The Rock Cycle & Earth's Structure (Years 7–10) — Science

Earth's layered structure in brief, the three rock types — igneous, sedimentary and metamorphic — and how the rock cycle transforms one into another over geological time.

Pick up almost any rock and you are holding a chapter of Earth's history — some rocks are frozen droplets of ancient magma, some are compressed layers of mud that once settled on a seabed, and some have been squeezed and cooked deep underground until they became something else entirely. None of these forms are permanent: given enough time, every rock type can transform into another.

Why this matters Rocks are the raw material of the planet's surface — building stone, metal ores, fossil fuels and fossils themselves are all found within specific rock types. Reading a rock correctly tells geologists how it formed, how old it might be, and even what the climate or environment was like when it was made.

Earth's layered structure — brief context

Before looking at rocks themselves, it helps to know roughly where they sit. Earth is built of layers: a thin, solid outer crust (the only layer directly studied by picking up rocks), a thick mantle of hot, semi-molten rock beneath it, and a core at the centre, made mostly of iron and nickel, that is liquid in its outer part and solid in its inner part. Almost every rock studied in school geology comes from the crust — the mantle and core are far too deep and hot to sample directly.

The crust is a thin solid skin; almost all rock studied at the surface comes from this outer layer.

The three rock types

Type	How it forms	Typical features
Igneous	Magma or lava cools and solidifies	Interlocking crystals; large crystals if cooled slowly underground, tiny crystals if cooled quickly at the surface
Sedimentary	Layers of sediment are compacted and cemented together	Formed in layers; often contains fossils; grains are usually rounded and visible
Metamorphic	Existing rock is changed by heat and pressure, without melting	Often has new banded or flattened crystal patterns; original rock structure altered

Igneous rock forms wherever molten rock cools: granite forms slowly deep underground, giving crystals time to grow large; basalt forms quickly at the surface from lava, giving tiny crystals. Sedimentary rock is built over millions of years as rock is broken down by weathering and erosion, the fragments are transported and deposited as sediment, and then buried, compacted and cemented into solid rock — this slow layer-by-layer burial is exactly why sedimentary rocks like limestone can preserve fossils. Metamorphic rock forms when existing rock — igneous, sedimentary, or even older metamorphic rock — is subjected to intense heat and pressure deep underground, changing its structure and crystals while staying solid: limestone becomes marble, mudstone becomes slate, and shale can become schist.

The rock cycle

None of these three rock types is permanent. Given the right conditions and enough time — often millions of years — each type can be transformed into either of the others. This ongoing transformation is the rock cycle.

Weathering and erosion turn any rock into sediment; burial under heat and pressure compacts sediment into sedimentary rock or metamorphoses existing rock; enough heat melts rock to magma, which cools to form new igneous rock.

Worked example. Trace one possible journey through the rock cycle, starting with granite (an igneous rock) exposed at the surface. Step 1: weathering and erosion break the granite down into loose sediment (sand and clay-sized particles). Step 2: rivers transport this sediment and deposit it in layers, e.g. on a seabed. Step 3: as more layers pile on top, pressure compacts the sediment and minerals cement the grains together, forming a new sedimentary rock. Step 4: if this sedimentary rock is later buried very deep by further geological activity, heat and pressure (without melting) transform it into a metamorphic rock. Step 5: if it is pushed deeper still and fully melted, it becomes magma again, which can cool to form new igneous rock — completing the cycle.

Common mistake: saying metamorphic rock is formed by "melting". It is not — metamorphic rock forms from intense heat and pressure while remaining solid throughout. If a rock actually melts, the molten material is magma, and whatever solidifies from it afterwards is igneous rock, not metamorphic.

Memory hook: "Igneous Is from heat" (in-from-melt), "Sedimentary Settles in layers", "Metamorphic is Made without melting". And remember the cycle has no fixed starting point or single direction — any rock type can eventually become either of the others.