rock cycle

The rock cycle is the planet's slowest recycling program. Every stone in your garden is just rock caught at one moment in an endless journey, forever being made, broken, buried, and remade. Rocks come in three families — igneous, born when molten rock cools and hardens; sedimentary, pressed together from sand, mud, and shells; and metamorphic, cooked and squeezed into a new form deep underground — and any one can, given time, turn into any other.

Picture a single mountain peak. Wind and rain crumble it grain by grain; rivers carry the grit to the sea, where it settles, piles up, and hardens into sedimentary rock. Bury that rock deep enough and heat and pressure bake it into metamorphic rock; push it deeper still and it melts, only to rise, cool, and harden as fresh igneous rock — perhaps the start of a brand-new mountain. The whole loop runs on two engines: the Sun, which drives the weather that wears rock down, and Earth's inner heat, which lifts and melts it from below.

The common misconception is that rocks are permanent — "solid as a rock." On a human lifetime they are. But over millions of years there are no final forms and no fixed order: a rock can skip steps, repeat them, or melt back to the start. The atoms in a pebble today may have been seafloor mud, then slate, then granite, many times over.