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9.2 Tectonic impacts: 3. Continents evolve
| Syllabus reference (October 2002 version) | ||
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3. Continents evolve as plate boundaries move and change |
Students learn to: | Students: |
Extract from Earth and Environmental Science Stage
6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit: 23 Jul 08]
Prior Learning: Preliminary module 8.5 (subsections 1, 2 and 3); Stage 5, Outcome 5.9
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
For this syllabus point go to
Australian
Evolution of a Continent, Australia through time 
at the Geoscience Australia website.
The site provides the capability to analyse the continent at different eras and periods of time. Look at the Geological Time Scale on the left and choose an era. Use the key on the left to choose the geological environments in time slices at different periods. The map will show the geological features during the different periods. You can then choose a different era and see how the geological features changed and the pattern of growth of the continent. The information you collect and analyse about the ages and rock structures from west to east, and from north to south, will allow you to describe the pattern of growth of the Australian continent.
outline the main stages involved in the growth of the Australian continent over geological time as a result of plate tectonic processes
It is important to understand that the Australian landmass has existed as an island as we know it since about 55 million years ago (mya). Any outline of how the Australian continent has grown must be set in the broader context of a smaller Australia linked to other landmasses, particularly to the west and south. Often the eastern border met ocean with island arcs or with shallow seas.
The oldest rocks of Australia are found in Western Australia and are 3800 million years old. They are found in cratons, areas that have been through a full cycle of continental crust building processes. An area is cratonised when it has been through stages of mountain building that includes folding, igneous emplacement and crustal thickening, and has become stable after continuous erosion and isostatic uplift until it is about 35 kilometres thick.
The general trend across Australia is that the rocks become younger as we move from west to east.
- Stage 1: Formation of cornerstone blocks (cratons)
- By 2500 mya, three large cratons were established in Western Australia.
- By 2500 mya, three large cratons were established in Western Australia.
- Stage 2: Welding the blocks together
- From 2500mya to 900 mya, the cratons were separated by active, linear mountain chains, known as mobile belts, that welded the cornerstone blocks together. These belts were highly deformed and folded and contain metamorphic rocks and granite.
- By 900 mya, the western two thirds of present day
Australia had been cratonised. Australia was still part of Gondwana.
- Stage 3: Subduction and accretion in the east.
- From about 500 mya to about 250 mya, the continent was developed further to the east in the formation of the Tasman Fold Belt. The rocks present in the eastern third of the Australian continent exhibit evidence of former island arcs and ocean trenches resulting from the subduction of an oceanic plate. Sediment accumulated between the continental edge and the island arc, filling the seaway.
- From 320 mya to 280 mya, major mountain building occurred in eastern and central Australia, including the formation of the Lachlan Fold Belt and the New England Fold Belt. These belts supplied the sediment for sedimentary basins that developed along the eastern flank of Australia. The active, or mobile, belt then moved eastward to produce the Lowe Howe Rise. The current mobile belt lies along the Tonga-Kermadec-New Zealand Line in the Pacific Ocean.
- By 200 mya, the eastern third of Australia was
cratonised.
- Stage 4: Shallow seas
- 160 mya, an area called Argoland rifted away to the northwest. Rift valleys formed down the Western Australian coast and between Australia and the Indian continent. This was the beginning of the breaking up of Gondwana. sea levels rose, flooding over the Greta Artesian Basin.
- 132 mya, a narrow seaway had developed separating Argoland. South and west of Australia, spreading began and marked out the continental shapes including India. Faster spreading between India, Antarctica and Australia continued to 118 mya, opening an ocean up to 600 kilometres wide.
- 96 mya, the Lord Howe rise began rifting south of Tasmania and westward, separating Antarctica. The rift that was moving India away was cut.
- 84 mya, the Indian continent moved further north with the same direction as the rift between Australia and Antarctica.
- 64 mya, the Tasman Sea continued spreading, until 49 mya when spreading stopped.
- From 45 mya to the present, the Southern Ocean continued
spreading. Resultant downwarping of the continent allowed shallow seas to
cover the Murray Basin.
- Stage 5: Intra-continental earthquakes and hot spot volcanoes
- As the continent (now the island we recognise) continued its northward drift, it passed over a number of mantle hot spots, resulting in a series of parallel lines of volcanoes which are younger towards the south. The largest of these include Mount Warning on the NSW/Queensland border and Mount Canobolas in the NSW Central West. The most recent volcanic eruption was at Mount Gambier in South Australia only 4000 years ago
- Tensional stresses acting within the continent as
the plate boundary to the north pushed against the Asian and Pacific plates
caused some very old faults to move periodically, and blocks to adjust isostatically.
The Great Dividing Range was uplifted to its present height by this process.
- Stage 6: Continuing northward
- Interaction between the converging Australian and Pacific plates has produced the current New Guinea mobile belt.
present information as a sequence of diagrams to describe the plate tectonic super-cycle concept
You should be able to gather information about the plate tectonic super-cycle concept from a range of resources including the sources referenced below.
Information to describe super-cycle concepts is presented in:
- The notes further below Scheibner, E., The Geological Evolution of New
South Wales - A brief review, Department of Mineral Resources 1999. Booklet
- Snowball Earth an online article by Paul F. Hoffman and Daniel P. Schrag. It includes
excellent illustrations.
- The
Wilson Cycle Lynn S. Fichter © 1999, (Fichtels@jmu.edu), Department of Geology and
Environmental Studies (Geology home page jmu.edu/geology/), James Madison
University, Harrisonburg, VA 22807, Spring, 1999
You could present a summary to show how a super-cycle operates using a cycle diagram like the one below.
summarise the plate tectonic super-cycle
The plate tectonic super-cycle is a theory to explain a sequence of events that have repeated at least three times. Formation of super-continents Pangea and Rodinia occurred 300 million years ago and 900 million years ago, suggesting a super-cycle time span for formation and breaking up of super-continents of about 600 million years.
The following is a very general description of possible super-cycles.
During plate tectonic development, a super-continent breaks up and the two new continents become separated by the new oceanic lithosphere that is produced at a mid ocean ridge between them. As the oceanic lithosphere grows, the continents drift further apart. If a subduction zone forms near the edge of one of the continents, the oceanic lithosphere will be consumed in the subduction zone. The continents will be drawn back together, eventually to collide producing a super-continent again.
If a subduction zone develops on the far side of one of the continents, oceanic lithosphere will be consumed. This may eventually cause the continent to collide with another continent producing a new super-continent.
The following is another super-cycle scenario, using Pangea as an example:
- Begin with a small super-continent, like Pangea, completely surrounded with ocean. (Pangea occupied 30% of the Earth's surface with the other 70% being ocean.)
- Spreading at a mid ocean ridge some distance from the super-continent will cause the oceanic lithosphere near the super-continent to begin to subduct beneath it.
- This subduction produces the characteristic andesitic volcanoes. The volcanism at the edges of the super-continent causes some weakness in the crust there.
- Subduction continues until the subduction zone becomes choked and ceases, causing a new subduction zone to develop a few hundred kilometres offshore. This new subduction zone will result in a chain of new andesitic volcanoes, and thus new continental material developing offshore. The weakness in the continental margin between the new island chain and the original super-continent allows spreading to occur creating a trough called a back-arc basin. The area west of the islands of Japan is an example of this.
- Now, marginal seas and island arcs surround the super-continent. Back-arc basins eventually fill with sediment, thus extending the size of the super-continent.
- Eventually, due to the presence of weaknesses in the zones that were once marginal seas, the super-continent is able to split up, allowing the formation of separate continents, like we see today.
- The cycles continue for each continent. If subduction of the ocean plates continues, it may bring continents together once again creating a supercontinent and thus the cycle can continue.
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