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.Continue Reading
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.Learn more about Geology
Some examples of intrusive igneous rocks are granite, diorite, syenite, gabbro and pyroxenite. These rocks were formed over eons in magma chambers deep in the earth under high pressure and high temperatures.Full Answer >
Marble is a soft metamorphic rock that forms at the edges of tectonic plates, where limestone is exposed to regional metamorphism. The heat and pressure of the metamorphic zone force calcite inside the limestone to recrystallize and merge into the classic marble consistency.Full Answer >
According to the University of California, Santa Cruz, most hydrothermal metamorphism occurs at the boundaries of oceanic plates. The plates move apart and allow seawater to percolate through the oceanic crust.Full Answer >
Rocks that are exposed to the high pressure and temperatures associated with mountain building are said to have undergone regional metamorphism. According to Columbia University, regional metamorphism takes place under mountain chains, at the collision point between continental plates and in other high-pressure, high-temperature zones.Full Answer >