Australia: The Land Where Time Began

A biography of the Australian continent 

Supercontinents

Blewett describes supercontinents as amalgamations of nearly all the continental blocks of the Earth. Geologists in India coined the name Gondwana in the late 1800s to describe the concept of a geological entity. This was first recognised from the floral Glossopteris assemblage dating from the Palaeozoic that was found only in the continents of the Southern Hemisphere. These continental landmasses were South America, Africa, Madagascar, India and Australia, with Antarctica being added later, and it was believed at the time that this widespread distribution could only be explained by the presence of ‘land bridges’ that are no longer present that connected all the continents. It has since become apparent the blocks of continental lithosphere have come together and then fragmented into separate entities that were subsequently dispersed by continental drift a number of times in the Proterozoic and the Palaeozoic since their earliest formation. The period of maximum packing of the continental blocks to form a supercontinent defines a time span, as the amalgamation of a supercontinent has been found to commonly overlap with the dispersal of the constituent continental blocks.

Fundamental to supercontinent formation and breakup is the interplay of continental blocks and the dynamics of the mantle, as it is believed likely that continents move from the locations of upwelling mantle and towards mantle cells that are downwelling. There are a number of mechanisms that could power dispersion. Warming of the mantle beneath the supercontinent could lead to uplift and lateral stresses, because the upper mantle cannot be cooled by subduction in the interior of the large landmass. According to an alternative proposal thermal plumes that are linked to reorganisation of mantle flow can drive dispersal of the constituent continental blocks. Blewett suggests these 2 mechanisms may be linked.

The global climate is influenced by the formation and breakup of supercontinents, as wind patterns and ocean circulation are affected by the location of continents. Separation of the continents can also affect eustatic sea level by the volume of the ocean basins being decreased by the rapid spreading of the seafloor, as occurred in the Eocene. This condition results from young oceanic crust being relatively buoyant as well as elevated when compared with old oceanic crust. Mid-ocean ridges are examples. Feedbacks also exist between global tectonics, climate and sea level. Major glacial accumulations (‘icehouse’ episodes, as have occurred in the end Precambrian, end-Ordovician and end-Carboniferous) have been linked to times when the continents were amalgamated, and therefore global sea levels were lowered. As occurred in the Silurian and mid-Cretaceous, supercontinental dispersal correlates with greenhouse regimes and continents that were flooded.

An intense monsoonal system was induced in the Triassic by the meridional distribution of continents in Pangaea, with the seasonal aridity being recorded in red beds on many continents. Glaciation at the poles can be driven by the opening of circumpolar circulation, such as resulted from the rifting in the Cainozoic of Australia from Antarctica, and the opening of Drake Passage, which led to the ice sheets that cover Antarctica. In the Northern Hemisphere, glaciation in the last 1-2 Myr is probably linked to the closure of equatorial circulation between the Pacific Ocean and the Caribbean Sea.

Sources & Further reading

1. Blewett RS (ed.) 2012. Shaping a Nation: A Geology of Australia, Geoscience Australia and ANU E Press, Canberra.