How do gneiss rocks form

Some of these bands or lenses contain granular minerals that are bound together in an interlocking texture. Other bands contain platy or elongated minerals that show a preferred orientation that parallels the overall banding in the rock. It is this banded appearance and texture - rather than composition - that define a gneiss. Gneissic Granodiorite: An outcrop of gneissic granodiorite in the Zarembo Island area of southeastern Alaska. The bands and lenses of foliation are clearly visible in this rock.

Image by the United States Geological Survey. 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. 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.

Numerous tight folds are visible in this rock. Click to enlarge. Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartz grains 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.

It can be easily split into thin, flakey pieces. Schists often have a high luster they are very shiny due to large crystals within the rock. Department of Earth, Atmospheric, and Planetary Sciences. Part of the EASP Geology Walk Gneiss is a metamorphic rock formed by changing schist, granite, or volcanic rocks through intense heat and pressure. During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes.

Gneisses from western Greenland comprise the oldest crustal rocks known more than 3. Gneiss is an old German word meaning bright or sparkling.