Igneous Rocks

Magma Type

Solidified Volcanic Rock

Solidified Plutonic Rock

Rock Color Plate Settings

Chemical Composition

Melting Temperature

Rock Viscosity

Magma Viscosity

Gas Content

Silica structures
Ultramafic    Peridotite Black/Green Upper mantle <45% SiO2    (1020 Pa s)     Isolated SiO4




Black Ocean crust, hot spot shield volcanoes

45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na

1000 - 1200° C


101 to 103 Pa s






Gray/green Subduction zone stratovolcanoes (composite cone)

55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K

800 - 1000° C


103 to 105 Pa s






Pink/White Subduction zone plutons

65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na

650 - 800° C

High (1024 Pa s)

105 to 109 Pa s


Interlocking SiO2

  Exrtrusive rocks cool at the surface, and have small crystals invisible to the naked eye.  Intrusive rocks cools slowly at depth, with time to form large crystals visible to the naked eye.

Pink fields are the common igneous rocks, and their plate settings.  The others are much less common, or essentially do not occur in nature.

Melting temperature depends on water content and pressure as well as composition.

The composition of the igneous rocks always gives percentages of the oxides--oxygen is by far the most common element.  The most common oxide is is silica (SiO2), but only in the felsic rocks is the silica in the form of quartz, and thus quartz only forms on or under the continents.

MORB (mid ocean ridge basalt) is often considered the most abundant rock on earth, but because it is underwater except for badly fragmented bits of ocean crust in ophiolites, it has less fame than continental rocks.

Last revision  1/8/2019