Answer
The four basic compositional groups of igneous rocks, in order from the group with the highest silica content to the group with the lowest silica content, are:
1. Felsic or Silicic: Felsic rocks have the highest silica content among the four groups, typically ranging from 65% to 77% or more. They are rich in light-colored minerals such as quartz, feldspar (orthoclase or plagioclase), and light-colored micas. Examples of felsic rocks include granite and rhyolite.
2. Intermediate: Intermediate rocks have a moderate silica content, ranging from around 53% to 65%. They are composed of a mix of light-colored and dark-colored minerals, including plagioclase feldspar, amphibole, and pyroxene. Examples of intermediate rocks include diorite and andesite.
3. Mafic: Mafic rocks, also known as basic rocks, have a lower silica content compared to felsic and intermediate rocks. Their silica content ranges from about 45% to 52%. Mafic rocks are rich in dark-colored minerals like pyroxene, amphibole, and olivine. Examples of mafic rocks include basalt and gabbro.
4. Ultramafic: Ultramafic rocks have the lowest silica content among the four groups, typically below 45%. They are composed mostly of dark-colored minerals like pyroxene and olivine, with very little feldspar. Ultramafic rocks are rich in iron and magnesium. Examples of ultramafic rocks include peridotite and komatiite.
The classification of igneous rocks into these four compositional groups is based on the relative proportions of different minerals and the overall chemical composition, particularly the silica content. This classification provides a framework for understanding the characteristics, formation, and geological significance of various igneous rocks.
Work Step by Step
The four basic compositional groups of igneous rocks, in order from the group with the highest silica content to the group with the lowest silica content, are:
1. Felsic or Silicic: Felsic rocks have the highest silica content among the four groups, typically ranging from 65% to 77% or more. They are rich in light-colored minerals such as quartz, feldspar (orthoclase or plagioclase), and light-colored micas. Examples of felsic rocks include granite and rhyolite.
2. Intermediate: Intermediate rocks have a moderate silica content, ranging from around 53% to 65%. They are composed of a mix of light-colored and dark-colored minerals, including plagioclase feldspar, amphibole, and pyroxene. Examples of intermediate rocks include diorite and andesite.
3. Mafic: Mafic rocks, also known as basic rocks, have a lower silica content compared to felsic and intermediate rocks. Their silica content ranges from about 45% to 52%. Mafic rocks are rich in dark-colored minerals like pyroxene, amphibole, and olivine. Examples of mafic rocks include basalt and gabbro.
4. Ultramafic: Ultramafic rocks have the lowest silica content among the four groups, typically below 45%. They are composed mostly of dark-colored minerals like pyroxene and olivine, with very little feldspar. Ultramafic rocks are rich in iron and magnesium. Examples of ultramafic rocks include peridotite and komatiite.
The classification of igneous rocks into these four compositional groups is based on the relative proportions of different minerals and the overall chemical composition, particularly the silica content. This classification provides a framework for understanding the characteristics, formation, and geological significance of various igneous rocks.
