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Zorluk: ZorRocks, Minerals, and Soil

During progressive regional metamorphism, a clay-rich sedimentary rock undergoes mineral recrystallization and textural changes under steadily increasing temperatures and pressures. Arrange the following foliated rocks in order of increasing metamorphic grade, from the lowest temperature and pressure conditions to the highest.

  1. 1Slate
  2. 2Phyllite
  3. 3Schist
  4. 4Gneiss

Cevap

The correct sequence from lowest to highest metamorphic grade is Slate, followed by Phyllite, then Schist, and finally Gneiss.
Regional metamorphism of clay-rich sedimentary rock proceeds through a predictable sequence as temperature and pressure increase: Slate forms under low-grade conditions with microscopic cleavage; Phyllite forms next as micas impart a satiny sheen; Schist develops under intermediate-to-high conditions with visible mineral flakes; and Gneiss forms at the highest grade with distinct light and dark compositional banding.

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1
Identify the foliated rock formed under the lowest temperature and pressure conditions.
Slate is the initial low-grade product resulting from low-temperature (200°C–300°C) alteration of shale, exhibiting planar slaty cleavage.
Slate requires the least thermal energy and differential stress to form from a clay-rich protolith.
2
Determine the next rock in the progression as conditions intensify moderately.
Phyllite represents the next stage where microscopic micas grow large enough to impart a satiny, wavy sheen across foliation planes.
Phyllite forms under low-to-intermediate grades above slate but below the threshold for coarse crystal growth.
3
Identify the rock that forms under intermediate-to-high metamorphic grade.
Schist forms under higher temperatures (350°C–500°C), characterized by coarse, macroscopic flakes of platy minerals such as muscovite and biotite.
Higher thermal energy and prolonged recrystallization allow mineral grains to become plainly visible to the unaided eye.
4
Identify the rock that forms under the highest grade conditions before melting occurs.
Gneiss represents high-grade metamorphism (exceeding 550°C–600°C), exhibiting distinct alternating bands of felsic (light) and mafic (dark) minerals.
Extreme heat and directed pressure cause minerals to segregate compositionally into gneissic banding.

Anahtar Kavram

Progressive regional metamorphism of pelitic rocks and the sequential development of foliation with increasing temperature and pressure
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Rocks, Minerals, and Soil

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