Rock Cycle
The rock cycle is the continuous set of processes by which igneous, sedimentary, and metamorphic rocks form, break down, and change into one another.
Definition
The rock cycle is the never-ending way rocks change from one kind to another: they melt, cool, break into pieces, pile up, get squeezed, and change again over millions of years. It links the three rock types through processes, since melting and crystallization form igneous rock, weathering, erosion, deposition, and lithification form sedimentary rock, and heat and pressure form metamorphic rock. Any rock type can become any other, and there is no required order; plate tectonics drives the cycle by uplifting, burying, and melting rock. This is the geosphere's material cycle, powered by internal heat (mantle convection, radioactive decay) and external energy (solar-driven weather and gravity). Its rates span orders of magnitude: a lava flow crystallizes in days, sediment accumulates at millimeters per year, and continental crust has a mean residence time on the order of a billion years. Subduction recycles oceanic lithosphere into the mantle within about $200$ million years, so no oceanic crust older than that survives.
Example
A mountain of granite crumbles into sand, the sand becomes sandstone, and buried deep and heated the sandstone becomes quartzite; if it melts and cools, it becomes igneous rock again. The Appalachians were once as tall as the Rockies, and weathering and erosion carried their sediment east to build the Coastal Plain and continental shelf, while the metamorphic roots of the old range are now exposed in the Blue Ridge. Zircon crystals in Western Australia's Jack Hills sandstone are $4.4$ billion years old, older than the sandstone that holds them by billions of years. They survived multiple passes through the cycle because zircon resists weathering and melting.
Key Insight
No rock is permanent, and the pebble in your hand has probably been several different kinds of rock before this one. The cycle has no start or finish and no single path: sedimentary rock can be weathered straight back into sediment without ever becoming metamorphic or igneous. The rock cycle is also coupled to the carbon cycle. Weathering of silicate rock draws down atmospheric $CO_2$, which is buried as carbonate sediment and returned by volcanic outgassing after subduction, a feedback that has regulated Earth's climate over geologic time.