The difference between erosion and weathering

Weathering and erosion: how rocks are broken down

Firstly it is important to point out that weathering is not the same as erosion.

Weathering comprises the processes that cause the structure of a rock to break, and that will be described in this page.

Erosion is the process by which the rock fragments ( produced by weathering) are carried away by the wind and water (rivers, sea).

Various forms of Weathering

Rocks are attacked chemically, physically and biologically. That means, in all possible ways. The diagram below represent various forms of weathering:

concept map rocks weathering

Physical processes:

Rocks may crack due to the force exerted by molecules of water trying to form a crystal (ice). Water is very unusual in the sense that its crystalline form is less dense than its liquid form. That means that when water freezes, it expands its volume . That explains why ice is so slippery! Water that has penetrated fissures in a rock will cause damage when it freezes ; it will cause a lot of strain, which will end up breaking the rock.

Rocks that are subject to big variations of temperature, in a short period of time, may also break because of tensions that arise as a result of the contraction and dilation of the stone, according to temperature variations. Some rocks may break when boiling water is poured over them!

Chemical processes

There are two main chemical processes: acid attack and rusting.

Rocks are attacked by acids that can originate in animals, plants and minerals . Limestone in particular is vulnerable to chemical attack. This reaction is used as a test for the identification of this class of rocks.

Acid rain has been causing an accelerated decomposition of important sculptures and monuments made of limestone (marble)

Rusting is another process involved in weathering of rocks containing elements like iron. Rocks corroded in this way show rust streaks identical to those commonly seen in boats, bridges etc...

Biological processes:

Some fungi and bacteria decompose rocks.

Fungi manage to extract minerals from the rocks. Certain elements that are essential for their metabolism are obtained this way. Liquens are associations between fungi and algae (symbiosis),where the fungi provide the minerals and the algae provides energy and carbohydrates(obtained by photosynthesis).We often see liquens stuck to rocks and buildings:

Liquens in roof tiles (Brighton-UK)

liquens in AMchu Picchu

Liquens in rocks (Machu Picchu - Peru)

Growing plant roots are very strong and hard. They also put a lot of strain on the structure of rocks, which eventually break into pieces.

Rock cycle index

Weathering and erosion: how rocks are broken down

Various forms of Weathering