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Earth and Environmental Science
HSC Course
Stage 6
Tectonic impacts
Part 4: Evolution of continents
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Number: 43181
Title: Tectonic impacts
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 Kay and Dick Alliband
Part 1 pp 11, 12
Part 2 p 18
Photographs © Raymond A. Binns, Chief Research Scientist, CSIRO Exploration and
Mining, taken from Jamstec’s submarine “Shinkai-6500”
Part 1 p 35
CSIRO Media Release 97/255 CSIRO in world’s richest undersea goldstrike, December 19th,
1997 found at http://www.csiro.au/communication/mediarel/mr1997/mr97255.htm
Part 1 p 37
Photographs courtesy of Tim Reid
Part 2 p 8, Part 2 pp
19, 20, 26 Part 5 pp
25, 29, 32
Photographs courtesy of Ric Morante
Part 3 p 3, Part 5 p 31
Photograph courtesy of Barbara Gurney
Part 3 p 19
Photographs courtesy of U. S. Geological Survey, Department of the Interior/ USGS
Part 5 pp 31, 33 Part 6
pp 7, 8
Text extract describing volcano Popocatepetl, found at
http://www.geo.mtu.edu/volcanoes/popocatepetl/updates/update.008.html
Part 5 p 39
Text extract from article Volcano: evacuation ordered, found in Sydney Morning Herald,
15/2/1993 quoted from Agence France-Presse
Part 5 p 40
Emerson, Tony (31st October, 1995) “A curse called lahar”, Newsweek Inc.
Part 6 p 13
All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith.
Published by
Centre for Learning Innovation (CLI)
51 Wentworth Rd
Strathfield NSW 2135
_______________________________________________________________________________________________
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Copyright of this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or
transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the
written authority of the Centre for Learning Innovation (CLI).
© State of New South Wales, Department of Education and Training 2007.
Contents
Introduction ............................................................................... 2
Where is Australia? ................................................................... 3
Geological history...................................................................... 4
Geological maps...................................................................................4
Evolution of the Australian continent ....................................... 11
The oldest parts of Australia ..............................................................11
Development of the craton.................................................................12
Building eastern Australia ..................................................................13
The super–continent cycle ...................................................... 19
Impacts of the super–continent cycle................................................21
Additional resources................................................................ 25
Suggested answers................................................................. 33
Exercises – Part 4 ................................................................... 37
Appendix ................................................................................. 41
Part 4: Evolution of continents
1
Introduction
Rocks hold evidence for the existence of past tectonic environments.
In this module you will be introduced to geological maps showing
different rock sequences that hold the key to past tectonic activity
in Australia.
You will then be able to deduce changes in tectonic activity and then go
on to predict possible future tectonic changes.
You will also study the super–continent cycle theory and analyse past,
present and future plate movements and the effect these movements have
on the Earth’s environment.
In this part you will be given opportunities to learn to:
•
outline the main stages involved in the growth of the Australian
continent over geological time as a result of plate tectonic processes
•
summarise the plate tectonic super–cycle
In this part you will b given opportunities to:
•
analyse information from a geological or tectonic map of Australia
in terms of age and / or structure of rocks and the pattern of growth
of the continent
•
present information as a sequence of diagrams to describe the plate
tectonic super–cycle concept.
Extracts 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_lista.html
2
Tectonic impacts
Where is Australia?
You should by now be familiar with the idea that Australia is moving
northwards as the Indo–Australian plate moves away from the
mid–oceanic ridge in the Southern Ocean.
Can you recall at what rate Australia is said to be moving?
_________________________________________________________
Check your answers.
In this section of the module, you will learn that the size and shape of
Australia, and all the other continents, has changed over time as a result
of tectonic processes. Every year, news reports of volcanic eruptions and
earthquakes remind us of the tectonic forces at work inside the Earth, and
we can see on television some of the catastrophic effects these forces
have had on local areas of the Earth’s surface.
In Australia, earthquakes do occur but they rarely cause great loss of life.
Eastern Australia now is not near a active plate boundary. The leading
margin of the Australian plate moving to the North – East is New
Guinea. This has not always been the case. In the central west of New
South Wales, there are extensive areas where andesite rocks outcrop (are
exposed at the surface suggesting subduction along the East coast).
1
What type of plate boundary does the existence of andesite indicate?
(Refer to the Volcanic mountains section in the last part of this module
if you have forgotten.)
_____________________________________________________
_____________________________________________________
2
Why is it that we do not have volcanic eruptions in modern
Australia? (A look back to the map showing plate boundaries in the
first part of this module may prompt your memory.)
_____________________________________________________
_____________________________________________________
Check your answers.
Part 4: Evolution of continents
3
Geological history
The process, of interpreting ancient environments by observing modern
day environments is based on a principle called uniformitarianism.
This just means that there have been the same (uniform) processes
operating in the past as there are operating today. This is often explained
by the saying, ‘The present is the key to the past’.
Geological maps
Geologists are not only interested in the type of rocks that are found both
on and beneath the Earth’s surface but also their age. From the age, they
can work out the geological history of a region.
A typical geological map shows two main features:
•
symbols showing the rock types
•
colours showing the ages of the rocks.
The geological time scale
Before you move on to look at a geological map, you need to know
something about how geologists determine the age of rocks.
Since you study this in detail in the module, Environments through time,
a quick review is all you need here.
Determining a rock’s age can be obtained by two different means:
•
absolute dating methods
•
relative dating methods.
You have already dealt in depth with absolute dating methods in Part 1 of
the Dynamic Earth module when you investigated radiometric dating.
This method gives a particular age and time of formation for a rock.
4
Tectonic impacts
Relative dating methods use the position of rock layers and igneous
intrusions to determine the order of their formation and therefore which
rock type is the youngest and which is the oldest.
When piecing together the geological history of an area, earth scientists
are generally more interested in the order of events rather than the actual
time in years when they occurred.
As an example, when you see a sequence of rock layers in the side of a
road cutting, it is reasonable to assume that the layer on the bottom was
formed first and therefore is the oldest. The rest of the layers are
progressively younger towards the top.
There are some tectonic processes that can make this assumption incorrect.
Can you think of any? (Recall structures that can occur in
continent–continent convergence zones or in volcanic areas.)
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Throughout the 1800s, scientist suggested a wide variety of names for
particular rock units which contained distinctive fossils. Eventually, a
naming scheme was agreed on which broke up Earth’s past into blocks
of time.
Long stretches of time were called eras, and these were subdivided into
shorter blocks called periods.
Some periods have been further subdivided into even smaller blocks
called epochs. For example, the Mesozoic era is subdivided into three
periods: Cretaceous, Jurassic and Triassic. Each of these periods are
further subdivided into epochs according to whether they occurred early,
middle or late.
Look at the diagram over the page and locate the Mesozoic era.
Part 4: Evolution of continents
5
Cainozoic
Tertiary
Quaternary
Pleistocene
Pliocene
Neogene
Miocene
Palaeogene
Oligocene
Eocene
Paleocene
Ma
65
Late
Cretaceous
Early
Mesozoic
141
Late
Jurassic
Middle
Early
200
Late
Triassic
Middle
Early
251
Phanerozoic
Late
Permian
Early
Carboniferous
Stephanian
Westphalian
Namurian
Visean
298
Tournaisian
354
Late
Devonian
Paleozoic
Middle
Early
Pridoli
418
Ludlow
Silurian
Wenlock
Llandovery
Ashgill
Caradoc
441
Llanvirn
Arenig
Ordovician
Tremadoc
490
Late
Cambrian
Middle
Early
Proterozoic
542
Neoproterozoic
1000
Mesoproterozoic
1600
Paleoproterozoic
2500
Archaean
6
Tectonic impacts
Interpreting geological maps
When you look at an area the size of NSW, it is not possible to show the
whole range of rock types in each region. So the main rock types in a
region are listed in the legend or key beside the map.
Below is an extract from a geological map of the area around Bathurst.
As it appears in black and white, it does not seem to make much sense.
The original map has various colours on it. Colour in the following map
according to the colours shown in the table following.
S-Dc
S
Dm
S-Dc
Dm
Cg
S
S-Dc
Dtn
Dtn
S-Dc
S
Dm
S
S-Dc
Cg
Cg
S
Orange
S
Dm
S
S-Dc
Cg
Bathurst
Tv
Dta
Tv
Pzg
Cg
Cg
Cg
S
S-Dc
Pzg
S
Pzg
c
S-D
Pzg
S
S-Dc
Cg
Pzg
S-Dc
S
S
S
S-Dc
Cg
Tv
S
S-Dc
Geological map: Bathurst
Part 4: Evolution of continents
7
Map symbol
Colour
Cg
red
Tv
yellow
Pzg
orange
Dm
purple
qv
pink
S
brown
Dtn, D–Sc
white
1
Some sort of pattern should become apparent. What is the general trend
or the overall direction and shape of the rocks whose symbols start with
q, S and D?
_____________________________________________________
2
What is the general trend and shape of the Cg rock mass?
______________________________________________________
3
Below is a modified extract from the legend of the same map.
It gives the symbol, the age (period), and the main rock types for
each of the areas you have just coloured in.
Symbol
Age
Name
Tv
Tertiary
basalt
Cg
Carboniferous
basalt, trachyte
Dm
Devonian
granite, granodiorite
Pzg
Devonian
tuff, dacite, slate
S–Dc
Siluro–Devonian
granite, diorite
Skc etc
Silurian
slate, tuff
qv
Ordovician
slate, limestone, tuff volcanics
Look at this table carefully.
8
Tectonic impacts
What does the first (capital) letter of the symbol for a rock unit
stand for?
_____________________________________________________
4
This table has the rock units arranged in the order in which they
formed. The rocks formed first, and therefore the oldest, are at the
bottom and the rocks formed last, these being the youngest are at
the top.
How do you think the rocks labelled Tv were formed?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
5
If you went to one of the areas where Tv occurs, would you find it:
above, cutting across or below the other rocks in that area?
Explain your answer.
_____________________________________________________
_____________________________________________________
_____________________________________________________
6
Next, look at the peculiar pattern made by the Dm formation (in
purple). What do you think has happened to this part of New South
Wales to produce this outcrop pattern?
_____________________________________________________
_____________________________________________________
_____________________________________________________
7
In your answer you should have mentioned that the region was
folded, can you work out from the map when the folding took place?
Use the following two clues to help you:
•
folding must have taken place after the rocks were deposited
•
look closely at the contact between the folds and the granite
(Cg), just to the north of Bathurst.
_________________________________________________
_________________________________________________
_________________________________________________
Check your answers.
Part 4: Evolution of continents
9
You can see that the granite cuts across the folds, so it would have
formed after the folding event. This means that the Dm rocks were
folded in the late Devonian or early Carboniferous Periods.
Now, do you recall in what tectonic environment the following are formed:
folded sedimentary rocks, particularly marine sedimentary rocks like
limestone, volcanic rocks like tuff, metamorphic rocks like slate,
and granite?
_________________________________________________________
_________________________________________________________
Check your answers.
This collection of rock types indicates that the region was once in a
convergence zone. This in turn tells us that the present–day NSW coast
was not always the eastern edge of the continent.
Turn to the back of this part and complete Exercise 4.1: Geological past.
This is very relevant to how the entire east coast of the Australian
continent was built up. Later in this part you will look at a map of all of
Australia and you will be asked to look for the evidence of how the
Australian continent was built.
10
Tectonic impacts
Evolution of the Australian continent
In Part 1 of Dynamic Earth you studied radiometric dating methods.
The oldest minerals on Earth have been found in Australia. You should
revise that learning now and then answer these questions.
1
Where in Australia were these minerals found?
_____________________________________________________
_____________________________________________________
2
What type of minerals are they?
_____________________________________________________
3
In what rocks were these minerals found?
_____________________________________________________
4
What age were these minerals?
_____________________________________________________
5
What age were the rocks where these minerals were found?
_____________________________________________________
Check your answers.
The oldest parts of Australia
Although the quartzite referred to above is about 3600 million years old,
the oldest rocks in Australia are found in the Yilgarn and Pilbara Blocks
in Western Australia. These rocks have been dated at about 3800 million
years old and include such rock types as granite, gneiss and greenstone
which is a metamorphosed basalt. (See the map on the following page.)
Over the next 2000 million years, a number of sedimentary basins built
up deposits in what is now South Australia, the Kimberley region and the
Northern Territory. These areas underwent compression and
Part 4: Evolution of continents
11
metamorphism and produced fold mountain ranges. All these ancient
rocks, where they outcrop form the Australian continental shield.
After erosion, much of these older areas became lowlands and were
covered by fairly thin sheets of sediment. Those areas where these
comparatively undeformed sedimentary rocks cover the older deformed
rocks are called the Australian craton.
Pilbara
Yilgarn
0
1000
Km
The Pilbara and Yilgarn areas are located in Western Australia.
Development of the craton
By the time the Palaeozoic era began, the Australian continent looked
like the following diagram.
0N
10 N
20 N
30 N
50 N
coastline
present day Australia
Continental
Plate
plate boundary
volcanoes
Oceanic
Plate
present day Antarctica
0
Km
1000
Australia at the beginning of the Palaeozoic era.
12
Tectonic impacts
What parts of present day Australia are missing on the Palaeozoic era map
of Australia?
_________________________________________________________
_________________________________________________________
Check your answers.
Building eastern Australia
This missing third of the present continent gradually built up over the
Palaeozoic and Mesozoic eras. In the Cambrian, the east coast of NSW
was in the area of Broken Hill, in the far west. A shallow sea existed
along the central Queensland/western NSW/South Australian coastline.
To the east, a long volcanic island arc ran from south to north, with a
trench on the eastern side of the arc.
What type of plate boundary do you think is indicated by these
structures? If you answered a convergent or destructive plate margin
then you would be correct.
Rock types and tectonic setting
The island arc referred to above wasn’t static. It shifted its position
several times. Each time the island arc reformed moving hundreds of
kilometres further east. As a result, we find north–south sequences of
particular rocks repeated as we move from west to east.
What sort of volcanic rocks are erupted at island arcs and volcanic chains on
continental crust?
_________________________________________________________
Check your answer.
Because of the explosive nature of andesite eruptions, volcanic ash
deposits cover large areas. This forms a layer that resembles sandstone.
These ash particles settle out of the air and form rocks such as tuffs and
pumice. The holes in this pumice are produced by the escaping gases.
Part 4: Evolution of continents
13
Sometimes tuffs are also referred to as ignimbrites. Tuffs or ignimbrites
are often a mixture of lava, ash and volcanic glass and can show flow
banding structures.
Tuff (ignimbrites). Photo: Tim Reid
Pumice. Photo: Tim Reid
Coral reefs may form around these volcanic islands. These coral reefs
are eventually preserved as limestone. And on the sea–floor, fine grained
material can build up and eventually turn into shale.
Look back now at the Bathurst map (page 7) and see if you can find a
sequence showing the following rock types: tuff, limestone and shale.
You should also be able to see more clearly the general north to south trend
of the rock formations. The exceptions should be the granites and the
Tertiary volcanics.
1
Can you recall what the environment must be to form limestone?
(Remember, limestone is composed of calcium carbonate from shells.)
_____________________________________________________
_____________________________________________________
2
Obtain a road map or road atlas of New South Wales. Look for the
following tourist sites. They are all limestone cave systems, most of
which are open to the public.
•
Wellington Caves
•
Molong Caves
•
Jenolan Caves
•
Tuglow Caves
•
Wombeyan Caves
•
Wee Jasper
•
Yarrangobilly Caves.
Mark their locations on the map of NSW following.
14
Tectonic impacts
Dubbo
Bathurst
Sydney
Canberra
0
100
200
Km
3
What comment can you make on their position?
_____________________________________________________
_____________________________________________________
_____________________________________________________
4
In what direction do you think Australia grew for most of the
Palaeozoic Era?
_____________________________________________________
Check your answers.
Note that this is the direction of plate convergence during that time. The
converging or subducting plate came from the east.
Australia’s current situation
In the last module Dynamic Earth you learned that the Australian
continent is moving northwards.
Where is the northern–most point of the continent?
On an atlas, the tip of Cape York peninsula appears to be as far north as
you can go. But, from a plate tectonic point of view, Australia extends a
lot further north. If we draw a map without worrying about the present
day sea–level, and instead draw the outline of the continental shelf,
Australia looks quite different.
Part 4: Evolution of continents
15
The shelves of continents slope very gently until they are about 200 m
below sea–level. Then the angle steepens. This change in angle marks
the boundary between the shelf and continental slope.
1
On the map below, go over the – 200 m (below sea–level) contour with
a dark pen.
2
Colour in all the areas that have an elevation above this –200 m contour.
Use a colour that would represent the Earth such as green or brown.
3
Colour in all the areas below this contour in another colour – blue
might be an appropriate colour.
Equator
–200 m contour
40∞ S
∞E
20
40∞ S
∞E
160
1
Australian continent showing –200 m contour.
This is the real shape of the Australian continent at present.
16
Tectonic impacts
1
List the major differences between your map above and the usual map
of Australia.
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
In what direction is the Australian continent presently moving.
_____________________________________________________
3
Name the plate that our continent is converging with in the north.
(Refer to the map near the start of this module if you have forgotten.)
_____________________________________________________
4
What type of plate boundary exists where these two plates are
meeting?
_____________________________________________________
Check your answers.
Australia’s future
The growth of eastern Australia throughout the Palaeozoic suggests that a
continent grows in the direction of plate convergence.
Where then do you predict that Australia will grow in the future?
Look at the north west part of Australia on the map you drew above, as well
as an atlas for the same area. If Australia continues moving northwards,
predict what landmasses may be added to our northern edge?
_________________________________________________________
_________________________________________________________
Check your answer.
Using NSW throughout the Palaeozoic as a model, sketch the trend of future
mountain ranges and folded rocks that may form to the north of Australia
over the next fifty to 150 million years.
To help you with this exercise there is an excellent website showing several
animations of past and future tectonic plate movements. These will help you
to visualise past and future possible movements of the Earth’s plates.
This site can be accessed via http://www.lmpc.edu.au/science
Part 4: Evolution of continents
17
Turn to the back of this Part and complete Exercise 4.2:
Australia in the Paleozoic.
18
Tectonic impacts
The supercontinent cycle
There are about 35° of latitude between the northernmost part of New
Guinea and the island of Shikoku in Japan, due north. This represents a
distance of roughly 4000 km.
Given that Australia is currently moving northwards at about 3 cm per year,
how long would it be before the Australian continent converged with Japan?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Australia would eventually join India as a fragment of Gondwanaland
that has rejoined Laurasia! Africa is already very close to Europe,
squeezing the Mediterranean Sea making it smaller and smaller and, in
the process, forming the active volcanoes of Etna, Vesuvius, Stromboli
and others.
Over a time period of about 200 million years, the super–continent
Pangaea broke apart and appears to be coming together again.
There is compelling evidence that this process occurred with North
America and Europe in the past. The Atlantic Ocean closed as continents
came together to form Pangaea, then opened as the super–continent
broke into fragments.
The convergence then breakup of a super–continent, which can also be
seen as a closing then opening of an ocean basin, was first described by
T J Wilson in 1968 and is known as the Wilson cycle.
There is an excellent animation of past plate movements and projected plate
movements showing this cyclic movement on the internet.
This site can be accessed via http://www.lmpc.edu.au/science
Part 4: Evolution of continents
19
Since then, the model has been revised and is now known as
The super–continent cycle.
You can find an article on the super–continent cycle in the Additional
resources. This article was written by Dr. R. Morante.
There are numerous examples around the world that support the
super–continent cycle model: let’s look at one of them.
The map in the Appendix shows the location of the Appalachian
mountains in North America and the Caledonian mountains in
north–west Europe.
The map has been divided into four areas.
1
Trace the outline of each of the four regions using tracing paper or
transparent plastic and mark in the location of these two mountain
ranges.
2
Cut out these four areas.
3
Move the section containing The Appalachian mountains eastwards
until it joins the British Isles on the second section. Now try and
position the other two sections so that one super–continent is formed.
4
What is the relationship between the two mountain ranges?
_____________________________________________________
_____________________________________________________
_____________________________________________________
5
As these ranges are fold mountains, what tectonic process formed them?
_____________________________________________________
_____________________________________________________
Check your answers.
You should see that when Pangaea formed, this one long mountain range
would have been thousands of kilometres inside the super–continent.
Today, they are both within a few hundred kilometres of their coastlines.
The super–continent broke apart at a new position, and sea floor
spreading has separated these sections of the mountain range by
thousands of kilometres.
20
Tectonic impacts
Impacts of the super–continent cycle
Before the super–continent Pangaea began to separate some 200 million
years ago, the number of mid–oceanic ridges existing and operating
would have been minimal.
Can you recall the massive size of the mountain ranges produced by
mid–oceanic ridges?
If you recall these mountain ranges are often over a thousand kilometres
wide, many thousands of kilometres long and regularly 3 to 4 km in
height from the bottom of the oceans.
Think of the massive volume that these mid–oceanic ridges have
collectively. This would have had a marked effect on the
sea–levels worldwide.
When the southern super–continent Gondwanaland split up into five
major continental plates there must have been new mid–oceanic ridges
produced to allow this separation.
1
How many ridges would have been produced? Name the five initial
plates produced.
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Assess the effect this break up of Gondwanaland would have had on
the world wide sea–level.
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
What do you think would have happened to continental shelves
worldwide?
_____________________________________________________
_____________________________________________________
4
Can you think of examples of organisms that would have flourished
as a result of this huge increase in area of shallow seas?
_____________________________________________________
_____________________________________________________
Part 4: Evolution of continents
21
5
Name a specific sedimentary rock type that you would expect to
form in much greater quantities as a result of this sea–level rise.
______________________________________________________
______________________________________________________
Check your answers.
The super–continent cycle model opens up many avenues for research
into how our planet works. While there is disagreement over many
suggested explanations of surface processes, the super–continent cycle is
an exciting development of plate tectonic theory.
Analysing a chronological geology map of
Australia
Key
Mesozoic to Cainozoic
Proterozoic to Palaeozoic
Mesozoic
Palaeozoic to Mesozoic
with some Cainozoic
Palaeozoic
Proterozoic
Archaean to Proterozoic
Archaean
Proterozoic to Mesozoic
Look at the map above. Check the key and see if you can identify where the
oldest regions of Australia are. These are the Archaean to Proterozoic areas.
Shade these areas on your map in on colour. These areas represent the
original size of the “Australian continent”. Do you notice that all these areas
are in one part of Australia?
22
Tectonic impacts
Which part of Australia is the oldest?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
The area of Palaezoic to Mesozoic rocks in the north west of Australia is
overlying Proterozoic basement rocks in the most part. This area was
part of a Y shaped rift valley that began to for in the late Palaeozoic time.
The other half of the Y is now on the Indian plate.
Look at the east coast of Australia.
How old are the rocks along the east coast in general?
_________________________________________________________
Check your answer.
These rocks along the east coast are much younger than the rocks found
in the west of Australia.
Suggest a reason why the east coast of Australia has no very old rocks?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
The largely Mesozoic cover of rocks in the eastern central part of
Australia represents rocks deposited when a shallow sea covered the area
between the Archaean to Proterozoic West Australia rocks and the
Palaeozoic east Australia rocks. These rocks overly Proterozoic to
Palaeozoic basement.
The rocks on the east coast are often folded like rocks that have been in a
collision zone.
Part 4: Evolution of continents
23
Can you suggest how the Australian continent might have grown to be its
current shape?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
24
Tectonic impacts
Additional resources
The super–continent cycle
By Dr. Richard Morante, 2001.
In the early 1970s J Tuzo Wilson while at the University of Toronto in
Canada, suggested that the thermal effects within the Earth might cause a
large continent or super–continent to disperse and then to reassemble as
oceans periodically open and close. This idea was revolutionary at the
time as scientists were just coming to grips with the concept of
plate tectonics.
The concept of a super–continent cycle was proposed in the late 1980s by
a team of geologists working in North America. That team consisted of
R. Damian Nance, Thomas R Worsley and Judith B Moody. Around the
same time in Australia Professor John Veevers from Macquarie
University in NSW was working on a similar idea. He also published a
scientific paper dealing with and describing the super–continent cycle.
The idea that the assembly of all, or most of the landmasses on the Earth
into a single entity or super–continent was cyclic was a relatively new
idea in the late 1980s. The science of plate tectonics was still relatively
young.
The details of the assembly of Pangea, the most recent super–continent,
were just emerging (although the idea was pivotal to the concepts of
Wegner on continental drift). The German meteorologist Alfred L
Wegener first proposed the theory of continental drift in 1912, describing
the assembly of Pangaea.
The thought that there was a cycle of around 500 million years that
would result in the assembly of a new super–continent was something of
a revolution. It may seem logical to you that if Pangaea's assembly and
breakup were recorded in rocks, that the probability was that a new
Pangaea like continent could assemble again and that an older Pangaea
like continent probably existed in the past.
Part 4: Evolution of continents
25
Rodinia: An ancient super–continent
Rodinia is the super–continent that existed in the super–continent cycle
before Pangea. This massive continent was named from the Russian
word for ‘motherland’.
The creation of Rodinia began due to the shifting of tectonic plates about
1100 million years ago during the late Proterozoic era. Reconstructions
of the Rodinian super–continent place North America at the core of
Rodinia with the east coast of North America adjacent to the western
coast of South America. The west coast of North America lay next to
Australia and Antarctica.
About 750 million years ago Rodinia began to break apart close to the
equator. This breakup stopped about the time that metazoan life emerged
at or near the Precambrian / Cambrian boundary about 540 million
years ago.
Identifying ancient super–continents
The problems faced by geologists working on plate tectonic
reconstructions of ancient super–continents are manyfold. The main
obstacle to the reconstruction of events in the past is that the longer ago
the events occurred, the less likely the evidence of their occurring will be
preserved in the rock record. This is true of all geological events.
The palaeontologist studying fossils from the distant past have less
material at their disposal for study simply because the chances of the
material being preserved decreases directly in proportion with the age of
the rocks. In the time frame of the ancient super–continent of Rodinia
(assembled around 800 million years ago) no metazoan life is known.
Since most palaeontology relies on the rapidly evolving metazoan life for
relative dating, palaeontological timing of the events surrounding the
assembly of Rodinia and even older super–continents is reliant upon
absolute dating, sequence stratigraphy and chemostratigraphic techniques
supporting paleomagnetic data.
Documentation of an ancient super–continent assembly and breakup is
hence not easy. By far the most well described super–continent cycle is
the one named by Wegner in 1912, Pangaea. Pangea is also the most
recent super–continent to have assembled! Even Rodinia, its predecessor
in the cycle, is far less certain in construction and event timing.
26
Tectonic impacts
What causes the super– continent cycle?
When dealing with a super–continent cycle two vital questions need to be
asked:
•
What drives the super–continent assembly?
•
What drives the super–continent break up?
Continental assembly
The answer to what drives continental assembly probably lies in the force
of gravity. It also involves the growth of one major ocean and the
disappearance of others. It may seem strange that continental assembly
is the direct result of actions that occur in the oceans but it is so.
You are all familiar with the concept of buoyancy. A material floats on
top of a liquid provided it is less dense than the liquid below.
The continental shields that form the core of the continents are granitic in
composition and are hence much less dense than the underlying
asthenosphere. Subduction of these continental shield areas is extremely
difficult because of their relatively high buoyancy
The oceanic crust is made from material from the asthenosphere that fills
the spaces between the continental fragments at mid ocean ridges. It is
inherently denser than the continental crust because the mantle is richer
in elements such as iron and magnesium. When it is hot and subjected to
a relatively high heat flow it remains less dense than the asthenosphere
material underlying it and hence it floats. When it cools after an
extended period of up to 250 Ma, the reverse is true, it becomes more
dense and sinks into the asthenosphere at subduction zones. After
subduction the minerals making up the slab undergo metamorphism and
increase in density. This further reduces buoyancy and encourages
further subduction dragging the oceanic plate behind with it.
This situation is best ascribed to the modern Atlantic Ocean that is
opening today with little obvious subduction on either its eastern or
western margin. Over time however such an ocean does develop
subduction at the oldest edges. These edges of the oceanic crust grow
colder and consequently denser the longer it is exposed to the surface.
The seafloor at the leading edge of the oceans next to the continents
becomes denser and eventually begins to subside as it migrates away
from the mid–ocean ridge. The tenuous connection to the lighter
continental crust at its leading edge breaks near or at the interface
between the less dense continental crust and the denser oceanic crust and
subduction begins.
Part 4: Evolution of continents
27
Andrew Hynes of Mcgill University in Canada suggested in the late
1980s that increasing oceanic crustal density overcomes any buoyant
resistance to subduction when the ocean floor reaches an age of about
200 million years. This is supported by the dating of oceanic crust which
rarely exceeds 200 million years even on the edges of oceans where the
crust is at its oldest.
Scientists studying the floor of the Atlantic have found that it grows older
and reaches its maximum depth at the farthest points from the
Mid–Atlantic Ridge. These oldest Atlantic seafloor rocks are about 180
million years old. When the oldest ocean floor along the margin of the
ocean grows denser than the asthenosphere below, deep trenches form
marking zones where subduction of old dense oceanic crust is occurring.
Permanent and non permanent oceans
The Atlantic Ocean is an example of a recycling ocean. It is of the type
that may open and close a number of times with new super–continent
cycles. The Pacific Ocean, on the other hand, is a permanent type ocean.
It will wax and wane in size depending upon the point in the
super–continent cycle but will always retain some presence.
The present size of the Pacific Ocean has been shrinking as the Atlantic
Ocean has been growing in the current phase of this super–continent
cycle which has been occurring over the past 180 million years.
Using the concept that the 200 million year old maximum age for the
Atlantic Ocean, it should be clear that the Atlantic is approaching the
point where its eastern and western margins will begin to subduct within
a few tens of millions of years. That subduction should begin where the
sea floor is oldest. That is, at the margins of the continents. When that
subduction happens, the rim of the Atlantic will become geologically
active just as the ‘Rim of Fire’ is active around the rim of the Pacific
Ocean today. The present outward drift of the continents on both sides of
the Atlantic should stop and reverse as the Atlantic Ocean begins to close
and marginal subduction zones on both sides of the Atlantic dominate
over any seafloor spreading occurring at the Mid–Atlantic Ridge.
The Pacific Ocean in this part of the cycle should then grow as
subduction on its margins declines. The result should be a new
super–continent assembly beginning. The timing of this should coincide
neatly with the beginning of breakup of Pangea some 250 million years
ago. If the time since the initiation of the breakup point is doubled, this
leads almost precisely to the 500 million year period of the
super–continent cycle.
28
Tectonic impacts
When continents hit continents
Reassembly of continents as ocean basins close means large compressive
forces. Rocks under compression bend and contort to produce massive
fold mountain chains similar to those on the Alps of Europe and the most
spectacular example on Earth today, the Himalaya. The resulting
mountain chains often bear relics of ancient ocean sediments scraped
literally to the top of the world. The physical expression of these ancient
mountain building episodes is not usually preserved except perhaps as
the exposed roots of the fold mountain chains.
The large quantities of sediments produced as a result of the erosion of
the ancient fold mountain belts are often preserved as thick sedimentary
sequences in ancient basin structures. These sedimentary sequences may
bear unique signatures of their origins that lead geologists to an
understanding of the sequence of events leading to their deposition. In
such cases, the mountain filling the hole in the ground is seen as the only
evidence of the existence of the eroded mountain belts having ever
existed.
The ancient super–continent researcher seeks these structures and
interprets the sediment flow back to the mountainous source in order to
develop a coherent picture of continental assembly in the ancient past.
Timing the super–continent cycle
The deduced history of Pangaea suggests the life expectancy of an
assembled super–continent is less than 100 million years and probably on
the order of 40 million years.
Pangea assembled during the Late Permian time and apparently began to
break up in the Triassic less than 40 million years later. The oceans, like
the Atlantic, apparently don't last much more than 400 million years if
they close at about the same rate as they open (although the age of the
oceanic crust rarely exceeds 200 million years old). Again given the
slow initiation of the ocean basins and the slowing rate as they wane in
their growth phase only a small extension of this time leads to a 500
million year cycle for super–continent breakup and assembly.
Continental breakup
The answer to what could have caused Pangaea (or any of the past
super–continents) to break apart has at least two complementary theories.
Part 4: Evolution of continents
29
Both take into account the evidence based largely on a reconstruction of
the Pangaean continent cycle that shows that no sooner had Pangaea
assembled than it almost immediately (in a geological sense anyway)
began to break apart.
Don L. Anderson of the California Institute of Technology suggested in
the 1980s that super–continents breakup because once assembled they
impede the flow of heat from the Earth's internal heat engine to the
surface. He suggested that because continental rocks are poor conductors
of heat they insulate the Earth causing a build up of heat beneath them.
The build up of heat actually causes the continental crust to rise much
like a crust on a pie cooked in the oven. This rise in the continental crust
puts the crust under strain and eventually causes it to split into three
segments at 120°, much like the appearance of the centre of a Mercedes
Benz car symbol. Molten rock from the overheated asthenosphere
beneath the continental lithosphere rapidly fills any resulting splits.
These splits are then widened as the super–continent fragments.
The splits become ocean basins as the fragments of the super–continent
are pushed and pulled apart.
The process of super–continent splitting in this theory comes about
because the continental crust is a better insulator of heat compared with
the thinner, denser basaltic ocean floors. The assembly of a large
continent or super–continent produces an effect like an insulating
blanket. It reduces the flow of heat from the mantle to the surface of the
Earth locally beneath the continental mass producing doming and
stresses in the continent that lead to splitting.
A proposal by Andrew Hynes of McGill University in the 1980s
suggested the breakup of a super–continent was due to the rotation of the
Earth. His idea was that super–continents riding high on the surface of
the Earth possess high angular momentum. This is a consequence of the
continental landmass consisting of an elevated mass that makes the
Earth's surface lopsided. As the Earth rotates, the effect of this
lopsidedness is the additional angular momentum produced as a result of
the higher mass of the continents at high elevation which produces
long–lived stresses within the super–continent. These stresses eventually
cause the super–continent to tear itself apart. Something that may
support this model is that both Rodinia and Pangaea split initially about
the Equator into northern and southern super–continents at the point
where the angular momentum would have been greatest for an elevated
super–continent.
Both models of continental breakdown most probably play some role in
the breakup of a super–continent although the restricted heat flow
scenario is probably given more credit. In both cases, however, the
super–continent itself produces the conditions that result in its own
destruction even as it forms. It is the assembly that actually leads to the
30
Tectonic impacts
inevitable breakup. That breakup in turn leads to the inevitable
reassembly.
superco
oceanic crust
ntinent assembled
oceanic crust
doming of supercontinent
heat flow to the surface restricted
by the insulating supercontinent
mid oceanic ridge
doming causes a split in the supercontinent
(similar to the current Atlantic Ocean)
A
A
B
B
mid oceanic ridge
heat flow to the surface
embryonic ocean
a fully grown ocean develops
(around 200 million years)
density at oceanic margins increases
(oceanic crust sinks at margins)
A
A
B
B
heat flow to the surface
oceanic sediments are scraped
off onto continental margins
A
A
seafloor
spreading
slows
subduction begins
B
B
subduction zone
fold mountain range forms as
the continents reassemble
compression
A B
A
compression
B
folded mountains
A sequence of events describing the super–continent cycle for the continents A
and B.
Part 4: Evolution of continents
31
32
Tectonic impacts
Suggested answers
Where is Australia?
The Australian continent is thought to have moved at about 7 cm/y,
although it is currently about 3 cm/yr.
1
Andesite is indicative of destructive plate margins.
2
The Australian continent is situated in the middle of a plate and not
at a plate edge – mountains and volcanoes are not being formed.
The geological time scale
Tectonic processes can lead to the deformation of rocks through folding
and faulting. As a result rocks that were initially deposited on the base of
a sequence can be overturned or lifted to be at the top of a sequence.
Interpreting geological maps
1
These rocks produce a linear north – south trend.
2
The Cg rock mass produces very circular outcrops.
3
The first letter for the rock represents the age of the rock type.
4
The rocks labelled Tv are basaltic. Therefore, some type of basaltic
eruption possibly involving ocean floor material could have occurred
to produce these rocks.
5
Because Tv is the youngest rock type in the area and it is volcanic,
it should be sitting on top of the other rock layers. On the map it
should be seen overlaying other rock types.
6
The granitic material labelled Dm appears to be folded. Therefore,
this material has been subjected to compressional forces.
7
Everything up and including the Devonian granites appears to have
been folded. The Carboniferous and Tertiary basalts have not.
Therefore, the folding must have taken place at the end of the
Devonian or early Carboniferous periods.
The tectonic environment that would produce all of these types of
features would be a destructive or convergent plate margin.
Part 4: Evolution of continents
33
Evolution of the Australian continent
1
The oldest minerals on Earth were found at Mt Narryer in the
Murchison region of Western Australia
2
The oldest minerals on Earth are zircons.
3
These minerals were found in quartzites.
4
The minerals are between 4100 and 4200 million years old.
5
The rocks are about 3600 million years old.
Development of the craton
The eastern one third of the Australian continent is missing.
Rock types and tectonic setting
Andesites are commonly produced at island arcs.
Rock types and tectonic setting
1
Shallow marine environments are conducive to the formation of
limestone because of the large amount of organisms containing
calcium in their shells and bones.
2
Dubbo
1
2
Bathurst
4
5
6
Canberra
7
0
100
Km
34
200
3
Sydney
Key:
1
2
3
4
5
6
7
Wellington
Molong
Jenolan
Tuglow
Wombeyan
Wee Jasper
Yarrongobilly
3
The cave locations are generally all in a line running north–south.
4
Australia grew towards the east during the Palaeozoic era.
Tectonic impacts
Australia’s current situation
1
The usual map of Australia is much smaller than the map just drawn.
The map just outlined includes New Guinea, Tasmania and
Australia’s mainland as one land mass.
2
Australia is presently moving northwards.
3
Australia is converging with the Pacific plate to the north.
4
A destructive plate margin where the Pacific plate is being subducted
beneath our own plate.
Australia’s future
Australia would collide with South East Asia and form fold mountains
pushing New Guinea and Indonesia up into mountain ranges. Eventually
Australia will collide with the Eurasian plate causing massive fold
mountains
The super–continent cycle
4000 km = 4000 x 1000 x 100 cm
= 400 000 000 cm
If Australia is moving at 3 cm every year then to move 400 000 000 cm
will take 400 000 000 every 3 years. This equates to 133 333 333 years.
This is roughly 130 million years.
n
lachia
Appantains
u
Mo
Part 4: Evolution of continents
35
4
The Caledonian mountains are really just an extension of the
Appalachian mountains.
5
Convergent plate boundaries must have existed between North
America and the Eurasian plate at some time in the past.
Impacts of the super–continent cycle
1
There would have been at least five new mid–oceanic ridges
produced. The five initial plates produced are: Indian, Australian,
Antarctic, South American and African plates.
2
With the massive size of each new mid–oceanic ridge produced
taking up increasing amounts of volume in the oceans, the sea–levels
around the world would rise.
3
Continental shelves would increase in size as coastlines retreated.
4
An increase in area of shallow seas would result in increased
numbers of shallow marine organisms, such as corals, pippies,
clams, sea snails and particular species of fish.
5
Limestone would become more common as the calcium from the
sea–shells and bones from the increased number of dead marine
organisms became concentrated and compacted.
Analysing a chronological geology map of Australia
The western part of Australia is the oldest.
The rocks in the east of Australia are mostly Palaeozic or Mesozoic.
The east coast of Australia has no very old rocks because it didn’t exist in
the Archaean or Proterozoic.
As subduction occurred along the east coast of Australia bits of land
were welded on to the older western Archaean and Proterozoic shield
areas. The mountains produced by these collisions were eroded and
form the sediment cover of the rest of central eastern Australia during
the Mesozoic.
36
Tectonic impacts
Exercises - Part 4
Exercises 4.1 to 4.2
Name: _________________________________
Exercise 4.1: Geological past
The following diagram represents a cross–section through a sequence of
rock beds.
sand
(unconsolidated)
shale
limestone
folded sequence
of rocks
basalt
metamorphosed
The geological sequence of events is as follows:
1
deposition of sediments (A)
2
folding event of these sediments (A)
3
intrusion of granite pluton (B)
4
erosion of surface (C)
5
deposition of limestone (shallow marine environment) (D)
6
deposition of shale (E)
7
basalt flow (F)
8
erosion of river channel (G)
9
deposition of sand in river channel (H)
Part 4: Evolution of continents
37
Create a geological history for the following diagrams:
A
sandstone
shale
limestone
conglomerate
B
sandstone
shale
limestone
conglomerate
basalt
metamorphosed
C
sandstone
shale
limestone
conglomerate
basalt
metamorphosed
_________________________________________________________
_________________________________________________________
_________________________________________________________
38
Tectonic impacts
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 4.2: Australia in the Palaeozoic
Draw a map of Australia as it was at the beginning of the Palaeozoic era.
Ensure that you label the following:
•
Palaeozoic coastline
•
present day coastline
•
any volcanoes (island arcs)
•
subduction zones
•
Yilgarn and Pilbara blocks
•
any adjoining landmasses.
Part 4: Evolution of continents
39
40
Tectonic impacts
Appendix
Location of Appalachian mountains and Caledonian mountains
Caledonian
Mountains
Appalachian
Mountains
0
2000
4000
Km
Part 4: Evolution of continents
41