HKDSE Geography/E1/Physical Weathering

Let's start by defining weathering:

Now for another definition:

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.

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.