Rocks
Three families, and the cycle that turns each into the others.
Rock is a naturally occurring aggregate of minerals. Every rock belongs to one of three families defined by how it formed, and the rock cycle describes how any one can become any other given enough time, heat and pressure.
The three types
| Type | Forms from | Recognise by | Examples |
|---|---|---|---|
| Igneous | Cooling magma or lava | Interlocking crystals, no layers | Granite, basalt, obsidian |
| Sedimentary | Compacted sediment or precipitation | Layers, often fossils | Sandstone, limestone, shale |
| Metamorphic | Existing rock altered by heat and pressure | Banding or foliation, aligned crystals | Slate, marble, gneiss |
Crystal size tells you cooling rate
In igneous rock, slow cooling deep underground grows large crystals — granite. Fast cooling at the surface gives small ones — basalt. Cooling so fast that crystals cannot form at all gives volcanic glass — obsidian. Same chemistry, three textures, and the texture is the record of the history.
The cycle
Igneous rock weathers into sediment, which lithifies into sedimentary rock, which under heat and pressure becomes metamorphic, which if it melts becomes magma and cools back into igneous. There is no fixed order — any step can be short-circuited.