Physical weathering

Physical weathering

In this answer I will explain the processes of physical weathering.

Physical or mechanical weathering is the breaking down of rocks into smaller particles called regolith.

Physical weathering does not change the chemical composition of the rocks.

Physical weathering generally occurs in areas that have little or no vegetation cover and extremes of climate such as in mountainous regions and deserts.

The main types of physical weathering are freeze-thaw action (frost action) and exfoliation (onion weathering).

Both these types of physical weathering occur as a result of a temperature change.

Freeze-thaw action

Freeze-thaw action is the most common type of physical weathering and occurs in mountainous areas where there are frequent temperature changes.

It is also known as frost action. It is most likely to occur in areas where temperatures frequently rise above and fall below freezing point and there is a plentiful supply of rainfall.

Process of freeze thaw action

During the day when it rains, water collects in the cracks or fissures in the rock.

During the night when temperatures drop below 0C the water freezes.

When the water freezes its volume expands by about 10%. This puts pressure on the surrounding rock and the cracks are widened.

When daytime comes the ice melts.

Over time this repeated freezing and thawing of rainwater eventually causes the rock to shatter and crumble until it is completely broken down.

This regolith moves down slope under the influence of gravity.

This is known as mass movement. This weathered rock is known as scree or talus.

Evidence of scree slopes can be seen on Croagh Patrick, Co. Mayo and The Sugarloaf, Co. Wicklow

Exfoliation

A second type of physical weathering is exfoliation.

It is also known as onion weathering. This is when the surface of the rock peels away layer by layer.

Exfoliation occurs in regions that experience a large diurnal temperature range such as deserts.

In deserts, temperatures may reach 50C during the day. This causes the rock to expand. At night, when the temperatures drop to below 0C, the rock cools and contracts.

Over time this repeated process of expansion and contraction weakens the rock and the outer layer of the rock shatters.

The outer layer of the rock will expand more than the inner layer causing the outer layer to peel off. The broken layers collect at the base of the rock to form scree.