Geology for construction: basalt belongs to which rock class based on its origin and mode of formation? (Select the correct category used in engineering geology.)

Difficulty: Easy

Correct Answer: Extrusive igneous rock

Explanation:


Introduction / Context:
Understanding rock origin helps predict engineering properties like strength, durability, and jointing. Basalt is common in many regions and is widely used as aggregate and road metal. The question asks for its geologic classification by origin.


Given Data / Assumptions:

  • Igneous rocks form from cooling magma or lava.
  • Extrusive rocks cool at or near the surface; intrusive rocks cool slowly at depth.
  • Basalt is fine-grained and typically forms from lava flows.


Concept / Approach:
Rapid cooling at the Earth’s surface leads to fine-grained textures. Basalt is the extrusive equivalent of gabbro (its coarse-grained intrusive counterpart). Its mineralogy (mafic) gives it dark color and high specific gravity, making it suitable for aggregates and railway ballast.


Step-by-Step Solution:

1) Determine origin: basalt forms from erupted lava.2) Cooling rate: fast at surface → fine-grained matrix.3) Classify accordingly: extrusive igneous rock.4) Rule out sedimentary/metamorphic: different formation mechanisms.


Verification / Alternative check:
Engineering geology texts list basalt under extrusive igneous rocks alongside andesite and rhyolite, with typical uses in roadwork and concrete aggregate after proper processing.


Why Other Options Are Wrong:

  • Sedimentary: requires deposition and lithification of sediments.
  • Metamorphic: requires transformation under heat/pressure of pre-existing rock.
  • Intrusive igneous: slow-cooled coarse-grained rocks like gabbro and granite.


Common Pitfalls:
Confusing basalt with dolerite (diabase), which is typically medium-grained and often considered hypabyssal/intrusive; basalt is characteristically extrusive.


Final Answer:
Extrusive igneous rock

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