HKDSE Geography/E1/Physical Weathering

Let's start by defining weathering:

Weathering - the disintegration of rocks in situ (on the site).

Regolith - A blanket of loose rock debris formed by weathering, acting as the parent material for soil development.

Now for another definition:

Physical weathering - the in situ distintegration of rocks into smaller fragments by physical means with no change in chemical composition.

There are a few important examples of physical weathering, in roughly decreasing order of importance:

• By pressure release
• By thermal expansion and contraction
• By biological activity
• By freeze and thaw action
• By salt crystallisation
• By alternate wetting and drying

By pressure release

Favourable conditions: Well-jointed plutonic rocks (e.g. granite), active denudation

1. Plutonic rocks, such as well-jointed granite, were developed under the earth's surface at great pressure from overlying rocks.
2. If the overlying rocks are removed by denudation, the rock's high pressure will be released.
3. The rock will expand to form cracks parallel to the surface.
4. Over time, the outer layers of the rocks split into sheets, break away and peel off like onions. This is sheeting.
5. The process leaves a rounded rock called an exfoliation dome on the surface. Other agents and denudation processes can then operate on the new surface.

By thermal expansion and contraction

Favourable conditions: large diurnal range of temperature, no trees to protect the surface, low albedo (i.e. darker rocks) which ensures that more of the solar energy is absorbed

There are three types of weathering by thermal expansion and contraction. Note the favourable conditions and appearance of each type.

Name	Favourable conditions	Process
Block distintegration	Homogeneous, well-jointed rock (granite)	In daytime, the rock absorbs intense solar radiation and expands. At night, the temperature drops and the rock contracts. Repeated expansion and contraction produces stress and enlarges joints in the rock. Eventually, the rock will split along the cracks into blocks.
Granular disintegration	Coarse-grained rocks with minerals of different expansion/contraction rates (granite, sandstone)	In daytime, the rock absorbs intense solar radiation. Some minerals expand faster than others and push adjacent minerals away. At night, the temperature drops. Minerals with faster contraction rates will detach from adjacent minerals. Over time, repeated expansion and contraction will loosen the grains, which detach from the rock one by one. Grains of weathered rock debris are scattered at the base of the rock.
Exfoliation	A uniform mass of rock with fine grains, (rhyolite)	In daytime, the rock absorbs intense solar radiation. The outer layer of rock is heated up and expands more intensely than the interior, forming concentric cracks. At night, the temperature drops. The outer layer of rock cools down and contracts more rapidly than the core, forming radial cracks. Over time, repeated expansion and contraction creates pressure and enlarges the cracks. Eventually, the outer layers peel off to leave slabs of rock on the ground. The new surface will then be exposed to heating and cooling.

By biological activity

Favourable conditions: Hot and wet climate

Here are two ways biological activity helps:

• Tree roots may grow into rocks and force open joints. Pressure accumulates and eventually breaks the rocks along the joints.
• Burrowing by earthworms and rabbits facilitates physical weathering.

By freeze and thaw action

Favourable conditions: High-altitude areas where the temperature fluctuates above and below 0°C. (HK hilltops in winter)

1. When water seeps into joints and cracks, it will freeze, expand by 9% and exert pressure onto the joints at night.
2. In daytime, the water will thaw again.
3. Over time, repeated freezing and thawing creates pressure and enlarges the joints.
4. Eventually, the rock will break along the joints.

By salt crystallisation

Favourable conditions: Coastal regions where pore spaces are frequently soaked with seawater.

1. Pores in rocks are filled up with saline solution.
2. When the water or solvent evaporates, salt crystals will grow.
3. Over time, repeated salt crystallisation enlarges the pores.
4. The growth of salt crystals will weaken the rock and leave it more vulnerable to erosion.

By alternate wetting and drying

Favourable conditions: Granite.

1. Certain minerals in rocks expand when they absorb water.
2. When they dry up, they contract again.
3. Over time, repeated wetting and drying creates pressure and enlarges the joints.
4. Eventually, the rock will break along the joints.