Formation of Rocks

Igneous rocks: granite and basalt

Igneous rocks form when molten rock cools and crystallises into a mass of interlocking crystals. The molten rock is called magma while it is below the surface and lava once it has erupted onto the surface. Nothing else decides the texture of an igneous rock so much as how fast it cools, because crystals need time to grow.

Granite forms from magma that cools slowly, deep underground, where the surrounding rock keeps the heat in. Slow cooling gives the atoms time to arrange themselves into large crystals, so granite is coarse-grained: you can see the crystals of quartz, feldspar and mica with the naked eye. A rock formed underground like this is intrusive. It reaches the surface only later, when uplift and the removal of the rock above expose it, which is why granite appears in high moorland and in cliffs such as those of Cornwall and Devon.

Basalt forms from lava that erupts at the surface and cools quickly, so the crystals are tiny and the rock looks almost uniform and dark, a rock formed at the surface is extrusive. Successive lava flows build up thick plateaux, as in the Deccan Traps of India, and most of the ocean floor is basalt. Where a thick flow cools and shrinks it may crack into columns, as at the Giant's Causeway in Northern Ireland. Escaping gas can leave small bubbles in the lava, giving a rock full of holes.

Both are impermeable: the crystals grow into each other and leave no connected pore spaces, so water cannot pass through the body of the rock. Water can only travel along joints, the cracks produced as the rock cooled and as it was later unloaded. Granite resists weathering strongly because it is hard and has few joints, and it weathers to a thin, stony and acid soil, while basalt breaks down more quickly in a warm wet climate into a clay-rich soil, which is why the Deccan plateau is heavily farmed.

• Granite: magma cooled slowly deep underground, so the crystals are large; intrusive.
• Basalt: lava cooled quickly at the surface, so the crystals are tiny; extrusive.
• Both are impermeable, with water moving only along joints and cracks.
• Slow cooling gives large crystals; fast cooling gives small crystals or glass.

Sedimentary rocks: limestone, sandstone and shale

Sedimentary rocks are built from material that was broken off older rocks, moved, dropped and then buried and turned solid. Every one of them records the same chain of events: weathering, erosion, transportation, deposition, sedimentation, compaction and cementation.

Limestone is mostly calcium carbonate. It forms mainly on the floor of warm, shallow seas, where the shells and skeletons of corals, molluscs and microscopic organisms collect as a limy mud, although some limestone is precipitated straight out of calcium-rich sea water. Chalk is a very pure limestone made almost entirely of the plates of microscopic algae. Limestone is permeable because water passes through its pore spaces and along its joints and bedding planes, and because the weak carbonic acid in rainwater slowly dissolves the calcium carbonate, widening those cracks into caves, sinkholes and underground drainage. That is what produces karst scenery such as the tower karst of Guilin in Guangxi, China.

Sandstone is made of grains of sand, mostly quartz, released when older rocks are weathered. Rivers or wind carry the grains away and deposit them in layers called beds. Buried under later layers, the sediment is compacted, and dissolved silica, calcite or iron oxide in the water between the grains crystallises and cements them together. Iron oxide gives sandstone its red or brown colour. The spaces between the grains survive and remain connected, so sandstone is porous and permeable and makes an excellent store for groundwater.

Shale is made of clay and silt, the finest of all sediment. Particles this small settle only in very still water, so shale forms from mud laid down on the floor of a lake, lagoon or deep sea, or on a floodplain. Compaction presses the tiny plate-like clay particles tightly together and drives the water out. The pore spaces left are extremely small and are not connected to one another, so shale is impermeable, and the alignment of the clay gives the rock a fabric that splits into thin sheets.

Metamorphic rocks: marble and slate

Metamorphic rocks form when an existing rock is changed by heat and pressure without melting. The rock stays solid, but its minerals recrystallise into new interlocking crystals, and the pressure usually makes them line up. Which metamorphic rock you get depends on the rock you started with, on how hot it became and on how much pressure it was under.

Marble is formed from limestone. During a plate collision, or where limestone is baked by a nearby body of magma, the calcium carbonate recrystallises into a mosaic of interlocking calcite crystals. Marble is harder and denser than the limestone it came from, is impermeable, and contains no fossils because recrystallisation destroys them. It takes a high polish and resists weathering, which is why it has been quarried for sculpture and for building stone since Roman times, most famously around Carrara in the Apuan Alps of Italy.

Slate is formed from shale or mudstone, at lower heat and pressure than marble needs. Under pressure the plate-like clay minerals recrystallise and grow at right angles to the direction the pressure is coming from, giving the rock a slaty cleavage. The result is a rock that splits into thin, flat, strong sheets, which is exactly what is wanted for roofing tiles, and it is dense and impermeable, so it keeps water out of a roof as well as out of the rock. The slate quarries of Blaenau Ffestiniog in north Wales once roofed much of Britain, and the waste tips are still part of that landscape.

Notice that the chemistry of the rock hardly changes: limestone and marble are both calcium carbonate, and shale and slate are both made of the same clay minerals. Only the texture, and with it the hardness, density and permeability, has changed. Learn each pair together, because reversing them is one of the commonest errors in this topic.

• Limestone -> marble: baked and squeezed so the calcite recrystallises; harder, denser, impermeable, no fossils.
• Shale or mudstone -> slate: low-grade heat and pressure align the clay minerals, so the rock splits into thin sheets.
• Both are impermeable, because the crystals interlock and leave no connected pore spaces.
• Metamorphism changes the texture and properties of a rock, not its chemical make-up.

The rock cycle: one set of processes, three rock types

The rock cycle is the set of processes that turn one rock type into another over millions of years. It has two sources of energy. Heat from inside the Earth drives plate movement, uplift, metamorphism and melting, while energy from the Sun drives the weather that breaks rock down and the water, wind and ice that move it. Follow the arrows on any rock-cycle diagram and you are following those two energies at work.

Rock exposed at the surface is broken down in place by weathering, and the loosened fragments are then eroded and transported downhill by a river, a glacier, the wind or the waves. When the moving agent loses energy, the load is deposited, and the grains settle and accumulate in layers, which is sedimentation. Deeper in the pile, the weight of everything above squeezes the grains together and drives the water out (compaction), and minerals dissolved in the remaining water crystallise and glue the grains together (cementation), so loose sediment becomes sedimentary rock.

Everything is then buried further, and burial or a plate collision subjects the rock to heat and pressure, which makes it metamorphic. Still deeper, or close to magma, it melts to form magma, and when that magma cools and crystallises it becomes igneous rock. Uplift and the erosion of the rock above bring the new rock back to the surface, and the story starts again. There is no fixed order in the cycle: a metamorphic rock may simply be weathered away and rebuilt into a sedimentary rock without ever melting, and sediment weathered from sandstone may form a new sandstone.

When you are asked to interpret a rock-cycle diagram, read the arrows as verb phrases. Decide for each one whether the rock has to be at the surface (weathering, erosion, transportation, deposition), buried and under pressure (compaction, cementation, heat and pressure) or molten (crystallisation). Almost every arrow on the diagram tests one of those three ideas.

Permeability: which rocks let water through

Permeability is the ability of water to pass through the pore spaces of rock and soil. It is one of the most useful properties a rock can have, because it decides whether rain sinks into the ground or runs off the surface, whether a rock can store water for wells and boreholes, how quickly pollution can travel underground, whether a slope drains or becomes waterlogged, and where a landfill or a reservoir can safely be placed.

Permeable rocks include most sedimentary rocks, and the syllabus names limestone and sandstone. Sandstone is permeable because the spaces between its grains are connected; limestone is permeable through its pore spaces and, more importantly, through joints, bedding planes and the channels dissolved by weak acid. Impermeable rocks include igneous rocks such as granite and basalt, metamorphic rocks such as marble and slate, and some sedimentary rocks, of which shale is the example. In all of these the crystals, or the minute clay particles, fit so closely together that there are no connected spaces, so water cannot pass through the rock itself and moves only along joints and cracks.

There are two traps here. The first is that porous is not the same as permeable: a rock can hold water in its pore spaces and still not let it through, because the pores are not connected, and that is exactly what happens in shale. The second is that hardness tells you nothing about permeability: marble and limestone are both made of calcium carbonate, but marble is impermeable and limestone is permeable, and chalk looks solid yet soaks up water. If you are asked to classify a rock, think about the pore spaces and the cracks, not about how strong it looks.

Permeability matters most where a permeable rock lies on top of an impermeable one. Rain soaks down through the permeable rock until it reaches the impermeable layer, where it collects, filling every pore and crack and forming an aquifer. Where the junction comes out at the surface, water escapes as a spring, which is why a line of springs often marks the boundary between a sandstone and a shale. A borehole sunk into the permeable rock can then be pumped, which is how a great many villages and cities obtain their water.

Why rock type matters: water supply, land use and management

Permeable rocks are the world's natural water stores, so the management of water supply begins with geology. Sandstone and limestone basins are tapped by wells and boreholes, and abstraction is licensed so that pumping does not exceed the rate at which rain refills the store. The limitation is that this balance is easily broken: if abstraction is faster than recharge, the water table falls, springs and shallow wells dry up, the ground may subside, and on a coast salt water is drawn in from the sea and ruins the supply. Once nitrate from fertiliser or fuel from a leaking tank has entered a permeable rock it travels quickly through the joints, and cleaning an aquifer is far more expensive than preventing the pollution in the first place.

Where an impermeable rock lies under the soil, water cannot drain downwards, so it collects above the junction. Flat ground on shale or clay becomes waterlogged after heavy rain, and on a slope the water running along the top of the impermeable layer can lubricate it and trigger a landslide. Drainage ditches, French drains and the avoidance of building on steep clay and shale slopes are the usual answers, but they all cost money, need maintaining, and can only take the water away; they cannot make the rock permeable.

Rock type also drives building and waste management. Granite, slate and marble are impermeable and hard, which is exactly what is wanted in a wall, a roof or a kerb, while sandstone and limestone are quarried for building stone, lime and cement. Impermeable clay is engineered into the base of landfill sites to stop leachate, the polluted liquid draining from the waste, from reaching the groundwater. The limitation is that every one of these choices trades one problem for another. A clay liner only slows water down and can dry out and crack; a quarry removes the permeable rock, the soil and the habitats above it; the stone is valuable, but the aquifer it sat in may be lost with it. That trade-off, not the rock itself, is what examiners want you to discuss.

• Permeable sandstone and limestone store groundwater; impermeable shale and clay hold it at the surface.
• Over-abstraction from an aquifer lowers the water table, dries up springs and allows salt water into coastal wells.
• Pollution travels fast through permeable rock, and groundwater is difficult and costly to clean.
• Landfills are lined with impermeable clay, but a liner can crack, tear or simply stop being maintained.