Intermediate describes the igneous rock composition between mafic and felsic. Mafic rocks are low in silica (in the 45-50% range), but they make up most of the oceanic crust and lithosphere. Such minerals are dark-colored and include pyroxene and olivine. Mafic refers to igneous rocks with an abundance of ferromagnesian minerals (those with the elements Mg and Fe in their chemical formulae) plus plagioclase feldspar. Ultramafic rocks are very poor in silica, in the 40% or less range (this means that the rock would be less than 40 weight percent silica). Ultramafic rocks are rare on the surface, but they make up the primary composition of the upper mantle. These rocks have even more magnesium and iron and even less silica than ‘ordinary’ mafic rocks. Ultramaficrefers to rocks composed of mostly olivine and some pyroxene. Y-axis refers to the abundance of Fe and Mg-containing minerals in each rock type. ![]() Darker rocks are ultramafic or mafic, whereas lighter rocks are intermediate/felsic. Classification of igneous rocks by composition. For an igneous rock, the composition is generally divided into four groups: ultramafic, mafic, intermediate, and felsic. These groups refer to differing amounts of silica (SiO 2), iron (Fe), and magnesium (Mg) found in the minerals that make up the rocks. Igneous Rock CompositionĬomposition refers to a rock’s chemical and mineralogical make-up. ![]() We identify these rocks by their composition and texture. ![]() The rock that results from this is an igneous rock from the Latin word ignis, meaning “fire.” Igneous rocks are traditionally defined as the solid products from the cooling and hardening of molten magma in many different environments. When magma or lava cools, it solidifies by crystallization in which minerals grow within the magma or lava. Magma forms under Earth’s surface at about 800 to 1300☌ in the crust or mantle and erupts on Earth’s surface as lava.
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