Igneous rocks

Obsidian is volcanic glass: lava that cooled so fast its atoms had no time to line up into crystals.

The word igneous comes from the Latin for fire. Igneous rocks form when hot, molten rock crystallizes and solidifies. The melt starts deep in the Earth, near the edges of tectonic plates or above hot spots, and rises toward the surface. Below the surface the melt is called magma; once it erupts it is called lava. Where it finally cools decides what kind of rock it becomes.

Cooling speed decides crystal size

As a melt cools, atoms start to lock together into mineral crystals. Given time, more atoms join each crystal and it grows. Most magma never reaches the surface. It stays trapped underground, where the rock around it is warm, and cools very slowly over many thousands or millions of years. The crystals have a long time to grow, so the rock is coarse grained: you can see separate grains with the naked eye. These are intrusive, or plutonic, rocks. Granite, diorite, gabbro and peridotite are examples.

Lava that pours out at a volcano or an open fissure meets air or water far colder than itself and solidifies almost at once. Crystals have little time to grow, so extrusive, or volcanic, rocks are very fine grained, and sometimes glassy with no crystals at all. Obsidian is that glass. Gas bubbles trapped in quickly chilled lava leave holes, a texture geologists call vesicular. Rhyolite, andesite, basalt and obsidian are common extrusive rocks.

Why we can see rocks that formed deep down

If granite forms kilometres underground, how can it make up mountains and city kerbs? The BGS answer is time. Granite intrusions did cool slowly at depth, but over millions of years the land above them was pushed up and the covering rock was weathered and eroded away. What we see today is the roots of old magma chambers brought to the surface. Smaller bodies of magma that squeeze into cracks and cool near the surface form thin sheets, such as the basalt dykes that cut across other rocks on BGS maps.

Reading an igneous rock is a matter of two questions. Are the grains big enough to see? Then it cooled slowly, underground. Are they too small to see, or is the rock glassy or full of holes? Then it cooled fast, at or near the surface. Colour can help too: the BGS describes granite as usually light in colour with clearly visible crystals. Colour alone can mislead, though, so a geologist checks the minerals as well.

Using the diagram

Open the rock cycle above and tap Cooling. The panel describes both outcomes: slow cooling deep down and fast cooling at the surface. Then tap Igneous rock and read its list of next steps. An igneous rock does not stay put. It can be weathered into sediment once it reaches the surface, or it can be buried and squeezed into a metamorphic rock without ever seeing daylight. Press Trace a path a few times, starting from Magma, and count how often the route passes through an igneous rock before it returns to magma.