaeontologists.
A fossil is evidence of life in the
past. Evidence of the very oldest
living things is buried within the
deepest and oldest layers of rock.
M AKI NG A F O S S I L
YOU WILL NEED
236
beneath the sea, get raised up as
much as 1000 metres? Slow
movements caused by the forces
beneath the surface have tilted,
curved and pushed up the layers.
Weathering and erosion have
also exposed some layers that
were below the ground millions of
years ago. This explains why sea
shells are sometimes found many
metres above sea level.
You have two records of the
seashell — the mould or imprint
in the sand and the plaster cast.
1. Which parts of animals are most
likely to be preserved as casts?
2. Is the fossil of a fern leaf more
likely to be found as a cast or a
mould? Why?
3. Dinosaur fossils are found in
casts and moulds. What
evidence of dinosaurs is likely to
be found as a mould?
CAUTION: Do not put plaster
of Paris down the sink.
How fossils
form
The remains of most animals
and plants decay or are eaten
by other organisms, leaving no
trace behind. However, if the
remains are buried in sediments
before they disappear they can
be preserved, or fossilised. Fossils can take several forms.
• The hard parts of plants and
animals are more likely to be
preserved than the softer
parts. Wood, shells, bones and
teeth can be replaced or chemically changed by minerals dissolved in the water that seeps
into them. Fossils formed in
this way are the same shape
as the original remains but are
made of different chemicals;
petrified wood is an example.
Animal bones and shells can
be preserved in sediments or
rock for many years without
changing. The types of bones,
shells and other remains found
in the layers of sedimentary
rock provide clues about the
environment, behaviour and
diets of ancient animals.
• Sometimes, fossils of whole
organisms, including the soft
parts, are preserved. Such
fossils are rare and valuable.
Insects that became trapped
in the sap (called amber) of
ancient trees have sometimes
been wholly preserved. Similarly, if the remains of animals
or plants are frozen and
buried in ice they can be fully
preserved. Whole bodies of
ancient woolly mammoths
(including skin, hair and
internal organs) have been
found trapped in the ice of
Siberia and Alaska. These
Text Area: 280 mm Deep x 215 mm Wide
remains provide clues to the
way that living things have
changed since ancient times.
Whole bodies and preserved
skulls of animals can even
reveal evidence of their last
meal before death.
• The remains of animals or
plants sometimes leave an
impression, or imprint, in
hardened sediments or newly
formed rock. It is also possible
for remains trapped in rock to
be broken down by minerals
in water, leaving a mould in
the shape of the organism.
• Some fossils, called trace
fossils, only provide signs of
the presence of animals or
plants. For example, footprints
preserved in rock can provide
clues about ancient animals,
This insect was trapped in the sap of a
tree millions of years ago.
An ancient woolly mammoth. Whole
bodies of these ancient animals have been
discovered in the ice of Siberia
and Alaska.
including dinosaurs, and how
they lived. By studying the
shape, size and depth of footprints, hypotheses can be
made about the size and
weight of extinct animals as
well as how they walked or
ran. Plant, leaf and root
imprints, and feather impressions are other examples of
trace fossils.
The imprint of the leaf of an ancient fern
left in mudstone is a trace fossil.
Activities
Remember
1. A road cutting reveals the layers of rock shown in
the diagram. Which of the rocks in the cutting is:
(a) the oldest rock?
(b) the youngest rock?
(c) evidence of volcanic activity?
Shale
Sandstone
3. How does the relative age of a rock differ from its
actual age in years?
4. What does a palaeontologist study?
5. What clues about life in the past do fossils
provide?
6. Explain the circumstances that might lead to a
whole living thing being preserved as a fossil.
7. What are trace fossils and how are they useful?
8. Distinguish between a cast and a mould.
Think
Basalt
Limestone
Mudstone
Layers of rock exposed by a road cutting
2. Why are some layers of sedimentary rock tilted,
even though the sediments that formed them were
laid in horizontal beds?
1. In which rocks, shown in the diagram, would you
be most likely to find the fossil of:
(a) a seashell?
(b) the leaf of a fern usually found in swamps?
2. Why are some layers in the diagram thicker than
others?
3. Why are the hard parts of plants and animals
more likely to be preserved than the softer parts?
4. Is an ancient Egyptian mummy found in a newly
discovered tomb a fossil? Explain your answer.
5. Normally, older layers of rock will be found below
younger layers. Sometimes, however, younger
layers are found beneath older layers. How could
this happen?
THE CHANGING EARTH
237
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Text Area: 280 mm Deep x 215 mm Wide
1
10.6
0.6
History in rock
It is about 65 million years since the last
non-flying dinosaurs existed on the Earth.
• What did they look like?
• What colour were they?
• How fast could they move?
• How did they behave?
• What did they eat?
Palaeontologists use fossils to try to
answer all of these questions and more!
Dinosaurs preserved in rock
After the death of a dinosaur, its
body would usually be eaten by
meat-eating animals (carnivores
or scavengers). Its bones would
be crushed or weathered, leaving
no remains. If, however, the
remains of a dinosaur were buried
in sediment, the bones could be
preserved.
If a dinosaur died near a
muddy swamp, shallow lake or
river bed, its remains sank in the
mud or were washed into a river
in a flood. The bones were
quickly buried in sediment.
Over millions of years, more layers of
sediment were deposited on top of the
buried remains. Chemicals dissolved in
the water seeping into the remains
changed their colour and chemical
composition. The shape, however, was preserved. The surrounding sediments were
gradually transformed into
sedimentary rock.
The layers of rock containing the fossilised remains were
pushed upwards, bent and tilted by forces beneath
the Earth’s surface. Weathering and erosion
by the wind, sea, rivers or glaciers might
expose one or more of the bones or
teeth. If the exposed fossils were
discovered before being buried
again, palaeontologists might
begin the search for the rest
of the skeleton and
other
dinosaur
remains in the
same area.
238
CORE SCIENCE 2
Not just a pile
of bones
Dinosaur fossils are not all
bones. They may include the
following:
• Fossilised teeth. The shape
of the teeth and the way they
are arranged provide vital
clues about the diets of dinosaurs. Flat-surfaced teeth that
are arranged so that they can
grind would have belonged to
a dinosaur with a plant diet
similar to that of horses and
cows. When fossilised teeth
like these are examined under
a
microscope,
scratches
caused by the grinding of the
teeth are sometimes visible.
Sharp-pointed teeth suited to
tearing flesh would have
belonged to a meat-eating
dinosaur.
• Footprints. Dinosaur footprints are often preserved in
rock. Footprints from a single
dinosaur provide clues about
the size and weight of the
dinosaur. They also indicate
whether the dinosaur walked
on two legs or four, and how
its weight was spread. The
distance between footprints
enables palaeontologists to
estimate how fast the dinosaur moved. Footprints also
provide clues about the
behaviour of dinosaurs and
whether they lived in herds or
alone.
• Impressions of skin left in
mud that has been hardened.
• Droppings (called coprolites), which provide clues
about the diet and behaviour
of the animals that left them.
• Unhatched eggs.
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ch10.book Page 239 Tuesday, June 15, 2004 2:18 PM
The dinosaur
jigsaw
puzzle
Dinosaur bone fossils were generally scattered before or while
being buried. The task of putting
them together to make a single
dinosaur skeleton is very much
like solving a jigsaw puzzle. In
fact, if several dinosaurs die in
one area and their bones are fossilised, the pieces of the skeleton
may be all mixed up. Putting
them together is like solving
several jigsaw puzzles at
the same time with
all of the pieces
mixed with each
other. To make
matters worse,
some parts of
the ‘jigsaw
puzzle’ are
likely to be
missing.
Every footprint tells a story
At Lark Quarry, near Winton in central Queensland, over 3000 footprints made by about 150 different dinosaurs provide evidence of a
brief encounter that took place about 100 million years ago. The footprints were made in a soft clay which has been transformed into sedimentary rock. They tell the story of a herd of small plant-eating
dinosaurs (ornithopods) and a nearby group of small meat-eating
coelurosaurs being attacked by a large meat-eating tyrannosaur.
The smaller dinosaurs must have been trapped between
the tyrannosaur and some large rocks. The footprints show that they attempted to escape
by running at great speed
around their attacker.
Footprints 100 million years old tell a story.
Activities
Remember
1. What information about dinosaurs can be
obtained from fossils?
2. Fossils of dinosaurs form when their remains are
buried under many layers of sediment. Why are
fossils often discovered in rocks and soil on the
surface of the Earth?
3. List five types of dinosaur fossils that can be found,
other than the remains of their bones.
Think
1. Explain why all dinosaurs’ bones weren’t
preserved as fossils.
2. Why is piecing together a dinosaur skeleton like
solving a jigsaw puzzle?
3. Explain how is it possible to use preserved
footprints to form hypotheses about:
(a) whether they lived alone or in herds
(b) the way that dinosaurs walked
(c) the weight of dinosaurs
(d) the speed of dinosaurs.
right were found in a layer of sedimentary rock. Use
the footprints to write a description about what might
have happened millions of years ago.
1. Compare your interpretation of the footprints with
others.
2. Does each person interpret the evidence in the
same way?
3. If there are differences of opinion about what
happened, is there any way of knowing who is
right?
4. List as many differences between the two types
of dinosaurs making these footprints as you
can.
Imagine
Imagine that the set of fossilised dinosaur
footprints shown in the diagram on the
What could have happened here
millions of years ago?
THE CHANGING EARTH
239
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Text Area: 280 mm Deep x 215 mm Wide
10.7
THE DINOSAUR RIDDLE
Between about 250 and 65 million years ago, dinosaurs were the
most successful animals on Earth. In fact, those years were known
as the age of the dinosaurs. Dinosaurs thrived and dominated the
land while mammals lived in their shadow. Fossil evidence indicates that the last of the dinosaurs died about 63 million years ago.
There are several theories about the extinction of the dinosaurs.
Scientists and others argue about whether the end of the dinosaurs was sudden or gradual. Scientists do generally agree that the
riddle of the dinosaur remains unsolved. Palaeontologists and
other scientists continue to look for clues that might provide the
final solution.
The asteroid theory
The most widely accepted solution to the dinosaur riddle is that
an asteroid collided with the Earth 65 million years ago. The
impact of the asteroid threw billions of tonnes of dust into the air,
blocking out sunlight and plunging the Earth into darkness for
two or three years. Plants stopped growing but their seeds
remained intact. The temperature dropped. The large plant-eating
dinosaurs would have quickly died of starvation. The meat-eating
dinosaurs would probably have died next, losing their main food
supply, but surviving for a while by eating smaller animals. Many
smaller animals would have survived by eating seeds, nuts and
rotting plants.
As the debris began to settle and sunlight filtered through the
thinning dust clouds, many of the plants began to grow again.
The surviving animals continued to live as they did before the
impact. The surviving mammals were no longer competing with
dinosaurs for food. It was the beginning of the age of mammals.
240
CORE SCIENCE 2
The volcano theory
The eruption of Mt Pinatubo in
the Philippines, in June 1991,
showed that ash and gases from
volcanoes could reduce average
temperatures all over the world.
The average global temperatures
during the years 1992 and 1993
were almost 0.2°C less than
expected. While this is not a
large drop in temperature, the
size of the eruption of Mt
Pinatubo was very much smaller
than those of many ancient volcanoes.
The ash from a large volcano
could have the same effect on
sunlight and the Earth’s temperature as an asteroid impact. If there
was an unusually large amount of
volcanic activity about 65 million
years ago, the extinction of the
dinosaurs could be explained.
The largest known volcanic eruption occurred about 250 million
years ago in what is now Siberia.
It is believed that many types of
marine animals became extinct at
about the same time.
The first dinosaur fossil was
found in 1822 in the Stonesfield
slate quarry at Oxfordshire,
England. This dinosaur was
a flesh-eater called
Megalosaurus. Later that year,
Iguanodon was found in Sussex.
Up until this time, people had
no idea that such extraordinary
creatures had existed.
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The cooling
climate theory
The gradual cooling of the Earth’s
climate due to changes in the
sun’s activity could have caused
the extinction of the dinosaurs.
Dinosaurs, with no fur or feathers,
were less protected from cold
weather than mammals and
birds. Larger dinosaurs would
have found it very difficult
to shelter from the cold
conditions. Smaller animals
could burrow below the
ground or shelter in the
hollow trunks of trees or in
caves. Many mammals and
birds would have been able
to migrate to warmer regions
closer to the equator.
The emerging
plants theory
During the Cretaceous period
(140 million to 65 million years
ago), new types of plants began
to appear. Flowering plants
evolved, competing with more
primitive plants such as ferns
for nutrients, water and sunlight. The plant-eating dinosaurs
did not eat flowering plants. As
their traditional food supply
became more scarce, the planteating dinosaurs could not survive, and the meat-eating dinosaurs that preyed on
them starved as
well.
Cold-blooded
or warm-
blooded?
Until recently, it was believed
that dinosaurs were ectothermic
(cold-blooded).
Ectothermic
animals have body temperatures
that depend on the temperature
of their surroundings. As the surrounding temperature decreases,
their body temperature decreases
and they become less active.
Mammals are endothermic
(warm-blooded). Endothermic
animals are able to maintain a
constant body temperature that
is usually above that of their surroundings. They remain warm
and active in lower surrounding
temperatures.
If dinosaurs were in fact
ectothermic, a cooler climate
would have made it more difficult for them to compete
with animals for food. However, many scientists now
believe that dinosaurs may
have been endothermic.
The question of whether
dinosaurs were ectothermic or
endothermic needs to be
answered before the riddle
of the dinosaurs can be
solved.
Activities
Remember
1. What is the most widely accepted theory about the
extinction of the dinosaurs?
2. Would smaller or larger animals be more likely to
survive the effects of an asteroid impact or big
volcanic eruption?
3. What is the difference between an ectothermic
animal and an endothermic animal? Give some
present day examples.
Think
1. In what ways were the dinosaurs different from
mammals?
2. How might volcanic eruptions affect life throughout
the whole world?
3. How might meat-eating dinosaurs be endangered
by the evolution of new types of plants?
4. Which group of animals benefited the most as a
result of the extinction of the dinosaurs?
5. List as many weaknesses as you can in each of the
four theories presented.
6. Which theory of the extinction of the dinosaurs do
you think is most likely to be correct? Explain.
Imagine
1. Imagine what it would have been like 65 million
years ago if an asteroid plunged into the Earth. Write
a story about the first 24 hours after the impact.
2. Which animals and plants do you think would be
most likely to survive if an asteroid struck central
Australia now? Explain your answer.
THE CHANGING EARTH
241
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1
10.8
0.8
Life on Earth
Scientists believe that early Earth
was a hostile place. Lightning
bolts blasted through an atmosphere of hydrogen, methane,
ammonia, water vapour and
carbon dioxide. There was no
oxygen in the atmosphere. Under
these conditions, about 3500 million years ago, a miraculous mix
of chemicals combined to form
an entity that could reproduce
itself. Welcome life!
Even though many forms of
primitive life no longer exist,
studies of fossils provide clues
about what they were like and
how they changed over time.
The five
kingdoms
emerge
Representatives of each of the
five kingdoms of living things
appeared on Earth at different
times. The five kingdoms, in
their order of appearance on
Earth, are:
• Prokaryotae monera (bacteria)
• Protista (algae and protozoa)
• Fungi
• Plantae (plants)
• Animalia (animals).
The very first organisms lived
in the sea. They were similar to
some of the bacteria that still
exist today. The cyanobacteria, sometimes called
blue-green algae, used the
light from the sun to make
their own food, as the
green plants do today.
During this process, called
photosynthesis, they produced oxygen. The atmosphere gradually changed,
making conditions more suitable
for more complicated forms of
life.
242
CORE SCIENCE 2
Earth was a hostile place 3500 million years ago. Fossils provide evidence of structures
called stromatolites. They existed in warm sea water and consisted of cyanobacteria,
one of the earliest forms of life.
The first fungi and primitive
plants and animals appeared
about 2700 million years ago.
They all lived in the sea. It was
not until 600 million years ago
that the first shelled animals
appeared: organisms called trilobites which had flexible outer
shells. They were able to cast
off their ‘armour’ and produce a
new, larger cover. Trilobites
were the first organisms that
were able to see.
Primitive fish with sucking or
scraping mouths and no jaws
were the first vertebrates. These
fish had an internal skeleton.
On land at last
As life forms in the sea continued
to grow and change, the atmosphere was changing. Plants in
the sea were producing more
oxygen through photosynthesis.
The ozone layer, which screens
out deadly ultraviolet radiation,
was beginning to form. The
conditions became suitable
for life to exist on land.
The first land plants,
insects and other airbreathing
animals
appeared about 430
million years ago.
A trilobite fossil is up to 600 million
years old.
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Era
Geological
period
Millions of
years ago
2
Fossil evidence of life
first humans
Cenozoic
(recent life)
Quaternary
Mesozoic
(middle life)
Cretaceous
140
first flowering plants,
primitive mammals
Jurassic
Triassic
180
245
first birds, flying reptiles
first mammals
Palaeozoic Permian
(ancient life)
275
first dinosaurs
first land vertebrates
modern insects appear
Carboniferous
350
first reptiles
first winged insects
Devonian
410
first conifers, first amphibians
Silurian
430
first evidence of land plants
first insects
Ordovician
500
first fish, first corals
Cambrian
600
first animals with shells
65
Tertiary
modern plants, birds and invertebrates
first primates and hoofed mammals
2700
fungi, primitive sea plants,
first protozoans
Archaeozoic
(primitive life)
3500
Azoic
(without life)
4600
first traces of life –
algae, bacteria and
cyanobacteria.
no life yet discovered
Proterozoic
(earlier life)
Precambrian
The development of life on Earth
Activities
Using data
Use the table to answer the
following questions.
1. Which era was the most recent?
2. About how many millions of
years ago did the first dinosaurs
appear on Earth? During which
period was this?
3. Did the first reptiles appear
before or after the first fish?
4. During which geological period
did each of the following
organisms first appear?
(a) corals
(b) winged insects
(c) primates
(d) humans
(e) bacteria
5. List the following organisms in
order of their first appearance on
Earth: animals with shells,
conifers, mammals, birds,
dinosaurs, insects, flowering
plants, fungi.
6. Draw up a table to show about
how many million years ago,
and during which period, the first
representatives of each of the five
kingdoms appeared on Earth.
Remember
1. What type of organism made up
the structures called stromatolites?
2. What type of organism was a
trilobite? What features did it
have that made it different from
earlier forms of life?
Think
There was not enough oxygen
in the Earth’s atmosphere to allow
air-breathing animals to exist until
about 440 million years ago.
What caused the amount of
oxygen in the atmosphere to
increase?
Create
Construct a class timeline showing
the geological periods of the Earth
and the life forms that emerged
during each period. You could use
butcher’s paper, with a scale of
1 metre = 1000 million years. Fill
in any gaps by researching using
the library or the Internet.
THE CHANGING EARTH
243
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Putting it all together
Summing up
Copy and complete the statements below to compile a summary of this unit. The missing words can be found
in the list below.
1. Rocks on the Earth’s surface are constantly being broken down into smaller rocks in a process called
.
2.
is the movement of weathered rock from one place to another. The main agents of
erosion are wind, running
, the sea and glaciers.
3.
are deposited by running water. The
particles settle first.
4. Most sedimentary rocks are formed from weathered rock that has been
due to erosion.
5.
and
form from sediments comprising the remains of living things.
6.
rocks form when igneous or sedimentary rocks are exposed to high temperatures or
.
7. Sedimentary rocks form in layers.
the order in which events on Earth took place.
preserved in these layers provide valuable clues about
8. A fossil is evidence of life in the past. Scientists who study fossils are called
9. The
.
age of a rock indicates whether it was formed before or after other rocks.
10. Fossils provide many
about the appearance and behaviour of dinosaurs.
11. Dinosaurs became
about 65 million years ago. The most widely accepted theory put
forward to explain their death is an
impact on Earth.
12. Fossil studies indicate that the five
of living things appeared on Earth at different times,
beginning with bacteria-like organisms about 3500 million years ago.
Word list
relative
kingdoms
deposited
weathering
water
limestone
244
CORE SCIENCE 2
fossils
clues
pressure
palaeontologists
extinct
erosion
asteroid
sediments
metamorphic
coal
larger
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Looking back
1. In the word puzzle below you will find two igneous
rocks, seven sedimentary rocks and four
metamorphic rocks. Their names may be spelt
across, down, diagonally or even backwards.
C
C
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Y
M
A
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B
L
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K
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Q
G
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2. What is the difference between erosion and
deposition?
3. What are sediments?
4. Describe the three different ways in which
sedimentary rocks can be formed.
5. While studying sedimentary rocks in a railway
cutting, a geologist discovers a bed of rock with
ripple marks in its surface. How could the ripple
marks have been made in the rock?
6. Copy and complete the table below to summarise
what you know about igneous, sedimentary and
metamorphic rocks.
Type of
rocks
How
they
are
formed
Special
features
Example Uses
igneous
sedimentary
metamorphic
7. The photograph is of a giant dinosaur footprint
that has been preserved in rock at Lark Quarry in
central Queensland.
(a) What type of fossil is it?
(b) Why is it classified as a fossil even though it
could be described as a dent in a rock?
(c) Have all dinosaur footprints been preserved?
Why has this one been preserved for hundreds
of millions of years?
(d) What can be learned about the features of the
dinosaur that left this footprint?
(e) What forms of evidence, apart from preserved
footprints, can be used to gather knowledge
about dinosaurs?
This dinosaur footprint has been preserved in rock for
hundreds of millions of years at Lark Quarry in central
Queensland.
8. The Earth is believed to be about 4600 million
years old. Imagine that the lifetime of the Earth
could be squashed into just one year: midnight
on 1 January is the beginning of the Earth, and
midnight on 31 December is AD 2000.
Construct a timeline to represent the
approximately 4600 million year history of the
Earth. Divide your timeline into twelve equal
sections to represent each of the twelve months
of the year. (It is reasonable to assume that the
months are equal for the timeline.) Label the first
day of each month on your timeline.
If the lifetime of the Earth is squashed into one
year, on what date (approximately) did each of the
following forms of life appear?
(a) the first traces of life
(b) the first fungi
(c) the first land plant
(d) the first dinosaurs
(e) the first mammals
(f) the first humans
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Extension
What’s the date?
Radiocarbon dating
In order to determine the absolute age of rocks
and fossils, geologists use a technique called radiometric dating. Radioactive elements decay at a
rate that is constant for each element. They emit
small particles and break down into different elements. The time taken for half of the atoms of a
radioactive element to decay is called its half-life.
One form of the element carbon, called radiocarbon or carbon-14, is radioactive. It has a halflife of 5730 years. Radiometric dating with carbon
is called radiocarbon dating. All living things
contain the element carbon. A small amount of the
carbon is radiocarbon, which can be detected
because it emits special rays. As long as organisms
are alive, carbon (along with radiocarbon) is being
replaced. Plants take in carbon dioxide, animals eat
plants, and micro-organisms consume plant and
animal matter or each other. All living things, therefore, contain a small amount of radiocarbon.
When living things die, the decaying radiocarbon
is no longer being replaced. Since all fossils were
whole
100
Percentage of radiocarbon remaining.
It is usually easy for geologists to work out the
relative age of sedimentary rocks and fossils. In
any particular location it is almost certain that a
layer of sedimentary rock is older than the rocks
above it and younger than those below it. It can
also be assumed that the fossils in lower layers are
older than those in the layers above. The relative
age of some igneous rocks and metamorphic
rocks can be determined in the same way.
75
half left
50
1/4
25
left
1/8
left
1/16
0
5730
11460
17190
Time in years
left
22920
1/32
28650
The decay of a sample of pure radiocarbon
once living, their age can be determined by
measuring the amount of radiocarbon remaining.
After 5730 years only half of the usual amount of
radiocarbon will be left. A graph like the one above
can be used to estimate the absolute age of a fossil.
After about 50 000 years, the amount of radiocarbon
becomes too small to measure accurately.
All rocks contain small amounts of radioactive
elements such as uranium and potassium. The
absolute age of older rocks, and the fossils within
them, can be estimated using radioactive elements
with longer half-lives.
Activities
Remember
1. What is meant by the term half-life?
2. What is the half-life of carbon-14?
Using data
Use the graph to answer the following questions.
1. Parts of the skeleton of a large animal are found buried
in sand dunes. The amount of radioactive carbon-14
in the bones is about one-eighth of what is found in
the skeletons of living animals. How long ago did the
animal probably die (to the nearest thousand years)?
246
CORE SCIENCE 2
left
2. Approximately what percentage of the original
amount of radioactive carbon-14 would you
expect to find in:
(a) a wooden Aboriginal spear 11 000 years
old?
(b) a skull 23 000 years old, found in a cave?
Think
Why is radiocarbon dating not suitable for dating
objects over 50 000 years old? How do scientists
date such objects?
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Reflection
1. Rocks and reptiles
4. First life
(a) Design a travel brochure for a ‘geological
discovery’ holiday in your local area. Use as many
terms from this chapter as possible. You may need
to research the rocks in your local area first.
(b) With a partner, build a model of your favourite
dinosaur. Design a way to provide commentary with
your model, to tell viewers all about your dinosaur.
How do you think life began on Earth? Write a recount
that tells the story of life early in the history of our
planet. A recount tells the story in the order in which it
occurred, and has two parts, the orientation, followed
by a series of events.
The table below shows a sample recount. Examine it
carefully, then plan and write your recount.
2. Dinosaur debate
You have learnt some of the theories about how
dinosaurs became extinct. The table below contains
these and other theories. Look at the table and
complete the class activity that follows.
Possible reasons why dinosaurs became extinct
Biological
Astronomical
They were killed by disease.
Other animals ate their
eggs.
There was too much
inbreeding in herds.
They ate poisoned plants.
Only male or female babies
were born.
Their eggshells thinned.
They were cold-blooded.
They were too stupid.
There was a change in
available plants.
Unable to adapt to changing
conditions, they suffered
death by specialisation.
Meteorites or a comet
explosion caused debris
to block the sun.
An exploding supernova
caused extra radiation to
reach Earth.
There was increased
volcanic activity on Earth.
There was an increase in
the amount of solar
radiation reaching Earth.
The inclination of the
Earth’s axis changed.
Large variations in
temperature occurred.
Earth passed through the
tail of a comet.
(a) Choose one of the possible reasons and develop
arguments for and against it. You may do research
at the library or on the Internet. Test your ideas by
discussing them with a classmate.
(b) Present your arguments to the class. As a class,
you can then rank the possible reasons in order of
most likely to least likely. (Remember, scientists
don’t know the answer to this riddle.)
(c) Which of the reasons would make a good hypothesis
for a scientist to test? How might a scientist test it?
3. This is your life!
Construct a timeline to show the major events of each
year of your life. You may ask your family for help.
Recount structure
Sample
Part 1: Orientation
• Introduce your reader
to the who, what,
where, when of the
story.
• Use past tense.
Late last year, Carmel and Lee
were curious about whether bean
seeds or radish seeds sprouted
most quickly. They decided to
carry out a controlled
investigation. Carmel’s back
verandah was chosen as the
best spot for their work.
Part 2: Series of events
• Retell the events in
order, organised into
time periods.
• Use past tense.
• Use words like ‘first’,
‘then’ and ‘finally’ to
shape the story.
• Use descriptive
language to make it
interesting and
informative.
First, Lee and Carmel met to
plan their investigation and
decide how to record their
observations. They agreed to
make a list of materials and to
use a logbook. The first entry in
their logbook follows:
Nov 1, Materials:
6 pots (same size)
bean and radish seeds
potting mix
They then decided who should
obtain each material and
planned to meet again the
following week.
At the next meeting, they
discussed the control of the
variables in their investigation.
They worked out how to control
soil type, light and the quantity
of water the pots would receive.
Finally, they decided to plant six
seeds in each pot, with three
pots of radish and three of
beans. The investigation was fully
planned!
5. News flash
Find out about either Bog Man or Graubelle Man. For
your choice, write a newspaper article which explains
what the man ate, his lifestyle or occupation and how
he died.
THE CHANGING EARTH
247