Metamorphic Series

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Metamorphism is the process where rocks – igneous, sedimentary, or metamorphic –are altered by temperature and/or pressure or infiltration by chemical solutions into different rock types, such as banded gneisses. The nature of the resulting metamorphic rock is dependent upon the parent rock and the extent to which it has been recrystallized. The degree of change is reflected in a metamorphic series in which different minerals arise as the result of greater and greater degrees of metamorphic change.

Banded Gneiss

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The image is considerably more colorful than gneiss, which is a high-grade metamorphic rock that is formed when large volumes of deeply buried rock are subjected to high temperatures and pressures. This pressure-cooker alteration of rocks in geological areas like tectonic subduction zones is called regional metamorphism.

Gneiss, which is pronounced like 'niece', often displays banding of light and dark layers. The bands may be linear and parallel, but they may be quite distorted as a result of the pressures to which they were subjected. The layers are often gray-black and white, though they may be colored, often pinkish. Lesser grades of metamorphic change can produce shiny schists.