Jawaharlal Nehru Technological University Hyderabad B.Tech II ENGINEERING GEOLOGY Previous Question Paper (September/October - 2023 - R16)

Petrology is the branch of geology dedicated to the scientific study of rocks, including their origin, composition, structure, distribution, and the processes that lead to their formation. It serves as a fundamental discipline in understanding the Earth's crustal evolution and the history of geological processes.

The study is generally divided into three major branches based on the mode of rock formation:

• Igneous Petrology: Focuses on rocks formed from the cooling and solidification of molten rock (magma or lava).
• Sedimentary Petrology: Examines rocks formed by the accumulation, compaction, and cementation of mineral particles or organic matter at the Earth's surface.
• Metamorphic Petrology: Investigates rocks that have undergone physical or chemical changes due to intense heat, pressure, or chemically active fluids within the Earth's crust.

Petrology integrates field observations with laboratory techniques, such as petrography (the microscopic description of rocks) and geochemistry, to interpret the environmental conditions under which these geological materials were created.

Chemical weathering, also known as decomposition, is the process by which the internal structure of minerals is altered by the removal or addition of elements, resulting in the formation of new minerals or substances. Unlike physical weathering, which only fragments rock, chemical weathering changes the chemical composition of the rock material.

The primary processes involved in chemical weathering include:

• Hydration: The chemical combination of water with minerals. This often leads to the expansion of the mineral, causing stress and eventual disintegration of the rock. A common example is the conversion of anhydrite to gypsum.
• Hydrolysis: The reaction between hydrogen ions (from water or acids) and mineral ions. This is the most important process for the breakdown of silicate minerals, such as the conversion of feldspar into clay minerals like kaolinite.
• Oxidation: The reaction of minerals with oxygen, particularly common in iron-bearing minerals like magnetite or pyrite. This process produces iron oxides (such as hematite or limonite), which give rocks a characteristic reddish or yellowish-brown color and weaken the mineral structure.
• Carbonation: The reaction of atmospheric carbon dioxide dissolved in rainwater to form weak carbonic acid. This acid is highly effective at dissolving carbonate rocks, particularly limestone and marble, through the following reaction:
CaCO₃ + H₂CO₃ → Ca²⁺ + 2HCO₃⁻
• Solution: The process where minerals are directly dissolved in water. Halite (rock salt) is highly susceptible to this process, leading to the rapid removal of material from the rock mass.

Chemical weathering is most intense in regions with high temperatures and high humidity, as these conditions accelerate the rate of chemical reactions.