Shaping the Landscape continued

Phase 3 Mountain Building

Mountains form as one continental plate slides under another Photographer - Luke Simpkin Source - Museum Victoria

The eastern margin of Gondwana was rimmed by a major subduction zone as in the South American Andes mountains of today. A subduction zone is where the continental edge uplifts as the colliding continental plate slides beneath it. This process reached a peak between about 260 - 230 million years ago and was the last major mountain building event to have affected the eastern Gondwana margin. It added the finishing touches to the creation of the highlands across south-eastern Australia. Ongoing and vigorous erosion of the highlands since they first emerged back around 370 million years ago, by rivers and glaciers, would have exposed some of the early-formed granite. Erosion would have continued at an even greater pace immediately following this final phase of mountain building as rivers quickly carved into the high topography.

Phase 4 Break-up

Gondwana breaks up forming the modern continents Artist - Karina H McInnes

At about 200 million years ago Gondwana began to break up into smaller continental fragments and the modern continents began to take on their present identity. Australia emerged from eastern Gondwana with the formation of the Tasman Sea and the separation and movement of Antarctica to the south.

The break-up process involves extension (stretching) and thinning of the crust (the opposite of the mountain building process) until it ruptures along a line called a rift zone. The crust is literally pulled apart within these zones and ultimately replaced by molten material that upwells to fill the gap from the mantle below. The best example of an active rift zone is the East African Rift Valley in Kenya or the Red Sea Rift and Gulf of Suez.

The rifting process produced elevated and steep escarpments facing the nascent oceans and new rivers draining these steep escarpments started a new phase of erosion along the margin of south-eastern Australia.