Consider the following statements: Statement-I: Rainfall is one of the reasons for weathering of roc

Question

Consider the following statements: Statement-I: Rainfall is one of the reasons for weathering of rocks. Statement II: Rain water contains carbon dioxide in solution. Statement-III: Rain water contains atmospheric oxygen. Which one of the following is correct in respect of the above statements?

Accepted Answer

Chemical weathering is the basic process by which denudation proceeds. It is the extremely slow and gradual decomposition of rocks due to exposure to air and water. Air and water contain chemical elements, which though they may be in small quantities, are sufficient to set up chemical reactions in the surface layers of exposed rocks. Such reactions may weaken or entirely dissolve certain constituents of the rock, thus loosening the other crystals and weakening the whole surface.There are three major chemical weathering processes:Solution: Many minerals are dissolved by water, especially when, as with rain-water, it contains enough carbon dioxide to make it a weak acid. Solution is the most potent weathering process in limestone regions because the rain-water attacks and dissolves the calcium carbonate of which the rock is chiefly formed. The dissolved calcium carbonate is carried away by the water, joints and cracks in the rock are quickly widened and whole systems of caves and passages are worn out. Limestone, however, is by no means the only rock to suffer from solution. All rocks are subject to solution to some extent, though the process is much slower than with limestone. Hence Statement II is correct and explains Statement I.Oxidation: Dissolved oxygen is present in all rainwater and surface supplies due to contact with the atmosphere. Oxidation is the reaction of oxygen in air or water with minerals in the rock. For example, most rocks contain a certain amount of iron, which when it comes in contact with air is changed to iron oxide, familiar brownish crust or rust. Iron oxide crumbles easily and is far more easily eroded than the original iron. It is thus removed, loosening the overall structure of the rocks and weakening them. Hence Statement III is correct and explains Statement I.Decomposition by Organic acids: Within the soil which covers most rocks are bacteria which thrive on decaying plant or animal material. These bacteria produce acids which, when dissolved in water, help to speed up the weathering of the underlying rocks. Hence option (a) is the correct answer.Source Attribution: This explanation is based on information from NCERT: Class XI Fundamentals of Physical Geography and Certificate Geography by GC Leong.