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Difficulty: Very hardRocks, Minerals, and Soil

A geologist is analyzing a region where a coarse-grained felsic igneous bedrock is exposed to a warm, continuously humid climate over a prolonged period. Which of the following best describes the resulting weathering sequence and the mineral composition of the overlying mature soil profile?

  1. Chemical weathering via hydrolysis transforms the feldspar into clay minerals, which leach downward to accumulate in the B horizon, leaving highly resistant quartz grains concentrated in the upper horizons.Answer
  2. B
    Physical weathering dominates due to the coarse crystalline structure, breaking the bedrock into smaller, unaltered mineral grains that form a thick soil profile rich in intact feldspar.
  3. C
    The continuous exposure to extreme surface heat and moisture subjects the igneous rock to low-grade pressure, causing its constituent minerals to recrystallize into a foliated metamorphic soil base.
  4. D
    Intense leaching caused by the humid climate dissolves both quartz and feldspar equally, precipitating them as horizontal chemical sedimentary rock layers directly above the bedrock.

Answer

Chemical weathering transforms feldspar into clay minerals that accumulate in the subsoil, while resistant quartz remains concentrated in the upper topsoil.
In warm, highly humid environments, chemical weathering is the dominant force acting on exposed rock. Hydrolysis chemically alters feldspar, a major component of felsic rocks, into fine clay minerals. As rainwater percolates through the ground, it leaches these microscopic clay particles downward, causing them to accumulate heavily in the B horizon (subsoil). Quartz, however, is exceptionally resistant to chemical weathering and remains physically intact in the upper horizons (topsoil) as sand grains.

Step-by-Step Solution

1
Determine the dominant weathering process based on the climate.
In warm, continuously humid climates, chemical weathering (such as hydrolysis, oxidation, and dissolution) vastly outweighs physical weathering.
Water and high temperatures accelerate chemical reactions that break down rock.
2
Analyze the susceptibility of the minerals in the felsic igneous bedrock.
Felsic rocks are rich in feldspar and quartz. Feldspar undergoes hydrolysis and chemically alters into soft clay minerals. Quartz is highly stable and resists chemical breakdown.
Minerals crystallize at different temperatures; quartz crystallizes last and is most stable at surface temperatures and pressures.
3
Track the movement of these altered minerals through the soil profile.
Heavy rainfall leaches the newly formed, fine clay particles downward from the upper layers (A horizon) and deposits them in the subsoil (B horizon).
Percolating rainwater physically carries microscopic particles deeper into the ground.
4
Conclude the final composition of the mature soil profile.
The upper horizons are depleted of clay and retain the larger, resistant quartz grains (sand), while the B horizon becomes heavily enriched with clay.
This matches the correct description of soil horizon development over a weathered felsic bedrock.

Key Concept

Chemical Weathering, Mineral Stability, and Soil Horizon Formation
Estimated Time:2m 0s
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Topics

Rocks, Minerals, and Soil

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