Types of Chemical Weathering

1. Spheroidal Weathering 球狀風化 2. Honeycomb Weathering 蜂窩狀風化

1. Spheroidal Weathering

What? It is a type of chemical weathering

caused by acidic rainwater

1. Spheroidal Weathering

Where? It is common in hot and humid areas. It takes place in well-jointed and

exposed rocks such as granite.

Photograph of Spheroidal Weathering in granite, Cheung Chau

1. Spheroidal Weathering

How to form? Rainwater absorbs carbon dioxide in the

atmosphere and form a mild carbonic acid. It falls on the ground and seeps into the joints

of the rocks. Chemical processes such as oxidation,

hydration and solution occur. These processes start on the land surface

and along the joints and then work into the inner part.

1. Spheroidal Weathering

In time, large rectangular granite blocks are reduced to small and round boulders (corestones) and at last to fine weathered particles

When large part of weathered layers are removed by wind or rain, corestones are exposed on the ground and called tors

Corestones 岩心 / Tors 石塔Photograph of corestones exposed in a bay northeast of Sham Chung

2. HoneycombWeathering

What? It is a type of chemical weathering

caused by acidic rainwater and salt spray.

It is a kind of selective weathering 選擇性風化 or differential weathering 差別風化

2. Honeycomb Weathering

Where? It is found in coastal area of Hong Kong. It is common in rocks with minerals of

different resistance.

Human Head Rock in Cheung Chau (an example of honeycomb weathering)

2. Honeycomb Weathering

How to form? Rocks contain minerals of differential

resistance. Less resistant minerals are easily weathered.

(1) Some minerals may react with sea water. Sea water may act like sea spray. 海水噴灑

2. Honeycomb Weathering

(2) Rainwater absorbs carbon dioxide in the

atmosphere and form a mild carbonic acid.

Rainwater react with some minerals e.g. feldspar / mica

Process of honeycomb weathering

2. Honeycomb Weathering

Selective weathering takes place. The chemical processes of solution,

oxidation, hydration and hydrolysis act on the rock surface.

The rock structure is weaken. Some minerals e.g. feldspar are

dissolved. Holes, hollows / pits / cavities are left on

the rock.