Rocks formed when magma solidifies are called igneous rocks. Their types, textures, and mineral compositions differ greatly depending on cooling rate and chemical composition, especially SiO₂ content.
Basic Knowledge
When magma cools rapidly at or near Earth's surface, it forms volcanic rocks; when it cools slowly deep underground, it forms plutonic rocks. Because volcanic rocks solidify before crystals have time to grow, they show a porphyritic texture, in which phenocrysts are mixed into a fine-grained groundmass. Plutonic rocks show a equigranular texture (holocrystalline texture), with large crystals packed uniformly together.
Classification by SiO₂ content (from high to low):
・Felsic rocks (SiO₂ > 66%): volcanic rock = rhyolite; plutonic rock = granite. Light-colored, highly viscous, explosive eruptions
・Intermediate rocks (52〜66%): volcanic rock = andesite; plutonic rock = diorite
・Mafic rocks (45〜52%): volcanic rock = basalt; plutonic rock = gabbro. Dark-colored, low viscosity, gentle eruptions
・Ultramafic rocks (SiO₂ < 45%): peridotite. A major rock of the mantle
As SiO₂ content decreases, the proportion of dark-colored ferromagnesian minerals rich in Fe and Mg, such as pyroxene and olivine, increases, making the rock darker. Many Japanese volcanoes erupt andesitic magma.
Explain the differences between granite and basalt in SiO₂ content, color, major minerals, and occurrence.
Answer: Granite: SiO₂ > 66% (felsic rock), light-colored (white to gray), mainly composed of quartz, potassium feldspar, plagioclase, and biotite, and is a plutonic rock (equigranular texture). Basalt: SiO₂ 45〜52% (mafic rock), dark-colored (black to dark green), mainly composed of pyroxene, olivine, and plagioclase, and is a volcanic rock (porphyritic texture).
- Volcanic rocks (rapid cooling): porphyritic texture (phenocrysts + groundmass)
- Plutonic rocks (slow cooling): equigranular texture (large crystals distributed uniformly)
- Felsic rocks (SiO₂ > 66%): rhyolite, granite, light-colored, highly viscous
- Intermediate rocks (52〜66%): andesite, diorite
- Mafic rocks (45〜52%): basalt, gabbro, dark-colored, low viscosity
- Ultramafic rocks (< 45%): peridotite, main component of the mantle
- Volcanoes in Japan: mostly andesitic magma
Points to Note
① Felsic and mafic here are geological terms referring to the amount of SiO₂, not chemical acidity or basicity. ② Granite (a plutonic rock) and rhyolite (a volcanic rock) have the same composition but different textures; remember their correspondence. ③ Basalt is associated with oceanic crust and Hawaiian-type eruptions, whereas rhyolite is associated with continental areas and explosive eruptions.
Practice
- What classification does granite belong to (volcanic or plutonic), and what is its range of SiO₂ content?
- Why do volcanic rocks show a porphyritic texture?
- Describe the viscosity of magma with low SiO₂ content (mafic magma).