Basalt

Fine-grained mafic extrusive volcanic rock of calcic plagioclase and pyroxene forming Earth's oceanic crust.

Specifications & Metrics

type: Igneous Rock
composition: Calcic plagioclase feldspar (labradorite), augite pyroxene, olivine, and magnetite/ilmenite
texture: Aphanitic (fine-grained), vesicular, or porphyritic; often forms hexagonal columns or pillows
silica content: 45–52 wt% SiO₂ (mafic)
density: ~2.90 g/cm³
tectonic setting: Mid-ocean ridges (MORB), oceanic islands, and continental flood-basalt traps

Basalt accounts for more than 90% of all volcanic rock on Earth, forming the upper oceanic crust via decompression melting of upper-mantle peridotite along mid-ocean ridges (MORB) as well as volcanic islands and lunar maria.
Because its low silica content (45–52 wt% SiO₂) gives molten basalt a low viscosity, subaerial flows spread rapidly into vast flood-basalt provinces (such as the Siberian and Deccan Traps) and contract upon cooling into polygonal columnar joints.
When erupted underwater at oceanic spreading centers, basalt lava chills instantly on contact with seawater into bulbous 'pillow basalts', while deep-seated slow cooling of the same magma produces coarse-grained gabbro.