The rock cycle is the slow, continuous process by which rocks are formed, broken down and re-formed as one of three types. Igneous rock forms when molten rock cools: magma underground makes coarse-grained rocks such as granite, and lava at the surface makes fine-grained rocks such as basalt. At the surface, weathering and erosion break rock into sediment, which is deposited in layers, compacted and cemented into sedimentary rock such as sandstone and limestone. Deep underground, heat and pressure change any rock into metamorphic rock without melting it: limestone becomes marble, shale becomes slate. Melting turns any rock back into magma. Plate tectonics drives the cycle, and most of it takes millions of years.

The word that matters is "cycle", but it is not a circle with one direction. It is a set of paths, and any rock type can become any other.

Three rock types, one question each

Each rock type answers one question: how did it form? Learn the process before the names.

Igneous rock: cooling speed decides the crystals

Molten rock below the surface is magma. Once it erupts, it is lava. The same melt makes different rocks depending on where it cools.

• Intrusive (plutonic) igneous rock cools slowly, deep underground, sometimes over thousands of years or longer. Crystals have time to grow large enough to see. Granite is the classic example, with visible pink or white feldspar, glassy quartz and black mica. The moors of Dartmoor in south-west England are the eroded top of a huge granite intrusion.
• Extrusive (volcanic) igneous rock cools quickly at the surface. Crystals are tiny, often too small to see without a lens. Basalt is the classic example: dark, fine-grained, and the rock of the ocean floor. The hexagonal columns of the Giant's Causeway in Northern Ireland are basalt that cracked as a thick lava flow cooled.
• Very fast cooling leaves no crystals at all. Obsidian is volcanic glass. Pumice is frothy lava full of gas bubbles, light enough to float.

The rule for an exam answer: slow cooling, large crystals; fast cooling, small crystals.

Sedimentary rock: five steps in order

Sedimentary rock is built from pieces of older rock, or from the remains of living things. The steps always come in this order.

1. Weathering. Rock is broken down where it is, without being moved. Physical weathering includes freeze-thaw, where water in cracks freezes, expands by about 9 percent and prises the rock apart. Chemical weathering includes slightly acidic rain dissolving limestone. Biological weathering includes plant roots growing into cracks.
2. Erosion and transport. The broken pieces are carried away by rivers, glaciers, wind, waves or gravity. Grains get rounder and smaller the further they travel.
3. Deposition. When the water or wind slows down, it drops its load, usually in a sea, lake or river bed. Layers build up over time, with the oldest at the bottom.
4. Compaction. The weight of the layers above squeezes the lower layers and pushes out water.
5. Cementation. Minerals dissolved in the water, such as silica or calcite, crystallise between the grains and glue them together.

Together, compaction and cementation are called lithification: sediment becoming rock.

There are three main kinds of sedimentary rock:

• Clastic, made of rock fragments: conglomerate (pebbles), sandstone (sand), shale and mudstone (fine mud).
• Biological or organic, made from living things: limestone, mostly from the calcium carbonate shells and skeletons of sea creatures; chalk, a soft limestone made of microscopic plankton, as in the White Cliffs of Dover; coal, from compressed plant material.
• Chemical, crystallised from water: rock salt, left when sea water evaporates.

Sedimentary rocks are where almost all fossils are found, because heat would destroy them in the other two types.

Metamorphic rock: changed, not melted

Metamorphic rock forms when an existing rock, of any type, is buried deep or comes close to magma. Heat and pressure rearrange its minerals into new crystals while it stays solid. If it melts, it is no longer metamorphism: it becomes magma, and the cycle goes back to igneous.

• Contact metamorphism happens where rock is baked by nearby magma, in a ring around an intrusion. Heat is the main agent.
• Regional metamorphism happens over huge areas during mountain building, when plates collide. Heat and pressure act together, and the pressure lines up flat minerals into layers, called foliation.

The shale to slate to schist to gneiss sequence shows the grade of metamorphism rising. It is a good example to learn whole.

Every path, not just the circle

The usual textbook diagram is a triangle with arrows between the three types and magma at the bottom. The important point is that every rock can take more than one route.

Uplift is the step people forget. Rocks formed deep underground only reach the surface to be weathered because tectonic forces push them up and erosion strips away what is above.

The link to plate tectonics

Plate tectonics is the engine of the cycle. Plate tectonics, explained visually covers the plates themselves; here is how each boundary feeds the rock cycle.

• Divergent boundaries, such as the Mid-Atlantic Ridge: magma rises as plates pull apart and cools into new basalt ocean floor.
• Subduction zones, such as the west coast of South America: an ocean plate sinks, partly melts, and the magma rises to form volcanoes such as those of the Andes, making both intrusive and extrusive igneous rock.
• Collision zones, such as the Himalayas: two continents push together, sedimentary rock is folded, buried and turned into metamorphic rock, and mountains are uplifted to be weathered again. Marine limestone with sea fossils is found near the summit of Everest, which shows how far rock can be lifted.

Energy for the cycle comes from two sources: the Earth's internal heat, which drives melting, metamorphism and plate movement, and the Sun, which drives weather and the water cycle that does the weathering and erosion. The water cycle, explained visually is the surface half of the same story.

The mistakes that lose marks

• Confusing weathering and erosion. Weathering breaks rock where it is. Erosion moves it.
• Saying metamorphic rock forms by melting. It changes while solid. Melting makes magma.
• Mixing up magma and lava. Magma is underground. Lava has reached the surface.
• Getting crystal size backwards. Slow, deep cooling makes large crystals.
• Thinking the cycle has one direction. Igneous rock can be metamorphosed without ever becoming sedimentary, and any rock can be melted.
• Leaving out uplift. Without it, deep rocks never reach the surface to be weathered.
• Saying fossils are found in igneous rock. Almost never. Look in sedimentary rock.

Worked example: one grain's journey, a three minute class script

Following one piece of rock is the clearest way to teach the cycle, because it forces the right order. Here is the opening of a script, about 450 words in full at 150 spoken words a minute.

A grain of quartz sits inside a block of granite under Dartmoor. It crystallised there from magma, slowly, deep underground, which is why it is big enough to see.Millions of years of uplift and erosion bring the granite to the surface. Rain, frost and plant roots weather it. The grain breaks free.A stream carries it to the sea. On the way it is knocked about until it is small and round. Where the water slows, it settles with billions of others on the sea floor.

The scene list for the rest:

1. Granite cross-section with one grain highlighted.
2. Uplift: the land rising, the rock above stripped away.
3. Freeze-thaw in a crack, the grain breaking loose.
4. River transport, the grain getting rounder.
5. Deposition: layers building up on the sea floor.
6. Compaction and cementation: the layers squeezed, minerals gluing the grains. Sandstone.
7. Burial during a plate collision: heat and pressure. Quartzite.
8. Deeper still at a subduction zone: melting into magma.
9. Back to the start, cooling into a new granite.

Make your own version for class

1. 1

   Choose a grain and a place

   Pick one real starting rock, such as granite from Dartmoor or basalt from the Giant's Causeway, and decide which route it will take through the cycle.

2. 2

   Write one paragraph per step

   Follow the scene list above. Use the exact terms your exam board uses: weathering, erosion, deposition, compaction, cementation. About 450 words fills three minutes.

3. 3

   Keep one diagram on screen

   Draw the rock cycle triangle once and move a highlighted dot around it in every scene, so viewers always know where the grain is.

4. 4

   Paste the script into openCanviz

   Choose Keep my wording, set a target length, and pick the whiteboard style so the cross-sections build up as the narration describes them.

5. 5

   Check the labels

   Pause on each scene. Drafted drawings can label lava as magma, draw fossils in granite or put slate where marble should be. Fix any scene in the editor before you hand it in.

For the wider assignment, how to make a science explainer video for class covers planning and what teachers look for.

Common questions

What is the rock cycle in simple terms? The way rocks change from one type to another over millions of years. Molten rock cools into igneous rock, rock is broken into sediment that becomes sedimentary rock, heat and pressure turn rock into metamorphic rock, and melting turns any of them back into magma.

What are the three types of rock? Igneous, formed from cooled magma or lava; sedimentary, formed from compacted and cemented sediment; and metamorphic, formed when existing rock is changed by heat and pressure.

What drives the rock cycle? The Earth's internal heat, which melts and metamorphoses rock and moves the plates, and energy from the Sun, which drives the weather and water that weather and erode rock at the surface.

How long does the rock cycle take? Usually millions of years. Some steps are fast, such as lava cooling into basalt in days to years, but burial, metamorphism, uplift and erosion take millions.

Can a rock skip a stage? Yes. An igneous rock can be metamorphosed without ever being weathered, and a metamorphic rock can be melted straight back into magma. The cycle has many paths.