The Rock Cycle: Igneous, Sedimentary, Metamorphic

One rock's trash is another rock's raw material — the great recycling loop that connects every rock on Earth.

Intro GeologyUni Year 1
⏱️ About 14 min
The Rock Cycle: Igneous, Sedimentary, Metamorphic — illustration
Illustrative image (AI-generated).

Stand on a mountain of granite and you might think rock is permanent. It is anything but. That granite was once molten magma; given time it will weather to sand, wash to the sea, harden into sandstone, get buried and cooked into quartzite, or melt entirely back to magma. No rock is the end of the line. They are all just stops on a slow conveyor belt — the rock cycle — that has been recycling the same material for the entire 4.54-billion-year life of the planet.

💡
The big idea: The rock cycle is the set of processes that transform rocks among three families — igneous, sedimentary, and metamorphic. Magma cools into igneous rock; weathering and erosion break any rock into sediment that lithifies into sedimentary rock; heat and pressure alter any rock into metamorphic rock; and deep burial can melt any rock back into magma. It is a network of paths, not a single loop — a rock can take many routes, and matter is endlessly reused rather than created or destroyed.
🎯 By the end, you'll be able to
  • Name the three rock families (igneous, sedimentary, metamorphic) and state how each forms
  • Trace a single rock through several possible paths in the rock cycle, naming the process at each step
  • Identify the key processes that connect the families: crystallisation (cooling), weathering–erosion–lithification, metamorphism, and melting
  • Explain why the rock cycle is a network rather than a single fixed loop
📎 Helpful to know first

Three families, one family tree

Every rock on Earth belongs to one of three families, defined by how it formed:

  • Igneous rocks form when molten rock (magma below ground, lava above) cools and crystallises (solidifies). Granite and basalt are igneous.
  • Sedimentary rocks form when sediment — broken fragments of older rock, or minerals precipitated from water — is deposited in layers and lithified (compacted and cemented into solid rock). Sandstone, shale, and limestone are sedimentary.
  • Metamorphic rocks form when an existing rock is transformed by heat, pressure, or chemically active fluids — in the solid state, without melting. Marble (from limestone) and schist are metamorphic.

The crucial point: a rock is never stuck in its family. Each family can turn into either of the others.

The rock cycle as a network The rock cycle — a network, not a single loop cool & crystallise weather & erode lithify heat & pressure heat & pressure weather melt Magma Igneous Sediment Sedimentary Metamorphic Solid arrows: the classic loop · dashed/extra arrows show a rock can take many paths — matter is recycled, not destroyed.
The rock cycle as a network. Notice the multiple paths — for example, igneous rock can become metamorphic rock directly (heat and pressure), or become sedimentary rock (via weathering and lithification). It does not have to pass through every stage.

The processes that do the work

A handful of geologic processes drive every arrow on the diagram:

  • Crystallisation (cooling): molten magma or lava solidifies into igneous rock. Slow underground cooling grows large crystals (granite); rapid cooling at the surface gives fine or glassy rock (basalt, obsidian).
  • Weathering, erosion, and deposition: rocks at the surface are broken down and their fragments transported and laid down as sediment.
  • Lithification: loose sediment is compacted and glued together (cemented) into solid sedimentary rock.
  • Metamorphism: heat and pressure rearrange a rock's minerals without melting it, producing metamorphic rock.
  • Melting: buried rock, heated enough, melts back into magma — closing the loop.
🔑 A network, not a fixed loop
Textbook diagrams can make the rock cycle look like a tidy circle: igneous → sedimentary → metamorphic → igneous. Real rocks rarely follow that neat route. An igneous rock might be metamorphosed directly, then melted, without ever becoming sedimentary. A metamorphic rock might be uplifted and weathered into sedimentary rock. The cycle is a web of possible paths, and any given atom of rock may travel only part of it over the planet's lifetime.

Where the cycle begins

The rock cycle has no true beginning, but on the early Earth the first crust had to be igneous — it solidified directly from a magma ocean, with no older rocks yet existing to weather or metamorphose. So igneous rock is the natural starting point, and magma is the great recycler at the centre: given enough heat and time, any rock can be returned to magma and reborn.

📝 Worked example: Trace one possible journey of an igneous granite through the rock cycle until it becomes a metamorphic rock, naming each process.
  1. Start: igneous granite (cooled from magma).
  2. Uplift exposes it; it is weathered and eroded into sand grains (mostly quartz), which are transported and deposited as sediment.
  3. The sediment is buried and lithified into the sedimentary rock sandstone.
  4. Still deeper burial subjects the sandstone to heat and pressure, metamorphosing it into the metamorphic rock quartzite.
  5. (If buried and heated further still, the quartzite could eventually melt, returning to magma — and the cycle continues.)
✓ granite (igneous) → [weather, erode, deposit] → sandstone (sedimentary) → [metamorphose] → quartzite (metamorphic).
✨ The same atoms, recycled
Nothing is created or destroyed in the rock cycle — matter is only rearranged. The silicon and oxygen atoms in a granite countertop may once have flowed as lava, lain as sand on a beach, or sat in the shell of a long-extinct creature. The rock cycle is the Earth's ultimate recycling system, and most of the rock near the surface has been through it many times.

Check your understanding

1. Which process turns sediment into sedimentary rock?
Sediment becomes sedimentary rock by lithification: the loose grains are compacted under burial and cemented together by minerals precipitating between them.
2. A metamorphic rock is buried deeper and heated until it melts, then cools. What is it now?
Melting produces magma, and magma that cools and solidifies forms igneous rock — regardless of what the rock was before. Melting resets the rock to igneous.
3. Why is the rock cycle better described as a network than a single fixed loop?
Rocks are not forced through every stage in order. Many transitions are possible between any two families, so the cycle is a web of paths a rock may or may not travel.
🔎 Related lessons
Previous: From Atoms to Minerals to Rocks Next: How Geologists Think Igneous Rocks & Volcanism (Module 4)
✅ Key takeaways
  • Rocks belong to three families — igneous (cooled from magma/lava), sedimentary (lithified sediment), and metamorphic (altered by heat/pressure in the solid state).
  • The rock cycle is the set of processes connecting them: crystallisation, weathering–erosion–lithification, metamorphism, and melting.
  • Magma is the great recycler: any rock, melted and re-cooled, becomes igneous rock.
  • The cycle is a network of possible paths, not a single fixed loop — a rock need not pass through every stage.
  • Nothing is created or destroyed; the same atoms are endlessly rearranged, and the earliest crust was necessarily igneous.
➡️ We now have Earth's layers, its building blocks, and the cycle that recycles them. The last step of this opening module zooms out to the single idea that ties all of geology together — and that organizes every module that follows.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of earth as a planet & geologic materials.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.