The Rock Cycle

The rock cycle is the long, slow loop by which rocks change from one type to another over millions of years. Rocks are not made once and left alone forever. Igneous, sedimentary and metamorphic rocks are constantly being recycled: melted, cooled, weathered, buried, heated, lifted and broken down all over again. The cycle is powered by the Sun (which drives weathering and erosion), gravity (which pulls eroded material into rivers and seas), and Earth's internal heat (which drives plate tectonics and melts rocks underground). One rock cycle can take tens of millions of years to complete.

  • Three rock typesIgneous, sedimentary, metamorphicAll linked by the cycle
  • Powered bySun, gravity and Earth's heat
  • Typical full cycleTens of millions of yearsSome bits much faster, others much slower
  • Most abundantIgneous rockAbout 90% of crust by volume
  • Most visibleSedimentary rockAbout 75% of land surface
  • Most resilient grainQuartzSurvives many cycles

The four main processes of the rock cycle

  1. Melting: any rock can be melted by enough heat to become magma. When magma cools, it becomes igneous rock.
  2. Weathering and erosion: any rock at the surface can be broken down by wind, rain, frost, plant roots and chemistry. The bits are carried away by rivers, glaciers and wind, eventually settling somewhere new.
  3. Burial and compaction: settled bits get buried under more layers of sediment, eventually being squeezed and cemented into sedimentary rock.
  4. Heat and pressure: any rock buried deep enough is heated and squeezed, recrystallising into metamorphic rock. If the heat and pressure get extreme enough, the rock melts back to magma and the cycle begins again.

A typical journey through the cycle

One possible journey for a chunk of rock might be:

  1. Magma cools deep underground to form granite (igneous).
  2. Millions of years later, tectonic forces push the granite up to form mountains. Wind and rain slowly break it down into bits of sand and clay.
  3. The sand and clay wash into a river, which carries them to the sea. They settle on the seabed and slowly become sandstone (sedimentary).
  4. Continental drift carries the sandstone deep underground at a plate boundary. Heat and pressure cook it into quartzite (metamorphic).
  5. If pushed deeper, the quartzite melts back into magma, completing the cycle.

But the cycle has no fixed order. Igneous rock can be cooked into metamorphic rock without ever becoming sedimentary. Sedimentary rock can melt back to magma. Metamorphic rock can be broken down and become sedimentary again. There are many possible paths, and a single chunk of rock might go through several before its atoms end up somewhere else.

How long does the cycle take?

Each step takes its own time.

  • Cooling magma into igneous rock: days for fast surface lava, millions of years for deep magma chambers.
  • Weathering rock into sediment: hundreds to tens of thousands of years.
  • Transport by rivers: weeks to years.
  • Lithification into sedimentary rock: thousands to millions of years.
  • Burial and metamorphism: millions to hundreds of millions of years.
  • Tectonic uplift back to the surface: tens to hundreds of millions of years.

So a typical full rock cycle might take tens to hundreds of millions of years. A grain of sand on your local beach is probably the result of multiple cycles over billions of years.

Fact Almost every quartz grain on every beach in the world has been through many rock cycles. Quartz is one of the toughest common minerals: it resists weathering and chemical attack, so it survives being broken down and re-formed many times. A typical quartz sand grain may be over 3 billion years old, having spent that time inside several different rocks at different times, with brief intervals as a free grain on a beach.

The rock cycle never stops

Earth's internal heat (from radioactive decay deep in the planet, and from leftover heat from when the planet formed) keeps the rock cycle running. As long as Earth has enough internal heat to drive plate tectonics, the cycle continues. Eventually, billions of years from now, Earth's core will cool enough that tectonic activity stops and the rock cycle effectively ends. But that is far in the future. For now, every rock is part of an ongoing cycle.

Did you know? Some scientists think other planets and moons may have their own kinds of rock cycle. Mars appears to have had a rock cycle in the distant past but lost it as the planet cooled. Venus has a strange "stop-start" cycle where the entire surface seems to resurface itself every few hundred million years. The icy moons Europa (Jupiter) and Enceladus (Saturn) have an "ice cycle" instead of a rock cycle: surface ice slowly recycles into the moons' subsurface oceans and back. Each is a different version of the same underlying process.
Deeper dive: tracing the journey of a single atom through deep time

One of the most beautiful ideas in geology is that the atoms in any rock today have been in countless different places over billions of years. Every atom of iron in a building beam, every atom of silicon in a window, every atom of calcium in a marble statue has its own deep time history.

Take a simple atom of silicon in a quartz grain on your local beach. Roughly 4.5 billion years ago, that atom was inside a star that had reached its old age and was puffing off its outer layers. The star's death dispersed the atom into space, where it joined a cloud of gas and dust. About 4.5 billion years ago, the cloud collapsed to form our Sun and a disc of planet-forming material. The silicon atom became part of the rocky inner planets.

On Earth, the atom may have spent a billion years as part of a deep mantle rock. Then it was carried up to the crust as part of an igneous intrusion 3 billion years ago. Weathering eventually broke that rock down, and the atom (locked into a quartz grain) was washed into the sea. It became part of a sandstone, then was buried, then was metamorphosed into quartzite, then was uplifted, then weathered again, then carried by a river to the sea, then back into a beach.

By the time the atom is rolling around on your beach, it has probably been in several different rocks over hundreds of millions of years. The same atoms that built the dinosaurs' bones, the trilobites' shells and the pyramids of Egypt are still part of today's rocks, soils and seas. Nothing is ever quite thrown away in geology. The rock cycle just keeps rearranging the same atoms into new forms, over and over again.

For more, see igneous rocks, sedimentary rocks and metamorphic rocks.