What is Basalt?
Basalt is a fine-grained, extrusive igneous rock. It forms when mafic lava erupts onto the Earth’s surface and cools rapidly — within hours to days. The rapid cooling means crystals have almost no time to grow, resulting in a rock so fine-grained that individual minerals are invisible to the naked eye.
Key Idea: Basalt and granite have similar overall chemistry within the broader mafic/felsic spectrum — but completely different textures. Texture tells you where the rock cooled, not what it’s made of.
Basalt is mafic — rich in magnesium and iron, low in silica (~45–52% SiO₂). This low silica content makes basaltic magma less viscous than granitic magma, allowing it to flow great distances before solidifying. Hawaiian lava flows vividly demonstrate this property.
Mineral Composition
The essential minerals of basalt:
- Pyroxene — dark, iron/magnesium silicate. The dominant mineral. Augite is the most common variety.
- Plagioclase feldspar — calcium-rich (labradorite/bytownite). Appears as tiny white crystals or laths.
- Olivine — green, iron-magnesium silicate. Common in the most primitive (least evolved) basalts.
Basalt is dark grey to black in colour — a direct reflection of its high iron and magnesium content and the absence of pale quartz and potassium feldspar.
Key Idea: Dark colour = mafic = high iron and magnesium = oceanic crust basalt. Light colour = felsic = high silica and aluminium = continental crust granite.
Types of Basalt and Lava Forms
Not all basalt looks the same. The eruption style and cooling environment produce very different structures:
Pahoehoe and Aa
Two Hawaiian terms now used worldwide:
- Pahoehoe — smooth, ropy surface. Forms from slow-moving, low-viscosity lava.
- Aa — rough, jagged, clinker surface. Forms from faster-moving, more gas-rich flows.
Pillow Lava
When basaltic lava erupts underwater — at mid-ocean ridges — it chills instantly. The outer surface solidifies while the interior is still molten, creating bulbous, pillow-shaped structures. Pillow lavas are diagnostic of submarine eruption and are found throughout ophiolite sequences exhumed onto land.
Vesicular Basalt
Gas bubbles in the lava that don’t escape before solidification leave vesicles — small spherical holes. If these vesicles are later filled with secondary minerals (calcite, zeolites, quartz), they become amygdales, and the rock is described as amygdaloidal basalt.
Columnar Basalt
When a thick lava flow or sill cools uniformly, contraction creates polygonal (usually hexagonal) columns perpendicular to the cooling surface. The Giant’s Causeway in Northern Ireland is the most famous example.
Global Importance of Basalt
Basalt underlies the entire oceanic crust — roughly 70% of the Earth’s surface. It is continuously produced at mid-ocean ridges as mantle melts and erupts, then spreads laterally. This is seafloor spreading, the engine of plate tectonics.
On continents, flood basalts (or large igneous provinces) record episodes of extraordinary volcanic activity — the Deccan Traps, the Siberian Traps, the Columbia River Basalts. These events, involving millions of cubic kilometres of lava, have been linked to mass extinctions and rapid climate change.
Key Idea: Basalt is not just a rock type — it is the material from which most of the outer shell of the Earth is made, and its production at mid-ocean ridges drives the entire plate tectonic system.
Key Terms
- Extrusive rock — igneous rock that cooled on the Earth’s surface
- Mafic — high iron and magnesium, low silica content
- Vesicles — gas bubble holes preserved in volcanic rock
- Pillow lava — rounded lava forms produced by submarine eruption
- Oceanic crust — the basaltic layer of crust beneath the oceans
- Olivine — magnesium-iron silicate; first mineral to crystallise from mafic magma
- Pyroxene — dark, calcium/iron/magnesium silicate; dominant in basalt
- Flood basalt — massive outpouring of basaltic lava covering huge areas