Weathering Processes

The in situ (in place) disintegration and decomposition of rocks and minerals at the Earth's surface by physical, chemical and biotic processes. 

Physical weathering: rock disintegration

• Stress applied, rock falls apart mechanically

Chemical weathering: rock decomposition

• Changes minerals within rock
• Much weaker than they were before

Biotic weathering: physical or chemical

• Some form of organism can weather rocks

Most rock breakdowns involves physical and chemical processes acting in combination.

• E.g granite (very strong)
• If weathers under chemical weathering, it will crumble under physical weathering.
• Physical weathering leads to cracks, which leads to chemical weathering.

Why do rocks weather?

They are no longer in their native environment - most are formed underground under high temperatures and pressure.

Over millions of years, through erosion, they become exposed to the surface and they are not used to this or the different types of weathering. 

Rocks that are exposed at the surface by erosion are no longer in equilibrium with their environment. 

Importance of weathering

First stage in cycle of erosion that causes major landscape changes.

It weakens rocks, making them more vulnerable to erosion.

Weathered rocks form the parent material (regolith) for many soils

Breakdown of rock by mechanical stress

• measured in pascals.

Stress may be: 

• intrinsic (within rock)
• extrinsic (applied by something else).

Nature of weathering depends on how stress operates:

• along joints - producing clasts.
• grain boundaries - producing granular disaggregation. 

In all cases, physical weathering results from stresses set up by:

• expansion of the rock
• minerals in the rock
• material introduced to the rock (e.g ice). 

Overburden stress compresses rock mass

Rock exposed at the surface dilates, causing the opening of stress-release joints. These are cracks that form in the rock because it is not longer under compression. 

In large rocks, this produces large scale sheeting joints parallel to the surface, e.g. Granite - Half Dome.

In bedded rocks, stress release also produces micro-cracks that cause the rock to crumble and act as a place for freeze-thaw weathering & salt weathering. 

On steep slopes, stress release can cause rock slope failure (rockfall, rockslides). 

Joints opened by stress release are exploited by other processes. 

Water expands by 9% when freezing.

Expansion can generate stressed of up to 200MPs in an enclosed space.

• This is very rarely reached
• It is an open system, and pressures can be relieved in other ways

Two forms:

Macrogelivation (frost wedging)

• Expands on freezing and expands cracks

Microgelivation (disaggregation)

• Water in pores in rocks freezes, expands and pushes pores apart - the rock crumbles. 

Minerals in rocks expand slightly when they are heated, and they contract when they cool.

Surface layer of rock expands/contracts the most, stressing the rock and causing rock fatigue (thermal stress) and detachment of rock flakes.

Different minerals expand by different amounts and may cause granular disaggregation.

It only affects exposed rock surfaces - important in hot deserts and possibly in arid polar environments.

Hydration and ADsorption of water

• Rocks often contain clay minerals
• Clay minerals have water shells with negative charges - they repel each other so push each other apart.
• Wetting and drying cause it to break down.

ABsorption of water and swelling

• Many clay minerals also absorb water and dilate on wetting, stressing the rock. 
• Kaolinite dilates by 5-60%.
• Such swelling is resisted by surrounding grains within a rock, but it sets up tensile stress.
• Repeated cycles of wetting and drying weaken the rock

Effects of hydration weathering:

• 1. Splitting of boulders on a seasonally-wetted lake shore
• 2. Disintegration of rock and formation of a sandy grus deposit
• 3. Exfoliation and spherodial weathering (chemical weathering). 

It is a form of granual disaggregation due to crystallisation of salts in rock pores.

Evaporation of water leaves salt behind.

Growing salt crystals exert pressures against pore walls, causing granular disaggregation.

Small-scale rain detachment by growth of salt crystals.

Sources of salt:

• Near sea - sea spray - high concentration
• Low concentrations of salt in precipitation
• Evaporation draws salt-rich groundwater to surface

Salt-weathering sometimes causes alveolar weathering of rock to form pitted surfaces called tafoni.

Physical weathering of rocks by biological agents.

Tree roots exert stresses up to 2.2 MPa (mega pascals)

Argued to be of limited importance.

Case Study: Uluru, Central Australia

• Hot, arid climate
• 100-200mm rain per year
• -5 to 40 degrees
• Sandstone - several million years old

Physical weathering types at Uluru

• 1. Unloading, stress-release and rock failure (sheeting joints due to long term unloading)
• 2. Aveolar weathering due to salt crystallisation (haloclasty) 
• 3. Disaggregation and flaking of rock surface to differential thermal expansion (thermoclasty)