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Difficulty: MediumRock Types, Formation, and Economic Importance

Match each rock specimen listed in Column I with its corresponding formation origin and structural characteristic in Column II.

  • DoleriteMedium-grained hypabyssal igneous rock formed within minor intrusions like dykes and sills
  • LigniteLow-grade organically formed sedimentary rock representing an early stage of carbonization
  • GneissHigh-grade metamorphic rock exhibiting distinct alternating light and dark mineral bands
  • Coral limestoneOrganically derived sedimentary rock formed from accumulated calcium carbonate marine skeletons

Answer

Dolerite matches with medium-grained hypabyssal igneous rock in dykes and sills; Lignite matches with low-grade organically formed sedimentary rock; Gneiss matches with high-grade metamorphic rock displaying alternating mineral bands; Coral limestone matches with organically derived sedimentary rock formed from marine skeletons.
The items match accurately based on fundamental petrological processes: Dolerite is a medium-grained hypabyssal igneous rock formed in minor intrusions; Lignite is an organically formed low-grade sedimentary coal; Gneiss is a high-grade regional metamorphic rock with banded foliation; Coral limestone is an organically formed sedimentary rock from marine calcium carbonate skeletons.

Step-by-Step Solution

1
Determine the mode of formation for Dolerite
Dolerite cools at intermediate depths within sills and dykes, producing a hypabyssal igneous rock with medium-sized mineral grains.
Hypabyssal rocks solidify beneath the Earth's crust but faster than deep plutonic masses.
2
Analyze the origin of Lignite
Lignite is formed from compressed plant material under anaerobic conditions, serving as an intermediate stage between peat and sub-bituminous coal.
Organic sedimentary rocks originate from accumulated plant or animal tissues.
3
Identify the structural feature of Gneiss
Gneiss displays foliation characterized by alternating bands of quartz/feldspar and ferromagnesian minerals.
Extreme directed pressure during regional metamorphism causes compositional mineral banding.
4
Examine the formation process of Coral Limestone
Coral limestone is a biogenic sedimentary rock composed of marine organism skeletons rich in calcium carbonate.
Living coral reefs secrete calcite which accumulates into solid sedimentary beds upon marine deposition.

Key Concept

Classification of rocks according to their genetic processes (igneous hypabyssal, organic sedimentary, regional metamorphic) and diagnostic textures.
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Topics

Rock Types, Formation, and Economic Importance

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