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Earth and Environmental Science
Preliminary Course
Stage 6
Dynamic Earth
Part 3: Plate movement
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Number: 43180
Title: Dynamic Earth
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 courtesy of Upgrade Business Systems and Ric Morante
Part 2 p 14,
Part 4 p 8
Photograph of tillite courtesy of Barbara Gurney
Part 2 p 14
Photograph courtesy of Tim Reid
Part 4 p 7
Photographs of Japanese mountains courtesy of Richard Alliband
Part 4 pp 16, 17
Diagram from Veevers, JJ et al (1991) Australian Journal of Earth Sciences 38 p 384, courtesy
of Geological Society of Australia
Part 6 p 25
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Contents
Introduction ............................................................................... 2
Moving continents ..................................................................... 3
Different names for plate margins .......................................................3
Divergent plate boundaries..................................................................5
Conservative plate boundaries ............................................................6
Destructive plate margins ..................................................................10
Mechanism for plate movement .............................................. 15
Suggested answers................................................................. 19
Exercises – Part 3 ................................................................... 21
Appendix ................................................................................. 25
Part 3: Plate movement
1
Introduction
In the previous part you analysed a number of different types of evidence
used to support the Plate tectonic theory. In this part you will gain
further insights into the causes of plate movement and the mechanisms
that are thought to be responsible for these movements.
Within this part you will be required to model interactions between plates
as well as model mechanisms for plate movement, by completing a series
of activities. In doing so you will develop a greater understanding of the
interaction between different plate boundaries and the processes that are
occurring at each of the three boundaries.
This part will provide a good grounding for the work you will undertake
in the first Higher School Certificate module Tectonic impacts.
In these parts you will be given the opportunities to learn to:
•
describe the processes that may occur when two plates collide
•
define the term ‘subduction zone’ and identify the geological
features that are characteristic of a subduction zone
•
describe the plate tectonic model and use it to explain the
distribution and age of continents and oceans.
Extract from Earth and Environmental Science Stage 6 Syllabus © Board of
Studies NSW, amended October 2002. The most up-to-date version can be
found on the Board’s website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html
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Moving continents
You have already been introduced to the concept that Earth’s lithosphere
is made up of a series of plates. In the section, Lithospheric plates you
labeled each of the major plates. Look closely at the map once again.
What are the two types of plate boundaries shown in this diagram?
_________________________________________________________
_________________________________________________________
Check your answer.
You should have noticed a series of shaded triangles representing
one type of plate boundary known as a destructive plate margin.
The other type of plate boundary is shown as a series of parallel lines.
This represents a constructive plate margin.
If you look at the shape of the destructive margin you will notice it is
more continuous and can curve. On the other hand, constructive margins
are shown as distinct straight sections.
The single line joining these straight constructive plate margins go to
make up the edges of the plates and represent the third type of plate
boundary. These boundaries are known as passive plate margins or
transform faults. Transform faults allow the constructive margins to
move around in a curve-like shape by sliding along these faults.
Different names for plate margins
Each plate margin is known by a different descriptive names.
These names are derived from the processes or structures present at
these sites.
For instance the three plate margins are referred to above as being
constructive, destructive and passive, depending on whether they create,
destroy or maintain the amount of existing crustal material.
Part 3: Plate movement
3
However plate margins can also be named according to their own type of
geological structure. For example:
•
constructive boundaries are also referred to as mid oceanic ridges,
•
destructive boundaries are also be referred to as subduction zones.
They are associated with trenches
•
conservative boundaries are referred to as transform faults or
passive plate margins.
Names have also been given to plate boundaries according to their
relative movement. On either side of the mid oceanic ridge the plates
move away from each other and are therefore also referred to as
divergent plate boundaries.
On the other hand at destructive plate boundaries plates converge on
each other and collide. These are often referred to as convergent
plate boundaries.
Conservative plate boundaries can also be referred to as strike-slip faults
because of the movement of the plates along the transform faults.
As you can appreciate there are quite a few different names for the
same type of plate boundary – all are correct for different reasons.
Complete the following self-correcting question to help summarise
these different names.
Fill in the missing names for the following plate boundaries.
Plate boundary 1
Plate boundary 2
Plate boundary 3
subduction zone
(or trench)
divergent
conservative
(or passive)
Check your answers.
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Divergent plate boundaries
The term divergent means to move away, and that is what is happening
at these boundaries. The two adjacent plates are forced in opposite
directions as new oceanic crustal material is being inserted and added to
the inside edge of each of the plates, forcing them to move away from
each other.
mid-ocean
ridge
rift
ocean crust
continental
crust
Movement at divergent
plate boundaries.
Mid oceanic ridge producing ocean floor material
between continental land masses.
Because new material is being added to the plates through the creation of
new oceanic floor, these boundaries are sometimes referred to as
constructive plate boundaries. The mountain range that is produced, as a
result of the magma upwelling from the upper mantle, is known as the
mid oceanic ridge.
These ridges make up the largest mountain ranges in the world, not only
in length but also in average height above the level of the surrounding
solid surface. It is not uncommon for these mountain ranges to rise some
two to three kilometres above the level of the ocean floor. Ridges can
range in width from one thousand to four thousand kilometres.
Sea floor spreading not only gives a mechanism for the movement of
plates, but it also gives an explanation as to why ocean floor crust is
much younger than any of the continental crustal material.
Mechanisms for plate movement will be dealt with in greater detail later
in this module.
Look again at the first section in Part 2 under the heading of Lithospheric
plates. Look at the map showing the location of Earth’s major plates and
observe the position of the constructive plate margins.
What comment can you make about the location of these margins?
_________________________________________________________
_________________________________________________________
Check your answer.
Part 3: Plate movement
5
Conservative plate boundaries
These plate boundaries are called conservative due to the fact that they
neither create or destroy crustal material. Conservative plate boundaries
form the edges of plates and occur in association with mid oceanic ridges.
These conservative boundaries occur along transform faults where one
plate will slide past another plate, separating sections of mid oceanic ridge.
Transform faults at passive plate margins allow sections of lithosphere to slide
past each other in opposite directions.
Although these boundaries can also be referred to as passive plate
margins, again because crustal material is not created or destroyed,
they are in reality far from passive.
What do you think will occur along the fault plane between the two
opposing slabs of lithosphere?
_________________________________________________________
_________________________________________________________
Check your answer.
These regions produce some of the world’s strongest earthquakes.
ocean ridge-rift
Plate
A
transform
fault
Plate
B
lithosphere
asthenosphere
va
rising la
Transform faults separating a mid oceanic ridge into sections.
Earthquakes occur along the transform fault between the mid oceanic ridge.
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The following activity will help demonstrate how plate movement at
the mid oceanic ridge and transform faults can lead to the production of
large earthquakes.
A mid oceanic ridge and transform fault model
What you will need:
•
scissors
•
sticky tape
•
coloured pencils
•
paper sheet printed with a model from the Appendix
•
a frame sheet from the Appendix.
Creating the model:
1
Use your coloured pencils to shade the numbered segments on the
sheet in the Appendix. Try to use a different colour for each number.
For example if you select red for the number 1 segments then you
must colour in the six segments numbered 1 in red. Use a different
colour for all the segments numbered 2 and so on.
2
Use your scissors to cut along the two transform fault lines.
Make sure you do not cut through either end of the sheet. Only cut
between the two scissors symbols.
➩
Part 3: Plate movement
➩
7
Use sticky tape to reinforce the handles at either end of the sheet.
This will ensure that the sheet does not tear at the ends of the
transform faults.
4
Get the sheet of cardboard from the Appendix.
5
Cut along the three lines to create three slits in the cardboard.
These three slits represent the mid oceanic ridge.
6
Return to the sheet that you had coloured in and fold along the three
dotted lines. Make sure you have the folds pointing down and away
from you and not sitting up.
7
Place these folds through the top of the slits in the cardboard and
then pull them through from behind until colour number 6 is just
beneath the cardboard.
➩
3
➩
8
➩
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Using the model:
Feature of model
What the model feature represents
three slits in the cardboard
mid oceanic ridge
two slits on the sheet of paper
transform fault
segments on sheet
segments of ocean floor crust
colour
each colour represents a period in time,
therefore all segments that are the
same colour are also the same age
region to the left of the three slits
plate A
region to the right of the three slits
plate B
8
Hold on to the edges of the sheet and slowly pull the sheet away
from the centre allowing the folded paper to come up through the
three slits.
➩
9
➩
Take note of the colours that emerge from the three slits.
Remember that these three slits represent three sections of the
same mid oceanic ridge.
10 Take note of the direction that each section of the ocean floor is
moving in relation to the other sections.
Part 3: Plate movement
9
Analysing the results:
11 Draw arrows either side of the transform fault (on the ocean floor
segments) indicating the direction the plate is travelling.
12 Mark in large letters on the ocean floor segments A for one plate and
B for the other plate.
Earthquakes mostly occur along the section of transform fault where plates
are moving in opposite directions.
13 Use your model to find out where these sections are and then mark these
sections with a red dot near the particular section of transform fault.
Check your answers.
Destructive plate margins
If material is being added to a rigid plate at one end, then it must either
be crumpled or destroyed at the other end. The term destructive plate
margin is used because crustal material is being destroyed (destructive)
over a wide area (margin rather than boundary).
Colliding plates
Crustal material is destroyed when two
plates collide. The denser crust will be
subducted down deep trenches underneath
the less dense crust to be melted into the
upper mantle. This is why these types of
plate boundaries are also given the name
of subduction zones.
A convergent plate boundary.
Describe the direction of the forces for each of the three plate boundaries.
•
divergent ____________________________________________
_____________________________________________________
•
convergent ___________________________________________
_____________________________________________________
•
transform or conservative _______________________________
_____________________________________________________
Check your answers.
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When plates collide they can generally fall into one of three categories:
•
continental – ocean collision
•
continental – continental collision
•
ocean – ocean collision.
Continental – ocean collision
This is where an oceanic plate collides with a continental plate. In these
situations the denser oceanic crust is subducted underneath the more
buoyant continental crust.
The subducted oceanic crust is melted and the resulting magma rises
back up through the overlying continental crust to form volcanoes.
oceanic crust
oceanic trench
subducting oce
ani
c lit
100 km
hos
200 km
asthenosphere
p he
volcanic arc
re
continental crust
asthenosphere
melting
continental lithosphere
An ocean – continent collision
One example where this is happening is along the west coast of South
America. Here the Nazca plate is being driven towards and subducted
beneath the South American plate. The force of these plates colliding
and the volcanism has resulted in the production of the Andes
Mountain range.
Continental – continental collision
In this category are collisions between two continental plates.
An example where this type of collision is occurring is in northern India.
Here the Indo-Australian plate has collided with the Eurasian plate. As a
result India has subducted underneath the Eurasian plate.
The crustal material making up both plates have a similar density.
Therefore instead of India being subducted deep into the upper mantle
it has continued to follow the underneath side of the continental crust on
the Eurasian plate.
Part 3: Plate movement
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100 km
asthenosphere
200 km
Continental lithosphere
Continental lithosphere
A continental – continental collision
This gives rise to continental crust reaching a thickness of about 70 km
where as the average thickness of continental crust is about 40 km.
The mechanism for why this type of subduction occurs is still being
debated. One theory suggests the attachment of the oceanic plates
either side of the Indian plate is strong so that the subduction of the
oceanic crust is literally dragging the Indian subcontinent plate forward
beneath Eurasia.
Compare the diagram showing an oceanic – continental boundary with
the diagram showing a continental – continental boundary.
List two differences between these two boundaries shown by the
diagrams.
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Ocean – ocean collision
This type of plate boundary occurs when two oceanic plates collide.
oceanic crust
100 km
200 km
oceanic trench
subducting oce
ani
c lit
hos
asthenosphere
p he
island arc
re
melting
continental crust
asthenosphere
continental lithosphere
Ocean – ocean collision
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The Tonga-Kermadec Trench is a subduction zone that has been created
from the collision of oceanic crustal material from different plates.
The Tonga-Kermadec Trench occurs to the north east of New Zealand.
Features of subduction zones
You’ve seen that collisions between lithospheric plates can produce
subduction zones where one plate moves under the other. What are the
typical features of a subduction zone?
The diagram following shows some typical features of an ocean –
continent collision.
accretionary wedge
at trench
oceanic crust
(MORB)
✳
✳
✳
lithosphere
continental crust
✳
✳
✳
✳
lithosphere
✳
✳
✳
✳
asthenosphere
✳✳
✳
✳
✳
✳
✳
asthenosphere
earthquakes (in ‘Benioff Zone’)
rising magmatic plumes
metamorphic rocks (folded)
Features of a typical ocean – continent collision.
Part 3: Plate movement
13
The features shown on the diagram are described below.
Trench
This is where the oceanic plate is being subducted. These trenches can
be up to 11 km deep.
Accretionary wedge
This is the wedge of oceanic sediments that have been scraped off the
subducting oceanic plate. This wedge of material accumulates in the
trench between the subducting oceanic plate and the overlying
continental plate.
Wadati-Benioff zone
The Wadati-Benioff zone (often abbreviated to Benioff zone) is a region
of earthquakes generated along the leading edge of a subducted plate.
The sites where earthquakes begin increase with depth and outline the
angle and depth to which the plate is being subducted.
Rising magmatic plumes
When one plate is subducted beneath another the heat and pressure
increases as the depth of subduction increases. Eventually the subducted
plate melts and forms pockets of magma known as magmatic plumes.
Because these plumes are much hotter than the surrounding country rock
they rise and force there way up through the overlying plate.
Metamorphic rocks
The tremendous amount of heat and pressure generated from the
contact of the colliding plates causes rocks to deform. During this
deformation the rocks are folded and the minerals recrystallise to form
metamorphic rocks. These minerals assemblages include the so called
blue-schist facies or collection of minerals. These minerals are found
only where high pressures are found with low temperatures.
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Mechanism for plate movement
Although Alfred Wegener had clarified the theory of continental drift and
was certain that continents that were once joined had since separated, he
was at a loss to explain the driving mechanism for his continental drift
theory. As a result Wegener received much criticism and his theory was
left to stagnate until the late 1950s when Professor Harry Hess from
Princeton University proposed his theory of sea floor spreading.
Sea floor spreading provided a mechanism for continental drift.
Hess proposed that mantle material had risen and erupted at mid oceanic
ridges as a result of upward moving convection currents, and was
deposited as new crustal material on either side of the ridge.
The addition of new oceanic crust was said to have forced the
lithospheric plates away from each other, thereby separating and
moving the continents.
continental crust
oceanic crust
rift valley
(divergent convergent
boundary) boundary
divergent
boundary
oceanic crust
convergent
boundary
transform fault
convection cells
plate movement
Hess’ model of a mechanism for plate movement.
As the oceanic crust moves away from the mid oceanic ridge the crustal
material is allowed to cool increasing its density. At the same time
oceanic sediments accumulate on top of the crust. Eventually this
dense crust subducts into the partially molten upper mantle known as
the asthenosphere in much the same way as an over loaded boat sinks
into water.
Part 3: Plate movement
15
This cool and dense subducted crust sets up a downward moving
convection current, which then acts to drag the dense oceanic crust down
further into the mantle where it is remelted. These convection currents
set up convection cells within the upper mantle that contribute to driving
the plates over the asthenosphere, similar to a conveyor belt system.
Although this model has been widely accepted there is still much debate
about where convection cells are set up, how large they are and
sometimes even if they are set up at all. Some scientists argue that
convection cells, if they are formed, would not be able to apply enough
force to move continental plates laterally across the globe.
One theory that has grown in popularity since the mid 1990s is that it is
subduction of the dense slab that drives plate tectonics. This is a cold
down driven convection rather than a warm up driven convection current.
The primary heat source in Earth and therefore for the warm convection
currents is thought to be from heat being generated by decaying
radioactive elements within Earth. This heat is thought to assist the
process of continental drift by heating up the mantle and allowing the
mantle to penetrate areas of geological weakness in continental crust.
As this molten magma from the mantle finally penetrates the crust along
fissures in the ocean floor it solidifies as it cools.
Oceanic crust is added as the continents drift away from each other.
This process will be dealt with in greater detail in the module
Tectonic impacts.
The role of gravity
A lot of emphasis in the past has been placed on the role of warm driven
convection currents as the main driving force for plate movement.
Much discussion and debate has taken place over the degree to which
convection currents operating at the MOR push the plates apart.
Another view is that the differences in crustal density result in the
dragging of oceanic crust down into the upper mantle at subduction
zones. If a continental plate is firmly connected to the other end of the
subducting plate it is dragged along too!
Oceanic crust is composed almost entirely of basalt. This basalt has a
greater density than most continental crustal material. Basalt produced at
mid oceanic ridges has a density of approximately 3.0 gcm–3 whereas
continental crustal material can vary considerably, but would perhaps
average somewhere near 2.7 gcm–3.
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When a lithospheric plate composed of oceanic crust confronts another
lithospheric plate composed of continental crust, gravity acting on the
denser oceanic plate would pull this plate beneath the less dense
continental plate.
closing ocean crust
(density 3.0 g/cm3)
continental crust
(2.7 g/cm3)
trench
lithosphere
asthenosphere
Density of oceanic and lithospheric plates.
It has been argued that this dragging force, and not convection currents,
is largely responsible for the movement of plates. In reality it is
probably a combination of both, but to what degree is a topic of further
geological debate.
A quick review
1
Look back through the parts of the module that you have completed.
In your own words and using dot points, summarise the main
features of the plate tectonic model.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 3: Plate movement
17
2
Do you think that crustal push at mid oceanic ridges is mostly
responsible for plate movement? Or do you think that crustal drag at
subduction zones is the main mechanism for plate motion?
Give evidence to support your opinion.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Turn to the end of this part and complete Exercise 3.1.
Congratulations! You have now come to the end of this Part.
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Suggested answers
Moving continents
The two types of plate boundaries shown are constructive plate
boundaries and destructive plate boundaries.
Different names for plate margins
Plate boundary 1
Plate boundary 2
Plate boundary 3
transform fault
subduction zone
(or trench)
mid oceanic ridge
strike-slip
convergent
divergent
conservative
(or passive)
destructive
constructive
Divergent plate boundaries
These margins are located within the oceans, usually away from
continental margins.
Conservative plate boundaries
Friction between the two opposing slabs of lithosphere will
produce earthquakes.
Part 3: Plate movement
19
A mid oceanic ridge and transform fault model
Your completed model should be similar to the diagram below.
MID OCEANIC RIDGE
➩
PLATE A
➩
PLATE B
✳✳✳✳✳
TRANSFORM FAULT
➩
PLATE A
✳✳✳✳✳
PLATE A
➩
➩
➩
PLATE B
TRANSFORM FAULT
PLATE B
✳
Earthquake foci
Colliding plates
•
At divergent plate margins the forces are tensional. They are pulling
away from each other.
•
At convergent plate margins the forces are compressional. They are
pushing towards each other.
•
At conservative plate margins the forces are moving side by side in
opposite directions to each other.
Continental – continental collision
20
•
The thickness of the lithosphere at the point of subduction is
much greater at continental – continental collisions than at
oceanic – continental collisions.
•
The subducting oceanic crust in an ocean – continental collision is
denser than the continental crust.
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Exercises – Part 3
Exercise 3.1
Name: _________________________________
Exercise 3.1
1
2
The following terms refer to the three types of plate boundaries.
Link names that refer to the same type of plate boundary with
an arrow.
constructive
transform fault
mid oceanic ridge
passive
diverging
oceanic trench
destructive
converging
When an oceanic plate converges up against a continental plate, the
oceanic plate will always subduct beneath the continental crust.
a) Define the term subduction zone.
_________________________________________________
_________________________________________________
b) Explain why continental crust doesn’t subduct beneath oceanic
crust.
_________________________________________________
_________________________________________________
c) Outline how this provides an alternative to convection currents
as a mechanism for continental drift.
_________________________________________________
_________________________________________________
d) Outline why oceanic crust is much younger than continental
crust.
_________________________________________________
_________________________________________________
Part 3: Plate movement
21
3
The diagram below shows a cross-section through a number
of plates. Look at this diagram carefully and answer the
questions that follow.
plate X
plate Y
plate Z
fold mountain
West
A
D
sea level
D
East
B
E
D
C
D
C
a) Name the features at:
A
_______________________________________________
B
_______________________________________________
C
_______________________________________________
b) Identify the type of crust shown at E.
__________________________________________________
c) State the direction of movement of the following plates:
X
_______________________________________________
Y
_______________________________________________
Z
_______________________________________________
d) What name is given to the zone beneath the lithosphere labelled
as D?
__________________________________________________
e) Explain how zone D is said to aid movement of the plates.
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
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4
In the following box, draw a labelled diagram showing convection
currents operating underneath a mid oceanic ridge. Ensure that you
label the following:
•
mid oceanic ridge
•
oceanic crust
•
continental crust
•
direction of convection currents
•
plate A
•
plate B
•
oceanic sediment (near the ridge and away from the ridge)
•
direction of plate movement
•
asthenosphere.
Diagram of convection currents underneath a mid oceanic ridge
Part 3: Plate movement
23
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Appendix
Materials for the ‘A mid oceanic ridge and transform fault model’
activity are on the following pages. These materials are:
•
a printed sheet to colour, cut and fold
•
a frame sheet for your model.
Part 3: Plate movement
25
✂
✂
8
13
10
7
12
9
6
11
8
5
10
7
4
9
6
3
8
5
2
7
4
1
6
3
5
2
1
4
1
2
3
3
2
4
1
FOLD 1
FOLD 3
1
2
3
FOLD 2
5
4
1
6
2
7
6
3
8
7
4
9
8
5
10
11
9
6
TRANSFORM FAULT
10
✂
✂
26
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MID OCEANIC RIDGE
✂
CUT OUT SLOT 3
✂
MID OCEANIC RIDGE
CUT OUT SLOT2
TRANSFORM FAULT
✂
CUT OUT SLOT 1
TRANSFORM FAULT
MID OCEANIC RIDGE
Part 3: Plate movement
27