The rock cycle: HSC Earth and Environmental Science Module 5
“Investigate the rock cycle, including but not limited to the formation of igneous, sedimentary and metamorphic rocks and the processes that transform one rock type into another in the Australian context”
The rock cycle continuously recycles crustal material between three families. Igneous rock forms by cooling magma or lava (granite of the Snowy Mountains, Newer Volcanics basalt). Sedimentary rock forms by weathering, transport, deposition and lithification (Hawkesbury Sandstone, Nullarbor limestone). Metamorphic rock forms by heat and pressure in the solid state (Broken Hill gneiss, Wombeyan marble). Weathering, burial, melting and uplift convert any rock type into any other, driven by internal heat and solar energy.
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What this dot point is asking
NESA wants you to describe the three rock families, explain the processes that form each, and show how any rock can be transformed into any other through the rock cycle. You need the processes (cooling, weathering, deposition, lithification, heat and pressure, melting) and at least one named Australian rock to anchor the answer.
The answer
The rock cycle is the continuous set of processes that creates, destroys and recycles the three rock types: igneous, sedimentary and metamorphic. It is driven by the two energy sources from earlier in this module: internal heat (melting, metamorphism, uplift) and solar energy (weathering, erosion, transport, deposition).
Igneous rocks
Igneous rocks form when molten rock cools and crystallises. If magma cools slowly deep underground, large crystals grow and the rock is coarse-grained, like the granite of the Snowy Mountains and much of the eastern Australian highlands. If lava cools quickly at the surface, crystals are tiny and the rock is fine-grained, like basalt; the basalt flows of the Atherton Tableland and the Newer Volcanics Province of western Victoria are Australian examples. Crystal size therefore records cooling rate.
An igneous rock forms by the cooling and crystallisation of magma or lava. Intrusive (plutonic) rocks cool slowly at depth and are coarse-grained; extrusive (volcanic) rocks cool quickly at the surface and are fine-grained.
Sedimentary rocks
Sedimentary rocks form at the surface in three steps. First, weathering and erosion break existing rock into sediment. Second, water, wind or ice transport and deposit that sediment in layers. Third, burial compacts the grains and dissolved minerals cement them together, a process called lithification. Clastic sedimentary rocks are made of fragments (the Hawkesbury Sandstone that underlies much of Sydney is a clastic rock of ancient river sands). Chemical and biochemical sedimentary rocks precipitate from solution or accumulate from organisms, such as the limestones of the Nullarbor Plain. Sedimentary rocks commonly preserve layering and fossils.
Metamorphic rocks
Metamorphic rocks form when existing rock is altered in the solid state by heat, pressure or chemically active fluids, without fully melting. The minerals recrystallise and often line up to give a banded or foliated texture. Shale can become slate then schist then gneiss as conditions intensify. The high-grade gneisses of Broken Hill in New South Wales are metamorphic rocks formed deep in the crust; the marble quarried at places such as Wombeyan formed by the metamorphism of limestone.
How the cycle connects them
No rock type is permanent. Igneous rock exposed at the surface weathers to sediment that becomes sedimentary rock. Deep burial can metamorphose any rock. Enough heat melts any rock back to magma, restarting the cycle as new igneous rock. Uplift and erosion can expose rock that formed kilometres down, which is why deeply formed granites and gneisses now sit at the surface in Australia. Because Australia is so old and stable, its rocks record many complete turns of this cycle.
Suppose a cliff shows tilted sandstone layers cut across by a dark, fine-grained sheet of basalt. Read the history in order.
Step 1: Identify the rock types and how each formed
The sandstone is a clastic sedimentary rock formed by deposition and lithification of sand; the basalt is a fine-grained extrusive igneous rock formed by rapid cooling of magma.
Step 2: Reconstruct the sedimentary stage
Sand was deposited in horizontal layers in a river or shallow sea, then buried, compacted and cemented to form sandstone, recording the sedimentary part of the cycle.
Step 3: Identify the deforming event
The sandstone beds are now tilted, so tectonic forces deformed them after they had lithified.
Step 4: Apply cross-cutting relationships
The basalt sheet cuts across the tilted beds, so by the principle that a feature cutting another is younger, the basalt intrusion is the youngest event. The outcrop shows the rock cycle and relative dating working together at one location.
- Saying metamorphic rocks melt
- If rock melts completely it becomes magma and forms igneous rock. Metamorphism alters rock in the solid state.
- Forgetting lithification
- Sediment is not yet a rock; it becomes sedimentary rock only after compaction and cementation.
- Treating the cycle as one-way
- Any rock type can become any other. Markers reward showing the alternative pathways, not just a single loop.
Try this
Q1. Describe the three processes by which loose sediment becomes a sedimentary rock. [3 marks]
- Cue. Deposition in layers, compaction by burial, and cementation by minerals precipitating between grains (lithification).
Q2. Explain how a sample of granite could eventually become a metamorphic rock and then an igneous rock again. [4 marks]
- Cue. Burial subjects the granite to heat and pressure, recrystallising it into gneiss; further heating melts it to magma, which cools to form new igneous rock.
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
HSC 2022-style3 marksDescribe the three processes by which loose sediment becomes a sedimentary rock.Show worked answer →
A 3-mark "Describe" wants the three steps of lithification named and explained in order.
First, deposition: water, wind or ice lays the sediment down in layers. Second, compaction: the weight of overlying material squeezes the grains together and reduces pore space as burial continues. Third, cementation: minerals such as silica, calcite or iron oxide precipitate from groundwater between the grains and bind them into solid rock.
Markers reward all three steps in the correct order and the point that sediment is not yet a rock until compaction and cementation (lithification) have occurred.
HSC 2023-style6 marksExplain how a sample of granite could be transformed into a metamorphic rock and then into an igneous rock again, with reference to the rock cycle.Show worked answer →
A 6-mark "Explain" wants the processes that drive each transformation, linked to the energy sources of the rock cycle.
Burial subjects the granite to increased heat and pressure; in the solid state the minerals recrystallise and align, producing a foliated gneiss (metamorphism). Further heating, driven by internal heat, eventually exceeds the melting point, so the rock melts to magma; when that magma cools and crystallises it forms new igneous rock.
The explanatory lift is that no rock type is permanent: internal heat (melting, metamorphism, uplift) and solar energy (weathering, erosion, deposition) recycle any rock into any other. Markers reward the named processes, the solid-state versus molten distinction, and the energy sources.