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Earth and Environmental SciencePreliminary CourseStage 6

Planet Earth and its environment

Part 2: The evolution of the Earth

Incorporating October 2002

AMENDMENTS

Number: 43177 Title: Planet earth and its environment

All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith.

This publication is copyright New South Wales Department of Education and Training (DET), however it may contain material from other sources which is not owned by DET. We would like to acknowledge the following people and organisations whose material has been used: Photographs © R.A. Binns, CSIRO, taken from JAMSTEC submersible “Shinkai-6500” Part 3 p 17

Photograph courtesy of Paul Brooks Part 4 p 7

Photograph courtesy of Upgrade Business Systems Pty Ltd Part 4 p 11

Photograph courtesy of NASA Part 6 p 18

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Phanerozoic

Cenozoic

Eon Era Period Quaternary

Holocene

Tertiary

Mesozoic

Cretaceous

Jurassic

Triassic

Palaeozoic

Permian

Carboniferous

DevonianSilurian

Ordovician

Cambrian

Hadean

Archaean

Proterozoic

Epoch

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Palaeocene

Ediacaran

massextinction

oldest stromatolitesoldest evidence indicating life

age of BIFs

Mill

ions

of y

ears

bef

ore

pres

ent (

Ma

BP

)

012345

10

20

30

40

50

60

70

massextinction

100

200

300

400

500

600

Changeof scale

Changeof scale

1000

2000

3000

4000

Changeof scale

Precambrian

(last 10 000years)

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Part 2: The evolution of the Earth 1

Contents

Introduction ............................................................................... 3

Evolution of the Earth ................................................................ 4

The earliest stage.................................................................................4

Fpormation of a layered Earth................................................... 7

The role of gravity.................................................................................9

Theory to explain a layered Earth .....................................................10

The Earth’s layers ..............................................................................12

A traditional Aboriginal perspective ......................................... 18

The evolution of the atmosphere............................................. 21

The composition of the atmosphere..................................................21

The early atmosphere ........................................................................21

Suggested answers................................................................. 23

Exercises–Part 2 ..................................................................... 25

2 Planet Earth and its environment

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Introduction

At the end of Part 2, you will have been given opportunities to learn to:

• compare cultural beliefs with the views of astronomers and otherscientists that may arise in discussion of the origins of the Earth

• explain the role of gravity in the formation of the Earth

• describe the relationship between the density of Earth materials andthe layered structure of the Earth

• describe the composition of the early (pre–oxygen) atmosphere andcompare it with the composition of the present atmosphere.

At the end of Part 2, you will have been given opportunities to:

• gather and process information that compares a cultural explanationwith an astronomical or scientific model of the origin of the Earth

• perform a first–hand investigation to measure the density of aselection of earth materials representative of core, mantle and crust

• identify data sources, process and present information fromsecondary sources to compare Earth’s earliest atmosphere with thepresent atmosphere.

Extract from Earth and environmental science Stage 6 Syllabus © Board ofStudies NSW, amended November 2002.The original and most up–to–date version of this document can be found on theBoard’s website at:http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .


Evolution of the Earth

How old do you think the Earth is and what would you use to measureits age?

If you take the age of the oldest known sedimentary rocks on the planetas being the age of the Earth, then the Earth would be approximately3 800 million (3.8 billion) years old. However, if you accept the positionof many of the world’s leading astrophysicists, that the Solar system,including the Earth and meteorites all formed at the same time, then theage of the Earth could then be estimated at approximately 4 700 million(4.7 billion) years old.

The age of the Earth has been derived from the dating of knownmeteorites that have landed on the Earth.

The earliest stage

For the first billion years of its life the Earth existed as a relatively cooland homogenous mass of silicon compounds; and iron and magnesiumoxides. These materials would have been distributed relatively evenlythroughout the Earth’s interior at this very early stage.

Temperature and pressure

As materials began to react with each other producing new material,planetesimals (small planets), began to accrete (gather and build uponeach other) and the temperature began to increase. This increase intemperature is due to the energy being carried and released upon theimpact of accreting material, and also due to the increase in pressure as aresult of the accretion itself.

This event occurred approximately 4 700 million (4.7 billion) years ago.Refer to the diagram on the next page.

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The accretionary model. Materials react to form new products which accrete(build on each other). This results in an increase in temperature.

It has been estimated that the average temperature reached as high as1000°C as a result of accretion and compression of material during thisearly stage of the Earth’s development.

(An increase in pressure causes an increase in temperature. Have youever felt how hot your push bike pump is after you have pumped up oneof your tyres on a pushbike?)

Temperature and radioactive decay

Radioactive elements such as uranium, thorium and potassium, onlymake up a small percentage of the Earth’s elements however, theseplay an important role as they contributed to the heating of the Earth’sinterior. How do you think radioactive elements are able togenerate heat?

When radioactive elements are first formed they are said to be ‘unstable’.This is because as soon as they are formed they begin to change into adifferent element, which is more ‘stable’. Part of this transformation ofan unstable radioactive element into a stable non–radioactive elementinvolves the production and emission of radiation and heat.

This process of change is called radioactive decay. The unstableradioactive element is also called the parent material, and the stablenon–radioactive element produced from the decay of the parent materialis known as the daughter product.


Parent material Daughter product

uranium lead

potassium argon

thorium lead

rubidium strontium

Radioactive elements decay to form daughter products.

Parent material and daughter products

The table above shows some examples of radioactive elements and theirdaughter products. Remember, when each of the above parent materialsdecay to their stable daughter products they give off radiation and heat.It is this heat which contributed to the Earth’s initial heat source and isstill contributing to heat the Earth’s core today.

The heat generated through accretion and the heat generated throughdecay of radioactive elements was being produced at a faster rate than itcould escape. Because rocks are poor conductors of heat, the Earth’sinterior began to increase in temperature.

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Formation of a layered Earth

Did you know that the Earth is made up of a series of layers? Try thisself–correct exercise to see what you already know. Draw and label adiagram showing a cross–section of the Earth.

Check your answer.

Did the Earth always have this layered structure? If it did not, then howdid it come to develop this structure?

What is density?

The ideas behind how the Earth formed in layers relies heavily on theconcept of density. It is therefore important to understand thoroughlywhat density is. The density of a substance is the amount of matter thatsubstance contains in a given volume.

1 tonne feathers 1 tonne steel

One tonne of feathers has the same mass as one tonne of steel.


Every one knows that one tonne of feathers weighs the same as one tonneof steel. However, the steel would take up considerably less volume thanthe feathers would. This is because the atoms that go to make up thematter in steel are packed much closer together than the atoms that goto make up the feathers. Therefore, steel is said to be more densethan feathers.

The diagram on the next page shows a range of substances. It comparesthe density of these different materials. Each of the boxes in the diagramhas the same volume–one cubic metre. The mass of the matter in each ofthe boxes is measured in kilograms and is shown on the top of each box.

incr

easing density

1.5 kg

air

feathers wood

water

coal

aluminiumsteel

lead

45 kg 750 kg

1000 kg

1600 kg

2700 kg7800 kg

11 300 kg

Different materials have different densities.

1 List the materials shown in the diagram in order from the least densethrough to the most dense.

a) ____________________ b)________________________

c) ____________________ d)________________________

e) ____________________ f) ________________________

g) ____________________ h)________________________

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2 The average density of the Earth is 5500 kg per cubic metre.Which of the materials in your list above would best represent theaverage Earth material?

_____________________________________________________

_____________________________________________________

Check your answers at the back of this part.

The role of gravity

What is gravity? Is gravity the same everywhere?

Gravity is the force that one object applies on another object and isdetermined by the amount of mass or matter in that object.

Isaac Newton proposed the Law of Gravitation, that states ‘every particlein the Universe attracts every other particle with a force that is directlyproportional to the combined masses of the particles and inverselyproportional to the distances between the particles’. In other words, themore massive the particles the greater is the gravitational force thoseparticles can supply.

For example, the Earth has more matter, and therefore has a greater massthan the Moon. As a result the Earth has a stronger gravitational force.In fact the Earth’s gravitational force is six times stronger than that ofthe Moon.

This force attracts or ‘pulls’ objects towards the centre of the mass thatcreated it. The Earth, therefore, attracts objects towards its centre.

If the Earth’s gravitational attraction is stronger than the Moon’s, whydoesn’t the Moon come crashing into the Earth?

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Check your answer.

We are also attracted towards the centre of the Earth but are preventedfrom accelerating towards it by a solid and rigid continental crust.However if we were to attempt to stand on something less rigid such asthe ocean we would be drawn closer to the Earth’s centre.


Implications for a developing Earth

One implication is that gravity formed the driving mechanism for theaccretionary model of the Earth. Material is attracted and accreted fromout of space due to the Earth’s gravitational force. As the Earth gained inmass so did the amount of gravitational attraction, which in turn led tomore material being attracted and accreted.

Another implication is that once the Earth heated up as a result of theimpact and pressure from this accretionary process, (and from the decayof radioactive elements) material became partially molten. This allowedthe denser materials to be drawn down more easily to the centre of theEarth by the Earth’s gravitational force. This attracting force allowed thevery first layering of Earth materials.

What possible conclusions can you make about the composition of theEarth’s interior, given that the average density of all the Earth’s material is5.5 grams per cubic centimetre, and the average density for continentalcrustal material is approximately 2.5 grams per cubic centimetre?

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Check your answers.

Theory to explain a layered Earth

As you know, heat was generated from the impact of material accretingas well as from radioactive elements decaying down to their daughterproducts. The temperature eventually rose enough for iron to melt.

The melted iron then began to accumulate and, due to its greater density,began to sink towards the centre of the Earth forcing other material not asdense to be positioned above it. This movement of iron in itself wouldgenerate heat due to the friction created during movement.

Approximately one–third of the Earth’s material sank towards the centreof the Earth. Due to this material being heated it was not in a completelysolid state. However, it was also not in a completely liquid state either,as we would perhaps imagine. This material is said to be in a partiallymolten state, which is somewhere in between liquid and solid.

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The lighter molten material rose to the surface and cooled in the process,forming an early type of crust. Between the iron core and this early crustremained a mantle. During this time gases were most probably releasedfrom the Earth’s interior which, in turn led to the formation of theatmosphere and oceans. The result is a layered planet.

The diagram below illustrates the basic sequence of events leading to theEarth’s layered structure.

Label the layers of the last diagram by referring to the previous information.As well, you may need to refer to the following information The Earth’slayers. Please note this diagram of the Earth’s layers does not include theatmosphere, and that the crust is included in the lithosphere layer.

The early Earth – a homogenous mixture.

Iron sank to the centre.

The result is a zoned planet.

Check your answers at the back of this part.


The Earth’s layers

The inner core is thought to be solid and consisting almost entirely ofiron. The outer core is thought to be liquid and is mostly composed ofiron and nickel based minerals.

The mantle is thought to be composed mainly from the oxides of silicaand magnesium as well as some oxides of iron and aluminium.The mantle is separated into upper and lower regions based on thedensity differences between minerals.

Since the lower mantle is deeper, minerals produced in this region havedone so under greater pressure than the minerals produced in the uppermantle. As a result their crystal structure will change but not necessarilytheir chemical composition, producing a more dense structure. Carbon isa well–known example of a material changing its structure according todiffering pressure at the time of its formation. If the pressure is highenough the carbon will take the form of diamond, if the pressure is lowerit will take the form of graphite.

The boundary between the upper and lower mantle is mainly based uponthe different structures and density of material produced rather thandifferences in chemical composition.

Asthenosphere

The asthenosphere is situated in the upper mantle and is thought tocontain between one and two percent molten rock. This small percentageof molten material reduces the friction between rocks and allows rocksabove it (the lithosphere) to move over the top. (See the diagram below)The asthenosphere has been likened to conveyor belt–creating a mobilezone for material to move above it.

asthenosphere (partially molten)

0

70

continentalcrust

100

250

200

150

oceanic crust

lithosphere(rigid – includes crust)

oceanic crust

Dep

th (

km)

50

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Lithosphere

The lithosphere is composed of the uppermost part of the mantle and theoverlying crust, and comprises of relatively cool and rigid rock situatedabove the asthenosphere. The lithosphere is a mobile layer and can moveacross the top of the partially molten asthenosphere.

The crust, which forms the upper most part of the lithosphere, is eithercontinental or oceanic.

The continental crust is less dense than the asthenosphere and thereforecannot be puled down into the lower mantle. Continental crustalmaterial generally ranges from 20 to 70 kilometres in thickness. In mostcases, however, the continental crust is approximately 35 km inthickness. This crustal material is largely composed of oxides of silica,aluminium, iron, magnesium, potassium and calcium. (See the diagramon previous page.) The minerals that make up the continental crust donot undergo transformation to new minerals that increase in density asthey are placed under greater pressure.

Oceanic crust does not vary in composition as much as continental crust.Oceanic crust is denser than continental crust and is composed of basalticrock that is rich in iron, magnesium and silica. The minerals that makeup the oceanic crust are converted to other more dense minerals whenplaced under great pressure. This increased density aids oceanic crust tobe subducted or sink into the lithosphere. Oceanic crust is relatively thinranging from approximately 6 to 10 km in most places. Its thickness ismuch more uniform than that of the continental crust. Lying on theoceanic crust are the oceans and seas that make up the majority of thehydrosphere. (The remainder of the hydrosphere is made up fromrivers, creeks and all other bodies of water on the Earth’s crust.)

The outermost layer of the Earth is known as the atmosphere, and iscomposed largely from nitrogen (78%) and oxygen (21%).The atmosphere is both the outermost and the least dense layer ofthe Earth.

Write a statement linking the different density of Earth’s materials with theirlocation, as described in the layered model of the Earth above.

_________________________________________________________

_________________________________________________________

_________________________________________________________

Check your answer at the back of this part.

Complete Exercise 2.1 at the end of this section.


Density of liquids and layering

Aim:

To create a model that represents the layering of the Earth based on densitydifferences between different materials.

Materials:

• small baby food jar

• oil (such as cooking oil)

• water

Procedure:

1 Fill approximately one third of the jar with oil.

2 Fill another third of the jar with water.

3 Allow the jar to sit until all the fluids in the jar have settled.

Results:

1 Make a labelled sketch of the jar immediately after you have pouredin the water and before the liquids have settled.

2 Make a labelled sketch after the liquids have settled.

Sketch before settling Sketch after settling

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Conclusion:

How can these results be used to model the formation of the Earth’sinterior?

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Density of Earth materials

Aim:

The purpose of this activity is to find material which would simulate thecomposition of the Earth’s crust and mantle, and then perform aninvestigation to compare their densities.

Background information:

The density of a material is determined by the amount of material in agiven volume. Density is often given in:

• kilograms per cubic metre (kg/m3)

• grams per cubic centimetre (g/cm3 or g/cc).

Therefore, the density of an object can be calculated by dividing the massof an object by its volume.

density (D) = mass (m) ∏ volume (V)

It is also important to note that, when dealing with volume there is adirect relationship between cubic centimetres (cm3)

and millilitres (mL).

1 cm3 = 1 mL 1000 cm3 = 1 L

The mass of an object can be found by using a mass balance or a set ofscales marked in kilograms or grams.

The volume of a small object can be found by lowering the object into acontainer of water with measuring graduations on its side.Remember the narrower the container (with finer graduations on its side)the more accurate the reading will be. That is why a measuring cylinderis normally used in laboratories. When the object is fully immersed, therise in water level gives the volume of the solid.


Method:

1 You need to collect the following:

• iron sample eg. a bolt or nails (to simulate core material)

• a rock sample preferably a non–porous rock such as granite(to simulate crustal material)

• graduated measuring flask or measuring cylinder eg. babybottle, measuring cup

• a set of scales eg. kitchen cooking scales.

Equipment required to determine the density of a bolt.

2 With the above information and the equipment you have collected,write out a series of steps which will enable you to calculate thedensity of your simulated core material sample, and your simulatedcrustal material sample. Use diagrams where appropriate.Place these steps under the heading of Procedure.

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

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Results

Record your findings under the heading of Results. Remember to use thecorrect units for density.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Conclusion

Under the heading of Conclusion, make a statement about the density ofeach of your samples linking them with the formation of the Earth.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________


A traditional Aboriginal perspective

The Aboriginal belief system is known as the Dreaming. In theDreaming, meaning is handed down from generation to generation by thetelling of mythological stories, which are often linked to the land.Their religion is the land and therefore has spiritual significance.

The following stages give a brief outline of the formation of the Earth asdescribed in one version of the Dreaming.

1 At the beginning the enormous flat mass of Earth was surrounded byevil and murky water and everything was enveloped in darkness.Sand and soil covered the surface and gigantic rocks were hiddenbeneath the surface. These rocks were supported by many huge treetrunks. Without this support the Earth would have collapsed in onitself and be lost forever.

2 Suddenly the Sun was born and he forced his way up through theland mass and still water. This was thought to be the first man andhis face was a blaze of fire which shed light on the very flat anddusty plains. The day ended when the Sun became weary andgradually sank underground leaving everything in darkness.

3 After resting the Sun returned to the east through an undergroundpassage which he had made. The Sun felt he must once again seethis strange place. The Sun’s habits have been regular since this firstday.

4 The Sun did not know that mysterious things were hiddenunderground and only needed light to make them flourish. The soilawakened and every day was different. Springs began to trickle outof the dirt and these springs eventually formed creeks, lagoons andlakes. Grasses, plants and trees rose from the soil covered with fruit,berries and flowers. The Sun saw more and different plants everyday, while the trees grew in height.

5 One day he saw an unbelievable change when the Earth beneath himappeared to erupt. Level ground was pushed toward him as hills andmountains appeared. Large trees and rocks rose from the soil andflowing water was spread in all directions. The Sun saw that livingcreatures were emerging from underground. They had been asleep,but now they had light, water and plant growth, and it was time towaken.

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In the boxes below make diagrammatic representations of how you visualisethe five stages described above.

1.

2.

3.

4.


5.

Discuss with members of your community, family or class why it isimportant to include a cultural perspective when considering the origin ofthe Earth.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Complete Exercise 2.2 at the end of this section.

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The evolution of the atmosphere

The composition of the atmosphere

As stated earlier, the Earth’s current atmosphere is largely composed oftwo gases:

• nitrogen (78%)

• oxygen (21%).

The remaining 1% is mostly made up of argon (0.93%), carbon dioxide(0.03%) and trace elements of other gases.

This composition is the result of a continual evolutionary process.

The early atmosphere

Our planet was unable to keep an atmosphere of gases such as hydrogen,as the radiation given off by the Sun would have driven these very lightgases away and they would have escaped into space.

Our early atmosphere is thought to have developed from within theEarth’s interior in a process known as outgassing or degassing.During this process gases are released from the Earth’s interior due tointernal heat gained from the process of accretion and the decay ofradioactive elements.

The earliest evidence we have for the composition of Earth’s primitiveatmosphere originates from rock samples. The oldest rocks on the planetare approximately 3800 million years old and they provide with someclues about the atmosphere at the time of their formation. This leaves aquestion mark over what our very first atmosphere was like, 800 to 900million years before these rocks were formed. We have nodirect evidence.

The current scientific view is that the early atmosphere probablyconsisted of methane (CH4), water vapour (H2O), nitrogen (N2), carbondioxide (CO2), ammonia (NH3), and hydrogen sulfide (H2S). Methane is


thought to have been the dominant gas at this early stage. The role ofmethane as a greenhouse gas in the early atmosphere was probablycritical to the development of the planet. In the early days of the planetthe Sun was much less luminous than it is today so the transfer of energyto Earth’s surface as solar radiation would have been much lower.Without the greenhouse effect of the methane atmosphere the presence ofliquid water essential to early life and sedimentary processes would nothave been possible on Earth’s surface.

The 3800 million year old rocks referred to above were found inGreenland and were originally deposited as sediments in water. It can beinferred that:

• the Earth had at least cooled to a surface temperature below 100°Cby this stage and that the greenhouse effect keeping the surfacetemperature in the range for liquid water was operating

• these sediments must have been derived from the erosion of evenolder rocks.

Comparison of atmospheric composition

Compare the composition of the early atmosphere to the gases present in ourcurrent atmosphere. Describe the similarities and differences between theearly atmosphere and the current one.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Check your answers.

Ideas and evidence for the composition of the Earth’s earliest atmosphere isan area of active research by scientists. Check out some of the links tostudies of this fascinating area on the EES website links page at:http://www.lmpc.edu.au/science.

Next…

So far you have learned about the origin of the Universe and the Earth.In the next part you will find out how scientists believe living cellsoriginated.

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Suggested answers

Formation of a layered Earth

lithosphere

asthenosphere

upper mantle

lower mantle

outer coreinner core

What is density?1 a) air

b) feathers

c) wood

d) water

e) coal

f) aluminium

g) steel

h) lead

2 A material more dense that aluminium and less dense that steelwould represent the density of the average Earth material.

The role of gravity

Even though the Earth has a stronger gravitational attraction than theMoon, the Moon will not be drawn into a collision with the Earth.This is because it also wants to be hurled outwards away from the Earthdue to its orbit around the Earth (centrifugal force). These two forces,gravity and the centrifugal force cancel each other out resulting in theMoon remaining in its present orbit.


Implications for a developing Earth

The fact that the Earth’s average density is virtually double that of therocks that go to make up continental crust, means that the Earth’s interiormust be made up of material that is far greater in density than thematerial in the Earth’s crust. This therefore supports the layered modelof the Earth.

Theory to explain a layered Earth

lithosphere(0–70 km)

asthenosphere(70–250 km)

upper mantle(250–700 km)

lower mantle(700–2900 km)

liquid outer core(2900–4980 km)

solid iron core(4980–6370 km)

The result is a zoned planet.

Lithosphere

The very dense elements such as iron, nickel, cobalt and the radioactiveelements are concentrated mostly in the inner and outer core.Material gradually becomes less dense in the mantle until the least densematerial is concentrated in the Earth’s crust which ‘floats’ on the moredense upper mantle. The atmosphere could even be considered theEarth’s outermost layer and is by far the least dense layer.

Comparison of atmospheric composition

It is difficult to say for certain what the early atmosphere was like,however, it did contain methane, nitrogen, carbon dioxide and watervapour like our current atmosphere. The present atmosphere containsmuch less methane, ammonia and hydrogen sulfide.

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Exercises – Part 2

Exercises 2.1 to 2.2 Name: _________________________________

Exercise 2.1: Formation of the Earth

Read carefully the previous sections on The earliest stage, The formationof a layered planet and Theory to explain a layered Earth. You mayeven wish to do some of your own research for this exercise.

Create a flow diagram summarising the events that led to the formationof the Earth. These diagrams should show a logical sequence from theEarth’s earliest beginnings through to its present day structure.Ensure these diagrams are well labelled and contain a summarisingcaption beneath each diagram.

Your flow diagram should contain at least six individual diagrams andshould not be too small (approximately 2 to 3 diagrams per page.)


Exercise 2.2: Research assignment

Research, and in no more than two A4 pages, report on the beliefs of aparticular cultural group on the Earth’s origin.

You may wish to take another story from the Dreaming, or you mightlike to take a particular religious viewpoint or perhaps even draw fromanother culture altogether.

In your two page report, include such things as relevant diagrams andquotes.

Your research may include: accessing the internet, visiting your locallibrary or even conducting an interview with an appropriate elder.

Below are some questions you may use to help guide your research.

• Where did the culture or religion you have chosen originate from?

• How is this perspective of the origin of the Earth communicated toothers? (eg. story, song, dance, symbols, pictures, rituals, writing)

• Has this means of communication changed over time?

• At what age and to whom is this perspective communicated?

• Does it have religious significance?

• Does it have any significance for the way people live from day today?

• Is there any similarity with any other culture (or religion)?

• Are there any other messages being communicated, apart from anexplanation for the origin of the Earth?

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