How Is Gneiss Formed?
Gneiss is formed from the high-temperature metamorphism of existing igneous rocks, generally granite or diorite. The rocks that form gneiss are exposed to extreme pressures and temperatures of between 600 and 700 degrees Celsius. These temperatures cause the individual minerals to migrate, forming distinct bands through the rock.
Gneiss does not have any particular mineral composition, and the term refers only to the banded texture. However, it only forms from igneous rocks and so tends to contain minerals not found in metamorphic rocks formed from sedimentary rock, such as marble. Despite the general appearance of banding, the distinct structures are elongated granular structures, so unlike schist, gneiss does not cleave along its bands. The processes that creates gneiss can recreate normal granite if carried on long enough, and gneiss can also form from gabbro or shale. Schist and quartzite may be formed alongside gneiss, along with other types of metamorphic rock formed from igneous rocks.
Gneiss is very common and actually forms the majority of the Earth's lower crust, so any deep tunneling encounters gneiss. The oldest rocks yet found are gneiss, with the oldest dated to over 4 billion years old, older than most estimates of the beginnings of life on Earth.