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Difficulty: MediumRocks, Minerals, and Soil

A student examines a specimen of pumice and observes that it is extremely lightweight and filled with numerous small cavities and pores. What process directly causes this porous, vesicular texture during the rock's formation?

  1. A
    Intense directional pressure flattened and realigned mineral crystals during regional metamorphism.
  2. Gas bubbles were trapped as gas-rich lava cooled and solidified rapidly at Earth's surface.Answer
  3. C
    Slow cooling of magma deep within Earth allowed large mineral crystals to dissolve surrounding rock.
  4. D
    Percolating acidic groundwater dissolved soluble carbonate minerals after the rock was buried.

Answer

Gas bubbles were trapped as gas-rich lava cooled and solidified rapidly at Earth's surface.
The correct answer identifies that vesicular texture forms during volcanic eruptions when dissolved gases expand within lava. Because the lava cools and hardens very rapidly upon exposure to the atmosphere, the gas bubbles cannot escape in time and remain preserved as holes and cavities throughout the rock.

Step-by-Step Solution

1
Identify the rock category and observed physical characteristics
Pumice is an extrusive (volcanic) igneous rock characterized by a lightweight sponge-like structure containing many open cavities (vesicles).
Recognizing the rock type determines the geological environment and formation mechanism responsible for its texture.
2
Determine the physical process that generates vesicular cavities in volcanic rocks
As gas-charged lava erupts onto Earth's surface, the sudden drop in confining pressure allows dissolved gases to rapidly expand into bubbles. The lava quenches so quickly that the rock solidifies around these gas pockets, preserving them as vesicles.
Connecting rapid cooling at the surface with trapped gas expansion directly explains the origin of the vesicular texture.

Key Concept

Formation of vesicular texture in extrusive igneous rocks
Estimated Time:1m 0s
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Topics

Rocks, Minerals, and Soil

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