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Gneiss

Pictures of Metamorphic Rock Types

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Gneiss ("nice") is a rock of great variety with large mineral grains arranged in wide bands. It means a type of rock texture, not a composition. (more below)
Makes up the lower crust
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
Gneiss is a typical rock type formed by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. Nearly all traces of the original structures (including fossils) and fabric (such as layering and ripple marks) are wiped out as the minerals migrate and recrystallize. The streaks contain minerals, like hornblende, that don't occur in sedimentary rocks.

In gneiss, less than 50 percent of the minerals are aligned in thin, foliated layers. You can see that unlike schist, which is more strongly aligned, gneiss doesn't fracture along the planes of the mineral streaks. And thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. With still more metamorphism, gneisses can turn to migmatite and then totally recrystallize into granite.

Despite its highly altered nature, gneiss can preserve chemical evidence of its history, especially in minerals like zircon which resist metamorphism. The oldest Earth rocks known are gneisses from Acasta, in northern Cananda, that are more than 4 billion years old.

Gneiss makes up the largest part of the Earth's lower crust. Pretty much everywhere on the continents, you will drill straight down and eventually strike gneiss.

Gneiss is an old German word meaning bright or sparkling.

For more photos see the Metamorphic Rocks Gallery.

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