Investigate the physical, chemical and biological processes of weathering and erosion, and how they interact with climate and organisms to form soils, in the Australian context

What this dot point is asking

NESA wants you to distinguish weathering (the breakdown of rock in place) from erosion (the removal of the resulting material), classify weathering as physical, chemical or biological, and explain how these processes combine with climate and organisms to build a soil profile. Australian examples should show why the continent's soils are old and often infertile.

The answer

Weathering is the breakdown and alteration of rock at or near the surface. Erosion is the subsequent removal and transport of that loosened material by water, wind, ice or gravity. Weathering prepares the material; erosion carries it away. Together with deposition they feed the sedimentary part of the rock cycle and create the parent material for soils.

Physical (mechanical) weathering

Physical weathering breaks rock into smaller pieces without changing its chemistry. Mechanisms include freeze-thaw, where water expands as it freezes in cracks and prises them open (important in the Australian Alps); thermal expansion and exfoliation, where repeated heating and cooling under intense sun flakes off curved sheets, seen on granite domes; and pressure release, where deeply formed rock expands and fractures as overlying material is eroded. Breaking rock into smaller pieces increases surface area, which speeds up chemical weathering.

Chemical weathering

Chemical weathering changes the minerals themselves. Water, oxygen and weak acids react with rock to form new, more stable minerals and soluble ions. Key reactions are hydrolysis (feldspar reacting to clay), oxidation (iron minerals rusting to form red and orange soils), and carbonation (carbonic acid dissolving limestone to form caves and the karst of the Nullarbor and Jenolan). Chemical weathering dominates in warm, wet climates, which is why much of tropical and ancient Australia is so deeply and thoroughly weathered.

Biological weathering

Living things both break rock physically and attack it chemically. Roots wedge into cracks and widen them; lichens, fungi and bacteria secrete acids that dissolve minerals; burrowing animals expose fresh rock to air and water. Biological activity also mixes organic matter into the developing soil.

Erosion and transport

Once loosened, material is moved downslope or downwind. Running water is the dominant agent across most of Australia, but wind erosion is severe in the arid interior, producing dust storms and shaping dunes. Erosion rates rise sharply where vegetation is cleared, which is why land clearing and overgrazing have driven serious soil loss and gully erosion in Australian agricultural regions.

Soil formation

Soil forms where weathered rock, organic matter, water, air and organisms interact over time. A mature soil develops horizons: an organic-rich topsoil (A horizon), a subsoil where leached clays and oxides accumulate (B horizon), and weathered parent rock below (C horizon). The five soil-forming factors are parent material, climate, organisms, relief and time. Because Australia is flat, dry and ancient, its soils have formed slowly, lost nutrients to long leaching, and are often shallow and fragile, making sustainable land management essential.

Try this

Q1. Distinguish between physical and chemical weathering, giving one example of each. [3 marks]

• Cue. Physical changes particle size only (freeze-thaw); chemical alters minerals (oxidation forming red soils).

Q2. Explain why many Australian soils are nutrient-poor compared with soils on younger landscapes. [4 marks]

• Cue. Prolonged chemical weathering on an old, stable, flat continent has leached soluble nutrients over tens of millions of years, leaving thin, deeply weathered profiles.