The rock cycle and rock classification: the threefold classification of rocks into igneous, sedimentary and metamorphic; the processes that link them (crystallisation, weathering, erosion, transport, deposition, compaction and cementation, burial, metamorphism, melting and uplift); and the role of the surface (external) and internal processes driven by solar energy and the Earth's internal heat.

What this dot point is asking

Eduqas wants you to classify rocks into the three families (igneous, sedimentary and metamorphic), to describe the processes that link them in the rock cycle (crystallisation, weathering, erosion, transport, deposition, compaction and cementation, burial, metamorphism, melting and uplift), and to identify the energy sources that drive the surface (external) and internal parts of the cycle. The rock cycle is the framework that ties together every later topic on weathering, sedimentation, metamorphism and igneous activity.

The answer

The three rock families

All rocks belong to one of three families, defined by how they form:

• Igneous rocks form by the crystallisation of magma or lava. They have interlocking crystals and no bedding. Intrusive rocks (granite, gabbro) cool slowly at depth and are coarse-grained; extrusive rocks (basalt, rhyolite) cool quickly at the surface and are fine-grained.
• Sedimentary rocks form by the deposition and lithification of sediment. They are made of grains, fragments or fossils, are usually bedded, and include sandstone, mudstone, limestone and conglomerate.
• Metamorphic rocks form by the recrystallisation of existing rocks in the solid state under heat and pressure, without melting. They may be foliated (slate, schist, gneiss) or non-foliated (marble, quartzite).

The processes that link them

The rock cycle is the set of processes that convert one family into another. Material can move around the cycle by many routes; the main processes are:

• Crystallisation: magma cools and solidifies into igneous rock.
• Weathering: rock at the surface is broken down physically and chemically.
• Erosion and transport: the loosened material is removed and carried by rivers, ice, wind and the sea.
• Deposition: sediment settles out when the transporting energy drops.
• Compaction and cementation (lithification): burial compacts the sediment and mineral cement binds it into sedimentary rock.
• Burial and metamorphism: deeper burial, or contact with an intrusion, raises temperature and pressure and recrystallises the rock into a metamorphic rock.
• Melting: still greater heat melts rock back into magma, completing the cycle.
• Uplift: tectonic forces raise rocks back to the surface, where weathering can begin again.

There is no single fixed path: an igneous rock can be weathered straight to sediment, or buried and metamorphosed, or melted; a sedimentary rock can be uplifted and re-weathered without ever being metamorphosed.

The energy that drives the cycle

The rock cycle is powered by two energy sources:

• Solar energy and gravity drive the external (surface) processes: weathering, erosion, transport and deposition. The Sun powers the water cycle and winds that erode and carry sediment, and gravity moves material downslope and downstream.
• The Earth's internal heat (from the core and radioactive decay) drives the internal processes: metamorphism, melting and the plate-tectonic burial and uplift that move rocks to depth and back. This is the same internal heat that drives plate tectonics.

This division (Sun and gravity outside, internal heat inside) is a favourite short exam question.

Examples in context

Example 1. Quartz, the great survivor. Because quartz resists weathering, it is recycled again and again: from granite, through sandstone, to quartzite and back to sand, which is why it dominates mature clastic sediments.

Example 2. A mountain belt as a rock-cycle machine. In a collision belt, burial drives metamorphism and melting at depth while uplift and erosion strip the top, so all three rock families form and interact in one place.

Try this

Q1. State how each of the three rock families forms in one phrase. [3 marks]

• Cue. Igneous from the crystallisation of magma or lava; sedimentary from the deposition and lithification of sediment; metamorphic from solid-state recrystallisation under heat and pressure.

Q2. Name the processes that turn sediment into a sedimentary rock. [2 marks]

• Cue. Compaction (burial) and cementation, together called lithification.

Q3. State which energy source drives weathering and which drives metamorphism. [2 marks]

• Cue. Weathering is driven by solar energy and gravity (a surface process); metamorphism is driven by the Earth's internal heat.