Igneous rock facts for kids

Magma is called lava when it reaches the surface.

Kimberlite (a type of crystallized igneous rock)

Olivine basalt (a non-crystallized type of igneous rock)

Head of Amenhotep II in granite. Brooklyn Museum.

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock.

Igneous rocks are rocks formed from molten magma. The material is made liquid by the heat inside the Earth's mantle.

When magma comes out onto the surface of the Earth, it is called lava. Lava cools down to form rocks such as tuff and basalt. Intrusive rocks are made when the magma slowly cools to form rocks under the surface. Granite is one of these.

Geological significance

The upper 16 km (10 miles) of the Earth's crust is composed of about 95% igneous rocks with only a thin, widespread covering of sedimentary and metamorphic rocks.

Igneous rocks are geologically important because:

• their absolute ages may be determined from various forms of radiometric dating. This gives information about the dating of adjacent non-igneous geological strata;
• their features are usually characteristic of a specific tectonic environment (see plate tectonics);
• in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.

Two types

Igneous rock occurs in two ways.

• Igneous rock formed from magma that has become solid inside the earth is called intrusive igneous rock. Examples of intrusive igneous rock types are gabbro, diorite, and granite.
• Igneous rock formed from lava that has become solid on the outside of the Earth is called extrusive igneous rock. This can happen because of the eruption of a volcano. Some extrusive igneous rock types are basalt, andesite, rhyolite, tuff, obsidian, and pumice.

Magma

Magma is a complex high-temperature fluid substance. Temperatures of most magmas are in the range 700 °C to 1300 °C. Magma can get forced into adjacent rocks (intrusion or plutonic), or forced out to the surface (extrusion or volcanic) as lava, or blown out in exposions which include rock pieces (tephra).

Magma is made up of atoms and molecules of melted minerals. When magma cools the atoms and molecules rearrange to form mineral grains. Rock forms when mineral grains (often crystals) grow together. Granite, diorite, gabbro and basalt are a few types of igneous rock. Quartz is one of the chief minerals produced by igneous action; it is made of silica (SiO₂), the most common molecule in igneous minerals. Some examples of igneous volcanic rock are pumice, obsidian (volcanic glass), and scoria and much more.

Minerals

Most of the minerals that make up igneous rock are of these types:^p12

1. Olivines
2. Pyroxenes
3. Amphiboles
4. Micas
5. Feldspars & feldspathoids
6. Quartz
7. Oxides

Chemical make-up

Igneous rocks can be classified according to their chemistry. The main types are:

• Mafic: a silicate mineral or igneous rock which is rich in magnesium and iron.
• Felsic: silicate minerals, magma, and igneous rocks which are rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.

The first six minerals in the list are silicates. Oxides are mostly of iron.

Related pages

• List of rocks
• List of minerals
• Sedimentary rock
• Metamorphic rock
• Geology

Images for kids

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  Volcanic eruptions of lava are major sources of igneous rocks. (Mayon volcano in the Philippines, erupting in 2009)

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  Natural columns of igneous rock separated from each other by columnar joints, in Madeira

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  Formation of igneous rock

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  Extrusive igneous rock is made from lava released by volcanoes

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  Sample of basalt (an extrusive igneous rock), found in Massachusetts

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  Close-up of granite (an intrusive igneous rock) exposed in Chennai, India

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  Gabbro specimen showing phaneritic texture, from Rock Creek Canyon, eastern Sierra Nevada, California

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  Kanaga volcano in the Aleutian Islands with a 1906 lava flow in the foreground

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  A "skylight" hole, about 6 m (20 ft) across, in a solidified lava crust reveals molten lava below (flowing towards the top right) in an eruption of Kīlauea in Hawaii

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  Devils Tower, an eroded laccolith in the Black Hills of Wyoming

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  A cascade of molten lava flowing into Aloi Crater during the 1969-1971 Mauna Ulu eruption of Kilauea volcano

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  Columnar jointing in the Alcantara Gorge, Sicily

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  A laccolith of granite (light-coloured) that was intruded into older sedimentary rocks (dark-coloured) at Cuernos del Paine, Torres del Paine National Park, Chile

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  An igneous intrusion cut by a pegmatite dike, which in turn is cut by a dolerite dike

See also

In Spanish: Roca ígnea para niños