Petrology – Rock classification by origin Which rock listed below is an igneous rock formed by the cooling and solidification of magma?

Introduction / Context:Rocks are classified by origin: igneous (from solidified magma/lava), sedimentary (from sediments), and metamorphic (from transformation of existing rocks under heat and pressure). Identifying the genesis helps infer texture, mineralogy, and where a rock forms within the crust.

Given Data / Assumptions:

• We must select the example that is igneous.
• Common rocks include granite, limestone, slate, and quartzite.
• Only one fits the igneous category.

Concept / Approach:Granite is a coarse-grained intrusive igneous rock composed mainly of quartz, feldspar, and mica. Limestone is sedimentary (often biochemical/chemical calcite). Slate is metamorphic (from shale). Quartzite is metamorphic (from quartz-rich sandstone).

Step-by-Step Solution:Recall definitions: igneous = crystallized from melt.Match composition and texture: granite → intrusive igneous, phaneritic texture.Eliminate sedimentary and metamorphic examples.

Verification / Alternative check:Field identification: granite shows interlocking crystals of light-colored minerals; limestone fizzes with dilute acid (calcite); slate shows slaty cleavage; quartzite is very hard and resists scratching.

Why Other Options Are Wrong:

• Limestone: sedimentary carbonate rock.
• Slate: low-grade metamorphic product of shale.
• Quartzite: metamorphosed sandstone.

Common Pitfalls:Confusing quartz-rich rocks (quartzite) with igneous due to hardness; hardness alone does not determine origin.

Final Answer:Granite