Left - click to enlarge image - texture and mineral composition of common igneous rocks:
Increasing to the left: SiO2 content, viscosity; Increasing to the right: darkness, mafic : felsic composition, (Fe, Mg, Ca) : (K, Na) ratio, temperature of melting:
◘ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava),
◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt
phaneritic texture, emplaced as magma (plutonic)
◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite
0-100: percentage mineral content:
◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite
◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine
Igneous rocks, predominantly plutonic, comprise about 95% of the Earth's crust, though their burial by sedimentary rocks and association with metamorphic rocks disguises their true extent.
The crystalline basement rock (shield) at the core of most continents is ancient, having arisen predominantly during a period from 3.0 to 2.5 billion years ago, which was the period of maximum continent formation. Most of the earliest rocks have been greatly altered through regional metamorphic processes, but later rocks (3.2-2.5 Ga) are mostly pillow-basalts that formed beneath the vast oceans. Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.