Volcanic rock
Everything a volcano throws out — lava, ash and fragments — from eruptions a few thousand or a few billion years ago.
How it forms
Magma rises until dissolved gas comes out of solution, the way a shaken bottle does when it is opened. Runny magma lets the gas go quietly and pours out as lava; sticky magma cannot, and blows itself into ash and pumice that settles in beds for hundreds of kilometres downwind.
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
- A catch-all for material erupted at the surface, from solid lava to airborne ash that settled and welded. What you get depends on how runny the magma was: thin magma flows, thick magma explodes.
- How to know it
- Ash beds are pale, soft and layered, sometimes with visible pumice fragments. Solid lava is dark and dense. Both often sit in obvious layers with sharp boundaries, unlike the uniform mass of an intrusive rock.
- How it wears
- Ash rots to soil remarkably quickly — decades to centuries. Solid lava takes far longer.
- What it means above ground
- Fertile, mineral-rich ground in country that is usually dramatic. Volcanic ash holds water far better than its texture suggests, which is why ash soils grow well through dry summers.
- Where you stand on it
- The Cascades, the Aleutians, the Yellowstone plateau, the Jemez of New Mexico, and the ancient volcanic belts of the Shield around Yellowknife and Timmins.
- What lives on it
- Fast-growing forest and productive farmland, colonised in stages — lichen, then moss, then fireweed and alder — after each new flow.
- What it's used for
- Pumice as an abrasive and a soil conditioner; welded ash cut as light building stone; historically, obsidian for blades.
- What it does to the soil
- Neither sweetens nor sours — the soil above takes its pH from elsewhere.
Next to it in the cycle
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.