The Rock Cycle

You've collected all three rock types: igneous, sedimentary, and metamorphic. Now here's the plot twist. They're not stuck that way. Any rock can turn into any other rock, given enough time. It's called the rock cycle, and Earth has been running it for about 4 billion years.

No rock is ever really new. The granite on a countertop might once have been sand on an ancient beach, or lava, or deep inside a mountain. Rocks are the planet's oldest recyclers.

Think of these as the moves in the game:

• Melting: heat deep underground turns rock into magma.
• Cooling and crystallizing: magma or lava hardens into igneous rock.
• Weathering and erosion: wind, water, ice, and even plant roots break rock into sediment and carry it away.
• Deposition: sediment settles and piles up, usually in water.
• Compaction and cementation: sediment gets pressed and glued into sedimentary rock.
• Heat and pressure: rock gets squeezed and baked into metamorphic rock.
• Uplift: forces inside Earth push buried rock back up to the surface, where the cycle can start again.

Let's follow a single rock around the cycle:

1. Magma rises under Idaho and cools into granite (igneous).
2. Millions of years later, the granite gets pushed up into mountains. Rain, ice, and wind break it into sand.
3. A river carries the sand to a lake, where it settles in layers and hardens into sandstone (sedimentary).
4. Plates collide, burying the sandstone deep. Heat and pressure turn it into quartzite (metamorphic).
5. It gets pushed deeper still and melts back into magma. Then it's back to step 1.

Here's the catch: there's no set path. A rock can skip steps, go backward, or take shortcuts. Igneous rock can go straight to metamorphic. Metamorphic rock can weather into sediment. It's less of a circle and more of a choose-your-own-adventure.

Two engines keep it running:

• Earth's internal heat: melts rock, drives metamorphism, and pushes rock upward through plate movement.
• The Sun: powers the weather and water cycle that wear rocks down at the surface. Gravity helps by pulling sediment downhill.

When you think about it, the rock cycle ties all four spheres together. Rocks (geosphere), water and ice (hydrosphere), wind and weather (atmosphere), and roots and living things (biosphere) all play a part.

Mostly, painfully slow. A full trip around the cycle can take hundreds of millions of years. But some parts are fast. A lava flow can become solid rock in hours. A landslide can move tons of sediment in seconds.