Igneous rocks are one of the three main types of rocks, formed by the cooling and solidification of molten rock material called magma (if beneath the surface) or lava (if erupted onto the surface). They can form either below the Earth's surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. Intrusive rocks cool slowly underground, allowing large crystals to form, while extrusive rocks cool quickly on the surface, resulting in fine-grained or glassy textures.
Formation and Types
- The word "igneous" comes from the Latin word "igneus," meaning fire, reflecting their fiery origin.
- Magma forms by partial melting of rocks in the mantle or crust, often due to temperature increase, pressure decrease, or change in composition.
- Intrusive igneous rocks include granite and gabbro; extrusive rocks include basalt and rhyolite.
- Textures vary from coarse-grained phaneritic (large crystals) to fine-grained aphanitic (small crystals) and glassy.
- Some rocks have a porphyritic texture, with large crystals (phenocrysts) embedded in a finer-grained matrix, indicating two-stage cooling.
Composition and Classification
- Mostly composed of silicate minerals like feldspars, quartz, olivine, pyroxenes, amphiboles, and micas.
- Classified broadly into felsic (high silica, light colored) and mafic (low silica, dark colored) based on mineral content and silica percentage.
- Mafic rocks are rich in magnesium and iron; felsic rocks have higher amounts of silica, sodium, and potassium.
- The silica content generally ranges between 45% to 78% by weight.
Geological Significance
- Igneous and metamorphic rocks make up about 90-95% of the Earth's crust by volume.
- Provide insight into the composition and conditions of Earth's mantle and crust.
- Used for radiometric dating and tectonic studies.
- Some host important mineral deposits like tungsten, tin, uranium, chromium, and platinum.
In summary, igneous rocks are formed by the cooling of molten material, have diverse textures and compositions, and play key roles in understanding Earth's geological processes and history.
