Golden Gneiss

What is Gneiss?

Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture. Other bands contain platy or elongate minerals with evidence of preferred orientation. It is this banded appearance and texture - rather than composition - that define a gneiss.

Gneissic Granodiorite

Gneissic Granodiorite: An outcrop of gneissic granodiorite in the Zarembo Island area of southeastern Alaska.

How Does Gneiss Form?

Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the size of the mineral grains and segregated them into bands, a transformation which made the rock and its minerals more stable in their metamorphic environment.

Gneiss can form in several different ways. The most common path begins with shale, which is a sedimentary rock. Regional metamorphism can transform shale into slate, then phyllite, then schist, and finally into gneiss. During this transformation, clay particles in shale transform into micas and increase in size. Finally, the platy micas begin to recrystallize into granular minerals. The appearance of granular minerals is what marks the transition into gneiss.

Intense heat and pressure can also metamorphose granite into a banded rock known as "granite gneiss." This transformation is usually more of a structural change than a mineralogical transformation. Granite gneiss can also form through the metamorphism of sedimentary rocks. The end product of their metamorphism is a banded rock with a mineralogical composition like granite.

photo of polished gneiss from the yard of a countertop vendor

Folded Gneiss: A photograph of polished gneiss from the stock of a countertop vendor. The view shown in the photo is about 12 inches across.

Composition and Texture of Gneiss

Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartzgrains in an interlocking texture. These bands are usually light in color and alternate with bands of darker-colored minerals with platy or elongate habits. The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism.

Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic environment. These minerals might include biotite, cordierite, sillimanite, kyanite, staurolite, andalusite, and garnet. Gneiss is sometimes named for these minerals, examples of which include "garnet gneiss" and "biotite gneiss."

garnet gneiss

Garnet Gneiss: A coarse-grained gneiss composed mainly of hornblende (black), plagioclase (white), and garnet (red) from Norway.

The Rocks

What is Shale?

Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles that we commonly call "mud." This composition places shale in a category of sedimentary rocks known as "mudstones." Shale is distinguished from other mudstones because it is fissile and laminated. "Laminated" means that the rock is made up of many thin layers. "Fissile" means that the rock readily splits into thin pieces along the laminations.

Shale

Shale: Shale breaks into thin pieces with sharp edges. It occurs in a wide range of colors that include red, brown, green, gray, and black. It is the most common sedimentary rock and is found in sedimentary basins worldwide.

What is Slate?

Slate is a fine-grained, foliated metamorphic rockthat is created by the alteration of shale or mudstone by low-grade regional metamorphism. It is popular for a wide variety of uses such as roofing, flooring, and flagging because of its durability and attractive appearance.

Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. The specimen shown above is about two inches (five centimeters) across.

What Is Phyllite?

Phyllite is a foliated metamorphic rock that has been subjected to low levels of heat, pressure and chemical activity. It is composed mainly of flake-shaped mica minerals in parallel alignment. The strong parallel alignment of the mica grains allows the rock to be easily split into sheets or slabs. The alignment of the mica grains gives phyllite a reflective sheen that distinguishes it from slate, its metamorphic precursor or protolith. Phyllite is usually gray, black, or greenish in color and often weathers to a tan or brown. Its reflective sheen often gives it a silvery, nonmetallic appearance.

Phyllite is a very common metamorphic rock, found in many parts of the world. It forms when sedimentary rocks are buried and mildly altered by the heat and directed pressure of regional metamorphism. These are almost always convergent plate boundary environments involving continental lithosphere.

Phyllite

Phyllite: A specimen of phyllite that exhibits the lustrous and wrinkled surface common to this rock type.

What is Schist?

Schist is a foliated metamorphic rock made up of plate-shaped mineral grains that are large enough to see with an unaided eye. It usually forms on a continental side of a convergent plate boundarywhere sedimentary rocks, such as shales and mudstones, have been subjected to compressive forces, heat, and chemical activity. This metamorphic environment is intense enough to convert the clay minerals of the sedimentary rocks into platy metamorphic minerals such as muscovite, biotite, and chlorite. To become schist, a shale must be metamorphosed in steps through slate and then through phyllite. If the schist is metamorphosed further, it might become a granular rock known as gneiss.

A rock does not need a specific mineral composition to be called “schist.” It only needs to contain enough platy metamorphic minerals in alignment to exhibit distinct foliation. This texture allows the rock to be broken into thin slabs along the alignment direction of the platy mineral grains. This type of breakage is known as schistosity.

muscovite schist

Muscovite schist: The dominant visible mineral in this schist is muscovite. Its platy grains are aligned in a common orientation, and that allows the rock to be split easily in the direction of the grain orientation. The specimen shown is about two inches (five centimeters) across.

Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartzand feldspar with minor amounts of mica, amphiboles, and other minerals. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock.

Coarse grained granite

Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar, and the clear to smoky grains are quartz or muscovite. The black grains can be biotite or hornblende. Numerous other minerals can be present in granite.

To claim credit for this EarthCache send the answers to these questions:

How high does the formation in front of you rise from where you are standing?
Name at least three types of rocks you see here making up this Gneiss.
What causes the distinct bands of color you see in this formation?
Bonus: Do you see any types of rocks not listed on this page?

References:

Gillen, Con (2003). Geology and landscapes of Scotland. Harpenden. Terra Publishing. ISBN 1-903544-09-2.
Harper, Douglas (ed.). "gneiss", Online Etymological Dictionary. Retrieved 2015-03-01.
Marshak, Stephen (2013). Essentials of Geology (4th ed.). W.W. Norton. ISBN 978-0-393-91939-4.
McKirdy, Alan, Roger Crofts and John Gordon (2007). Land of Mountain and Flood: The Geology and Landforms of Scotland. Edinburgh. Birlinn. ISBN 978-1-84158-357-0.
Murray, W.H. (1966). The Hebrides. London. Heinemann.
Sacks, Paul E. and Donald T. Secor (1990). "Kinematics of Late Paleozoic continental collision between Laurentia and Gondwana". Science, 250 (4988): 1702–05. doi:10.1126/science.250.4988.1702.

Additional Waypoints

0186Z9M - Gneiss
N 39° 42.420 W 105° 15.850
Gneiss Formation
0286Z9M - Parking
N 39° 42.426 W 105° 15.868
N 39° 42.426' W 105° 15.868'