Chemical Weathering Processes

Chemical Weathering:

• Occurs when rocks go under a chemical reaction when they come into contact with water
• This changes the composition of rock which makes it break down
• This type of weathering is highly dependent on climatic conditions in hot, humid climates where it is active at a much faster rate than in cooler climates
• The most common forms of chemical weathering in Ireland are Carbonation and Hydrolysis.

Carbonation:

• Occurs when rainwater mixes with carbon dioxide in the air and from organic material in the soil
• This combines to form a weak carbonic acid
• This acid reacts with the minerals present in the rocks and dissolves them or changes their chemical formation
• In Limestone, this weak carbonic acid combines with the calcium carbonate (from the bones and shells that lithified to form limestone) present in the limestone
• This combination creates calcium bicarbonate which is soluble in water
• As rain falls on the permeable limestone surface, it passes through its lines of weakness – joints and bedding planes – dissolving the calcium bicarbonate
• This process is very active in exposed limestone or karst regions such as the Burren, Co. Clare

Hydrolysis:

• This is the introduction of water into the minerals in granite to create new substances
• Hydro means Water and lysis means 'to break down'
• Feldspar, the cementing agent in Granite, absorbs water
• Through a chemical process involving the carbonic acid in water, this feldspar is then converted into Kaolin clay
• This clay is unable to hold the other minerals (mica and quartz) together and it causes the granite rock to crumble and slowly disintegrate
• This is an important process in the formation of soils as hydrolysis leads to the creation of clay in the soils that cover the hillsides in areas of Granite, such as the Wicklow Mountains – this gives the area a smooth, rounded appearance.
• The rate at which Hydrolysis occurs is influenced by the temperature
• For every 10°C increase in temperature, the rate at which this process occurs doubles
• This process is very active on the Wicklow mountains, the exposed region of the Leinster Batholith

Other types of chemical weathering:

• Hydration: Water chemically combines with some minerals in rock, causing them to expand and put pressure on the rock

• Oxidation: Water reacts chemically with iron compounds breaking them down into iron oxide (rust) and causing rocks to crumble