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Handout 7 of 14
(Topic 2.2)
Plate Tectonics
Earth (http://veimages.gsfc.nasa.gov/2429/globe_east_2048.jpg)
Global Processes
Plate Tectonics
Key Ideas
Intended Student Learning
Plate Tectonics
Continental drift provides evidence to support the
theory of plate tectonics.
Explain how continental drift is supported by
matching:
•
•
•
•
the margins of continents;
the continuation of geological
structures;
rock types and fossils;
palaeoclimatic zones on widely
separated continents.
Explain how the evidence given above was used
to reconstruct Gondwana.
Sea-floor spreading results in the generation of
new oceanic crust at mid-ocean ridges.
Explain how each of the following forms of
evidence supports the observation that new
oceanic crust is being generated at mid-ocean
ridges:
•
•
•
The plate tectonics theory is a model that
explains global tectonics in terms of the
generation and subduction of lithospheric plate
material.
Palaeomagnetic striping
Symmetry of age
Thickness of sediment.
Describe, by means of well-labelled diagrams, the
processes that occur at the following types of
plate boundary:
•
•
•
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 earthquake
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.
Topic 2.2
Plate Tectonics
Page 2 of 28
2.2 - Global Processes
PLATE TECTONICS
Above: Plate motion based on Global Positioning System (GPS) data by NASA
(http://en.wikipedia.org/wiki/Plate_tectonics).
Topic 2.2
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 Earth - 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 South America provide the best evidence
of matching continental shelves.
Continuation of Geological Structures
Geological structures, such as fold mountain ranges,
continue from one continent to another, although the
continents are now many thousands of kilometres
apart. For example, the Adelaide Orogenic Belt (or
Adelaide Geosyncline) continues into Antarctica.
Rock Types and Fossils
Fossils of a late Palaeozoic reptile, Mesosaurus, have been found on both sides
of the South Atlantic Ocean, but nowhere else in the world. 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 Africa and South America were joined when
this reptile was alive.
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Plate Tectonics
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Above: Fossil Evidence for continental drift (http://en.wikipedia.org/wiki/Continental_drift)
Dinosaur remains found in Australia 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 time.
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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Plate Tectonics
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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 climate.
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 fauna.
The above evidence indicates
that, at about 200 Ma, all
Earth’s land masses were joined
to form a supercontinent,
Pangaea. This broke into two
continental blocks, the smaller
supercontinents of Laurasia and
Gondwana.
Gondwana,
consisted of the present land
masses of Australia, Antarctica,
India, South America and
Africa.
Eventually
this
landmass 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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Plate Tectonics
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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 basalt 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 sediments (as distinct
from sediments derived from the land) follows a similar pattern in all oceans.
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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Plate Tectonics
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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 field - a region where forces are exerted on
magnetic materials, such as iron. Earth’s magnetic field 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 magma or lava cools, grains of magnetic
minerals (e.g. magnetite, 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 times in the past several million years. Some of the cores'
"slices" indicate that the North Pole 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
magnetic anomaly is a small change in Earth's magnetic field that may be
detected by an instrument called a magnetometer, 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 coast of North America 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 Wegener, 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
mantle - called the lithosphere) 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
earthquakes, 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 Tectonics
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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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Plate Tectonics
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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 rift 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 sea level in Iceland, which contains a rift
valley 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 friction
between the edges of the plates. There is no volcanic activity.
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Plate Tectonics
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The San Andreas Fault
is
the
best-known
example
of
a
conservative
plate
boundary. In this region
the Pacific plate is
moving
northwards
relative to the North
American plate.
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 Asia. 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. Geologists 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 mountains - the Himalayas, the highest
mountain range 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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Plate Tectonics
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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 fold mountains. 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
Andes Mountains, is an example of a continental plate-oceanic plate boundary.
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Plate Tectonics
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iii. Oceanic Plate - Oceanic Plate Boundary
A trench - island-arc margin is the other example of an active continental
margin.
Again, a deep ocean trench is formed where one plate dips under the other. An
island-arc - a chain of volcanic islands (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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Plate Tectonics
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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 convection 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
Topic 2.2
Plate Tectonics
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EXERCISES
Continental Drift
led
1.
Several forms of evidence that
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
EVIDENCE
4.
DESCRIPTION OF
EVIDENCE
EXAMPLE/S
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.
Topic 2.2
Plate Tectonics
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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
landmasses 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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Plate Tectonics
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2.
3.
Explain how evidence to support the idea of sea floor spreading is provided
by:
a.
The ages of the ocean floor basalts.
b.
The thickness of non-terrestrial sediments on the ocean floor.
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 volcano’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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Plate Tectonics
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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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Plate Tectonics
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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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Plate Tectonics
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iii. Convergent (destructive) plate boundaries.
Topic 2.2
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
Pacific Ocean.
b.
the Andes, the Himalayas, the Hawaiian Islands, the Mid-Atlantic
Ridge, Iceland and the San Andreas Fault, the Japanese island-arc.
c.
the 'ring of fire' around the Pacific Ocean.
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Plate Tectonics
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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.
Radioactive decay 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
UNIT
% 'PARENT'
ELEMENT
A
238
60.0
B
235
U
25.0
C
87
Rb
97.5
X
235
12.5
Y
238
U
95.0
Th
87.5
Z
Topic 2.2
RADIOACTIVE
ELEMENT PRESENT
U
U
232
Plate Tectonics
AGE OF ROCK
UNIT (x109 yrs)
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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 space 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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Plate Tectonics
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Topic 2.2
Plate Tectonics
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