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Difficulty: MediumRocks, Minerals, and Soil

A field geologist investigating a dried desert basin discovers extensive horizontal strata composed of rock salt (halite) and rock gypsum. Which geological process is primarily responsible for the formation of these rocks?

  1. A
    Accumulation and compaction of plant debris in an anoxic wetland environment
  2. Precipitation of dissolved minerals from water as it evaporates in an arid basinAnswer
  3. C
    Rapid cooling and solidification of molten lava on Earth's surface
  4. D
    Recrystallization of pre-existing mineral grains under intense directional tectonic pressure

Answer

Precipitation of dissolved minerals from water as it evaporates in an arid basin
The correct answer accurately identifies the formation mechanism of chemical sedimentary rocks known as evaporites. Halite (rock salt) and gypsum form when restricted bodies of saltwater or desert playa lakes undergo high rates of evaporation, causing dissolved ions to crystallize and precipitate out of solution onto the basin floor.

Step-by-Step Solution

1
Identify the rock types and mineral composition mentioned in the scenario
Rock salt (halite) and rock gypsum are classified as chemical sedimentary rocks (evaporites).
Recognizing the chemical nature of halite and gypsum narrows down the genetic category of the rock.
2
Determine the formation mechanism for evaporite sedimentary rocks
Evaporites form through the chemical precipitation of soluble salts out of aqueous solutions when water evaporates in enclosed or restricted basins.
As evaporation concentrates the dissolved mineral ions beyond their saturation point, solid crystals precipitate and settle.

Key Concept

Chemical sedimentary rocks (evaporites) form by the inorganic precipitation of minerals from concentrated aqueous solutions as water evaporates.
Estimated Time:1m 0s
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Topics

Rocks, Minerals, and Soil

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