What Is Chemical & Mechanical Weathering

Small rocks are often formed by the weathering of large rocks.

Weathering is when natural forces break down rock. Weathering is not the same as erosion, although both can work together to break down the same rock. The two main types of weathering are chemical and mechanical.

Chemical Weathering

Chemical weathering is when a chemical reaction changes the composition of the rock that causes it to break down.

Types of Chemical Weathering

Chemical weathering includes oxidation, which is when oxygen reacts with iron to form rust. When carbon dioxide dissolves in water, it creates an acid that weathers some types of rock. When water combines with other substances, it can cause other types of chemical weathering as well, called hydration or hydrolysis.

Mechanical Weathering

Mechanical weathering is when the rock is broken down into smaller pieces, but the composition of the smaller rocks remains the same.

Types of Mechanical Weathering

Frost expansion is when water enters a crack in a rock then expands when it freezes to break apart the rock. Extreme temperature changes can cause weathering. Growing plant roots and branches can cause weathering.


The main difference between erosion and weathering is that the particles of rock are transported away by the force that is eroding the rock, usually moving water or wind. The particles are usually much smaller in erosion as well.

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