CHAPTER 6: WEATHERING AND EROSION 2. Chemical weathering
Chemical weathering happens because water contains many dissolved substances, such as carbon dioxide from the air, that can chemically attack rock. Chemical weathering is important for the formation of soils across the world. Sedimentary rocks have grains bonded together during lithification with
cementing agents such as silica and calcite. These cementing agents are easily weathered chemically so that some sedimentary rocks such as limestone and sandstone weather quickly. Quartz and mica (the glassy and silvery minerals common in granite) are the only minerals resistant to chemical weathering.
There are four processes of chemical weathering. (a) Carbonation (b) Hydration (c) Oxidation (d) Hydrolysis
(a) Carbonation Rainwater absorbs carbon dioxide from the air and becomes weak carbonic acid. As the rainwater seeps through the soil it may absorb humic acids created by rotting plant and animal remains. By the time the rainwater has reached the rock below the soil, it is quite acidic and dissolves the rock, especially limestone. This occurs when the acid reacts with the calcium carbonate in the limestone to form soluble calcium bicarbonate.
Limestone regions found in Ireland, Jamaica, Yorkshire and parts of Spain and Italy are weathered by carbonation, producing unusual and beautiful surface and underground landscapes called karst landscapes.