2-1 earths crust and interior

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Student: Date received:

Handout 6 of 14

(Topic 2.1)

Earths Crust and Interior

Seafloor topography around Iceland in the North Atlantic Ocean(http://en.wikipedia.org/wiki/Image:N-Atlantic-topo.png). Iceland has formedabove the Mid-Atlantic Ridge, on the boundary between the North Americaand Eurasian plates. Iceland is located above a plume of anomalously hotrock near the core-mantle boundary.
http://en.wikipedia.org/wiki/Image:N-Atlantic-topo.pnghttp://en.wikipedia.org/wiki/Image:N-Atlantic-topo.png


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Global PatternsThe Earths Crust and the Earths Interior

Key Ideas Intended Student LearningThe Earths Crust

Continental crust is different from oceanic crust. With the aid of maps and sectional diagrams,

compare continental crust and oceanic crust in termsof their:

global distribution;

thickness;

composition and density;

topographical features; age.

Continental crust consists of shields, orogenicbelts, and sedimentary basins.

Describe the typical ages, processes of formation,and topographic features of, and the rock typesassociated with, shields, orogenic belts, andsedimentary basins.

On a map of Australia, mark the locations of theWestern Australian, Gawler, Adelaide, Tasman, andEromanga crustal elements.

State the ages of each of the crustal elements listedabove.

List the distinguishing rock types in each crustalelement listed above.

Identify the tectonic crustal type in each crustalelement listed above.

Use the information above to explain how the

Australian continent has developed.

The Earths Interior

Evidence for the nature of the Earths interiorcan be obtained from seismic waves.

Explain the meaning of the terms focus andepicentre as they apply to an earthquake.

Describe the properties of P-waves and S-waves.

State the relative arrival times of P-waves and S-waves as shown by a typical seismogram.

Explain how the different arrival times of P-wavesand S-waves can be used to find the epicentre of an

earthquake.

Explain how the presence of shadow zones providesinformation about the layered structure of the Earth.

Using a diagram, describe the structure of theEarths interior, showing the crust, mantle, outercore, and inner core.

Describe the relative thickness, composition, andstate of each layer.

Topic 2.1 Earths Crust and Interior Page 2 of 28


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EARTHS CRUST

The crust is Earths outermost layer. It is a thin skin of relatively cool, brittlerock on which we live.

Continental and Oceanic CrustContinental crustand oceanic crustare very different in nature. Continentalcrust has a very complicated structure and variable composition, whereasoceanic crust has a simple layered structure and uniform composition.Differences between continental and oceanic crust are summarised in the tablebelow.

COMPARISON BETWEEN CONTINENTAL & OCEANIC CRUSTFeature Continental Crust Oceanic crust

Global distribution 35% of Earths surface - mainly in thenorthern hemisphere.

65% of Earths surface - mainly inthe southern hemisphere.

Average thickness 35 km 5 km

Maximum thickness 70 km 12 km

Topographical featuresFold mountain ranges, extensive areas oflow relief

Mid-ocean ridges, abyssal plains,trenches

Composition Sial (Silicon and aluminium) Sima (Silicon and magnesium)

Average density (gcm-3) 2.7 3.3

Age Up to 3800 Ma Up to 250 Ma

NB: The terms sima and sial are generic terms which describe the overallcomposition of continental and oceanic crust.

Sima - Silicon and m agnesium- is the material of the oceanic crust andupper mantle.Sial - Silicon and aluminium- is the material of the continental crust.

Global DistributionThe map of Earths landmasses on the left shows that oceanic crust occupiesthe majority of Earths surface, and that most of the continental crust lies inthe northern hemisphere


2.1 - Global Patterns


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The diagram below shows therelationship between oceanic andcontinental crust. The following pointsshould be noted:

Continental crust (sial) is muchthicker under mountain ranges

than beneath flat areas Highmountains have deep roots!

Oceanic crust (sima) is thought tolie beneath the continents as wellas forming the ocean floors.

Continental ShelvesAll continents are surrounded by continental shelves - regions of submergedcontinental crust where the water is comparatively shallow. Continentalshelves extend seawards from the shoreline to the upper edge of the continentalslope, where the depth of the water is usually about 200 metres. The shelfusually has a seaward slope of less than 1. At the outer edge of the continentalshelf there is an increase in slope which marks the beginning of the continentalslope. The continent-ocean boundary is half-way down the continental slope.

Structural Units of Continental CrustEarths continents consist essentially of three structural units - shields,orogenic belts and sedimentary basins.ShieldsShields are the oldest regions of continents. They are stable areas of thickcontinental crust - landmasses which have been severely folded and

metamorphosed, and have eroded for hundreds of millions, even thousands ofmillions of years. At least two thirds of Australia became a shield area by 1000



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Ma. The oldest rocks in Australia are in the Pilbara and Yilgarn Blocks ofWestern Australia where there are sedimentary strata as old as 3800 Ma.

In contrast to the mountainous nature of orogenic belts, shields are areas of lowrelief (essentially flat by world standards), possessing a thin surface cover ofunfolded sediments of terrestrial or marine origin.

The western part of Western Australia (i.e.Pilbara and Yilgarn Blocks), andthe Eyre Peninsula area of South Australia (Gawler Craton) are examples ofshields.

Predominant rock types in shields are schists,gneisses and granites.Orogenic Belts(also called Fold Belts, or Geosynclines)

An orogenic belt is a long linear area of Earth's crust which is undergoing, orhas undergone, intense deformation (i.e. folding) accompanied by seismic and

volcanic activity.

These are areas of fold mountain ranges, which may include both intrusive andextrusive igneous activity.

Such an orogenic belt may be formed when two continents collide and very highfold mountain ranges, such as the Himalayas, are formed.The Mount Lofty and Flinders Ranges in South Australia, and the GreatDividing Range of eastern Australia are examples of linear orogenic belts,

although they are much older and more eroded than the fold mountain rangesfound on other continents. Other orogenic belts include the Himalayas, the Alpsof Europe and the Andes of South America.

Ages of orogenic belts vary considerably from late Proterozoic, such as theMount Lofty and Flinders Ranges; to Cainozoic such as the Himalayas, Alpsand Andes (Earths highest mountain ranges are its youngest!).

A wide variety of rock types may be found in orogenic belts, including:

sedimentaryrocks such as sandstone, shale, and limestone. rocks produced by regional metamorphism- slate, schist, and gneiss. igneousrocks - granite and basalt.Sedimentary BasinsSedimentary basins are regions where thick layers of sediments have beendeposited on an older, eroded, 'basement' and where there has been nosignificant orogenic activity.

Most of the Australian continent consists of sedimentary basins, which vary inage from late Proterozoic to Cainozoic.



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All the rocks are sedimentary and include varieties of sandstone, shale andlimestone.

The map below shows the approximate locations of some of the most importantcrustal elements of the Australian continent.

Important Crustal Elements of the Australian ContinentThe table below summarises the significant features of the crustal elementsshown on the map:

Crustal element Age Distinguishing rock typesTectonic crustal

type

Eromanga (Basin)

Late Proterozoic to

Cainozoic Sandstone, limestone, shale Sedimentary basin

Tasman (Fold Belt) Palaeozoic

sandstone, shalelimestone, slate,schist, gneissgranite, basalt.

Orogenic belt

Adelaide(Geosyncline)

Late Proterozoic

Gawler (Craton) Early Proterozoic schist, gneissShield

Western Australian

(Shield)

Archaean granites

Development of the Australian Continent1. The Australian continent is one of Earth's

oldest and most stable land masses - the mostrecent, significant orogenic activity ended bythe beginning of the Mesozoic era (i.e. at 250Ma).

2. For most of its history, the Australian continentwas part of a much larger land mass. At around200 Ma the supercontinent Pangaea began to



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break apart into Laurasia and Gondwana. The shape of the present-dayAustralian continent did not finally come into existence until around 60Ma, when Australia and Antarctica began to move apart.

3. (The western and central parts of) Australia once formed part oflandmasses which existed even before the formation of Pangaea.

4. There is evidence that Australia moved extensively across the surface ofthe globe since 3800 Ma. Palaeomagnetic studies show that some rockswere formed when the continent was near the North Pole, others when itwas in the tropics and others when it was near the South Pole.

5. There is evidence that, during the Permian period, much of what is nowsouthern Australia was covered by a huge ice sheet, like the one thatcovers the Antarctic continent today.

6. In the Cretaceous Period and again during the Palaeogene and NeogenePeriods (formerly known as the Tertiary Period), the sea invaded largeareas of inland Australia, resulting in the deposition of marinesedimentary strata and the formation of sedimentary basins.

Processes in the Growth of the ContinentThe land which makes up the present-day Australian continent has beenaccreted (built-up) from the west. At the same time a series of orogenic beltshas been eroded to form shields, and sediments were deposited in long narrowtroughs (sometimes referred to geosynclines, e.g. the Adelaide Geosyncline).Eventually orogenesis turned these sediments into fold mountain ranges whichwere 'welded' onto the older continental nucleus in the west. In turn, thesemountain ranges have been eroded to form younger shields (e.g. the GawlerCraton).

The oldest rocks on the continent are found in the Pilbaraand YilgarnBlocks,which together comprise the Western Australian Shield, the nucleus of the

Australian continent.

The diagrams on the next page show the sequence of events by which the

Australian continent has, since 3800 Ma, 'grown' (accreted) progressively fromthe west by means of a succession of mountain ranges eroding to shields.

The most recent orogenic activity began early in the PalaeozoicEra (~ 530 Ma)and continued until the end of the Triassic Period (~ 180 Ma). It resulted in theformation of the Great Dividing Range from sediments deposited in theTasman Geosyncline. Essentially, there has been no orogenic activity withinthe Australian continent for since 180 Ma (i.e. the landmass has beentectonically stable), and consequently it has gradually eroding to form arelatively flat topography.



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EARTHS INTERIOR

The Nature of Earth s InteriorWe know more about outer space than

we do about the interior of our ownplanet. All our knowledge about Earth'sinterior comes from indirect evidence,such as seismic waves and thecomposition of meteorites. The adjacentdiagram shows that Earth consists offour major layers.

The diagram below shows the internal structure of the Earth in more detail,including the approximate depths of the boundaries between the layers.

The following table summarises the essential properties for each of Earth'slayers.

Name of Layer Thickness (km)Physical

stateComposition

Crust: Continental 25 - 40 solid granitic (sial)

Oceanic Average 12 solid basaltic (sima)Mantle 2900 solid peridotite

Outer core 2100 liquid alloy of Fe & Ni

Inner core 1400 solid same as outer core.



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EarthquakesThe passage of earthquake waves through Earth provides valuable informationabout the nature of its interior.

Earthquakes occur in areas where rocksare subject to directed pressure, whichcauses stress in the rocks. Thelithosphere (Earths solid outer layer)may bend until the stress exceeds thestrength of the rocks.

The lithosphere then breaks, or 'snaps'into a new position. In the breakingprocess, vibrations generated at thefracture travel through the rocks asearthquakes.

The focus of an earthquake is the location inside Earth of the fracture orfaulting which caused the earthquake.

The epicentreof the earthquake is the point on the surface of Earth situateddirectly above the focus.



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Earthquake WavesThe waves produced by earthquakes may be divided into two groups. They are:

1. Body waves travel through Earths interior. There are two types of bodywaves:a. rimary, or push-pull waves (P-waves).b. Secondary, or shear waves (S-waves).

2. Surface waves or L-waves, which travel around Earths surface. These are thewaves which cause earthquake damage.Primary WavesThese are the fastest waves produced by the earthquake. They travel throughEarth's interior, and reach recording stations first. They are longitudinalwaves, in which the particles of the medium (the material through which thewave is travelling) vibrate backwards and forwards along the line ofpropagation of the wave forming a series of compressionsand rarefactions.Compressionsare regions of the wave in which the particles of the medium areclosetogether.Rarefactionsare regions of the wave in which the particles of the medium arefurther apart.The diagram below shows the behaviour of the particles of a medium as a P-

wave passes through the medium.

Secondary WavesSecondary waves also travel through Earths interior. These are transversewaves in which the particles of the medium vibrate perpendicular to thedirection of propagation of the wave. A transverse wave consists of a series of

crestsand troughs, as shown in the diagram below.



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Refraction of Earthquake WavesThe speed at which a wave travels depends on the medium through which it is

travelling. As a wave passes from one medium to another its speed changes,and the direction in which it travels also changes. All kinds of waves undergo achange in direction, or refraction, as they pass from one medium to another.For example water waves are refracted as they pass from deep water intoshallow water, since their speed is less in shallow water. Light waves arerefracted as they pass from water into glass.

The density of Earth's mantle increases with depth, so that earthquake wavesare gradually refracted towards Earth's surface as they travel through themantle.

Shadow ZonesWherever an earthquake occurs, there are always some seismic stations aroundthe world which receive no waves at all from that earthquake. There are alsostations which receive only P-waves. This is because of the behaviour of thewaves as they pass from one of Earth's layers to the next one.

The P-Wave Shadow ZoneAs well as being gradually refracted as they pass through the mantle, P-wavesundergo refraction at the boundary between the mantle and the outer core. For

this reason, no P-waves are received by seismic stations in a band aroundEarth extending between 103 and 145 from the earthquake's epicentre. Thisregion is known as the P-wave shadowzone.Topic 2.1 Earths Crust and Interior Page 13 of 28


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The diagram below shows how P-waves are refracted at boundaries betweenEarth's layers to produce the P-wave shadow zone.

The adjacent diagram shows the P-wave shadow zone

produced by an earthquake which occurred at the NorthPole.

The extent of the P-wave shadow zone - between 103 and145 from the earthquake s epicentre - enables the depth ofthe boundary between the mantle and the outer core to becalculated.The S-Wave Shadow ZoneThe S-wave shadow zone is much more extensive than the corresponding P-wave zone. This is because S-waves are unable to travel through liquids andare therefore absorbed by the liquid outer core. The S-wave shadow zonetherefore extends from 103 on one side of the earthquaketo 103 on the other. Existence of the S-wave shadow zoneprovides evidence that Earth's outer core is liquid.

The diagram below shows the paths of the S-wavesthrough Earth's interior, and hence the S-wave shadow

zone.



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The adjacent diagram shows the S-wave shadow zone produced by anearthquake which occurred at the North Pole. It is much larger than the P-wave shadow zone produced by the same earthquake.



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EXERCISESEARTHS CRUST

Continental and Oceanic Crust1. Compare the essential features of continental and oceanic crust by

completing the table below.

Feature Continental crust Oceanic crustGlobal distribution

Average thickness

Maximum thicknessTopographical features

Composition

Average density (gcm-3)

Age (Ma)

2. Explain the meanings of the terms simaand sial.

3. The diagram below shows the relationship between oceanic andcontinental crust. On the diagram, label the following features:

continental shelf, continental slope, abyssal plain, continental crust,oceanic crust.



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4. Describe the essential features of a continental shelf.

Structural Units of Continental Crust1. In the table below, summarise the essential features of the principalstructural units of continental crust.

STRUCTURAL

UNITTYPICAL

AGESPROCESS OF

FORMATIONTOPOGRAPHIC

FEATURESPREDOMINANT

ROCK TYPES

2. On the map of the Australian continent shown below:

a. Identify the tectonic crustal types indicated in the key.

b. Name the five crustal elements which make up the continent.



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3. The diagram below contains a blank stratigraphic column. Complete thisstratigraphic column to show the periods of rock formation on the

Australian continent, and name the features which were formed.

Ma Era Period FEATURES OF AUSTRALIAN CONTINENT

0Neogene

24Cainozoic

Palaeogene

65Cretaceous

145Jurassic

210

Mesozoic

Triassic

250Permian

300 Carboniferous

350Devonian

400Silurian

440Ordovician

500

Palaeozoic

Cambrian

540Ediacaran

600

Proterozoic

2500

Archaean

STRATIGRAPHIC

COLUMN

CRUSTAL ELEMENTS

FORMED



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4. In the space provided below, write the names of the crustal elements inorder of their ages.

YOUNGEST

OLDEST

5. Use the table below summarises the significant features of the crustalelements shown on the map:

Crustal element Age (Ma)Distinguishing rock

types

Tectonic crustal

type

Development of the Australian Continent

1. When did the most recent major orogenic activity on the Australiancontinent end?

2. Explain why the Australian continent is one of Earth's most stable landmasses.

3. Name the super-continent which once encompassed all of Earth's landmasses.

4. When did this super-continent begin to break up?

5. Name the two land masses which were formed.



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6. When and how did the present-day Australian continent come intoexistence?

7. Is it true to say that Pangaea represents the distribution of land and sea

on Earth's surface from the formation of Earth until about 200 Ma?Explain your answer:

8. Has Australia always occupied its present position on Earth's surface?Explain your answer.

9. What are palaeomagnetic studies?

10. During which period did a large ice sheet cover much of southernAustralia?

11. Name some South Australian locations where there is evidence of

glaciation.

12. Name two periods in which the sea invaded large areas of inland

Australia.

13. What features are the results of these incursions?

14. Describe, in general terms, the processes by which the Australiancontinent has developed since 3800 Ma. Use diagrams to illustrate youranswer.



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15. Give the geological and geographic names of the feature which is a resultof the most recent orogenic activity on the Australian continent.

16. Name the periods during which this activity beganand ended.

17. The adjacent diagram shows thestructural features of an imaginarycontinent Walfordaria. It comprises afold mountain range and an erodedregion of low relief, where the rocksare mainly schists and gneisses.

In the space below, draw a series ofdiagrams showing the geologicalhistory of Walfordaria.



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18. Connect the appropriate pairs of words or phrases from the two listsbelow.

A range of fold mountainsPilbara and YilgarnBlocks

Pangaea Great Dividing Range

A very long period oferosion

Late in the Triassic Period

Show no signs of orogenicactivity

Cretaceous and TertiaryPeriods

Provides evidence for the'wandering' of the continent

The 'ancestor' of a shield

The Tasman crustalelement

Sedimentary basins

When much of inlandAustralia was under the sea

Leads to formation ofshields

The most recent orogenicactivity in Australia

Palaeomagnetic studies

The 'nucleus' of the

Australian continent

Formed by all the

continents joined together

The Nature of Earth's Interior

1. The diagram below shows a section through Earth. Give the names of thelayers numbered 1 to 4.

1:

2:

3:

4:



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2. The diagram below shows a section through Earth's interior. Draw theboundaries between the major layers in the appropriate places, and namethe material of which each layer is made.

3. Use the following table to summarise the essential properties of each of

Earth's layers.

Name of layer Thickness (km) Physical state Composition

Crust: Continental

OceanicMantle

Outer core

Inner core



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Earthquakes

1. The adjacent diagram shows anarea of Earths surface whererocks are being subjected todirected pressure causing stressto build-up.

a. In what way are rocksreacting to the appliedpressure?

b. Explain, with the aid of a seconddiagram, what happens whenthe applied force exceeds thestrength of the rocks.

2. Explain, with the aid of adiagram, the difference betweenthe focusand the epicentreof anearthquake.



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2. Use the table below to summarise the properties of P and S-waves.

TYPE OF

WAVE

MOVEMENT OFPARTICLES DIAGRAM

P-waves

S-waves

3. The diagram below shows a typical seismogram. Label it to show thearrivals of the different types of earthquake wave, and give the arrival

time of each wave type.



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4. Explain why earthquake waves are refractedas they pass through Earthsmantle.

5. Use the diagram of a part of Earths interior below to indicate the paths

of some of the body waves produced by an earthquake as they travelthrough the mantle.

6. Show on the diagram below the refraction of an earthquake wave passingfrom one of Earths layers to another.



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7. Use the two diagrams of Earth's interior provided below to show how the P

and S-wave shadow zones are formed.

P-wave shadow zone S-wave shadow zone

5. On the diagrams below, indicate the extent of the P and S wave shadowzones associated with an earthquake which occurred at the North Pole.

P-wave shadow zone S-wave shadow zone

6. What information is provided by the P-wave shadow zone?

7. What do we learn from the S-wave shadow zone?

2-1 earths crust and interior

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