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:
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:
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:
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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