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.Learn More
Mica is a silicate mineral that can be found in nature as metamorphic or igneous rocks. Mica can also be found in sedimentary rocks depending on the types of minerals left behind by water and what minerals are in the area.Full Answer >
Metamorphic rocks are made by physically and chemically changing the characteristics of sedimentary or igneous rocks. Heat, pressure under the Earth's surface or friction from colliding tectonic plates can cause these changes.Full Answer >
Igneous rocks that are allowed to cool more slowly form larger crystals, while igneous rocks that cool quickly form smaller crystals. It is simply a matter of time. The longer it takes for magma to cool, the more time is allowed for the crystals to form.Full Answer >
Sedimentary, metamorphic and igneous rocks are found in Georgia. The state has four distinct geographic regions, and different types of each rock can be found throughout the land.Full Answer >