JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MINING GEOLOGY Previous Question Paper (July/August - 2021 - R13)

Igneous rocks are primary rocks formed through the cooling and solidification of molten rock material known as magma (when below the surface) or lava (when extruded onto the surface). They are characterized by their crystalline structure, lack of fossils, and absence of layered bedding.

Igneous rocks are classified based on their mode of occurrence, chemical composition, and mineralogical content:

1. Mode of Occurrence (Depth of Formation):
   • Plutonic (Intrusive) Rocks: Formed at great depths where cooling is slow, resulting in coarse-grained (phaneritic) textures. Examples include granite and gabbro.
   • Hypabyssal Rocks: Formed at intermediate depths in fissures or cracks, often displaying medium-grained textures. Examples include dolerite.
   • Volcanic (Extrusive) Rocks: Formed on the surface from lava, cooling rapidly to produce fine-grained (aphanitic) or glassy textures. Examples include basalt and obsidian.
2. Chemical Classification (Based on Silica Content):
   • Acidic Rocks: Contain high silica (>65%), are light-colored, and have low density (e.g., granite, rhyolite).
   • Intermediate Rocks: Contain silica between 52% and 65% (e.g., diorite, andesite).
   • Basic Rocks: Contain silica between 45% and 52%, are dark-colored, and have high density (e.g., basalt, gabbro).
   • Ultra-basic Rocks: Contain less than 45% silica (e.g., peridotite).
3. Mineralogical Classification: Based on the Color Index (proportion of light-colored felsic minerals to dark-colored mafic minerals). Leucocratic rocks are light-colored, mesocratic are intermediate, and melanocratic are dark-colored.

Sedimentary rocks are secondary rocks formed by the accumulation, compaction, and cementation of mineral particles or organic matter that have been deposited by water, wind, or ice. They typically form in layers called strata and are the primary source of fossils and many economic mineral deposits.

The formation process, known as lithification, occurs through several sequential stages:

1. Weathering and Erosion: Pre-existing igneous, metamorphic, or older sedimentary rocks are broken down by physical and chemical weathering processes into smaller fragments (clasts) or dissolved ions.
2. Transportation: These sediments are transported by agents such as running water, wind, glaciers, or gravity. As the energy of the transporting medium decreases, the sediments are deposited.
3. Deposition: Sediments settle in basins, lakes, riverbeds, or ocean floors. Heavier particles settle first, leading to sorting.
4. Compaction: As layers of sediment accumulate over time, the weight of the overlying material exerts pressure on the lower layers, reducing pore space and squeezing out water.
5. Cementation: Dissolved minerals (such as silica, calcium carbonate, or iron oxide) precipitate from groundwater into the remaining pore spaces, acting as a natural glue that binds the particles together into solid rock.

Based on their mode of origin, sedimentary rocks are classified into three main types: clastic (formed from mechanical fragments), chemical (formed by precipitation from solution), and organic/biogenic (formed from the accumulation of plant or animal remains).