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Difficulty: MediumRocks, Minerals, and Soil

As fine-grained sedimentary rock is buried and subjected to progressively greater temperatures and tectonic pressures, it undergoes regional metamorphism that alters its mineralogy and texture. In what order do the following foliated metamorphic rocks form, from lowest metamorphic grade to highest metamorphic grade?

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

Answer

The correct sequence from lowest to highest metamorphic grade is Slate, Phyllite, Schist, and Gneiss.
The standard sequence of pelitic regional metamorphism advances through slate (low grade, dull surface, slaty cleavage), phyllite (low-to-intermediate grade, glossy sheen, crenulated foliation), schist (intermediate-to-high grade, visible mica flakes), and gneiss (high grade, compositional mineral banding).

Step-by-Step Solution

1
Identify the rock formed under the mildest metamorphic conditions.
Slate is the lowest-grade metamorphic rock in this sequence, developing directly from shale under relatively low temperature and pressure.
Slate exhibits microscopic grain alignment and slaty cleavage without significant crystal enlargement.
2
Identify the transitional rock formed as temperature and pressure increase above the conditions that produce slate.
Phyllite represents low-to-intermediate metamorphic grade.
Increased heat allows white mica and chlorite crystals to grow larger than those in slate, producing a glossy, satiny luster.
3
Identify the rock formed under intermediate-to-high regional metamorphic conditions.
Schist represents intermediate-to-high metamorphic grade.
Substantial heat and pressure promote extensive crystal growth, making coarse-grained platy minerals such as biotite and muscovite readily visible.
4
Identify the rock formed under the most extreme metamorphic conditions before melting.
Gneiss is the highest-grade metamorphic rock in this sequence.
Intense directional pressure and high temperatures cause ion migration and complete textural reorganization into alternating light felsic and dark mafic bands.

Key Concept

Metamorphic grade progression and foliation development in regional metamorphism
Estimated Time:1m 0s
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Topics

Rocks, Minerals, and Soil

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