103 - Gneiss

Hello friends! Welcome to the Rock Repository at the Department of Geology, Shivaji Science College, Nagpur. I am Gneiss, I am a common and widely distributed type of metamorphic rock. I was formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. I form at higher temperatures and pressures than schist. I nearly always show a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage. I am common in the ancient crust of continental shields. Some of the oldest rocks on Earth are gneisses, such as the Acasta Gneiss. The minerals in me are arranged into layers that appear as bands in cross section. This is called gneissic banding. The darker bands have relatively more mafic minerals. The lighter bands contain relatively more felsic minerals. I have been used as a building material, such as the Facoidal gneiss, used extensively in Rio de Janeiro. I am also used as construction aggregate for asphalt pavement.