Australia Yearbook - 2009-10

Brindabella Ranges, the highest ranges do not
coincide with the Great Dividing Range (which in
that area is east of Canberra).
The article Landforms and their history in Year
Book Australia 1988 provides a more detailed
description of Australia's landforms.
The history of Australia's landforms
As noted earlier, much of the Australian
landscape is many millions of years old. The
Western Plateau is especially old, and includes
some of the oldest rocks on earth, more than
3,500 million years old. Most of this region has
existed as a landmass for over 500 million years.
The present topography results from a long
landscape history which can be considered as
starting about 290 million years ago, the last time
Australia was subjected to large-scale glaciation.
Once the ice melted, parts of the continent
subsided and were covered with sediment to
form sedimentary basins such as the Great
Artesian Basin. By early-Cretaceous times, about
140 million years ago, Australia was already so flat
and low that a major rise in sea level divided it
into three landmasses as the shallow Cretaceous
sea spread over the land. The main separation of
Australia from Antarctica took place between 100
and 80 million years ago.
In the following Tertiary times, Australia can be
regarded as a landscape of broad swells varied by
a number of sedimentary basins (Murray,
Gippsland, Eucla, Carpentaria, Lake Eyre and
others). These slowly filled up and some are now
sources of coal or oil. Most of the Eastern
Highlands were uplifted at about this time,
although a few parts were still experiencing uplift
as recently as one million years ago. The central
Australian region was also uplifted, and then
eroded, leaving remnant mountains and
individual peaks such as Uluru (Ayers Rock),
which was exposed about 65 million years ago.
Another feature of this era is the Nullarbor Plain,
an uplifted limestone sea floor dating to about 25
million years ago.
Throughout the Tertiary, volcanoes erupted in
eastern Australia. Some individual volcanoes were
the size of modern Vesuvius, and huge lava plains
covered large areas. Volcanic activity continued
up until a few thousand years ago in Victoria,
south-east South Australia and Queensland, and a
resumption at some time in the next few
thousand years cannot be ruled out. Australia’s
youngest volcano is Mt. Gambier in South
Australia, about 4,600 years old.
Between 55 and 10 million years ago, Australia
drifted across the surface of the Earth as a plate,
moving north from a position once adjacent to
Antarctica. During much of this period the Earth
was much warmer and wetter than it is today,
with little or no ice cover even at the poles, and
hence Australia retained a warm, relatively moist
climate through most of this period despite its
latitudinal shift. It was probably under this climate
that the deep weathered, iron-rich profiles that
characterise much of Australia were formed.
Aridity only seems to have set in after Australia
reached near its present latitude range about five
million years ago, with no known landforms
(such as dunes or salt lakes) associated with
aridity that are more than one million years old,
and the northern part was probably never arid.
Today a large part of Australia is arid or semi-arid
(see the article Australia's deserts in Year Book
Australia 2006). Large parts of the arid zone are
covered with sand dunes, which are typically
aligned longitudinally according to prevailing
wind directions (south-east to east in the north,
north-west to west in the south). These dunes
were formerly mobile but are now mostly fixed.
Plains covered with small stones (stony deserts or
gibber plains) are found in areas without a sand
cover. Salt lakes are found in many low positions,
in places following lines of ancient drainage. They
are often associated with lunettes (dunes formed
on the downwind side of lakes), which have been
the location of many important finds of Aboriginal
prehistory. In addition to the present arid zone,
some of these landforms are found in areas which
were formerly arid but have become wetter, such
as parts of western Victoria and south-eastern
South Australia.
On a global scale, the last few million years were
notable for the Quaternary ice age. There were
many glacial and interglacial periods (over 20)
during this time, with the last ending about
12,000 years ago. As in the rest of the world,
Australia’s climate during this time was much
cooler (and probably generally drier) than it is
today, but only small parts of the continent were
glaciated – the Central Plateau of Tasmania and an
area of about 25 sq km around the summit of
Mount Kosciuszko, above 1,800 metres elevation.
These ice sheets disappeared about 20,000 years
Chapter 1 — Geography and climate 57