western australia

Geology

The bulk of Western Australia consists of the extremely old Yilgarn craton and Pilbara craton which merged with the Deccan of India, Madagascar and the Karoo and Zimbabwe cratons of Southern Africa, in the Archean Eon to form Ur, one of the oldest Supercontinents on Earth (3,200-3,000 million years ago). Because the only mountain-building since then has been of the Stirling Range with the rifting from Antarctica, the land is extremely eroded and ancient, with no part of the state above 1,245 metres (4,085 ft) AHD (at Mount Meharry in the Hamersley Range of the Pilbara region). Most parts of the state form a low plateau with an average elevation of about 400 metres (1,200 ft), very low relief, and no surface runoff. This descends relatively sharply to the coastal plains, in some cases forming a sharp escarpment (as with the Darling Range/Darling Scarp near Perth).

The extreme age of the landscape has meant that the soils are remarkably infertile and frequently laterised. Even soils derived from granitic bedrock contain an order of magnitude less available phosphorus and only half as much nitrogen as soils in comparable climates in other continents. Soils derived from extensive sandplains or ironstone are even less fertile, being even more devoid of soluble phosphate and also deficient in zinc, copper, molybdenum and sometimes potassium and calcium.

Western Australian cities, towns, settlements and road network

The infertility of most of the soils has required heavy inputs of chemical fertilisers, particularly superphosphate, insecticides and herbicides, which—with the ensuing damage to invertebrate and bacterial populations, and compaction of soils through heavy machinery and hoofed mammals—has done great damage to the fragile soils. Large-scale land clearing for agriculture and forestry has damaged habitats for native flora and fauna. As a result, the South West region of the state has a higher concentration of rare, threatened or endangered flora and fauna than many areas of Australia, making it one of the world's biodiversity "hot spots". Large areas of the state's wheatbelt region have problems with dryland salinity and the loss of fresh water.

Climate

The southwest coastal area is relatively temperate and was originally heavily forested, including large stands of the karri, one of the tallest trees in the world. This agricultural region of Western Australia is in the top nine terrestrial habitats for terrestrial biodiversity, with a higher proportion of endemic species than most other equivalent regions. Thanks to the offshore Leeuwin Current the area numbers in the top six regions for marine biodiversity, containing the most southerly coral reefs in the world.

Hay Street, Perth

Average annual rainfall varies from 300 millimetres (12 in) at the edge of the Wheatbelt region to 1,400 millimetres (55 in) in the wettest areas near Northcliffe, but in the months of November to March evaporation exceeds rainfall, and it is generally very dry. Plants must be adapted to this as well as the extreme poverty of all soils. A major reduction in rainfall has been observed, with a greater number of rainfall events in the summer months.

The central four-fifths of the state is semi-arid or desert and is lightly inhabited with the only significant activity being mining. Annual rainfall averages about 200 to 250 millimetres (8–10 in), most of which occurs in sporadic torrential falls related to cyclone events in summer months.

An exception to this is the northern tropical regions. The Kimberley has an extremely hot monsoonal climate with average annual rainfall ranging from 500 to 1,500 millimetres (20–60 in), but there is a very long almost rainless season from April to November. Eighty-five percent of the state's runoff occurs in the Kimberley, but because it occurs in violent floods and because of the insurmountable poverty of the generally shallow soils, the only development has taken place along the Ord River.

Occurrence of snow in the state is rare, and typically only in the Stirling Range near Albany, as it is the only mountain range far enough south and with sufficient elevation. More rarely, snow can fall on the nearby Porongurup Range. Snow outside these areas is a major event; it usually occurs in hilly areas of southwestern Australia. The most widespread low-level snow occurred on 26 June 1956 when snow was reported in the Perth Hills, as far north as Wongan Hills and as far east as Salmon Gums. However, even in the Stirling Range, snowfalls rarely exceed 5 cm (2 in) and rarely settle for more than one day.^[9]

The highest observed maximum temperature of 50.5 °C (122.9 °F) was recorded at Mardie, Pilbara, 61.6 kilometres (38.3 mi) from Barrow Island on 19 February 1998.^[10] The lowest minimum temperature recorded was -7.2 °C (19.0 °F) at Eyre Bird Observatory, 49 km (30 mi) south of Cocklebiddy.^[11]

Biota

The Black Swan is the state bird of Western Australia

The Red and Green Kangaroo Paw is the floral emblem of Western Australia

Western Australia is home to around 540 species of birds (depending on the taxonomy used). Of these around 15 are endemic to the state. The best areas for birds are the southwestern corner of the state and the area around Broome and the Kimberley.

The Flora of Western Australia comprises 9437 published native vascular plant species of 1543 genera within 226 families, there are also 1171 naturalised alien or invasive plant species more commonly known as weeds.^[12] In the southwest region are some of the largest numbers of plant species for its area in the world.

William Henry Harvey published a five-volume Phycologia Australia which was issued in parts between 1858 and 1863. He earned the title of father of Australian Phycology. His main collection is in the herbarium of Trinity College Dublin, there is also a large collection of his specimens in the Ulster Museum, Belfast.