The rock cycle links igneous, sedimentary and metamorphic rocks through the processes of weathering, erosion, transport, deposition, burial and lithification, melting and crystallisation, and metamorphism; the cycle is driven by energy from the Sun (at the surface) and from the Earth's interior (at depth), and any rock can be changed into any other given time and the right conditions.

What this dot point is asking

Eduqas wants you to explain the rock cycle: how the three rock families (igneous, sedimentary and metamorphic) are linked by a continuous set of processes, what those processes are, the two energy sources that drive the cycle, and the central idea that any rock can be turned into any other given enough time and the right conditions. This is the framework that ties together everything you learned about the three rock families, so it is examined as a way of connecting and applying that knowledge rather than just recalling it.

The answer

The three rock families

Every rock belongs to one of three families, defined by how it forms:

• Igneous rocks crystallise from a melt (magma or lava): interlocking crystals, no bedding.
• Sedimentary rocks build up at the surface from grains, fragments or fossils that are deposited then cemented: bedded, often fossil-bearing.
• Metamorphic rocks form when an existing rock recrystallises in the solid state under heat and pressure, without melting: often foliated.

The rock cycle is the model that shows these three are not fixed end points but stages a material can pass through again and again.

The processes that link them

The families are connected by a set of processes, each of which converts one kind of material into another:

• Weathering breaks rock down in place (physical, chemical or biological).
• Erosion and transport wear away and carry the loose fragments (by water, wind or ice).
• Deposition drops the sediment where the carrying energy falls.
• Burial and lithification compact and cement the sediment into solid sedimentary rock.
• Metamorphism recrystallises a buried or compressed rock in the solid state under heat and pressure.
• Melting turns rock into magma when the temperature rises past its melting point.
• Crystallisation forms igneous rock as that magma cools.
• Uplift and exposure raise deep rocks to the surface, where weathering can start the cycle again.

Reading the cycle

The cycle has no fixed start, but a typical loop runs like this. An igneous rock is uplifted and weathered; the fragments are eroded, transported and deposited, then lithified into a sedimentary rock. Deep burial with heat and pressure turns it into a metamorphic rock. Further heating melts it to magma, which crystallises back into an igneous rock. At any point the route can short-circuit: a sedimentary rock at the surface can be weathered straight back to sediment, and a metamorphic rock can be uplifted and weathered without ever melting.

The big idea Eduqas wants is that any rock can become any other: there is a pathway from each family to each of the others, given time and the right conditions.

The two energy sources

The cycle is driven by two energy sources, one at the surface and one at depth:

• Energy from the Sun powers the surface processes. Solar heating drives the weather, the wind and the water cycle, which cause weathering, erosion, transport and deposition.
• Energy from the Earth's interior (heat from radioactive decay and the planet's original heat) powers the deep processes. It drives mantle convection and plate movement, and so burial, metamorphism, melting and the rise of magma.

Gravity assists throughout, pulling eroded material downhill and dense sediment down to be buried.

Examples in context

Example 1. A pebble on a beach. Many beach pebbles are fragments of older rock that have been weathered, eroded and transported. Buried and cemented they could form a conglomerate; the pebble is mid-cycle.

Example 2. Recycled crust at a subduction zone. Where one plate sinks beneath another, sedimentary and igneous rocks are dragged down, heated and partly melted, feeding volcanoes whose lava crystallises into new igneous rock. The cycle runs on plate tectonics.

Try this

Q1. Name the process that turns loose sediment into solid sedimentary rock. [1 mark]

• Cue. Lithification (compaction and cementation).

Q2. Explain why uplift is an important part of the rock cycle. [2 marks]

• Cue. Uplift raises deeply formed rocks (igneous and metamorphic) to the surface, where weathering and erosion can act on them and feed the cycle again.

Q3. State which energy source drives weathering and erosion. [1 mark]

• Cue. Energy from the Sun (it powers the weather and the water cycle).