Basalt from Ethiopia - the columnar jointing is a result of slow cooling, allowing to distribute contraction fractures to arrange in a hexagonal columnar pattern, the geometry that requires the least energy to provide the necessary space when the rock slowly contracts.
Rhyolite from Milos island, Greece, erupted approx. 84,000 years ago. Rhyolite is the extrusive equivalent to the plutonic rock type granite - both form from the same magma. Rhyolite lava flows are very viscous. Due to their high viscosity, they only move through laminar flow along sheer planes that form where gas bubbles concentrate. These flow structures are usually visible in the cooled rock. They are also at the Greek origin of the name of the rock, stemming from rheo ("flowing") and lithos ("rock").
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 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 |
|45-53 ||Gabbro ||Basalt |
|53-63 ||Diorite ||Andesite |
|63-68 ||Granodiorite ||Dacite |
|68-75 ||Granite ||Rhyolite |
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.
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.
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