What is the rock cycle?
The rock cycle describes how Earth’s rocks are continually changed from one type into another. There are three main types: igneous, sedimentary and metamorphic. No rock is created from nothing or destroyed: the same material is recycled again and again by processes on the surface and deep inside the Earth.
Igneous rocks
Igneous rocks form when magma (molten rock underground) or lava (molten rock at the surface) cools and crystallises. Intrusive igneous rocks such as granite and gabbro cool slowly underground and have large crystals you can see. Extrusive rocks such as basalt and rhyolite cool quickly at the surface, so their crystals are tiny; lava that cools almost instantly forms volcanic glass (obsidian). The ocean floor is made mostly of basalt, and the continents contain a lot of granite.
Sedimentary rocks
Weathering breaks rocks into fragments and dissolved minerals; erosion carries them away by rivers, glaciers, wind and waves. When the sediment settles, usually in lakes and seas, it builds up in layers. Over time the weight of new layers compacts the older ones and minerals cement the grains together. Sandstone (from sand), shale (from mud) and limestone (largely from shells) are common examples. Sedimentary rocks cover about three quarters of the land surface and hold almost all fossils.
Metamorphic rocks
When rocks are buried deeply or caught between colliding plates, high temperature and pressure change their minerals and texture without melting. This is metamorphism. Limestone becomes marble, sandstone becomes quartzite, and shale becomes slate, then schist, then gneiss as conditions get more extreme. Pressure often lines minerals up in layers, a texture called foliation.
The processes
| Process | Changes | Typical time |
|---|---|---|
| Weathering and erosion | Rock → sediment | Thousands to millions of years |
| Deposition, compaction, cementation | Sediment → sedimentary rock | Millions of years |
| Heat and pressure | Any rock → metamorphic rock | Millions to tens of millions of years |
| Melting | Any rock → magma | Depends on depth and heat |
| Cooling and crystallisation | Magma → igneous rock | Days (lava) to a million years (large bodies of magma) |
| Uplift | Buried rock → surface | Millions of years |
What drives the cycle
Two engines power the rock cycle. Energy from the Sun drives the weather and water cycle that weather and erode rocks at the surface. Heat from Earth’s interior drives plate tectonics, which buries rocks, melts them, builds mountains and lifts deep rocks back to the surface. Without plate tectonics, Earth’s surface would gradually wear flat.
Identifying rocks
- Interlocking crystals with no layers: probably igneous. Large crystals mean slow cooling.
- Grains, layers or fossils, sometimes crumbly: probably sedimentary. Limestone fizzes in dilute acid.
- Bands, sheets or shiny aligned crystals: probably metamorphic.
Interesting facts
- The oldest known pieces of Earth are tiny zircon crystals from the Jack Hills of Western Australia, about 4.4 billion years old. They survived as grains inside much younger sedimentary rock.
- Marble is metamorphosed limestone, and slate is metamorphosed shale: the same atoms, rearranged by heat and pressure.
- Most rock cycle changes take thousands to millions of years, but a volcanic eruption can create new igneous rock in a few days.
- Pumice is an igneous rock so full of gas bubbles that it can float on water.
Key terms
- Weathering / erosion
- Breaking rock down where it is / carrying the pieces away.
- Lithification
- Turning sediment into rock by compaction and cementation.
- Intrusive / extrusive
- Igneous rock that cooled underground / at the surface.
Common misconceptions
- “Rocks always go igneous → sedimentary → metamorphic in order.” There are many shortcuts in every direction.
- “Metamorphic rocks are melted rocks.” Metamorphism happens in the solid state; melting makes magma.
- “Rocks never change.” They change constantly, just very slowly.
Key takeaways
- Igneous rocks come from cooling magma; sedimentary rocks from compacted sediment; metamorphic rocks from heat and pressure.
- Cooling speed controls crystal size in igneous rocks.
- Any rock type can become any other through different processes.
- The Sun’s energy and Earth’s internal heat keep the cycle going.
Model note: real rocks follow many more paths and form mixtures; the times shown are rough orders of magnitude.