Q17.Describe the characteristics and types of primary rocks.

Primary rocks are igneous rocks, formed from the cooling and solidification of molten magma or lava. They represent the Earth's initial crustal material, foundational to other rock types.

Key Characteristics

• Possess a crystalline structure, varying in size based on cooling rates.
• Lack stratification or layering, distinguishing them from sedimentary rocks.
• Are devoid of fossil remains due to their high-temperature formation.
• Generally hard, compact, and resistant to weathering and erosion.

Classification by Formation

• Intrusive (Plutonic) Rocks: Form underground from slow-cooling magma, leading to large, coarse crystals (e.g., granite, gabbro).
• Extrusive (Volcanic) Rocks: Form on the surface from rapid-cooling lava, resulting in fine-grained or glassy textures (e.g., basalt, obsidian).

Classification by Chemical Composition

Primary rocks are also categorized by chemical composition into acidic (felsic, high silica content) and basic (mafic, low silica content) types, influencing their mineralogy and density.