The geological context of the Australian continent

This notion of Australia as a "timeless land" and an "ancient continent" forms a persistent theme in the Australian psyche. These notions are founded in part in our understanding of the geological framework of the continent; a continent that in geological terms has been "relatively" inert for the last several hundred million years. Indeed, much of the western half of the continent has been quiescent for up to a billion years or more. This quiescence is reflected in the generally subdued landscapes (we have no great mountain ranges), relatively low earthquake (or, in geological-speak, seismic) activity and absence of active volcanoes.

The lack of geological activity can be understood in the context of the broader geologic setting of the continent. The Australian continent forms part of the Indo-Australian plate. The Indo-Australian plate forms one of a seven major tectonic plates that comprise the great proportion of the earth's crust. Even though the tectonic plates move across the surface of the Earth (at velocities of up to about 15 cm per year), they do so individually in a coherent fashion with relatively little internal distortion. However, neighboring plates move at different directions at different velocities, with the resulting "collisions", "shearing" and "tearing apart" at plate boundaries resulting in geological activity at least several orders of magnitude more intense than in the late interiors.

 

Global map showing the structure of the ocean floor derived form Geosat synthetic topogrpahy and gravity data

The boundaries of the Indo-Australian plate occur

Image of the Australian plate constructed from gravity and bathymetric data. Stresses in the Australian continent arise from interactions along the plate margins including the mid-Ocean Ridge to the south (in the pink), the New Guinea collision and subduction systems of the Banda arc to the north, and the New Zealand collision to the east, as well as interactions with the convective mantle over which the Australian lithosphere is traveling.

The focus on the ancient and the timelessness of the Australian continent, and particularly its landscape, has detracted form many of the subtle young features suggesting our continent is stirring from its geological "slumbers". Earthquake activity does occur as evident by the Newcastle quake in 199? While it low in comparison to many parts of the world, such activity provides evidence that significant forces are operating on the Australian continent.

 Shaded topography of the Australian continent and surrounding oceans derived from the global ETOPO5 dataset
 

Perhaps the most dramatic indication of this stirring is to be found in the landscape of the Mount Lofty Ranges (aka the Adelaide Hills) which point to a recent resurgence in tectonic activity in this region; a resurgence which may well be the "harbinger" of greater things to come. In these pages we provide a visual guide to some of the features of the landscape of the Adelaide Hills which point to its geological youth. These images have been produced from digital elevation models (DEM's) derived forma number of different sources including the South Australian Department of Environment and Natural Resources (DENR) and the Australian Geological Survey Organisation (AGSO).

 Shaded topography of the Australian continent derived from the TerrainBase dataset.