ROCK LANDFORMS of Australia & NZ - TECTONIC LANDFORMS

This site examines several areas of interest in southern Australia that illustrate aspects and results of the
breakup of the ancient supercontinent, Gondwana - when segments of the crust separated (i.e. rifted).

Theme Study (B): GONDWANA BREAKUP (SITE: 1) Location: Perth Plain / Darling Scarp, WA Feature: Depositional plain below scarp
KEY POINT: This landform (Perth) owes its major features to continental separation (major fault line and sediment plain)
This photo hints at several key events in Australia's long geological history. In the distance (east) is a feature known as the Darling Scarp, a long (almost 1000km), north-south fault-related feature of 300-400m height. Beyond the scarp, the land continues half way across Australia as the Yilgarn Plateau, which was formed when the Earth was young. Some mineral fragments have been dated at 4.2 b.y., possibly making this the oldest material yet discovered. The foreground shows the Perth Plain, composed of sediment deposited after the land mass on this side (west) of the fault subsided and slumped into the sea. This took place as major crustal rifting split Gondwana, forcing the separation of the land mass to the west of what is now southern WA. Major marine areas appeared off WA by 180 m.y. ago. By 125 m.y. ago Australia and the fragment now called India had finally parted. India moved away, colliding with the Asian land mass. top of page
Theme Study (B): GONDWANA BREAKUP (SITE: 2) Location: Cradle Mountain, central Tasmania Feature: Igneous activity resulting from crustal movements
click on photo for enlargement KEY POINTS: Crustal rifting initiated volcanic activity (in Tasmanian highlands)
Cradle Mountain, 1545m elevation, protrudes above the central Tasmanian Plateau. It is composed of a dark, low-quartz rock called dolerate. Crustal movements about 180 m.y. ago (Jurassic Period) permitted the intrusion of molten magma into large underground fractures. This magma became the rock dolerite which now forms many prominent mountain and rugged coastal areas of the state. These events resulted from tensional forces associated with the beginning of Antarctica's break from Australia. Dolerite exposures often reflect the magma cooling conditions; weathering occurs along shrinkage joint planes to reveal 'organ pipe' and spire-like features. After time, the the vertical columns separate and collapse. This igneous activity was the early manifestation of crustal movements in Gondwana that would eventually split Antarctica and Australia. Since separation a water body has filled the widening space between these two continents. top of page
Theme Study (B): GONDWANA BREAKUP (SITE: 3) Location: Port Campbell coast, western Victoria Feature: Erosion in former rift basin
KEY POINTS: Parts of the southern coastline of Australia were continuous with Antarctica, prior to separation (rifting)
Continents, on large crustal plates, move slowly across the surface of the Earth due to sub-crustal convection currents. Interestingly, about 200 m.y. ago eastern Australia was over the South Pole! During such movements, plates may collide destructively, join and travel together, or separate. Fossil and coastline matching support the hypothesis that Australia and Antarctica were once joined. It is believed that these two masses met and joined well over a billion years ago and later formed a major portion of the supercontinent Gondwana. After the Indian portion separated from Australia's west, southern Gondwana began splitting with crustal movements in Tasmania. Separation (rifting) from Antarctica occurred along the zone which eventually widened (from the west) between the two masses. (A similar rifting process is taking place in northeastern Africa at present, with a large crustal segment slowly separating from the main mass of Africa.) The separation of Australia and Antarctica was completed about 55 m.y. ago. Marine conditions became established in the rift, along with sediment build up and rock formation. Ultimately uplift and erosion shaped the new southern coastline (see photo). Australia now moves northwards at the rate of several centimetres per annum. top of page

to Tectonic Page		Tectonic features have resulted from earth movements Tectonic landforms are found across a wide range of rock types. On the largest scale, continental movements reflect movements of huge portions of the Earth's crust, known as plates. These plates may collide or they may separate (a process called rifting). The ancient supercontinent of Gondwana contained components that we now call Antarctica, South America, India, Africa, and Australia. Australia separated from the large Indian and Antarctic blocks millions of years ago.
TECTONIC LANDFORMS		- Theme Study (B) © C.Grant Continental Breakup (Sites 1-3)

Map of Tectonic Sites	go to Tectonic Landforms	go to Main Landform Page

depositional plain igneous activity former rift basin