Gneiss Rock Type, Composition, Formation, Occurrence & Uses

Gneiss is a common and widely distributed foliated metamorphic rock. It is identified by its bands and lenses of varying mineral composition.

Some bands may contain granular minerals bound together in an interlocking texture. Some bands show a parallel orientation with overall banding in the rock because of elongated minerals.

Gneiss rock’s texture is defined by its banded appearance and texture rather than composition. The word gneiss is taken from the German word Gneiss. It has been used in English since at least 1757.

Sample of gneiss exhibiting "gneissic banding".

Table of Contents

Rock Composition

Gneiss rock is usually defined by its appearance and not its composition. Most specimens contain feldspar and quartz grains in an interlocking texture. These bands are light-colored and found with elongated dark-colored bands. The orientation of darker minerals is often determined by metamorphism pressure.

Some specimens contain distinctive minerals found in metamorphic environments. These minerals usually include sillimanite, kyanite, staurolite, andalusite, biotite, cordierite, and garnet. You may also find gneiss named after minerals like garnet gneiss and biotite gneiss.

Gneiss Formation

It usually forms by regional metamorphism at convergent plate boundaries. The rock requires intense heat and pressure to form. The minerals recrystallize under intense heat and pressure to form gneiss, a high-grade metamorphic rock. The formation happens at temperatures above 600 °C (1,112 °F) and pressure from 2 to 24 kbar.

Petrologists say that gneiss can form in several different ways. The most common path starts from shale (a sedimentary rock). Regional metamorphism converts shale to slate, then pyrite, schist, and finally, gneiss is formed.

The clay particles in shale transform into micas, and their size increase. The platy micas then recrystallize due to intense heat and pressure, giving the rock a granular appearance.

The intense heat and pressure also convert granite into a banded rock known as granite gneiss. This conversion doesn’t affect the mineralogical composition but the rock’s structure.

Pure shear deformation of rock producing gneissic banding. The undeformed rock is shown at upper left, and the result of pure shear deformation at upper right. At lower left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as shown by the arrows. The rock is simultaneously rotated to produce the final configuration, repeated at lower right.

Where is Gneiss Found?

Gneisses are found in areas of regional metamorphism, continental shields, greenstone belts, and orogenic belts (regions of mountain formation). Large reserves of gneiss have been found in the following areas.

The Acasta gneiss in the Northwest Territories, Canada, one of the most ancient intact crustal fragments, metamorphosed 3.58 to 4.031 billion years ago.
The Lewisian gneiss in Outer Hebrides of Scotland.
The Morton gneiss in the Minnesota River Valley, Minnesota, is considered the oldest intact block of continental crust in the United States.
The Peninsular gneiss in Indian Shield, some 2500 – 3400 million years old.

Augen gneiss from Leblon, Rio de Janeiro City, Brazil

Gneiss Uses

Gneiss is a durable and super-sturdy rock. It doesn’t break along planes like other metamorphic rocks. These properties make gneiss a valuable rock. It is frequently used as a crushed stone in road construction, buildings, and landscaping projects.

Gneiss is also used as a dimension stone. The rocks are shaped into blocks and slabs to be used in various buildings, paving, and curbing projects. Some gneiss specimens can be polished and used as architectural stone. Polished gneiss is often used to make beautiful floor tiles, window sills, countertops, cemetery monuments, and stair treads. Commercially, gneiss is also labeled as granite because of the visible and interlocking grains of feldspar.