Types of Chemical Weathering n 1 Spheroidal Weathering

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

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

1. Spheroidal Weathering What? n It is a type of chemical weathering caused by

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

1. Spheroidal Weathering Where? n It is common in hot and humid areas. n

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

Photograph of Spheroidal Weathering in granite, Cheung Chau

Photograph of Spheroidal Weathering in granite, Cheung Chau

1. Spheroidal Weathering How to form? n Rainwater absorbs carbon dioxide in the atmosphere

1. Spheroidal Weathering How to form? n Rainwater absorbs carbon dioxide in the atmosphere and form a mild carbonic acid. n It falls on the ground and seeps into the joints of the rocks. n Chemical processes such as oxidation, hydration and solution occur. n These processes start on the land surface and along the joints and then work into the inner part.

1. Spheroidal Weathering n In time, large rectangular granite blocks are reduced to small

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

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

2. Honeycomb. Weathering What? n It is a type of chemical weathering caused by

2. Honeycomb. Weathering What? n It is a type of chemical weathering caused by acidic rainwater and salt spray. n It is a kind of selective weathering選擇 性風化or differential weathering差別風 化

2. Honeycomb Weathering Where? n It is found in coastal area of Hong Kong.

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

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

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

2. Honeycomb Weathering How to form? n Rocks contain minerals of differential resistance. n

2. Honeycomb Weathering How to form? n Rocks contain minerals of differential resistance. n Less resistant minerals are easily weathered. (1) n Some minerals may react with sea water. n Sea water may act like sea spray. 海水噴灑

2. Honeycomb Weathering (2) n Rainwater absorbs carbon dioxide in the atmosphere and form

2. Honeycomb Weathering (2) n Rainwater absorbs carbon dioxide in the atmosphere and form a mild carbonic acid. n Rainwater react with some minerals e. g. feldspar / mica

Process of honeycomb weathering

Process of honeycomb weathering

2. Honeycomb Weathering n Selective weathering takes place. n The chemical processes of solution,

2. Honeycomb Weathering n Selective weathering takes place. n The chemical processes of solution, oxidation, hydration and hydrolysis act on the rock surface. n The rock structure is weaken. n Some minerals e. g. feldspar are dissolved. n Holes, hollows / pits / cavities are left on the rock.