09/10/2025
CLASSIFICATION OF IGNEOUS ROCKS BASED ON MINERAL COMPOSITION
INTRODUCTION
Igneous rocks are formed through the cooling and solidification of magma or lava. They are classified into different types based on their mineral composition and texture. The classification ranges from felsic (silica-rich) to ultramafic (silica-poor, magnesium- and iron-rich). This compositional variation influences not only the appearance of the rocks but also their physical properties, density, and geological setting.
MINERAL COMPOSITION TRENDS
~ Felsic rocks (e.g., Granite, Rhyolite) are rich in quartz, potassium feldspar, sodium-rich plagioclase, and muscovite. They are light in color and silica-rich.
~ Intermediate rocks (e.g., Diorite, Andesite) contain moderate amounts of plagioclase feldspar, biotite, and amphibole, with less quartz.
~ Mafic rocks (e.g., Gabbro, Basalt) are dominated by pyroxene, calcium-rich plagioclase, and amphibole, giving them darker colors and higher density.
~ Ultramafic rocks (e.g., Peridotite, Komatiite) are composed mainly of olivine and pyroxene, with very low silica content and a greenish to dark coloration.
TEXTURAL CLASSIFICATION
° INTRUSIVE (PLUTONIC) ROCKS: Formed from slow cooling beneath the Earth’s surface, producing coarse-grained textures. Examples include Granite, Diorite, Gabbro, and Peridotite.
° Formed from rapid cooling at or near the surface, resulting in fine-grained textures. Examples include Rhyolite, Andesite, Basalt, and Komatiite.
GEOLOGICAL IMPORTANCE
• Composition determines magma viscosity, eruption style, and tectonic setting.
• Felsic rocks are commonly associated with continental crust, while mafic and ultramafic rocks dominate the oceanic crust and mantle.
Image Credit📷:
R. Weller / Cochise College, K. Panchuk (Source: Diagram showing igneous rock classification by mineral composition).
Geology forum 1
