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Large-scale processes and features of the Australian coast

The coast of Australia displays enormous variety, from tropical rainforest with fringing reefs in the Daintree of North Queensland to the arid sands at Coffin Bay in South Australia; from the hot arid cliffs of the Kimberley coast of northwest Western Australia to the cool, wet, cliffed coast of Western Tasmania; from the alluvial flats and mangroves of the Gulf of Carpentaria (Northern Territory) to the saline samphire flats of the Gulf St Vincent (South Australia). Its marine waters (excluding Antarctic and offshore territories), range from tropical at 10° south to cool temperate at 45° south, and extend over 55° of longitude. A survey of the terrestrial coast by aerial photography shows the dominance of sand coasts; division by states shows the importance of rock coasts in Tasmania and mud in the Northern Territory.

The coastal zone of Australia is a dynamic meeting area of earth (lithosphere) and atmospheric forces (atmosphere), of oceans and rivers (hydrosphere), and of terrestrial and marine life (biosphere). Changes, over a variety of time scales, are a characteristic of all coasts, and are an important driver of coastal management.

There is no universally accepted classification of coasts that would enable Australian coasts to be neatly categorised and comparisons to be made on a world scale. However, a number of biological, lithological, atmospheric and hydrological characteristics of the Australian region provide the framework for a basic description of the coast that is relevant to its management.

Tectonics

Compared with other continents, Australian coasts are relatively stable tectonically, undergoing few episodes of crustal movement or vulcanism. This may be explained by the continent's central position on its lithospheric plate. Inman and Nordstrom (1971) produced an important classification system based on plate tectonics and the relationship of the coastline to global plate margins and movements (figure 2.1). This system divides coasts into the following types:

1 Collision coasts, on the collision and subduction sides of continents and island arcs; these are the young mountain zones, with narrow continental shelves and high seismicity and volcanicity. These active areas are characterised by the delivery of coarse sediments from mountain catchments to a narrow or absent coastal plain and narrow continental shelf. Shorelines are often rocky with only small beaches.

2 Trailing-edge coasts, on the sides of continents and island arcs that are moving away from the rising and spreading ocean ridges. Generally wide coastal plains, fine sediments from large catchments delivered to broad shallow continental shelves; deltas and barrier islands are common:

(a) neo-trailing edge coasts, where new rifts have recently opened and only a small amount of separation has taken place, e.g. Red Sea

(b) Afro-trailing edge coasts, where both east and west coasts are trailing, as in Greenland and Africa; these have plateaux, plains and narrow continental shelves

(c) Amero-trailing edge coasts, where the other side of the continent has a collision coast. These are characterised by extensive coastal plains, broad continental shelves, and low seismicity and volcanicity.

3 Marginal seacoasts that lie on the protected side of island arcs.

Figure 2.1 Inman and Nordstrom's classification of coasts based on plate tectonics with some modification by Davies 1972

Source: Davies 1972

This classification does not separate Australian coasts in any significant way, as the continent is relatively remote from plate boundaries, tectonically stable throughout, and characterised by unusually ancient landforms. The classification thus points to an important characteristic of Australian coasts. Air and ocean climate variables appear more significant around the Australian shores. However, Australian coastal areas are subject to warping due to loading effects that are a consequence of sea level change. These movements are small in amplitude, but significant in the management of low-lying coastal areas. (See section 'Sustainable coastal management and scientific uncertainty'.)

 
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