What are igneous rocks?Igneous rocks are the most basic type of rocks. They are formed when magma (molten rock, typically derived from the earth's mantle) solidifies. This can happen beneath or above the surface, resulting in 2 subtypes:
1) Intrusive rocks or Plutonic rocks
When magma never reaches the surface and cools to form intrusions (dykes, sills etc) the resulting rocks are called plutonic. Depending on their silica content, they are called (in ascending order of silica content) gabbro, diorite, granite and pegmatite. By quantity, these are the by far most common rock types. Most magmas actually never reach the surface of the earth.
2) Extrusive rocks or Volcanic rocks
When magma does reach the surface during a volcanic eruption, the rocks that form there are called lavas or volcanic rocks. The basic classification is the same as for plutonic rocks: with increasing silica content, they include: basalt, andesites, dacites, rhyolite, pumice and obsidian.
Main types of igneous rocksThe most widely used and simplest classification of igneous rocks is according to the silica (SiO2) content in the bulk rock composition. The most common types are shown in this table:
|Weight % of SiO2||Plutonic rock type||Volcanic rock equivalent|
Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California. (Wikimedia Commons)
Diorite sample (image: Michael C. Rygel via Wikimedia Commons)
Granodiorite from Massif Central, France (image: Rudolf Pohl / Wikimedia Commons)
The volcanic rock equivalent of granodiorite is dacite.
Granite (image: Friman / Wikimedia Commons)
Granites can be predominantly white, pink, or gray in color, depending on their mineralogy.