There are three types of rock, defined by how they form: igneous rock solidifies from molten magma or lava, sedimentary rock forms when fragments and dissolved minerals accumulate and harden, and metamorphic rock is existing rock transformed by heat and pressure without melting. Every rock on Earth belongs to one of these three families, and the rock cycle continuously converts each into the others.
The Three Families and What Separates Them
The classification is based entirely on formation process, not on appearance, hardness or age. Two rocks that look nearly identical can belong to different families, and two rocks in the same family can look nothing alike.
The proportions are counter-intuitive. Igneous and metamorphic rock together make up roughly 90 to 95 per cent of the Earth's crust by volume. Sedimentary rock accounts for only about 5 to 10 per cent — but it covers around 75 per cent of the continental land surface, forming a comparatively thin veneer over igneous and metamorphic basement. Most rock you actually see at the surface is sedimentary; most rock that exists is not.
A useful diagnostic question for any hand specimen: does it contain interlocking crystals, cemented grains, or aligned bands? Interlocking crystals with no preferred orientation suggest igneous. Visible grains or fragments held in a matrix, or fossils, suggest sedimentary. Minerals flattened into parallel bands or sheets suggest metamorphic.
Igneous Rock: Melt That Cooled
Igneous rock — from the Latin ignis, fire — crystallises from molten rock. The single most important variable is cooling rate, because it controls crystal size.
Intrusive (plutonic) igneous rocks cool slowly deep underground, giving ions time to migrate and build large crystals. The result is coarse-grained rock with crystals visible to the naked eye. Granite is the classic example, typically composed of quartz, feldspar and mica. Gabbro and diorite are its darker, more iron- and magnesium-rich relatives.
Extrusive (volcanic) igneous rocks cool rapidly at or near the surface, leaving no time for large crystals. Basalt, the most abundant rock in the Earth's crust and the floor of every ocean basin, is fine-grained to the point of appearing structureless. Obsidian is the extreme case: cooled so fast that no crystal lattice forms at all, making it technically a volcanic glass rather than a mineral aggregate. Pumice is froth — lava quenched while saturated with gas, leaving a rock full of voids and light enough to float.
Composition matters alongside cooling rate. Silica-rich (felsic) magmas are viscous and produce granite and rhyolite, while silica-poor (mafic) magmas are runny and produce gabbro and basalt. This is why Hawaiian volcanoes ooze basaltic lava in flowing rivers while silica-rich volcanoes such as Mount St Helens erupt explosively — the viscosity traps gas until it fails catastrophically. See our guide to how volcanoes form for more.
Sedimentary Rock: Compressed Debris and Chemistry
Sedimentary rock forms at low temperatures near the Earth's surface through weathering, erosion, transport, deposition and lithification — the compaction and cementation of loose sediment into solid rock. It divides into three subgroups.
Clastic sedimentary rocks are made from physical fragments of older rock, classified by grain size. Conglomerate contains rounded pebbles; breccia contains angular fragments, indicating little transport. Sandstone is sand-sized grains, typically quartz, cemented by silica or calcite. Shale is the finest, formed from clay and silt settling in still water, and is the most abundant sedimentary rock of all.
Chemical sedimentary rocks precipitate directly from solution. Rock salt (halite) and gypsum form as evaporites where enclosed water bodies dry out, which is why thick salt beds indicate ancient shallow seas. Some limestones form chemically as calcium carbonate precipitates.
Organic (biochemical) sedimentary rocks are built from biological remains. Most limestone consists of accumulated shells, coral and marine skeletal debris. Chalk is a fine-grained limestone made largely of coccolith plates from single-celled algae. Coal is compressed plant matter, mostly from Carboniferous swamp forests.
Sedimentary rock is the only family that preserves fossils in useful quantity, because igneous and metamorphic processes destroy organic structures. It is also where essentially all oil, natural gas, coal and groundwater are found, which makes an economically minor fraction of the crust the one that matters most commercially.
Metamorphic Rock: Changed Without Melting
Metamorphism alters a rock's mineralogy and texture in the solid state, under heat, pressure or chemically active fluids. Crossing into melting produces magma and takes the rock back to the igneous family, so metamorphism operates in a window — broadly above about 200 °C but below the melting point of the rock in question.
Regional metamorphism affects enormous volumes of rock during mountain building, where continental collision generates sustained directed pressure. This produces foliation: platy minerals such as mica rotate and grow perpendicular to the pressure, creating the banding or sheeting that makes many metamorphic rocks instantly recognisable. The classic progression is shale to slate to phyllite to schist to gneiss, with grain size and mineral segregation increasing at each step.
Contact metamorphism occurs where magma intrudes cooler rock and bakes a zone around it. Because the pressure is not directional, the result is typically non-foliated: limestone becomes marble, sandstone becomes quartzite, and both develop interlocking recrystallised textures without banding.
Grade is the other axis of description. Low-grade metamorphism, at the cooler end of the range, produces slate — fine-grained, still recognisably derived from mud, and splitting into flat sheets along a cleavage that has nothing to do with the original bedding. High-grade metamorphism produces gneiss, in which minerals have segregated so thoroughly into light and dark bands that the rock is approaching partial melting. Rocks that have crossed that threshold and begun to melt in part are called migmatites, and they sit genuinely on the boundary between the metamorphic and igneous families.
A key point often missed: metamorphism does not change bulk chemistry much. Marble has the same calcium carbonate composition as the limestone it came from; the calcite crystals have simply recrystallised larger and interlocked, destroying any fossils in the process.
The Rock Cycle Connects All Three
The three families are stages in a continuous system rather than fixed categories, and the cycle can run in any order.
Magma cools to igneous rock. Weathering and erosion break it into sediment, which is deposited, buried and lithified into sedimentary rock. Deeper burial or tectonic collision converts that into metamorphic rock. Further heating melts it back to magma. But shortcuts are the norm: metamorphic rock can be uplifted and eroded straight into sediment, sedimentary rock can be melted directly, and igneous rock can be metamorphosed without ever becoming sediment.
The engine is plate tectonics. Subduction drives rock down into zones of high temperature and pressure; collision builds mountains and drives regional metamorphism; rifting and mid-ocean spreading generate new igneous crust; and uplift exposes deep rock to weathering. Without tectonics the cycle would stall, which is broadly what has happened on Mars.
Timescales vary enormously. Volcanic rock forms in hours. Sedimentary sequences accumulate over millions of years — the strata visible in the Grand Canyon span a very large fraction of Earth's history, with a substantial unconformity where hundreds of millions of years of record are simply missing.
Telling Granite from Gneiss in the Field
This is the most common identification error, and the pair makes a good worked example because the two rocks can share nearly identical mineral content — quartz, feldspar and mica — while belonging to different families.
The distinguishing feature is texture, not composition. Granite has a randomly oriented, evenly distributed interlocking crystal fabric: the minerals are scattered with no directional pattern, because they crystallised freely from melt with no directed stress. Gneiss shows gneissic banding — light and dark minerals segregated into alternating layers, often contorted and folded — because directed pressure sorted them.
Two further contrasts worth internalising:
- Limestone vs marble: limestone is dull, often fossil-bearing, and made of fine carbonate grains; marble is sugary and crystalline, with no fossils. Both fizz in dilute acid.
- Sandstone vs quartzite: sandstone breaks around its grains and feels gritty; quartzite breaks straight through them, giving a glassy fracture, because the cement and grains have fused.
Where Each Type Dominates the Map
Rock type governs landscape, soil and land use, so the distribution is visible in the geography of whole regions.
Ancient continental cores — the shields of Canada, Scandinavia, Western Australia and central Africa — are dominated by igneous and metamorphic rock, some of it over 3 billion years old. These regions typically have thin soils, abundant lakes scoured by glaciation, and significant metal ore deposits.
Sedimentary basins cover the great plains and lowlands: the US Midwest, the Paris Basin, the North European Plain, the Ganges basin. These are the world's agricultural heartlands, because sedimentary parent material weathers into deep, fertile soil, and they hold the hydrocarbon reserves.
Metamorphic rock dominates mountain belts and their eroded roots — the Alps, Himalayas, Appalachians and Scottish Highlands — where collision drove burial and deformation, and subsequent erosion has exposed rock once many kilometres deep. To explore related processes, see how mountains are formed, or test your physical geography knowledge in our geography games.