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Difficulty: HardRock Types, Formation, and Economic Importance

During a geological survey in a mountainous terrain, an engineering team extracts a fine-grained, foliated metamorphic rock characterized by distinct slatey cleavage that allows it to split easily into thin, flat, durable slabs suitable for high-grade roofing tiles. Which parent rock underwent low-grade regional metamorphism to produce this specific rock?

  1. Clay or shaleAnswer
  2. B
    Limestone or dolomite
  3. C
    Pure quartz sandstone
  4. D
    Intrusive coarse-grained granite

Answer

Clay or shale is the parent rock of slate.
Slate is a fine-grained, foliated metamorphic rock produced by the low-grade regional metamorphism of clay, shale, or mudstone. The intense directional pressure aligns microscopic mica and clay minerals, creating planar cleavage that allows the rock to split into thin sheets used commercially for roofing and flooring tiles.

Step-by-Step Solution

1
Identify the metamorphic rock described in the stem.
The rock with fine grains, prominent cleavage enabling thin splitting, and commercial utility for roofing is identified as slate.
Slatey cleavage and fine-grained foliation are defining physical diagnostic properties of slate.
2
Determine the metamorphic grade and origin process.
Slate forms under low-temperature and low-pressure conditions (low-grade regional metamorphism).
Low-grade metamorphism re-aligns microscopic clay minerals perpendicular to directional pressure without growing large visible crystals.
3
Match the rock type to its protolith (parent rock).
The original protolith containing abundant fine clay minerals is shale or mudstone.
Argillaceous sedimentary rocks (clay/shale) contain the necessary phyllosilicate minerals that transform into slate.

Key Concept

Metamorphic Rock Protoliths and Transformation Grades
Estimated Time:1m 30s
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Topics

Rock Types, Formation, and Economic Importance

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