landscape, climate and tectonics
and Jim Bowler, Geological
Society of Australia,
School of Earth Sciences, Public Lecture Series, 2003
University of Melbourne
1. Image: Australian continent image-oblique view showing continent and surrounding ocean floor
We live on an ancient continent, in many ways the most ancient of all the continents! We are home to the most ancient minerals, some 4300 million years old, and some of the most ancient rocks. In many of our landscapes we tread in the footsteps of the dinosaurs, more than 100 million years old, when Australia was part of the last great super-continent, Gondwana.
2. Image: Gondwana landscape relics - ??Jim what do you suggest
The notion of Australia as an ancient continent pervades the way we view our landscapes, and our place them. While much of the Australian landscape is indeed ancient, there is also an immensely important record change in our landscapes since the fragmentation of Gondwana. This record provides a unique catalogue of global change. To understand this catalogue, we must track the movement of Australia northward following its final break from Gondwana some 45 million years ago. We will explore the forces that drive Australia northwards and the way in which these forces have tilted and bent the land. We will explore one of the great paradoxes of the Australian landscape, magnificently preserved in the landscapes of the Murray Basin. As it has moved northwards to lower latitudes it dried and cooled; a trend that has culminated in the wildly oscillating climatic conditions that have accompanied the arrival of humans, and provided a legacy that impacts on, and informs, current land management issues.
Finally, we will debate the connection between Australia’s motion and the factors that have precipitated the climate change, in the context of the emerging realisation of our capacity to influence global climates.
3. Image: Australian plate reconstruction movie showing Australia’s northward motion over the last 45 ma.
Gondwana began fragmenting around 150 million years ago, progressively giving birth to various southern continents as we know them today. The fragmentation was complete by around 45 million years ago, when Australia started to move rapidly north leaving Antarctica as the lone Gondwana relic. Prior to this, Australia and Antarctica were separated by a narrow ocean only a few hundred kilometres wide. Goondiwindi was then at latitude ?60°S, well south of Hobart. Since then, Australia has been the moving faster than any other continent, travelling north at around 6.5 cm/year and traversing some 3000’s kms and x° of latitude. At current rates Goondiwindi will be in the northern hemisphere in another x° million years.
45 Million years ago was a period of massive change in the arrangement of the tectonic plates. As Australia began its northward drift, India, which had been sprinting north at 18 cm/yr, collided with Asia, slowed to around 5 cm/yr and began to build the Himalayas mountain chain and Tibet. The Pacific plate took a left hand bend of about 45°, reflected in the dramatic bend in the Emporer Hawaiin seamount change.
4. Image: of the 45 Ma old world.
45 million year ago saw the amalgamation of the Indian and Australian plates to from the Indo-Australian plate, the combined force of which helped drive India northwards into Asia. The northward motion of Australia has seen the opening of the southern ocean which, by ?30 million years ago, had opened sufficiently to isolate Antarctica with circum-polar currents. The motion has progressively closed the seaways that separated Australia from SE Asia. Simultanaeously, SE Asia itself was being squeezed eastwards like a lemon pipe from the vice formed by collision of India with Asia, further restricting the ocean currents flowing between the Pacific to the Indian Oceans.
As Australia moved northwards it began to gently tilt and sink. First, the southern margin sank more than one hundred metres contributing to the flooding that saw seas extend inland hundreds of kilometres across the Nullabor Plain and the Murray Basin. More recently, the northern margin has begun to sink, drawn down by flow in the underlying mantle beneath Indonesia, so that over the period of its northern motion Australia has sunk some 200 metres or so more-or less keeping pace with a global reduction of sea-levels over the last 65 million years.
5. Image: stress in the Indo-Australian plate: model and observation
The northward motion of Australia is a dramatic manifestation of “plate tectonics”, a kind of a planetary-scale thermoregulation mechanism. At great depths, where the rocks are made malleable by high temperatures, heat is carried towards the surface by a slow, steady upward flow of rock (termed convection). This motion displaces shallower rocks sideways, towards regions of down-flow where rocks made dense by heat loss descend back into deep Earth. The surface tectonic plates forming the cold, rigid outer 150 kms or so of the Earth form an important part of the deeper flow system being pulled and pushed towards regions of down flow termed subduction zones.
In the modern Earth, one of the main zones of mantle down-flow is to the north of Australia extending from northwards from beneath Indonesia along the western margin of the western Pacific Basin. Here the dense rock that floor the ocean basins are carried down “subduction zones” to depths of 600 kms. Continental crust drawn into subduction zones by plate motion, is too buoyant to be carried down into the deep interior. Consequently, continents tend to “clog up” subduction zones as they are forced into them, building huge mountain ranges and resistance to plate motion. In the Indo-Australian plate we can see the balance between the forces driving plate northwards towards Indonesia, and its resistance, generated by the mountain systems of the Himalaya, New Guinea and, to a lesser extent, New Zealand. This is revealed in the pattern of the tectonic stress field within the plate, this pattern sourcing the earthquakes that occasionally shake our continent.
6. Image sequence: earthquakes in the Indo-Australian plate, strain rate map homing into earthquakes in SE Australia and their relationship to topography
Unusually high stress levels in our plate Stresses are building sufficiently in the Indo-Australian plate has begun to fragment, in a kind of “plate-tectonic” divorce. The strategic allegiance between the Indian and Australian plates forming the Indo-Australian plate 45 million years ago.
The dynam, was a geologically short but incandescent affair!
The Gondwana world was a climate.. --- Southern Australia then at latitudes of 70°S was home of dinosaurs. Seas were some 200m higher, in part because polar ice caps did not exist, in part because the ocean basins were slightly shallower.
By virtue of its voyage across the southern hemisphere, opening ocean basins and closing others as it goes. Australia occupies a unique position.
7. Image: Murray Basin record
This northward journey has been accompanied by the drying and, somewhat paradoxically, cooling of our continent.
Tectonics and climate are intimately linked through changes in the ocean and atmospheric circulation patterns. The northward motion of the Indo- Australian plate has been a driving force for such change.
The stories of the Earth Sciences provide the context for the essentially moral question of how we should allocate resources between
Š present and future generations,
Š developed and developing societies,
Š human societies and natural ecosystems.
all central to questions of sustainability. It raises the question of what are the uniquely Australian Earth Science stories that empower us (that is, provide the context) in our quest to arrive at balanced answers to these questions.
The earth sciences provide the "context" for understanding global change.
The Australian continent has played a crucial role in global change and contains a special record of this change. The nature of the changes that have provided the opportunities for human evolution but now so endanger our future are prime concerns of the earth sciences.