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(Topic 2.2)

Plate

Earth (http://veimages.gsfc.nasa.gov/2429/globe_east_2048.jpg)

Global Processes Plate Tectonics Key Ideas Intended Student Learning Plate Tectonics provides evidence to support the Explain how continental drift is supported by of plate tectonics. matching: • the margins of ; • the continuation of geological structures; • types and ; • palaeoclimatic zones on widely separated continents. Explain how the evidence given above was used to reconstruct . -floor spreading results in the generation of Explain how each of the following forms of new oceanic at mid- ridges. evidence supports the observation that new is being generated at mid-ocean ridges: • Palaeomagnetic striping • Symmetry of age • Thickness of . The plate tectonics theory is a model that Describe, by means of well-labelled diagrams, the explains global tectonics in terms of the processes that occur at the following types of generation and of lithospheric plate plate boundary: material. • Constructive or divergent

• Conservative or transform • Destructive, convergent, or collisional: Oceanic–oceanic Oceanic–continental Continental–continental. Explain the contribution of each type of boundary to the overall movement of the plates, and describe the forms of igneous and activity that occur. Explain how the existence of a Benioff zone contributes to the understanding of the process of subduction. Discuss mechanisms that have been suggested as explanations of plate movement.

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2.2 - Global Processes

PLATE TECTONICS

Above: Plate based on Global Positioning System (GPS) data by NASA (http://en.wikipedia.org/wiki/Plate_tectonics).

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Continental Drift

The idea of continental drift - that the continents have not always occupied their present-day positions but have moved across - led to the theory of plate tectonics. This theory explains why the continents have moved, and are still moving.

Many features of widely separated continents can be matched, providing evidence that the continents were once joined together.

Matching Continental Margins

The edges of the continental shelves (not the actual coastlines) fit together like the pieces of a jigsaw puzzle.

Africa and provide the best evidence of matching continental shelves.

Continuation of Geological Structures

Geological structures, such as ranges, continue from one to another, although the continents are now many thousands of kilometres apart. For example, the Adelaide (or Adelaide ) continues into .

Rock Types and Fossils

Fossils of a late Palaeozoic reptile, Mesosaurus, have been found on both sides of the South , but nowhere else in the . If Mesosaurus were able to swim well enough to cross the ocean we would expect that its fossils were more widespread. Since it is confined to the two locations shown in the diagram, we must assume that and South America were joined when this reptile was alive.

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Above: Evidence for continental drift (http://en.wikipedia.org/wiki/Continental_drift)

Dinosaur remains found in show that the same types of dinosaurs existed here as in the rest of the world. (e.g. the copy of an Allosaurus skeleton in the Adelaide museum was made from remains found in America. Only a fossilised Allosaurus anklebone indicates that it once lived in what is now Australia.)

Palaeoclimatic Zones

Evidence of extreme cold or warm conditions can also be used to help establish the fact that the present-day continents have moved through geological .

1. Rocks showing evidence of the Permian glaciation, such as those at Victor Harbor and Port Elliot, are now widely distributed on continents that are separated by thousands of kilometres, as shown in the adjacent map.

If the southern continent of Gondwana is reconstructed by fitting these continents together, there is evidence that a single huge ice cap existed during the Permian.

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2. Coal is found in Antarctica. Obviously neither tropical nor temperate swamps and/or forests could exist in today's Antarctic .

Reconstruction of Gondwana

What did Earth look like near the close of the Proterozoic eon? Below is a reconstruction of the southern hemisphere at 550 Ma (http://en.wikipedia.org/wiki/Gondwana), the time of the Ediacaran .

The above evidence indicates that, at about 200 Ma, all Earth’s masses were joined to form a , . This broke into two continental blocks, the smaller of and Gondwana. Gondwana, consisted of the present land masses of Australia, Antarctica, , South America and Africa. Eventually this began to break into the continents we know today. The final separation, between Australia and Antarctica occurred at approximately 50 Ma.

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Above: Sequence of maps showing how the supercontinent Pangaea began to separate at approximately 225 Ma (Source: http://geology.com/pangea.htm).

Sea Floor Spreading

Plate tectonics theory proposes that a process known as sea floor spreading causes movement of the continents. New oceanic crust is continually generated at mid-ocean ridges forcing the existing crust to move away from the ridge. Three major lines of evidence support this concept.

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Symmetry of Age

The age of the ocean floor increases with increasing distance from the mid-ocean ridges. It also increases symmetrically on either side of a ridge. This provides evidence that basalt is continually being produced at mid-ocean ridges, as shown in the above diagram.

Nowhere is the age of the ocean floor greater than 200 million years.

Above: Age of ocean floor (http://en.wikipedia.org/wiki/Sea_floor_spreading)

Thickness of Sediment

It has been found that the distribution of deep-water (as distinct from sediments derived from the land) follows a similar pattern in all . The thickness of sediments increases with distance from each mid-ocean ridge, and this pattern is symmetrical on both sides of the ridge, providing further

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evidence that the ocean floor near the mid-ocean ridges is very young, while the age of the floor increases with increasing distance from the ridge.

Magnetic Striping on the Ocean Floor

Earth possesses a magnetic - a region where forces are exerted on magnetic materials, such as iron. Earth’s is similar to that of a bar magnet. In other words, Earth behaves as if it has a bar magnet at its centre.

The adjacent diagram shows the pattern of lines of force of Earth’s magnetic field.

Igneous rocks show the direction of Earth's magnetic field at the time of their formation. As or lava cools, grains of magnetic (e.g. , ilmenite) align themselves in the direction of Earth's magnetic field.

Drill cores recovered from the ocean floor show that Earth's magnetic field has reversed many in the past several million years. Some of the cores' "slices" indicate that the was in its present position when the sediments were laid down. We call this a time of normal polarity. Other "slices" indicate that, when the sediments were laid down, the South magnetic pole was where the North Pole is today. We call this a time of reverse polarity. Periods of normal and reverse polarity have alternated, at irregular intervals, throughout much of Earth's history. No clear reasons for these reversals are known.

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A is a small change in Earth's magnetic field that may be detected by an instrument called a , carried in a plane.

A magnetic anomaly is detected when a magnetometer passes from a region in which igneous rocks solidified during a period of normal magnetic polarity (i.e. as it is today) to one in which the rocks solidified during a time or reverse polarity.

In 1961, a magnetic survey off the of showed a pattern of magnetic anomaly stripes over the floor of the Atlantic Ocean.

The stripes show that some of the ocean floor basalt was formed when Earth's magnetic field was as it is today - normal polarity - while adjacent 'stripes' of basalt were formed during periods when the magnetic poles were reversed. This pattern continues along the whole length of the ridge, and is symmetrical on both sides of the ridge, indicating the basalt is actually formed along the ridge.

Similar magnetic anomaly patterns are associated with all mid-ocean ridges.

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Therefore magnetic striping on the ocean floor provides evidence that the sea floor is spreading away from the mid-ocean ridges and explains how the continents move. New oceanic crust is formed when basalt erupts along the ridges, and existing crust is forced away from the ridge, causing sea floor spreading.

Discovery of magnetic striping on the ocean floor in the 1960s was historically very significant to the development of plate tectonic theory, because it suggested a mechanism by which the continents can move. During the 1920s Alfred , a meteorologist, collected and published evidence for continental drift, but he had not been believed - partly because he could not suggest a mechanism by which the continents could move. Magnetic striping, leading to the idea of sea floor spreading, provided the missing evidence!

The Theory of Plate Tectonics

The plate tectonics theory encompasses the following major ideas:

• The outer 50 to 150 km of Earth (i.e. the crust and part of the upper - called the ) is broken up into a number of rigid plates that move with respect to each other.

• Interaction between these plates at their boundaries results in , vulcanism and crustal deformation.

• Most of the boundaries occur in the oceans, or on the ocean-continent margins, but some plate boundaries are on continents.

• At the boundaries, the plates are converging, diverging or moving laterally past each other.

• Each type of boundary is characterised by distinctive tectonic features.

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Plate boundaries can be divided into three main types:

The three types of plate boundaries are illustrated in the diagram above

1. Divergent (or constructive) - the plates are moving apart, new crust is being created.

2. Conservative - plates are sliding past each other. Crust is not being created or destroyed.

3. Convergent (or destructive) - plates are moving together, crust is being destroyed.

There are three types of convergent plate boundary (or subduction zone).

i. Continental plate - continental plate ii. Continental plate - oceanic plate iii. Oceanic plate - oceanic plate.

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1. Divergent Plate Boundaries

Mid-ocean ridges are divergent, or constructive, plate boundaries. New lithosphere is formed by eruption of basaltic magma at these boundaries, and the plates move apart at ~ 5 to 10 cm a year. The ridges contain valleys where magma is welling up from the mantle. They are areas where basaltic volcanoes are common, and shallow-focus earthquakes occur due to the movement of up-welling magma.

The Mid-Atlantic Ridge rises above in Iceland, which contains a rift where basaltic eruptions and earthquakes occur.

2. Conservative Plate Boundaries

Transform Faults

The mid-ocean ridges are not continuous structures. All along their lengths, they are offset by a series of faults, known as transform faults.

At conservative plate boundaries, lithosphere is neither created nor destroyed. Two plates slide laterally past each other, and earthquakes occur due to between the edges of the plates. There is no volcanic activity.

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The San Andreas is the best-known example of a conservative plate boundary. In this region the is moving northwards relative to the .

3. Convergent Plate Boundaries

Convergent, or destructive plate boundaries are also known as subduction zones. They are regions where one plate is dipping, or subducting, under another and lithosphere is being destroyed. There are three types of divergent plate boundary. i. Continental Plate - Continental Plate Boundary

In this scenario, one continental plate dips and pushes beneath the other. The best-known example of this type of boundary is that between India and the rest of . The formation of this boundary is described on the next page.

As the two continents approached one another, sediments were constantly filling the sea between them. refer to this ancient ocean as the Sea of Tethys, or simply Tethys.

When the two continents actually collided, orogenesis occurred. Consequently these sediments became a range of fold - the , the highest on Earth. Intrusive igneous activity occurs at this type of plate boundary, but there are very few (if any) volcanoes, as the magma cannot move up through the thick layers of sediments. Both shallow and deep focus earthquakes occur in this kind of tectonic setting.

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ii. Continental Plate-Oceanic Plate boundary

A deep ocean trench and subduction are formed where the oceanic plate is subducted under the continental plate. The edge of the continental plate is crumpled to form a range of . As the descending plate melts, both intrusive and extrusive volcanic activity (andesitic lava) occurs in the fold mountain range. Both shallow and deep-focus earthquakes are associated with these boundaries. Shallow-focus earthquakes occur where the subducting plate bends before dipping into Earth. Deep-focus earthquakes are due to friction between the subducting plate and the lithosphere past that it is moving.

The western seaboard of South America, with a deep ocean trench and the Mountains, is an example of a continental plate-oceanic plate boundary.

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iii. Oceanic Plate - Oceanic Plate Boundary

A trench - -arc margin is the other example of an active .

Again, a deep ocean trench is formed where one plate dips under the other. An island-arc - a chain of volcanic (andesitic lava) - is formed where magma from the subducting plate rises to the surface. Both shallow and deep-focus earthquakes also occur at these boundaries.

The Japanese islands represent an example of an oceanic plate - oceanic plate boundary as is shown in the following diagram.

The Benioff Zone

This zone is associated with all destructive plate boundaries. It is the zone of deep-focus earthquakes caused by the friction between the subducting plate and the lithosphere against which it is moving. It extends to a depth of

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about 700 km below the earth's surface. Consequently, earthquake foci can also be up to 700 km deep.

Mechanisms Causing Plate Movement

These mechanisms are still not fully understood, but the most important mechanism is thought to be currents in the mantle. The continents are thought to be 'passengers' on a conveyer belt of oceanic crust. Since new crust is formed at the mid-ocean ridges, and crust is destroyed in subduction zones, the continents move away from mid-ocean ridges and towards subduction zones.

Below: Convection currents in the mantle are the driving force of plate tectonics and mountain building.

Source: http://en.wikipedia.org/wiki/Image:Oceanic_spreading.png

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EXERCISES

Continental Drift

1. Several forms of evidence that led eventually to the theory of plate tectonics.

2. The evidence for continental drift may be grouped into four categories. Name these four types of evidence.

3. Use the table below to summarise the essential features of each type of evidence for continental drift.

TYPE OF DESCRIPTION OF EXAMPLE/S EVIDENCE EVIDENCE

4. The adjacent diagram shows a late Palaeozoic reptile Mesosaurus, of which fossils are found on both sides of the South Atlantic, but nowhere else in the world.

Do you think Mesosaurus was able to swim well enough to cross the Atlantic Ocean?

Suggest its likely distribution if it had been a good swimmer.

Suggest why fossils of Mesosaurus are confined to Africa and South America.

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5. Rocks showing evidence of the Permian glaciation, such as those we saw at Victor Harbor and Port Elliot, are now widely distributed on continents which are separated by thousands of kilometres, as shown in the adjacent map.

Suggest an explanation for this distribution of glacial evidence.

6. Coal is found in Antarctica. What can you deduce from this information?

7. The diagram below shows Earth’s grouped as they were at about 150 Ma.

a. On the diagram label the present day landmasses of Australia, Antarctica, India, Africa and South America.

b. Name the continent that all these landmasses once formed.

Sea Floor Spreading

1. Describe, with the aid of a diagram, the process of sea floor spreading.

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2. Explain how evidence to support the idea of sea floor spreading is provided by: a. The ages of the ocean floor .

b. The thickness of non-terrestrial sediments on the ocean floor.

3. The adjacent diagram shows a volcanic cone with a flow of basalt from a past eruption.

Explain why this basalt provides information about the direction of the Earth’s magnetic field at the time of the ’s last eruption.

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4. The adjacent diagram shows a drill core obtained from sediments on the ocean floor.

Use the diagram to answer the following questions:

a. How many times did the earth's magnetic poles reverse during the time indicated by the drill core?

b. Ten million years ago, was the earth's polarity normal (i.e. like it is today) or reversed?

c. Give the ages of the earliest and most recent reversal of polarity shown on the diagram, and state the direction of the reversal in each case. Earliest reversal: Most recent reversal:

6. What is a magnetic anomaly?

7. Draw a diagram showing the linear magnetic anomalies that would be detected by a magnetometer in an aircraft flying over a mid-ocean ridge.

8. Draw a labelled diagram explaining how linear magnetic anomalies provide evidence for sea floor spreading.

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The Theory of Plate Tectonics

1. Write down five major ideas of the plate tectonics theory.

A.

B.

C.

D.

E.

2. Use the diagram below to summarise the different types of plate boundary.

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3. Describe, with the aid of diagrams, the significant features of each of the following types of plate boundary:

i. Divergent boundaries (mid-ocean ridges).

ii. Conservative boundaries

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iii. Convergent (destructive) plate boundaries.

a. Continental Plate - Continental Plate

b. Continental Plate - Oceanic Plate

c. Oceanic Plate - Oceanic Plate

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3. Draw a diagram to show the most significant mechanism that is thought to control the movement of the continents.

4. On the map of the world drawn below, mark the following features: a. the names of the major land masses, the Atlantic Ocean and the .

b. the Andes, the Himalayas, the Hawaiian Islands, the Mid-Atlantic Ridge, Iceland and the , the Japanese island-arc.

c. the '' around the Pacific Ocean.

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DRIFTING CONTINENTS PUZZLE

The next page shows a map of two imaginary continents, Walfia and Ordic.

1. Describe two forms of evidence that could be used to determine whether the continents Walfia and Ordic were once joined.

2. curves for three different decay processes are shown below. Use these radioactive decay graphs and the table below to determine the ages of the different rock units on the two continents.

AGES OF THE ROCK UNITS

ROCK RADIOACTIVE % 'PARENT' AGE OF ROCK UNIT ELEMENT PRESENT ELEMENT UNIT (x109 yrs) 238 A U 60.0 235 B U 25.0 87 C Rb 97.5 235 X U 12.5 238 Y U 95.0 232 Z Th 87.5

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2. Using the ages of the rocks recorded in your table, draw lines between the continents on p. 21 joining rock units of similar ages.

In what way do the ages of corresponding rock types support the idea that these two continents were once joined?

3. To see if the continents fit, cut them out and glue them in the below, to make the continent that may once have existed.

Suggest a name for this continent.

4. Rock units D and W were not suitable for radiometric dating, but their likely ages can now be inferred. D. W.

THE DRIFTING CONTINENTS

• When you have found the ages of the rock units A, B, C, X, Y and Z, draw lines connecting rock units of similar ages.

• Cut the continents out and glue them in the space provided on page 20, to show that the two continents were once joined.

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