Basalt
Lava that made it to the surface and cooled fast — dark, fine-grained, and unusually generous to whatever grows on it.
How it forms
Magma rich in iron and magnesium reaches the surface and spreads as a flow. The top chills into a crust while the inside is still moving; as the mass finally sets it contracts and cracks, often into the five- and six-sided columns that make basalt cliffs look built rather than grown.
Igneous — cooled from molten
Rock that was liquid. Cooled slowly at depth it grows crystals big enough to see; cooled fast at the surface the crystals never get the chance, and the rock comes out fine and dark. Everything else on this page started as one of these.
- What it is
- The commonest rock on Earth's surface, and the floor of every ocean. It is dark and fine-grained because it cooled in the open air or under water in days rather than millennia, leaving crystals too small to see.
- How to know it
- Dark grey to black, heavy for its size, and so fine-grained you cannot pick out a single crystal. Often pocked with small round holes where gas bubbles were trapped. Columns, when you find them, are unmistakable.
- How it wears
- Faster than most hard rock, and generously — it is full of iron, magnesium and calcium, which are exactly what plants want. The iron rusts, so weathered basalt goes red-brown at the surface.
- What it means above ground
- Deep, dark, fertile soil where it has had time to rot, and stepped terraces and cliffs where flows stacked one on another. Basalt country is famously good farmland once the rock has broken down.
- Where you stand on it
- The Columbia River flood basalts of Washington and Oregon, the Snake River Plain, the Cascades, Hawaii entirely, and the ancient flows along Lake Superior's north shore.
- What lives on it
- Rich mixed forest, or grassland and orchard where the climate is dry. The Columbia Basin's fruit and wine country is basalt soil.
- What it's used for
- Crushed for road base and railway ballast, cut for paving setts, and spun into rock wool insulation.
- What it does to the soil
- Neither sweetens nor sours — the soil above takes its pH from elsewhere.
Next to it in the cycle
Granite
the same heat, cooled slowly at depth instead
Volcanic rock
the wider family basalt belongs to
Schist
what basalt can become under pressure
Is this what you're standing on?
Find the rock under your town
Enter a town or postal code and we'll show you the rock beneath it and how old it is, the soil it makes, and the chain of rivers your water joins on the way to the sea.
General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.