Types of Chemical Weathering

1. Types of Chemical Weathering • 1. Spheroidal Weathering球狀風化 • 2. Honeycomb Weathering蜂窩狀風化

2. 1. Spheroidal Weathering What? • It is a type of chemical weathering caused by acidic rainwater

3. 1. Spheroidal Weathering Where? • It is common in hot and humid areas. • It takes place in well-jointed and exposed rocks such as granite.

4. Photograph of Spheroidal Weathering in granite, Cheung Chau

5. 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.

6. 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

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

8. 2. HoneycombWeathering What? • It is a type of chemical weathering caused by acidic rainwater and salt spray. • It is a kind of selectiveweathering選擇性風化or differential weathering差別風化

9. 2. Honeycomb Weathering Where? • It is found in coastal area of Hong Kong. • It is common in rocks with minerals of different resistance.

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

11. 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

12. 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

13. Process of honeycomb weathering

14. 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.