The rock cycle is the never-ending process by which Earth's three rock families, igneous, sedimentary and metamorphic, slowly turn into one another. Melted rock cools into igneous rock, weathering breaks any rock into grains that pile up and harden into sedimentary rock, heat and pressure deep underground squeeze rock into metamorphic rock, and if it gets hot enough it melts and the cycle starts again.
Three Rock Families: Igneous, Sedimentary and Metamorphic
Every rock on Earth belongs to one of three families, and the family tells you how the rock was made.
Igneous rocks: born from fire
"Igneous" comes from the Latin word for fire. These rocks form when melted rock cools and hardens. If it cools slowly, deep underground, big crystals have time to grow and you get a speckled rock like granite. If lava pours out of a volcano and cools in hours or days, the crystals are too small to see and you get dark basalt, the rock that makes up most of the ocean floor and the islands of Hawaii. Cool it so fast that no crystals form at all and you get obsidian, a natural black glass.
Sedimentary rocks: built from layers
Wind, rain, rivers and ice grind rocks down into sand, mud and pebbles called sediment. Rivers carry the sediment to lakes and seas, where it settles in flat layers. Over millions of years the weight of new layers presses the old ones flat, and minerals in groundwater glue the grains together. Sand becomes sandstone, mud becomes shale, and the shells and skeletons of tiny sea creatures become limestone and chalk. Sedimentary rocks are the only family that can hold fossils.
Metamorphic rocks: changed by heat and squeezing
"Metamorphic" means changed in form. When a rock is buried many kilometres down, or caught between two colliding continents, heat and pressure rearrange its crystals without melting it. Limestone becomes marble, shale becomes slate, sandstone becomes quartzite, and granite becomes a banded rock called gneiss.
Melting, Cooling, Crushing: How One Rock Becomes Another
The arrows on the diagram are the important part. Each one is a process, and any rock can take any path.
- Weathering and erosion break rock apart. Water freezes in cracks and splits boulders, roots pry stones open, and rivers tumble pebbles into sand. This can happen to any of the three families.
- Deposition, compaction and cementation turn loose sediment into sedimentary rock as layers pile up and get glued together.
- Heat and pressure turn any buried rock into metamorphic rock.
- Melting turns rock into magma if it is hot enough, usually more than about 700 to 1,300 degrees Celsius depending on the rock.
- Cooling and crystallisation turn magma or lava back into igneous rock.
Follow one journey. A granite mountain in the Rockies is slowly worn down by rain and frost. Its quartz grains wash down a river to the coast and settle as a beach. Buried under later sand, the beach hardens into sandstone. Millions of years later that sandstone is dragged deep under a growing mountain range and baked into quartzite. If the mountain-building goes deep enough, the quartzite melts, rises as magma and cools into new granite. The journey can then begin all over again.
Rocks can also skip steps. Sedimentary rock can be lifted straight back to the surface and eroded into new sediment without ever being cooked. Metamorphic rock can be re-squeezed into a different metamorphic rock. The only rule is that there are no dead ends.
How Long Does One Trip Around the Cycle Take?
Usually a very, very long time. The oldest known rocks on Earth, the Acasta Gneiss in northern Canada, are about 4 billion years old. Tiny zircon crystals found inside sandstone at Jack Hills in Western Australia are even older, at about 4.4 billion years, and they survived being recycled through several rounds of the cycle. Earth itself is around 4.54 billion years old, so the cycle has been running almost since the planet formed.
Some steps are quick. In Hawaii, lava flowing into the sea makes brand-new igneous rock every day. A flash flood can lay down a layer of mud in an afternoon. But turning that mud into shale takes millions of years of burial, and turning shale into slate takes millions more. The limestone near the summit of Mount Everest formed on a sea floor roughly 450 million years ago and was pushed almost 9 kilometres into the sky when India collided with Asia, a collision that began about 50 million years ago and is still going.
So when you pick up a pebble on a beach, you might be holding something that has been a mountain, a sea floor and a volcano, several times each.
Rocks You Can Spot at Home and Outside
- Granite (igneous): kitchen worktops, the cliffs of Yosemite in California, and many old bridges and kerbstones. Yosemite's granite cooled underground around 100 million years ago.
- Basalt (igneous): the hexagonal columns of the Giant's Causeway in Northern Ireland, formed from lava about 60 million years ago.
- Pumice (igneous): the grey foot-scrubbing stone in the bathroom. It is frothy lava so full of gas bubbles that it floats on water.
- Sandstone (sedimentary): Uluru in central Australia, a single block of sandstone about 550 million years old, and the red rocks of Arizona and Utah.
- Chalk (sedimentary): the White Cliffs of Dover in England, made from the shells of plankton that lived 70 to 100 million years ago. Blackboard chalk is usually gypsum, a different sedimentary mineral.
- Limestone (sedimentary): the Great Pyramid of Giza, most caves, and many school buildings.
- Marble (metamorphic): the Taj Mahal in India, built from Makrana marble, and Michelangelo's statues, carved from Carrara marble quarried in Italy.
- Slate (metamorphic): old roof tiles and the original school slates children wrote on.
A full list of rock types, with pictures and how to tell them apart, is in our guide to the types of rocks.
The Rock Cycle Compared With the Water Cycle
You probably already know the water cycle: water evaporates from the sea, falls as rain, flows down rivers and returns to the sea. The rock cycle works on the same idea, a loop with no beginning or end, but with three big differences.
- Speed. A water droplet can go round its whole cycle in about nine days. A grain of rock may take hundreds of millions of years.
- Energy source. The water cycle is powered by the sun, which lifts water into the air. The rock cycle is powered by two engines: the sun and gravity drive weathering and erosion at the surface, while heat from deep inside Earth drives melting, metamorphism and the movement of tectonic plates underneath.
- Changing substance. Water stays water whether it is ice, liquid or vapour. Rock changes its minerals and crystals completely as it moves round the cycle. A piece of marble is not just squashed limestone; its crystals have regrown into something new.
The two cycles also feed each other. Rain and rivers are the main tools that break rock down and carry sediment, so without the water cycle the sedimentary half of the rock cycle would barely work. You can read about the faster loop in our water cycle for kids guide.
Who Figured Out the Rock Cycle?
In the 1700s most people in Europe believed Earth was only about 6,000 years old and that rocks had been made once and never changed. A Scottish farmer and doctor named James Hutton disagreed. He noticed that the rivers on his farm carried soil to the sea a little at a time, and worked out that if that process made rock, it would need an almost unimaginable amount of time.
In 1788 Hutton took friends by boat to Siccar Point, on the coast east of Edinburgh. There, flat layers of red sandstone lie on top of older grey layers that stand nearly on end. Hutton explained that the grey rock had been laid down flat, tilted and lifted into mountains, worn away, and then buried under new sand, all before the red sandstone formed. One of his companions wrote that "the mind seemed to grow giddy by looking so far into the abyss of time". Hutton summed it up by saying he could see "no vestige of a beginning, no prospect of an end". That is the rock cycle in one sentence, and Hutton is remembered as the father of modern geology because of it.
Make Your Own Rock Cycle
You can run the whole cycle on a kitchen table with a box of wax crayons.
- Weathering: use a pencil sharpener or grater to shave three colours of crayon into flakes. These are your sediment.
- Sedimentary rock: pile the flakes in layers inside a square of foil, fold it up and press down hard with your hands. Open it and you have a layered "sandstone" that crumbles easily.
- Metamorphic rock: refold the foil and press it under a heavy book, or stand on it, and dip the packet in warm (not hot) water for a minute. The flakes squash and swirl together into a harder, streaky rock, just as real rock does under heat and pressure.
- Igneous rock: with an adult, melt the packet in a dish of hot water until the wax runs, then let it cool. The crystals and layers vanish and you get one solid lump with no structure at all.
Shave the igneous lump back into flakes and you are at the beginning again. Real volcanoes do the melting step on a much larger scale, as our post on how volcanoes form explains, and real rivers do the weathering, as you can see in erosion for kids.