Transcript Types of Chemical Weathering

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