Rock cycle

The rock cycle is the continuous natural process by which rocks of one type are changed into another type through geological processes over millions of years. Any rock may become any other, and no rock is permanent.

            MAGMA <-----------------+
              |                     |
              | cooling             | melting
              v                     |
           IGNEOUS                  |
              | weathering,         |
              | erosion, deposition |
              v                     |
          SEDIMENTS                 |
              | compaction,         |
              | cementation         |
              v                     |
         SEDIMENTARY                |
              | heat, pressure      |
              v                     |
         METAMORPHIC ---------------+

Sequence

1. Igneous rocks form when magma or lava cools and solidifies (granite, basalt).
2. Exposed rocks are broken into fragments by weathering and erosion; these are transported by water, wind and ice and deposited as sediments.
3. Sediments undergo compaction and cementation (lithification) into sedimentary rocks (sandstone, shale).
4. Under high heat, pressure and chemical fluids, igneous or sedimentary rocks recrystallise to metamorphic rocks (gneiss, schist, slate), without melting.
5. If temperature rises further, metamorphic (or any) rocks melt into magma, which cools again as igneous rock.

Shortcuts

Igneous rocks may be metamorphosed directly; metamorphic and sedimentary rocks may be weathered into new sediments; and uplift brings buried rocks to the surface.

The cycle is driven by the Earth's internal heat and by solar energy through the water cycle.