Gneiss is a common and widely distributed type of rock formed by high-grade regional
metamorphic processes from pre-existing formations that were originally either igneous or sedimentary
rocks. It is often foliated. The foliations are characterized by alternating darker and
lighter colored bands, called "gneissic banding".
Etymology The etymology of the word gneiss is disputed.
Sources say it comes from the Middle High German verb gneist. It has occurred in English
since at least 1757. Composition
Gneissic rocks are usually medium- to coarse-foliated and are largely recrystallized but do not
carry large quantities of micas, chlorite or other platy minerals. Gneisses that are
metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses,
etc. Gneiss rocks may also be named after a characteristic component, such as garnet
gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from
an igneous rock, and paragneiss is one from a sedimentary rock.
Not all gneiss rocks have detectable banding however. Kyanite gneiss crystals look like
random clumps of kyanite in what is mainly a plagioclase matrix.
Gneissic banding Gneiss appears to be striped in bands, called
gneissic banding. The banding is developed under high temperature and pressure conditions.
The minerals are arranged into layers which appear to be bands, when the rock is broken
and viewed from the side. This is because of a different composition of each layer,
or band, called compositional banding. The darker bands have relatively more mafic minerals.
The lighter bands contain relatively more felsic minerals.
A common cause of the banding is the subjection of the protolith to extreme shearing force,
a sliding force similar to the pushing of the top of a deck of cards in one direction,
and the bottom of the deck in the other direction. These forces stretch out the rock like a plastic,
and the original material is smeared out into sheets.
Some banding is formed from original rock material that is subjected to extreme temperature
and pressure and is composed of alternating layers of sandstone and shale, which is metamorphosed
into bands of quartzite and mica. Another cause of banding is "metamorphic differentiation",
which separates different materials into different layers through chemical reactions, a process
not fully understood. Types
Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from
a sedimentary rock. Gneissose rocks have properties similar to gneiss.
Lewisian Most of the Outer Hebrides of Scotland have
a bedrock formed from Lewisian gneiss. This bedrock contains rocks that are among the
oldest in Europe and indeed the world, having been formed in the Precambrian "super-eon"
up to three billion years ago. In addition to the Outer Hebrides, they form basement
deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll
and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble,
quartzite and mica schist with later intrusions of basaltic dykes and granite magma. The gneiss's
delicate pink colours are exposed throughout the islands.
Augen gneiss
Augen gneiss, from the German: Augen [ˈaʊɡən], meaning "eyes", is a coarse-grained gneiss
resulting from metamorphism of granite, which contains characteristic elliptic or lenticular
shear-bound feldspar porphyroclasts, normally microcline, within the layering of the quartz,
biotite and magnetite bands. Archean and Proterozoic gneiss
Archean and Proterozoic gneisses appear in Scandinavia.
Henderson Gneiss Henderson Gneiss is located in North Carolina
and South Carolina, USA, east of the Brevard Shear Zone. It has deformed into two sequential
forms. The second more warped form is associated with the Brevard Fault and the first deformation
results from displacement to the southwest. See also
List of rock types Footnotes
References Blatt, Harvey and Robert J. Tracy. Petrology:
Igneous, Sedimentary
and Metamorphic, 2nd ed. Freeman, pp. 359–365. ISBN 0-7167-2438-3.
Gillen, Con. Geology and landscapes of Scotland. Harpenden. Terra Publishing. ISBN 1-903544-09-2.
McKirdy, Alan Gordon, John & Crofts, Roger. Land of Mountain and Flood: The Geology and
Landforms of Scotland. Edinburgh. Birlinn. ISBN 978-1-84158-357-0.
Murray, W.H.. The Hebrides. London. Heinemann. External links
"Gneiss". Encyclopædia Britannica. 1911. "Gneiss". New International Encyclopedia.
1906.