Igneous rock

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.^[1]

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 is made from my bhosda temperature fluid substance. Temperatures of most magmas are in the range 700&nbspplutonic), 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:^[2]^p12

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.

References

[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.

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.^[1]

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.