How igneous rock forms from volcanoes
Chemistry · Key Stage 2 · The School
Rock that was once liquid
Deep underground, rock gets so hot it melts. This melted rock is called MAGMA. When magma reaches the surface, we call it LAVA. As lava or magma cools, it turns solid again. This new rock is IGNEOUS rock. The word means "from fire".
Cooling fast, cooling slow
Lava that cools fast on the surface makes tiny crystals, or none at all. Basalt is like this. Magma that cools slowly underground makes big crystals. The pieces have more time to grow. Granite is like this. Same start, different speed, different rock.
Suppose a class has two igneous rocks, but the labels fell off. One is basalt and one is granite. The crystals are the clue to how each one cooled. Step 1: they look closely. The first rock has crystals too small to see. The second rock has crystals about 6 millimetres wide, easy to see. Step 2: the slower a rock cools, the bigger its crystals grow. So the second rock cooled more slowly, deep underground. The first rock cooled fast, in the open air. Step 3: they check real figures. The Geological Society says basalt crystals are usually under 1 millimetre. It says rock from large masses of magma that cooled underground has crystals over 5 millimetres. The first rock fits the first figure. The second rock has 6 millimetres, which is more than 5, so it fits the second. Step 4: slow cooling makes granite and fast cooling makes basalt. So crystal size can tell these two apart. Some other rocks have big crystals too. So the first rock is basalt and the second rock is granite. The labels go back on.
Volcanoes make igneous rock today
Volcanoes are still doing this now. When a volcano erupts, magma reaches the surface as lava. The lava cools fast in the open air. Much of it turns to basalt. Iceland and Hawaii are built from layer after layer of it. Every eruption adds a fresh layer of new rock. Old islands can even grow bigger this way, one eruption at a time.