What is Granite?
Granite is a coarse-grained, intrusive igneous rock. It forms deep within the Earth’s crust when magma cools slowly over millions of years — slow enough that large mineral crystals have time to grow. The result is a visibly crystalline rock with interlocking grains of quartz, feldspar, and mica that you can see with the naked eye.
Key Idea: Slow cooling = large crystals. Fast cooling = small crystals. This is the fundamental rule of igneous texture.
Granite is felsic — high in silica (SiO₂, typically 65–77%) and aluminium, low in iron and magnesium. This makes it lighter (less dense) than basalt, which is why continental crust, largely composed of granitic rock, floats higher than oceanic basaltic crust.
Mineral Composition
Granite is defined by its essential minerals:
- Feldspar (50–60%) — both potassium feldspar (pink/white, orthoclase) and plagioclase feldspar (white/grey). Gives granite its dominant colour.
- Quartz (20–30%) — glassy, grey, extremely hard (Mohs 7). Fills the spaces between feldspars.
- Mica (5–15%) — either muscovite (silver, glittery) or biotite (black, dark). Splits into flat sheets.
Key Idea: If you can see quartz and two types of feldspar, you are almost certainly looking at granite or a close relative.
Accessory minerals — zircon, apatite, hornblende — appear in small quantities and give different granite varieties their distinct appearance.
How Does Granite Form?
Granite forms from magma that intrudes into the continental crust and cools at depth. This creates a pluton — a large body of solidified igneous rock. Where multiple plutons coalesce into a huge mass, the structure is called a batholith.
The Dartmoor granite in Devon, England, is the surface exposure of a single vast batholith that intruded during the Variscan orogeny (~300 million years ago). The overlying rock has since been eroded away, exposing the granite at the surface.
The cooling process:
- Magma intrudes into country rock
- Heat slowly dissipates over millions of years
- Minerals crystallise in sequence (see Bowen’s Reaction Series)
- Final result: interlocking mosaic of crystals — the phaneritic texture
Granite Landforms
When erosion removes the rock above a batholith, granite is exposed. Its resistance to erosion (due to hardness and lack of foliation) produces distinctive landforms:
- Tors — rounded, isolated stacks of jointed granite (e.g., Haytor, Dartmoor)
- Exfoliation domes — smooth, rounded domes formed by pressure release as overlying rock erodes (e.g., Half Dome, Yosemite)
- Moorland — shallow, acidic soils over granite produce open, boggy upland landscapes
Key Idea: Granite’s joint pattern (sets of vertical and horizontal cracks) controls how it weathers and what landforms develop.
Key Terms
- Intrusive rock — igneous rock that cooled inside the crust
- Felsic — high silica and aluminium content, light-coloured
- Phenocrysts — large crystals in an igneous rock
- Batholith — a large mass of intrusive igneous rock
- Silica content — SiO₂ percentage; controls magma viscosity and rock type
- Feldspar — the most common rock-forming mineral group
- Quartz — pure SiO₂; the hardest common mineral in granite
- Mica — sheet silicate minerals; muscovite and biotite