Gneiss

Lore & Background

Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage. The darker bands contain more mafic minerals, while the lighter bands contain more felsic minerals. The banding develops at high temperature when the rock is more strongly compressed in one direction than in others, often due to extreme shearing force. Some banding is formed from original rock material with alternating layers of sandstone and shale, while another cause is 'metamorphic differentiation', a process not fully understood. Gneisses that are metamorphosed igneous rocks are termed granite gneisses, diorite gneisses, and so forth; orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Augen gneiss contains characteristic elliptic or lenticular shear-bound grains, normally feldspar, surrounded by finer grained material. Migmatite is a gneiss consisting of two or more distinct rock types, often interpreted as rock that has been partially melted. Gneisses are characteristic of areas of regional metamorphism that reaches the middle amphibolite to granulite metamorphic facies, at temperatures in excess of 600 °C and pressures between about 2 to 24 kbar. They are common in granite-greenstone belts and gneiss domes in orogenic belts.

Reader's Guide

Gneiss is significant as a major rock type of the Earth's ancient continental crust, providing a record of high-grade metamorphic processes. Its banded texture preserves evidence of the direction and intensity of tectonic forces, such as shearing and compression, that shaped the crust over billions of years. The presence of gneiss in continental shields and orogenic belts helps geologists understand the thermal and pressure conditions of deep crustal metamorphism. Notable examples like the Acasta Gneiss, the Lewisian gneiss, the Morton Gneiss, and the Peninsular Gneiss are among the oldest intact crustal fragments on Earth, offering insights into early Earth history. The rock is also used as a building material, such as the Facoidal gneiss used extensively in Rio de Janeiro, and as construction aggregate for asphalt pavement.

Did You Know?

• The Acasta Gneiss in Canada is one of the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago.
• Augen gneiss gets its name from the German word for 'eyes', referring to elliptic feldspar grains surrounded by finer material.
• Gneiss is used as a building material, such as the Facoidal gneiss used extensively in Rio de Janeiro.
• The word 'gneiss' is probably derived from the Middle High German noun 'gneist' meaning 'spark', because the rock glitters.