Basalt
The dark fine-grained volcanic rock that forms the ocean floor — by area the most abundant rock at the Earth's surface.
Basalt is what most of the Earth's surface is actually made of. It floors every ocean, and though granite dominates the continents, the ocean basins are larger — which makes basalt the commonest rock on the planet by area by a wide margin.
It is the fine-grained equivalent of gabbro: the same magma composition, cooled fast enough at the surface that crystals had no time to grow visible. That fine grain is the identifying feature and the practical one, because it makes basalt tough, dense and awkward to break along any preferred direction.
How it forms
Basalt erupts from partial melting of the mantle, most prolifically at mid-ocean ridges where plates pull apart and new sea floor is continuously created. It also builds shield volcanoes such as those of Hawaii and Iceland, and floods enormous areas in the rare events that produce continental flood basalts.
Cooling geometry produces its most recognisable form. A thick flow contracting as it cools cracks into polygonal columns perpendicular to the cooling surface, which is what makes the Giant's Causeway and similar formations look built rather than erupted.
Uses
Crushed basalt is a premium aggregate — hard, tough and angular, which is what a road base and a railway ballast need. It is used as concrete aggregate where strength matters and as armour stone on coastal defences.
Melted and spun it becomes basalt fibre, used as a reinforcement and insulation material and marketed as an alternative to glass fibre. Basalt is also the rock most studied for mineral carbonation, because its calcium and magnesium silicates react with dissolved carbon dioxide to form solid carbonate — a route to permanent carbon storage rather than mere containment.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Olivine — mineral · an early-crystallising constituent of basaltic magma
- Plagioclase — mineral · calcium-rich plagioclase is a defining constituent of basalt
- Sapphire — mineral variety · carried up as xenocrysts in alkali basalt and recovered from the gravels the basalt weathers into, rather than mined from the rock itself
- Amethyst — mineral variety · lining vugs and geodes in weathered basalt flows, which is where nearly all of the world's amethyst comes from — the cavity is a gas bubble the lava left behind
is used in
- Construction — industry · crushed as a premium aggregate for road base and ballast
is an input to
- Quarrying — process · for aggregate rather than dimension stone — hard, tough and angular, which is what a road base needs
is sourced from
- Peridotite — rock · partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt
is a source for
- Bauxite — rock · the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several
- Mineral wool — material · melted at around 1500 °C and flung into fibres by spinning wheels
Sources
- Material WorldOur own writing