Fig. 1 shows a process that contributes to the formation of soil from rock. (a) State what is meant by the term weathering. [2] (b) The diagram shows water ...

Labelled answer diagram:

(a) Weathering is the breaking down of rocks [1] at or near the Earth's surface by physical, chemical, or biological processes, without the material being transported away (it occurs in situ). [1] Weathering is the first step in soil formation: it converts solid rock into smaller fragments and minerals that eventually become the mineral component of soil.

(b) How freeze-thaw (frost shattering) leads to the breakdown of rocks:

1. Water enters cracks, joints, or pores in the rock surface. [1] Rainwater or meltwater seeps into every available opening in the rock.
2. When the temperature drops below 0 degrees C, the water freezes and expands by approximately 9% in volume. [1] This expansion exerts enormous outward pressure on the walls of the crack, forcing it to widen.
3. Repeated cycles of freezing and thawing progressively widen and deepen the crack. [1] Each cycle pushes the crack walls a little further apart. Eventually, the stress exceeds the rock's tensile strength, and pieces of rock break off (a process sometimes called frost shattering or frost wedging). Over many cycles, large rocks are broken down into smaller and smaller fragments.

(c) Two ways in which chemical weathering breaks down rocks to form soil:

1. Carbonation / acid dissolution: Rainwater absorbs carbon dioxide from the atmosphere and soil air, forming a weak carbonic acid. This acid reacts with and dissolves minerals in rocks such as limestone (calcium carbonate). [1] The calcium carbonate is converted into soluble calcium bicarbonate, which is washed away in solution. Over time, the rock structure weakens and crumbles, contributing calcium-rich mineral particles to the developing soil. [1]
2. Oxidation: Minerals containing iron in the rock react with oxygen and water (rusting). [1] The iron changes from its reduced (ferrous) state to its oxidised (ferric) state, forming iron oxides. These new compounds occupy a larger volume and have a weaker crystal structure than the original minerals, causing the rock to expand, crack, and eventually break apart into fragments. [1] The characteristic red or orange colour of many tropical soils comes from these iron oxide products of oxidation.

(d) Why biological weathering is more common in tropical forests than in hot deserts:

1. Tropical forests have abundant vegetation, and plant roots grow into cracks in rocks, exerting physical pressure that forces the cracks apart over time. [1] The dense, fast-growing vegetation of the tropics produces far more root biomass than the sparse plants of a desert.
2. Tropical forest soils support vastly more organisms (bacteria, fungi, earthworms, insects) than desert soils. [1] These organisms produce organic acids as by-products of decomposition, and these acids chemically dissolve rock minerals. The warm, moist conditions of the tropics accelerate decomposition and microbial activity.
3. Hot deserts have very little vegetation and very few soil organisms because of the extreme lack of water. [1] Without moisture, plants cannot establish, roots cannot penetrate rock, and microbial populations remain negligible. Biological weathering therefore barely occurs in arid environments.