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Difficulty: MediumRocks, Minerals, and Soil

An outcrop of dark igneous rock containing iron-bearing silicate minerals gradually develops a reddish-brown crust after prolonged exposure to rainwater and atmospheric oxygen. This reaction alters the rock's mineral composition and weakens its outer structure. Which geological process is primarily responsible for this change?

  1. A
    Mechanical weathering
  2. Chemical weatheringAnswer
  3. C
    Metamorphic recrystallization
  4. D
    Sedimentary deposition

Answer

Chemical weathering
Chemical weathering occurs when rock-forming minerals react chemically with water, oxygen, or acids in the environment. In this scenario, iron within the rock oxidizes upon contact with oxygen and moisture, creating new reddish-brown iron oxide compounds and structurally weakening the rock.

Step-by-Step Solution

1
Analyze the geological mechanism and material transformation described in the stem.
Atmospheric oxygen and moisture react with iron-bearing minerals to produce a new chemical compound (iron oxide) that alters the rock's original mineral composition.
Identifying whether the material's chemical identity changes distinguishes chemical weathering from physical disintegration.
2
Classify the observed chemical alteration under standard Earth science weathering categories.
Processes that break down rock through chemical reactions at or near Earth's surface belong to chemical weathering (specifically oxidation).
Oxidation directly alters mineral chemistry and contributes to rock degradation and soil genesis.

Key Concept

Chemical weathering alters the internal atomic structure and chemical composition of minerals through atmospheric reactions such as oxidation, carbonation, and hydrolysis.
Estimated Time:1m 0s
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Topics

Rocks, Minerals, and Soil

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