earth’s crust and interior - marden senior collegegeoscience.msc.sa.edu.au/library/2-1 earths...

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(Topic 2.1)

Earth’s Crust and Interior

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

http://en.wikipedia.org/wiki/Image:N-Atlantic-topo.png

Global Patterns The Earth’s Crust and the Earth’s Interior

Key Ideas Intended Student Learning The Earth’s Crust

Continental crust is different from oceanic crust.

With the aid of maps and sectional diagrams, compare continental crust and oceanic crust in terms of their:

• global distribution; • thickness; • composition and density; • topographical features; • age.

Continental crust consists of shields, orogenic belts, and sedimentary basins.

Describe the typical ages, processes of formation, and topographic features of, and the rock types associated with, shields, orogenic belts, and sedimentary basins.

On a map of Australia, mark the locations of the Western Australian, Gawler, Adelaide, Tasman, and Eromanga crustal elements.

State the ages of each of the crustal elements listed above.

List the distinguishing rock types in each crustal element listed above.

Identify the tectonic crustal type in each crustal element listed above.

Use the information above to explain how the Australian continent has developed.

The Earth’s Interior

Evidence for the nature of the Earth’s interior can be obtained from seismic waves.

Explain the meaning of the terms ‘focus’ and ‘epicentre’ 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-waves and S-waves can be used to find the epicentre of an earthquake.

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

Using a diagram, describe the structure of the Earth’s interior, showing the crust, mantle, outer core, and inner core.

Describe the relative thickness, composition, and state of each layer.

Topic 2.1 Earth’s Crust and Interior of 28

EARTH’S CRUST

The crust is Earth’s outermost layer. It is a thin skin of relatively cool, brittle rock on which we live. Continental and Oceanic Crust

Continental crust and oceanic crust are very different in nature. Continental crust has a very complicated structure and variable composition, whereas oceanic crust has a simple layered structure and uniform composition. Differences between continental and oceanic crust are summarised in the table below.

COMPARISON BETWEEN CONTINENTAL & OCEANIC CRUST

Feature Continental Crust Oceanic crust

Global distribution 35% of Earth’s surface - mainly in the northern hemisphere.

65% of Earth’s surface - mainly in the southern hemisphere.

Average thickness 35 km 5 km

Maximum thickness 70 km 12 km

Topographical features Fold mountain ranges, extensive areas of low 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 overall composition of continental and oceanic crust.

Sima - Silicon and magnesium - is the material of the oceanic crust and upper mantle.

Sial - Silicon and aluminium - is the material of the continental crust.

Global Distribution The map of Earth’s landmasses on the left shows that oceanic crust occupies the majority of Earth’s surface, and that most of the continental crust lies in the northern hemisphere


2.1 - Global Patterns

The diagram below shows the relationship between oceanic and continental crust. The following points should be noted:

• Continental crust (sial) is much thicker under mountain ranges than beneath flat areas High mountains have deep roots!

• Oceanic crust (sima) is thought to lie beneath the continents as well as forming the ocean floors.

Continental Shelves

All continents are surrounded by continental shelves - regions of submerged continental crust where the water is comparatively shallow. Continental shelves extend seawards from the shoreline to the upper edge of the continental slope, where the depth of the water is usually about 200 metres. The shelf usually has a seaward slope of less than 1°. At the outer edge of the continental shelf there is an increase in slope which marks the beginning of the continental slope. The continent-ocean boundary is half-way down the continental slope. Structural Units of Continental Crust Earth’s continents consist essentially of three structural units - shields, orogenic belts and sedimentary basins. Shields Shields are the oldest regions of continents. They are stable areas of thick continental crust - landmasses which have been severely folded and metamorphosed, and have eroded for hundreds of millions, even thousands of millions of years. At least two thirds of Australia became a shield area by 1000


Ma. The oldest rocks in Australia are in the Pilbara and Yilgarn Blocks of Western Australia where there are sedimentary strata as old as 3800 Ma. In contrast to the mountainous nature of orogenic belts, shields are areas of low relief (essentially flat by world standards), possessing a thin surface cover of unfolded sediments of terrestrial or marine origin. The western part of Western Australia (i.e. Pilbara and Yilgarn Blocks), and the Eyre Peninsula area of South Australia (Gawler Craton) are examples of shields. 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, or has undergone, intense deformation (i.e. folding) accompanied by seismic and volcanic activity. These are areas of fold mountain ranges, which may include both intrusive and extrusive igneous activity. Such an orogenic belt may be formed when two continents collide and very high fold mountain ranges, such as the Himalayas, are formed. The Mount Lofty and Flinders Ranges in South Australia, and the Great Dividing Range of eastern Australia are examples of linear orogenic belts, although they are much older and more eroded than the fold mountain ranges found on other continents. Other orogenic belts include the Himalayas, the Alps of Europe and the Andes of South America. Ages of orogenic belts vary considerably from late Proterozoic, such as the Mount Lofty and Flinders Ranges; to Cainozoic such as the Himalayas, Alps and Andes (Earth’s highest mountain ranges are its youngest!). A wide variety of rock types may be found in orogenic belts, including: • sedimentary rocks such as sandstone, shale, and limestone.

• rocks produced by regional metamorphism - slate, schist, and gneiss. • igneous rocks - granite and basalt. Sedimentary Basins

Sedimentary basins are regions where thick layers of sediments have been deposited on an older, eroded, 'basement' and where there has been no significant orogenic activity.

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


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

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

Important Crustal Elements of the Australian Continent

The table below summarises the significant features of the crustal elements shown on the map:

Crustal element Age Distinguishing rock types Tectonic crustal type

Eromanga (Basin) Late Proterozoic to Cainozoic Sandstone, limestone, shale Sedimentary basin

Tasman (Fold Belt) Palaeozoic sandstone, shale limestone, slate, schist, gneiss granite, basalt.

Orogenic belt

Adelaide (Geosyncline) Late Proterozoic

Gawler (Craton) Early Proterozoic schist, gneiss Shield

Western Australian (Shield) Archaean granites

Development of the Australian Continent

1. The Australian continent is one of Earth's oldest and most stable land masses - the most recent, significant orogenic activity ended by the beginning of the Mesozoic era (i.e. at 250 Ma).

2. For most of its history, the Australian continent was part of a much larger land mass. At around 200 Ma the supercontinent Pangaea began to


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

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

4. There is evidence that Australia moved extensively across the surface of the globe since 3800 Ma. Palaeomagnetic studies show that some rocks were formed when the continent was near the North Pole, others when it was 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 now southern Australia was covered by a huge ice sheet, like the one that covers the Antarctic continent today.

6. In the Cretaceous Period and again during the Palaeogene and Neogene Periods (formerly known as the Tertiary Period), the sea invaded large areas of inland Australia, resulting in the deposition of marine sedimentary strata and the formation of sedimentary basins.

Processes in the Growth of the Continent

The land which makes up the present-day Australian continent has been accreted (built-up) from the west. At the same time a series of orogenic belts has been eroded to form shields, and sediments were deposited in long narrow troughs (sometimes referred to geosynclines, e.g. the Adelaide Geosyncline). Eventually orogenesis turned these sediments into fold mountain ranges which were 'welded' onto the older continental nucleus in the west. In turn, these mountain ranges have been eroded to form younger shields (e.g. the Gawler Craton).

The oldest rocks on the continent are found in the Pilbara and Yilgarn Blocks, 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 from the west by means of a succession of mountain ranges eroding to shields.

The most recent orogenic activity began early in the Palaeozoic Era (~ 530 Ma) and continued until the end of the Triassic Period (~ 180 Ma). It resulted in the formation of the Great Dividing Range from sediments deposited in the Tasman Geosyncline. Essentially, there has been no orogenic activity within the Australian continent for since 180 Ma (i.e. the landmass has been tectonically stable), and consequently it has gradually eroding to form a relatively flat topography.


EARTH’S INTERIOR

The Nature of Earth's Interior

We know more about outer space than we do about the interior of our own planet. All our knowledge about Earth's interior comes from indirect evidence, such as seismic waves and the composition of meteorites. The adjacent diagram shows that Earth consists of four 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's layers.

Name of Layer Thickness (km) Physical state Composition

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.


Earthquakes

The passage of earthquake waves through Earth provides valuable information about the nature of its interior.

Earthquakes occur in areas where rocks are subject to directed pressure, which causes stress in the rocks. The lithosphere (Earth’s solid outer layer) may bend until the stress exceeds the strength of the rocks. The lithosphere then breaks, or 'snaps' into a new position. In the breaking process, vibrations generated at the fracture travel through the rocks as earthquakes. The focus of an earthquake is the location inside Earth of the fracture or faulting which caused the earthquake. The epicentre of the earthquake is the point on the surface of Earth situated directly above the focus.


Earthquake Waves

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

1. Body waves travel through Earth’s interior. There are two types of body waves:

a. Primary, or push-pull waves (P-waves).

b. Secondary, or shear waves (S-waves). 2. Surface waves or L-waves, which travel around Earth’s surface. These are the

waves which cause earthquake damage. Primary Waves

These are the fastest waves produced by the earthquake. They travel through Earth's interior, and reach recording stations first. They are longitudinal waves, in which the particles of the medium (the material through which the wave is travelling) vibrate backwards and forwards along the line of propagation of the wave forming a series of compressions and rarefactions.

Compressions are regions of the wave in which the particles of the medium are close together.

Rarefactions are regions of the wave in which the particles of the medium are further apart.

The diagram below shows the behaviour of the particles of a medium as a P-wave passes through the medium. Secondary Waves

Secondary waves also travel through Earth’s interior. These are transverse waves in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave. A transverse wave consists of a series of crests and troughs, as shown in the diagram below.

Topic 2.1 Earth’s Crust and Interior Page 12 of 28

Refraction of Earthquake Waves

The 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 a change in direction, or refraction, as they pass from one medium to another. For example water waves are refracted as they pass from deep water into shallow water, since their speed is less in shallow water. Light waves are refracted as they pass from water into glass.

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

Shadow Zones

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

As well as being gradually refracted as they pass through the mantle, P-waves undergo 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 around Earth extending between 103° and 145° from the earthquake's epicentre. This region is known as the P-wave shadow zone. Topic 2.1 Earth’s Crust and Interior Page 13 of 28

The diagram below shows how P-waves are refracted at boundaries between Earth'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 North Pole.

The extent of the P-wave shadow zone - between 103° and 145° from the earthquake's epicentre - enables the depth of the boundary between the mantle and the outer core to be calculated. The S-Wave Shadow Zone

The 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 and are therefore absorbed by the liquid outer core. The S-wave shadow zone therefore extends from 103° on one side of the earthquake to 103° on the other. Existence of the S-wave shadow zone provides evidence that Earth's outer core is liquid.

The diagram below shows the paths of the S-waves through Earth's interior, and hence the S-wave shadow zone.


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


EXERCISES EARTH’S CRUST

Continental and Oceanic Crust

1. Compare the essential features of continental and oceanic crust by completing the table below.

Feature Continental crust Oceanic crust Global distribution Average thickness Maximum thickness Topographical features Composition Average density (gcm-3) Age (Ma)

2. Explain the meanings of the terms sima and sial.

3. The diagram below shows the relationship between oceanic and continental crust. On the diagram, label the following features: continental shelf, continental slope, abyssal plain, continental crust, oceanic crust.


4. Describe the essential features of a continental shelf.

Structural Units of Continental Crust

1. In the table below, summarise the essential features of the principal structural units of continental crust.

STRUCTURAL UNIT

TYPICAL AGES

PROCESS OF FORMATION

TOPOGRAPHIC FEATURES

PREDOMINANT 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. Topic 2.1 Earth’s Crust and Interior Page 17 of 28

3. The diagram below contains a blank stratigraphic column. Complete this stratigraphic 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 0

Neogene

24 Cainozoic

Palaeogene

65 Cretaceous

145 Jurassic

210

Mesozoic

Triassic

250 Permian

300 Carboniferous

350 Devonian

400 Silurian

440 Ordovician

500

Palaeozoic

Cambrian

540 Ediacaran

600

Proterozoic

2500 Archaean

STRATIGRAPHIC

COLUMN

CRUSTAL ELEMENTS FORMED


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

YOUNGEST

OLDEST

5. Use the table below summarises the significant features of the crustal elements 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 Australian

continent end?

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

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

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

5. Name the two land masses which were formed.


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

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 southern Australia?

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 Australian continent has developed since 3800 Ma. Use diagrams to illustrate your answer.


15. Give the geological and geographic names of the feature which is a result

of the most recent orogenic activity on the Australian continent.

16. Name the periods during which this activity began and ended.

17. The adjacent diagram shows the structural features of an imaginary continent Walfordaria. It comprises a fold mountain range and an eroded region of low relief, where the rocks are mainly schists and gneisses.

In the space below, draw a series of diagrams showing the geological history of Walfordaria.


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

A range of fold mountains Pilbara and Yilgarn Blocks

Pangaea Great Dividing Range

A very long period of erosion Late in the Triassic Period

Show no signs of orogenic activity Cretaceous and Tertiary

Periods

Provides evidence for the 'wandering' of the continent The 'ancestor' of a shield

The Tasman crustal element Sedimentary basins

When much of inland Australia was under the sea Leads to formation of

shields

The most recent orogenic activity 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 the

layers numbered 1 to 4.

1:

2:

3:

4:


2. The diagram below shows a section through Earth's interior. Draw the boundaries between the major layers in the appropriate places, and name the 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

Oceanic

Mantle

Outer core

Inner core


Earthquakes 1. The adjacent diagram shows an

area of Earth’s surface where rocks are being subjected to directed pressure causing stress to build-up.

a. In what way are rocks

reacting to the applied pressure?

b. Explain, with the aid of a second

diagram, what happens when the applied force exceeds the strength of the rocks.

2. Explain, with the aid of a

diagram, the difference between the focus and the epicentre of an earthquake.


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

TYPE OF WAVE

MOVEMENT OF PARTICLES DIAGRAM

P-waves

S-waves

3. The diagram below shows a typical seismogram. Label it to show the

arrivals of the different types of earthquake wave, and give the arrival time of each wave type.


4. Explain why earthquake waves are refracted as they pass through Earth’s mantle.

5. Use the diagram of a part of Earth’s interior below to indicate the paths of some of the body waves produced by an earthquake as they travel through the mantle.

6. Show on the diagram below the refraction of an earthquake wave passing

from one of Earth’s layers to another.


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 shadow

zones 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?


earth’s crust and interior - marden senior collegegeoscience.msc.sa.edu.au/library/2-1 earths...

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