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 rocks
The 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)
Gabbro is a silica-poor intrusive igneous (plutonic) rock chemically equivalent to basalt. It is normally coarse-grained, dark and typically contains feldspar, augite and sometimes olivine.
Diorite sample (image: Michael C. Rygel via Wikimedia Commons)
Diorite is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate between gabbro and granite. It corresponds to the volcanic rock type andesite formed when the same magma erupts to the surface and cools quickly.
Andesite from Methana, Greece
Andesite is a volcanic (=extrusive igneous) rock with an intermediate to high silica content. It is the eruptive equivalent of diorite,- the same magma that produces diorite as an intrusion will become andesite if erupted.
Andesite is usually light to dark gray or pale-red in color and typically consists of a fine-grained or glassy groundmass with embedded crystals (phenocrysts) of plagioclase, pyroxenes and/or amphiboles. It is one of the most common magmas erupted from subduction zone volcanoes and is usually involved with both effusive and explosive eruptions.
Granodiorite from Massif Central, France (image: Rudolf Pohl / Wikimedia Commons)
Granodiorite is an intrusive igneous rock in composition intermidiate between diorite and granite. It typically contains more than 20% quartz by volume, a large amount of sodium (Na) and calcium (Ca) rich plagioclase, minor amounts of muscovite mica, and biotite and amphiboles as the darker minerals.
The volcanic rock equivalent of granodiorite is dacite.
Granite (image: Friman / Wikimedia Commons)
Granite, the equivalent of its extrusive (volcanic) rock type rhyolite, is a very common type of intrusive igneous rock. It contains more than 68% weight % of silica in composition and is granular and coarse-grained in texture. Its principal minerals are feldspars, quartz, and mica.
Granites can be predominantly white, pink, or gray in color, depending on their mineralogy.
Ethiopia and the Danakil Ethiopia is a very diverse and beautiful country. Thanks to our numerous expeditions to Erta Ale and Dallol and beyond, we have an extensive collection of images showing the volcano's lava lake, the desert, the colorful hot springs of Dallol, the vast salt lakes, camel caravans and more.
Exploring the unknown We sometimes invite you to "test" new itineraries or destinations with us. Such trips are a reduced cost and require a higher degree of flexibility, but often offer a great deal of adventure.
Fuego volcano photos One of Central America's most active volcanoes, Fuego in Guatemala is in near-permanent explosive eruption with intermittent phases of increasing activity that generate lava fountains, lava flows and pyroclastic flows.
Copyright:VolcanoDiscovery ou autres sources indiquées. Utilisation des documents: Les textes, les images et les vidéos de ce site internet sont protégées par copyright. Toute reproduction et tout usage sans accord préalable ne sont pas autorisés. Si vous désirez acquérir pour certains documents (photos, vidéos, textes ou autres) le droit de reproduction, d'édition et, plus généralement, de toute utilisation destinée à un usage public, qu'elle soit commerciale ou pas, vous êtes priés de nous contacter.