Gneiss facts for kids

Outcrop of weathered Lewisian gneiss, 5 km NW of Loch Inver, Isle of Lewis

Gneiss rock

The stone circle at the centre of the Standing Stones of Callanish ("Callanish I"), Isle of Lewis. These rocks are thought to be 3 billion years old, making them some of the oldest in the UK.

Gneiss (pronounced "nice") is a type of metamorphic rock. This means it's a rock that has been changed by extreme heat and pressure deep inside the Earth. The minerals in gneiss can come from rocks that were originally igneous (formed from cooled magma or lava) or sedimentary (formed from layers of sand, mud, or other bits of rock).

When these original rocks are heated and squeezed, their minerals recrystallize. This means they form new crystals or rearrange themselves. In gneiss, the minerals often line up in distinct bands or layers. This layered look is called foliation. Imagine thin seams of different minerals, like quartz or mica, all lined up together.

Types of Gneiss

Gneiss can be divided into two main types, depending on what kind of rock it started as:

• Orthogneiss is gneiss that formed from an igneous rock. A common example is gneiss that started as granite.
• Paragneiss is gneiss that formed from a sedimentary rock. For instance, it might have begun as sandstone.

Lewisian Gneiss: An Ancient Rock

The Lewisian complex or Lewisian Gneiss is a group of very old metamorphic rocks found in the northwestern part of Scotland. These rocks are part of the Hebridean Terrane, a large block of Earth's crust.

These rocks are incredibly ancient. They formed during the Precambrian time, which is a very early period in Earth's history. Their ages range from about 3.0 billion to 1.7 billion years old. This makes them some of the oldest rocks in the United Kingdom! Studying them helps scientists understand what the Earth was like billions of years ago.

Images for kids

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  Orthogneiss from the Czech Republic

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  Pure shear deformation of rock producing gneissic banding. The undeformed rock is shown at upper left, and the result of pure shear deformation at upper right. At lower left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as shown by the arrows. The rock is simultaneously rotated to produce the final configuration, repeated at lower right.

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  Dark dikes (now foliated amphibolites) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes

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  Contact between a dark-colored diabase dike (about 1100 million years old) and light-colored migmatitic paragneiss in the Kosterhavet National Park in the Koster Islands off the western coast of Sweden.

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  Sample of Sete Voltas gneiss from Bahia in Brazil, the oldest rock outcropping in the crust of South America, c. 3.4 billion years old (Archean)

See also

In Spanish: Gneis para niños