化學風化 CHEMICAL WEATHERING ６Ａ　江鼎立 　　　廖世偉 　　　溫愷琦 　　　雷寶瑤 　　　徐玉靈

Definition Chemical weathering is the decomposition or decay of rocks in situ due to the chemical reactions between rock minerals and air, moisture, rainwater, or seawater.

Factors causing chemical weathering

Humidity high humidity or abundant rainfall dissolves rock minerals and removes them in solution 濕度高或多雨水能溶解及帶走岩中礦物

Temperature high temperature speeds up the chemical reaction 高溫加速化學反應

Composition and structure of rocks rocks with many joints or soluble minerals are easily weathered 多節理或含可溶解礦物的岩石容易被風化

Organic acids These are released by decaying organic matter, e.g. decaying plants or animals. When in contact with rocks, these acids act on rock minerals and lead to chemical changes.

Bare/expose/unvegetated surface When rock minerals come into contact with air or gases like oxygen, they are also weathered.

The texture of Rocks Coarse-grained rocks are likely to weather more rapidly than fine-grained rock

Slope aspect In cold but not generally frosty areas, south-facing slopes will face more sunshine, chemical weathering will be greater

Human beings Cause large scale disintegration of rocks , e.g. deforestation

Processes of chemical weathering

氧化作用

碳化作用

水解作用

水化作用

Types of chemical weathering

Spheroidal weathering 球狀風化 also called onion-skin weathering or concentric weathering takes place in well-jointed and exposed rocks such as granite 活躍於多節理和露出地面的岩石，例如花崗岩 。

Chemical processes occur. 化學過程開始 Rainwater falls on the ground and seeps into the joints of the rocks. 雨水滲入岩中裂縫和節理 Chemical processes occur. 化學過程開始 Rocks adjacent to the joints are weakened.節理旁的岩石被減弱 Those processes start on the land surface and along the joints and then work into the inner part. 化學風化過程先由表面開始，接著深入岩石內部。

In time, large rectangular granite blocks are reduced to small and round boulders surrounded by concentric weathered layers (corestones) and at last to fine weathered particles. 經長時間，岩石被減少成為被同心風化層包圍的細小及圓形石礫(即核心石)。

When large parts of weathered layers are removed by wind or rain, corestones are exposed on the ground and called tors. 當大部分風化層被風或雨帶走，核心石露出地面成突岩。

Tors突岩 (石塔) 風化殼剖面第一及第二層的風化物質均易被侵蝕及搬運，使第三以至第四層出露地面；那些露出及散佈地面的個體岩石稱為突岩。

In this basalt outcrop, groundwater penetrates along joints and fractures, loosening and decaying the rock layer by layer. As the process progresses into a block, the surface of weathering grows more and more rounded. 在這玄武岩的露出面，地下水沿著節理和裂縫滲透，一層層地分解和腐朽岩石。 當過程深入岩石，風化表面變得越來越圓。

Honeycomb weathering蜂窩狀風化 Commonly found in rocks with different structure which contain soluble minerals 常見於構造不同而含有可解礦物的岩石

Also found in coastal rocks where sea water sprays onto rock surface

Water falls or sprays onto rock surface, and it removes some rock minerals in solution. 水落到或濺灑於岩面上溶解及帶走岩中礦物 Pits are left on rock surface, and rock is like a honeycomb. 岩面上剩下孔洞，岩石似蜂窩。

Karst Landscape 喀斯特地形

出現的地方…… often associated with limestone regions岩溶地區常與石灰岩地區有關 limestone is a well-jointed rock which contains insoluble calcium carbonate石灰岩是多節理及含有不溶解碳酸鈣的岩石

Formation rainwater combines with carbon dioxide to form carbonic acid雨水二氧化碳組合成碳酸 when carbonic acid enters limestone along the joints, it converts calcium carbonate into soluble calcium hydrogen carbonate當碳酸沿節理進入石灰岩, 它轉變碳酸鈣成碳酸氫 limestone is dissolved and removed石灰岩被溶解及帶走 joints of limestone are widened and deepened to form grikes while the remaining parts of limestone higher up form clints石灰岩節理被擴闊及加深成岩溝, 另外岩溝間殘留的石灰岩成岩脊

石灰岩地表地形 岩溝 滲穴 伏流 窪盆︰錐丘、殘丘；殘丘在廣西俗稱峰林。

Doberdò del Lago (Italian Karst)

Malham Cove near Malham in the Yorkshire Dales, United Kingdom

桂林 陽朔 廣西

形成喀斯特地形的條件： 需具有可溶性岩層 可溶性岩層需緻密且具有高度節理 多雨 缺乏植被覆蓋 地表起伏不大 地下水面甚低

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