Intrusive Igneous Rocks Volcanic Igneous Rocks Igneous Intrusions/Plutons
Volcanoes and Lava Flows Igneous Rocks Quiz Credits
Formation of Igneous Rocks
Igneous rocks form when molten (hot, liquid) elements cool and solidify into crystals. The crystals that form are rich in the elements silicon and oxygen, by far the most abundant elements comprising Earth's crust and mantle. When bonded together, silicon and oxygen form the silicate ion, the building block for almost all igneous minerals. The igneous rocks pictured in this web site are composed of silicate minerals.
The classification of igneous rocks is based on two important aspects of all rocks on Earth, their texture and composition, which are discussed in detail below. Use this classification chart to help you identify your igneous rocks.
Textures of Igneous Rocks
The texture of an igneous rock is determined by its rate of cooling. Molten elements trapped underground, called magma, requires from several days to thousands of years to crystallize. To learn more about magma and its effects on rocks comprising Earth's crust, see the intrusions/plutons page. Molten rock erupted at Earth's surface, referred to as lava, can solidify in few seconds or over the course of several days. For more information about lava and volcanoes, see the volcanoes and lava flows page. The very different rates of cooling for magma and lava lead to the development of two basic textures of igneous rocks, described below. For convenience of presentation the igneous rocks are divided into intrusive rocks (also called plutonic rocks) which form underground, and volcanic rocks (also called extrusive rocks) which form at Earth's surface.
coarse-grained texture Intrusive rocks, formed due to slow cooling of magma underground, are composed of large crystals because crystals had a long time to grow before the crystallization process was finished. This forms a coarse-grained or phaneritic texture. The pictures below illustrate this coarse-grained texture. Also, note the interlocking nature of the silicate mineral crystals comprising this rock.
Click on photos to see enlarged views.
granite hand sample composed of large, interlocking mineral
crystals
granite outcrop in San Gabriel Mountains, California
fine-grained texture Extrusive rocks, formed due to rapid cooling of lava at Earth's surface, are composed of tiny crystals
or even unordered atoms because crystallization was completed within a few seconds, hours, or perhaps days.
The resulting fine-grained or aphanitic texture is characteristic of volcanic
igneous rocks. The rocks shown below show this fine-grained texture.
Click on photos to see enlarged views.
basalt hand sample composed of tiny, interlocking
mineral crystals
rapidly cooling hot lava
special igneous textures
glassy texture - forms as lava cools very rapidly at Earth's surface, in a matter of seconds or a few minutes. The resulting rock is composed of unordered atoms forming a non-crystalline mass, such as the volcanic rock obsidian shown below. Since obsidian is not composed of crystals, it is not composed of minerals, technically speaking. This makes it difficult to assign obsidian to a particular composition (felsic, intermediate or mafic), confounding geologists who feel compelled to categorize mother nature!
Click on photos to see enlarged views.
obsidian handsample
volcanic rock outcrop with both glassy and frothy textures;
Jeff Brenner for scale
frothy texture - forms as gas-charged lava cools very rapidly at Earth's surface. An example of such a volcanic rock is pumice, which is glassy like obsidian, but full of holes formed when trapped gasses expanded as the lava solidified. Like obsidian, pumice is difficult to classify according to standard compositional categories.
Click on photo to see enlarged view.
pumice hand sample; so full of air that it floats
on water
porphyritic texture - forms due to two cycles of cooling, initially very slowly underground, then rapidly at Earth's surface. The rocks shown below each contain a combination of large and small crystals. In the first, some large white crystals formed as magma began to slowly crystallize underground. The rest of this rock, the gray matrix, is composed of tiny crystals which formed as the still-molten magma was erupted at Earth's surface where it cooled very rapidly.
Click on photos to see enlarged views.
andesite hand sample illustrating the combination
of coarse and fine igneous-rock textures
another igneous-rock hand sample illustrating the porphyritic texture
pyroclastic texture - forms as an explosive volcanic eruption mixes fragments of the volcano with hot ash in the atmosphere. As this hot combination of tiny ash particles and larger, angular fragments settles to the ground, they blanket the Earth's surface and weld together. Such spectacular events can wipe out all life forms in the affected region surrounding a volcano! The volcanic tuff shown below is mainly composed of ash, and the volcanic breccia contains a lot of large, angular fragments.
Click on photos to see enlarged views.
handsamples of rhyolite - volcanic tuff and breccia
volcanic outcrop in New Mexico showing the
rough, pyroclastic texture
Composition of Igneous Rocks
The composition of an igneous rock is determined by the minerals present in a sample. The mineralogy of an igneous rock is a product of the combination of silicon dioxide with other elements originally present within a body of magma or lava. The variations in compositions of igneous rocks is, in part, a result of the gradual cooling of a body of magma underground. This slow process allows minerals that form at a high temperature, like olivine and calcium-rich plagioclase feldspar, to crystallize and separate from the remaining magma. Over time, this process alters the magma's composition, forming progressively lower-temperature minerals like quartz and potassium feldspar. This series of chemical changes was observed by N.L. Bowen, and it is referred to as Bowen's Reaction Series.
Igneous rocks are divided into three basic compositional groups, with the rare ultramafic rocks not included in the descriptions below. Refer back to the Bowens Reaction Series chart, as you scroll through the descriptions and photographs of the different igneous-rock compositions.
felsic composition - composed mostly of the minerals quartz and potassium feldspar which are generally white to pink in color. Minor mineral components of granite can be amphibole and biotite mica, both of which are black in color. The rocks below are felsic in composition.
granite hand sample with minerals identified
granitic landscape; Diane Kawahata relaxing in Joshua Tree
National Park
intermediate composition - composed mostly of the minerals amphibole and sodium-rich plagioclase feldspar, with some quartz and pyroxene possible. Intermediate igneous rocks usually
contain a combination of light and dark-colored minerals, so they have a salt
and pepper coloration (diorite), or they are simply gray in color (andesite).
The rocks shown below are intermediate in composition.
diorite hand sample with minerals identified
andesite hand sample
rhyolite outcrop; Bandelier National Monument cliff
dwellings; Bob Careless for scale
mafic composition - composed mostly of the minerals pyroxene, calcium-rich plagioclase feldspar and olivine. Mafic rocks are usually black to dark green in color. The rocks
below are mafic in composition.
gabbro hand sample with minerals identified
basalt hand sample with its characteristic black color