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9.4 Caring for the country: 1. Weathering and erosion in Australia
| Syllabus reference (October 2002 version) | ||
|---|---|---|
1.
Australia’s land surfaces exhibit the effects of long periods of
weathering and erosion |
Students learn to: | Students: |
Extract
from Earth and Environmental Science Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW
[Edit: 21 Jul 09]
Prior Learning: Preliminary module 8.3 (subsection 2).
Background: Soil is produced from weathered bedrock and from the remains of living organisms. Weathering is the physical or chemical breaking down of the rock and erosion is the carrying away of these weathered products by agents of erosion, such as wind, water and ice. The fertility of the soil is heavily influenced by the climate, terrain, amount and type of plant cover and the bedrock from which it is formed.
When compared with northern hemisphere soils, most Australian soils are relatively infertile. There are some very fertile soils in a few locations such as regions of weathered basalt and on flood plains where silt has been deposited during floods.
present information comparing the fertility of a basalt-derived soil from recent volcanic activity in eastern Australia with a deeply weathered lateritic soil from Western Australia
- Gather information about the long-term stability
of the Australian continent and the great age of Australian soils and landscapes
by looking in books and journals, such as Ecos, produced by the CSIRO,
Habitat, produced by the Australian Conservation Foundation and agricultural
journals such as The Land and Australian Landcare. You could
also look on the Internet and see if there are any CD-ROMs in the library.
Also view Landscapes and Soils
NSW Department of Environment and Climate Change.
- Process the information by comparing different sources. If one source has quite different information than another, check a third source or ask your teacher. Organise the data in a way that helps you understand the relationship between the original
rocks and the present soil.
- Present your information including an explanation of the cross-sections and the age of the soils.
describe the low fertility of most Australian soils in terms of:
- long period of depletion of nutrient ions
- stability of the Australian continent
- the low relief of the Australian continent
Long period of depletion of nutrient ions
- The land making up the eastern quarter of Australia has only been consistently above sea level for the past 60 million years, but parts of Western Australia and Central Australia are about 900 million years old and large sections have been exposed for much of this time. Thus Western Australia and Central Australia have soils that are much more depleted of nutrients by this long period of leaching than soils in the eastern states.
Stability of the Australian continent
- Most of Australia’s
landmass is away from plate margins and has been exposed above sea level
for a very long time, allowing weathering to occur over this long period.
Stability
without recent folding or volcanic activity, and a relatively dry climate,
ensured that soil-producing activity was very slow.
- Volcanic rock is often the basis of rich soils. However, only small regions of such soils exist in Australia. The liverpool Plains, near Gunnedah and north to Narrabri, are perhaps the best example of soils produced from volcanic lava flows and other volcanic activity in Australia (350–330 million years ago). This area has some of the deepest and most fertile soils in NSW.
The low relief of the Australian continent
- Australia is the flattest continent, with no high mountains,
as well as the lowest, with a mean elevation of 300m, compared with the world's
mean of 700m.
- Low relief contributes to low rainfall. There is little
water available for weathering of rocks, and for transporting weathered material.
This means that there has been little renewal of soils across most of the
continent for millennia. During long arid periods when soils in the Northern
Hemisphere were renewed by glacial weathering, Australian soils were depleted
by wind erosion.
- Australia’s largely arid climate slows down the process of soil formation. More dense vegetation
cover leads to a faster rate of soil formation. Roots slowly break down rock and
force their way into tiny cracks, slowly widening them, aiding in their
breakdown. Moss and lichen begin the chemical breakdown of rock by producing
organic acids and therefore contribute to soil formation. In much of Australia,
the vegetation cover is sparse, contributing to a slow rate of soil formation.
- Arid climate results in lower levels of organic matter being available to enrich soils.
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