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Difficulty: MediumRocks, Minerals, and Soil

An igneous rock specimen discovered in a mountain range features large, coarse-grained mineral crystals that are easily visible without magnification. What geological process was primarily responsible for this coarse texture?

  1. Slow cooling and crystallization of magma deep beneath the Earth's surfaceAnswer
  2. B
    Rapid cooling and solidification of lava flowing across the Earth's surface
  3. C
    Recrystallization of solid rock caused by extreme tectonic pressure without melting
  4. D
    Accumulation, compaction, and mineral cementation of eroded rock fragments

Answer

Slow cooling and crystallization of magma deep beneath the Earth's surface
Igneous rocks with coarse-grained textures form from magma that cools very slowly deep underground, providing sufficient time for large mineral crystals to grow.

Step-by-Step Solution

1
Identify the rock type and described texture
The rock is identified as an intrusive igneous rock with a coarse-grained (phaneritic) crystal structure.
Igneous rock textures are determined directly by the rate of cooling of the molten material.
2
Relate cooling rate to crystal grain size
A slow rate of cooling allows mineral crystals to grow large enough to be seen with the unaided eye.
Rapid cooling quickly solidifies the melt, restricting crystal growth to microscopic sizes or volcanic glass.
3
Identify the geological environment that permits slow cooling
Deep underground environments provide the thermal insulation necessary for magma to cool slowly.
Intrusive (plutonic) bodies cool over thousands to millions of years beneath the Earth's crust.

Key Concept

Igneous rock textures and cooling rates (intrusive vs. extrusive)
Estimated Time:1m 0s
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Topics

Rocks, Minerals, and Soil

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