Gneiss facts for kids
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Sample of gneiss exhibiting "gneissic banding"
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Gneiss (pronounced "nice") is a very common type of metamorphic rock. Metamorphic rocks are rocks that have changed due to extreme heat and pressure. Gneiss forms deep inside the Earth. It starts as other rocks, like igneous rocks (formed from cooled magma) or sedimentary rocks (formed from layers of sediment). These original rocks get squeezed and heated a lot. Gneiss forms under pressures from 2,000 to 15,000 times atmospheric pressure. Temperatures are usually hotter than 300°C (572°F). Gneiss almost always has a striped look. It shows alternating dark and light bands. It does not easily split along flat surfaces.
Gneiss rocks are often found in the very old parts of continents. These old parts are called continental shields. Some of the oldest rocks on our planet are gneisses. A famous example is the Acasta Gneiss.
Contents
What is Gneiss?
Gneiss is a metamorphic rock with large mineral grains. You can easily see these grains without a magnifying glass. These minerals are arranged in distinct layers, creating a striped look. This striped pattern is called gneissic banding. Gneiss does not easily split into thin sheets. This is different from rocks like schist, which split more easily.
Geologists use the term "gneiss" for rocks with this specific texture. The layers in gneiss are usually thicker than 5 millimeters. The rock tends to break into pieces thicker than 1 centimeter. The main thing that makes a rock gneiss is its texture, not what it's made of. Gneiss rocks have been largely recrystallized. This means their minerals have reformed under heat and pressure. They usually don't have many flat, flaky minerals like mica or chlorite.
We can name gneiss rocks based on what they were before they changed. If a gneiss came from an igneous rock, it's called an orthogneiss. For example, a granite that changed into gneiss is a granite gneiss. If it came from a sedimentary rock, it's called a paragneiss. Gneiss can also be named after special minerals it contains. Examples include garnet gneiss or biotite gneiss.
How Gneiss Gets Its Stripes: Gneissic Banding
The stripes you see in gneiss are called gneissic banding. These bands are layers of different minerals.
- Darker bands usually have more "mafic" minerals. These minerals contain more magnesium and iron.
- Lighter bands usually have more "felsic" minerals. These include minerals like feldspar or quartz. They contain lighter elements like aluminium, sodium, and potassium.
This banding forms when the rock is very hot and squeezed unevenly. Imagine squeezing a sponge more in one direction than another. This uneven squeezing is called nonhydrostatic stress. The mineral layers line up perpendicular to the strongest squeeze. Flat minerals rotate or recrystallize into these parallel layers.
One way this happens is through a strong "shearing force." Think of pushing the top of a deck of cards forward and the bottom backward. This force stretches the rock, making the original materials spread out into sheets. The rock becomes somewhat plastic, like clay.
Sometimes, the original rock already had layers. For example, alternating layers of sandstone (lighter) and shale (darker). When these layers are put under extreme heat and pressure, they transform. They become bands of quartzite and mica, forming gneiss.
Another way banding can form is through "metamorphic differentiation." This is when different materials separate into layers through chemical reactions. Scientists are still studying exactly how this process works.
Special Types of Gneiss: Augen Gneiss
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Augen gneiss gets its name from the German word "Augen," which means "eyes." This type of gneiss often forms from granite. It has distinctive eye-shaped or lens-shaped mineral grains. These "eyes" are usually made of feldspar. They are surrounded by finer-grained material. The softer, finer material flows around the harder feldspar grains. This creates the unique eye-like texture.
Migmatite: A Mix of Rocks
A migmatite is a special kind of gneiss. It looks like it's made of two or more different rock types mixed together. One part often looks like regular gneiss. This part is called the mesosome. Another part looks like an intrusive rock, such as granite. This lighter-colored part is called the leucosome. Sometimes, there's also a darker part called the melanosome.
Scientists believe migmatites form when a rock partially melts.
- The leucosome is thought to be the melted part, rich in silica.
- The melanosome is the solid rock left behind after some melting.
- The mesosome is the original rock that hasn't melted yet.
Where Can We Find Gneiss?
Gneiss is found in places where rocks have undergone intense regional metamorphism. This means large areas of rock were subjected to very high temperatures and pressures. Gneiss typically forms at temperatures above 600°C (1112°F). It also forms under pressures between 2,000 and 24,000 times atmospheric pressure.
Many different kinds of rocks can turn into gneiss. Geologists often add details to the name of a gneiss. This helps describe its color and the minerals it contains. For example, they might say "garnet-biotite paragneiss." This tells us it has garnet and biotite minerals and came from a sedimentary rock.
Gneiss in Ancient Continental Shields
Continental shields are very old, stable parts of continents. They are like the ancient foundations of our landmasses. The oldest rocks in these shields are from the Archean age, over 2.5 billion years old. These ancient rocks often form "granite-greenstone belts."
Greenstone belts contain metavolcanic and metasedimentary rocks. These rocks experienced milder metamorphism. They formed at temperatures between 350-500°C (662-932°F). They also formed under pressures of 200-500 times atmospheric pressure.
These greenstone belts are surrounded by areas of high-grade gneiss. These gneiss areas show intense deformation. They formed at high temperatures (over 500°C or 932°F) but lower pressures. Gneiss makes up most of the exposed rock in these ancient continental cores, called cratons.
Gneiss Domes in Mountain Ranges
Gneiss domes are common in orogenic belts. These are regions where mountains have formed. A gneiss dome looks like a dome-shaped mass of gneiss. It often has younger granite and migmatite mixed in. Layers of sedimentary rock cover the dome.
Scientists have different ideas about how these domes form. Some think they show two separate mountain-building events. The first event created the granite base. The second event then deformed and melted this base, forming the domes. Other scientists believe gneiss domes might be the cores of metamorphic core complexes. These are deep crustal rocks brought to the surface when the Earth's crust stretches apart.
Famous Gneiss Examples Around the World
- The Acasta Gneiss is found in the Northwest Territories, Canada. It is on an island about 300 kilometers (186 miles) north of Yellowknife. This is one of the oldest pieces of Earth's crust. It was metamorphosed between 3.58 and 4.031 billion years ago.
- The Lewisian gneiss is found in Scotland. You can see it in the Outer Hebrides and on the Scottish mainland. These rocks mostly started as igneous rocks. They are mixed with changed marble, quartzite, and mica schist. Later, basaltic dikes and granite magma pushed into them.
- The Morton Gneiss is an Archean-age gneiss. It is exposed in the Minnesota River Valley in the United States. It is believed to be the oldest intact block of continental crust in the United States.
- The Peninsular Gneiss is a series of Archean gneisses. They are found across the Indian Shield. Their ages range from 3.4 billion to 2.5 billion years old.
What Does the Word "Gneiss" Mean?
The word gneiss has been used in English since at least 1757. It comes from the German word Gneis. This German word probably came from an older word meaning "spark." This is because the rock often has a shiny, glittering appearance.
How Do People Use Gneiss?
Gneiss is a strong and durable rock. People use it as a building material. For example, a type called Facoidal gneiss is used a lot in Rio de Janeiro, Brazil. Gneiss has also been crushed and used as construction aggregate. This means it's mixed into materials like asphalt pavement for roads.
See also
In Spanish: Gneis para niños
- List of rock types
- Glossary of geology
