Igneous rock

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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. Also, the magma may slowly cool down to form rocks under the surface. An example is granite.

Geological significance[change | edit source]

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.[1]

Igneous rocks are geologically important because:

  • their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;
  • 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.

Igneous rock occurs in two ways. The first, extrusive, is above the Earth's crust. The second, intrusive, is at or below the Earth's crust.

Magma[change | edit source]

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[change | edit source]

Most of the minerals that make up igneous rock are of these types:[2]p12

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

Chemical make-up[change | edit source]

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

The first six are silicates. Oxides are mostly of iron.

Related pages[change | edit source]

References[change | edit source]

  1. Klein, Cornelis and Cornelius S. Hurlbut, Jr. 1985. Manual of mineralogy, Wiley, 20th ed, p275 ISBN 0-471-80580-7
  2. Smith P. 1900. Volcanoes: a planetary perspective. Oxford.