Australia: The Land Where Time Began

A biography of the Australian continent 

Recycling and mountain uplift  see Tethys Ocean

Thousands of kilometres of mid-ocean ridge continued producing new ocean crust as India moved towards Eurasia, steadily increasing the size of the Indian Ocean. As in all other major oceans, the process in the Indian Ocean has been continuing for more the 100 My. If ocean crust continued to be produced with no way of removing crust at the same rate the surface of the Earth would need to continually increase to accommodate it, but it is now known that oldest crust, the furthest from the spreading centre, is being continually taken back down in to the Earth at subduction zones at the bottom of deep trenches near the edges of continents. The heavy oceanic crust dives beneath the lighter continental crust, after which it is melted and merges with the mantle rock. At the surface of the descending plate melting occurs due to extremely large levels of friction in the upper levels of the subduction zone. Rocks carried down on the plate surface and wet sediments, as well as in the zone of friction just above, begin melting at depths of 50-100 km, where by this depth the temperatures have risen to 1200-1500o C. Molten rocks accumulate in chambers (localised pockets) and begin rising as Plutons (intrusions) into the lower crust where they form granites as they cool and crystallise, the main rock type of continents. Where some of the molten rock breaks through the surface the reduced pressure allows the magma to erupt with explosive force on land and beneath the sea. Over time these can form archipelagos (island arcs) that are strung out behind the subduction zone.

The highest number of earthquakes and volcanoes in the world occur around the edges of the Pacific Ocean. The Ring of Fire, as it is surrounded by subduction zones, trenches and island arcs. Some of the greatest earthquakes and eruptions ever recorded have occurred on the eastern margin of the Indian Ocean where the oceanic plate plunges into the subduction zone under the Indonesian arc. According to the author3 the same would have occurred on the northern margin of Tethys, a line of trenches being strung out in an east-west direction around ¼ of the circumference of the globe, close to the continent, while in other places separated from the continent by an island arc and a marginal sea. There would have been a subtropical climate along its the full length and there would have been islands surrounded by coral reefs, though it would also have had what the author3 calls a Girdle of Fire, the Tethys Ocean version of the Ring of Fire around the Pacific Ocean, the whole region being subjected to powerful earthquakes, violent volcanic eruptions and very large tsunamis.

As a result of this geological violence the land above the subducting plate would have been pushed up, as frictional heat above the subduction zone caused the formation of deep-seated granite melts that are less dense than the mantle rock that surrounds them causing them to rise up through the crust until they are above the land surface. As the granite plutons are emplaced they are accompanied by volcanic eruptions producing copious volumes of granite, the rock of the continents that is lighter than the basalt lavas that form the oceanic plates. Some of the most common rock types composing granite are andesite, rhyolite, dacite and others that are rich in quartz.  The pressure required to trigger a volcanic eruption of granite is much high than that required for basalt and as a result the eruptions of granitic lava are much more violent than those of basaltic lava. Granitic eruptions occur far less frequently than basaltic eruptions but that are far more explosive, spewing out not only lava but also volcanic dust and ash, but also blocks and bombs. The author3 suggests that similar mountains would have been thrust up along the northern margin of Tethys.

The material that is eroded and weathered from the rising mountains as they are gradually denuded is eventually carried by the torrential rivers to the ocean where most is plastered against the continental margin. The small amount that reaches the true ocean far from shore is eventually returned to the continent as the plates inexorably move towards the subduction zones. Huge slices of sediment, as well as parts of the ocean crust are scraped off and added to the overriding continent, being thrust up away from the plunging slab. This scraped off material is badly deformed, being crumpled and folded and broken up by faults, and extreme pressures and temperatures partly convert them to metamorphic rocks. The rocks of the margin of the continent also fold and buckle upwards as a result of the huge and relentless pressure of the subducting oceanic plate. Exotic terranes, giant rafts of oceanic and continental material, foreign to the local area, are also fused to the growing belt of mountains. Exotic terranes are often of unknown origin as their source land mass may have been previously destroyed. The growing mountains preserve at least some clues to their history.

Eventually the ocean closes and there is a continent to continent collision, this happened when the northern part of the Tethys Ocean between Eurasia and the approaching Indian Plate was squeezed out of existence once they had come into contact. The northern Tethys Ocean Trench was overridden as the rocks of the Eurasian continent, as with all continents, was too light and buoyant to be subducted. The rocks of both continents folded and buckled more than they had done in their previous history, with whole fragments of the continental margin being thrust up over the growing mountains. The superimposed slices of rock are called nappes (from the French word for ‘sheet’) The extreme complexity of the Himalayas has meant that there is still a lot to understand, exactly what happened after the collision, such as the great height of the mountains, the elevation of the vast Tibetan Plateau, the over-thickening of their roots that extend far into the mantle. It has been found that the Indian Plate has penetrated nearly 2000 km into Asia following the first contact. The Asian crust was bent and almost flowed as it was pushed aside forming a great tectonic arc from the Tibetan Plateau to the Three Gorges region of western Yunnan.

Continued slippage and plate movement still occurs at the line of fusion of the plates, the Shan Shear Zone.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press. 



Author: M. H. Monroe
Last Updated 10/04/2012


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