How rocks form

Three classes

Rocks are classified by how they are sourced and formed: sedimentary, igneous and metamorphic.^[1,2]

Igneous: a record of cooling rate

Igneous rocks form when molten rock crystallises. Where it solidifies decides everything: magma trapped at depth cools over thousands to millions of years and grows large crystals; magma erupted at the surface solidifies almost instantly.^[1,2]

The texture is therefore a direct measurement of process rate. Granite, gabbro and peridotite are coarse-grained intrusive rocks. Rhyolite, andesite and basalt are fine-grained extrusive ones, and obsidian is glass — cooled too fast for any crystal to form.^[1,2]

Sedimentary: a record of transport

Sedimentary rocks form from pre-existing rocks or from the remains of organisms, out of deposits accumulating at the surface, and they commonly show layering.^[1]

Clastic rocks follow one connected pathway: weathering loosens fragments, transport carries them to a basin, burial compacts and cements them into rock. Their names come from grain size — clay to shale, silt to siltstone, sand to sandstone, pebbles to conglomerate — so the name records the energy of the water that carried the material.^[1,2,3]

Not all sedimentary rock is clastic. Limestone can form from accumulated organic remains, or by precipitating directly out of water: two routes to one rock.^[1]

Metamorphic: a record of conditions

Metamorphic rocks began as something else — igneous, sedimentary, or an earlier metamorphic rock — and were changed by high heat, high pressure, hot mineral-rich fluids, or a combination, in conditions found deep in the crust and where plates meet.^[1,2]

Crucially, metamorphism does not melt the rock. It transforms it into something denser and more compact and grows new minerals; a rock hot enough to melt becomes igneous instead.^[1]

Foliation records the direction pressure came from: platy minerals rotate into alignment under directed stress. Where the minerals are not platy, as in limestone, no foliation develops however hard the rock is squeezed. Limestone becomes marble; shale becomes slate.^[1,2,3,4]

Heat alone can do it too: where hot igneous rock intrudes into cooler rock, the surrounding material is baked without any added pressure.^[1]