Your turn
Each lab is one part of the video. Tap, drag and slide, and see what happens.
Each lab isolates one step. They are simplified teaching sketches: settling and burial are sped up and not to scale.
Tap the buttons and slide the sliders.
Drag the magnifier (or tap a button) over the canyon wall. What is each layer made of?
Each bed has its own grain size and composition. Move the lens to compare sandstone, shale and limestone.
Tap a button. What is inside each stripe?
Tap the crack (or the button) to freeze it, then thaw it. How many cycles until the rock splits?
Water in the crack expands as it freezes, prying the crack wider. Repeat the freeze–thaw cycle until the block fails.
Tap to freeze the water. Tap again. Crack!
Press Flood! What sinks first? What settles on top?
Coarse grains settle fastest, clay slowest, so each flood leaves a graded layer. Stack a few.
Press Flood! Watch the pebbles, sand and mud sink.
Sped up: in a glass of water sand sinks in a second or two, but fine clay can stay floating much longer.
Slide the burial deeper. Watch the grains squeeze together, then get glued by crystals.
Increase burial: grains pack closer and pore water is expelled (compaction), then mineral cement grows in the pores (cementation).
Slide it down. Squeeze and glue!
Illustration: real burial is hundreds of metres or more.
Pick an ingredient. Which rock does it become? Is it clastic, chemical or biological?
Choose a starting material and see its lithified product and group.
Pick one. What rock does it make?
Slide down through the layers (or tap one). Which layer is the oldest? What fossil is inside?
Step down through an example sequence: each bed records an environment, and lower means older.
Slide down. The deepest layer is the oldest!
An example stack, not a real place.
Look at a big canyon wall. Can you see the stripes? Each stripe is a layer of rock. Many of these rocks are made of tiny bits, like grains of sand, all stuck together.
High on a mountain, rain and wind wear a big rock away. Water gets into a crack and freezes into ice. The ice pushes, and the rock breaks. The little bits are called sediment: pebbles, sand and mud.
A river carries the bits to a calm lake or sea. There, they sink. Big pebbles sink first, then sand, then soft mud. Each flood makes a new flat layer on top of the last one.
More and more layers pile on top and squeeze the bottom ones tight. Water brings dissolved minerals, which grow into crystals between the grains and glue them together. Squeeze and glue, and the sand turns into stone!
The oldest layers are at the bottom, and the newest are on top. Some layers hold fossils, like shells and leaves from long, long ago.
Short answer: sedimentary rocks form when rocks on land are broken into small pieces by weathering, the pieces (called sediment) are carried by rivers, wind or ice to lakes and seas, and they settle there in layers. Over thousands to millions of years, new layers bury the older ones. The weight squeezes out the water (compaction) and minerals dissolved in the water glue the grains together (cementation), turning loose sediment into solid rock.127
That's why sedimentary rocks usually come in layers, called beds or strata, like the stripes in a canyon wall.13 And not all of them start as broken rock: some are made from shells, coral or plants, and some from salt left behind when water dries up.2
A sedimentary rock is a rock made from pieces of older rock or from the remains of living things, laid down at or near Earth's surface.1 It is one of the three main rock types, along with igneous rock (made from molten rock that cooled) and metamorphic rock (rock changed by heat and pressure).3
Sedimentary rocks are the most common rocks you'll see at the surface. They cover about three-quarters of the land, but they're only a thin skin: roughly 5% of Earth's crust by volume.17 Sandstone, shale, limestone, conglomerate, rock salt and coal are all sedimentary rocks.12
Most sedimentary rock is made in the same order. Here are the steps you can watch in the video and try in the labs above:
| Step | What happens | Example |
|---|---|---|
| 1. Weathering | Rain, wind, waves and ice break rock into smaller pieces.4 | Water freezes in a crack, expands and splits a boulder.10 |
| 2. Erosion and transport | Rivers, wind and ice carry the pieces away. Faster water can move bigger pieces.7 | A muddy river after a storm. |
| 3. Deposition | When the water slows down, the sediment drops and settles in a layer.5 | Sand and mud settling where a river meets the sea. |
| 4. Compaction | New layers bury the old ones. The weight packs the grains together and squeezes out water and air.7 | Loose mud pressed flat. |
| 5. Cementation | Minerals such as calcite and quartz crystallise in the gaps and glue the grains.7 | Sand becomes sandstone. |
Geologists call steps 4 and 5 together lithification, which just means "turning into stone".2
The sand grains in a rock like sandstone were once part of a bigger rock. Weathering is the slow breaking down of rock where it sits. Rain and waves wear it away, wind blasts it with sand, and rainwater, which is slightly acidic, slowly dissolves some minerals.4
Ice is especially good at it. Water seeps into a crack, freezes and expands, pushing the crack wider. When it thaws, more water gets in. After many freeze-thaw cycles, chunks break off. Geologists call this frost wedging.410
The loose pieces are named by size: clay is the smallest, then silt, then sand. Anything bigger than 2 millimetres is a pebble, and boulders can be enormous.17
Rivers, wind and glaciers carry sediment away. The faster the water flows, the bigger the pieces it can carry. When a river reaches a calm lake or sea, it slows down and can't hold on to its load, so the sediment sinks. That's deposition.57
Big grains sink fastest. In a glass of water, a tiny pebble (a granule) hits the bottom in under half a second, a sand grain takes a second or two, silt takes several seconds, and fine clay may never settle at all.7 So each flood tends to leave coarse stuff at the bottom and fine mud on top, and the next flood adds a new layer. Sediment settles in flat, horizontal layers, which geologists call the principle of original horizontality.9
Note: the settling tank in the explainer is sped up and not to scale. It shows the order grains settle in, not real speeds.
You've just done deposition. If you want to try the chemical recipe too, dissolve two teaspoons of salt in a little hot water, pour it onto a plate and leave it for a few days. The water evaporates and salt crystals grow, just like on a drying salt lake.8
A pile of sand on the sea floor is not rock yet. Two things change that.
Compaction. As more layers pile on top, the buried sediment carries more and more weight. The grains are pressed closer together and much of the water and air between them is squeezed out.7 Mud, with its tiny flat clay particles, gets squashed especially flat, which is why shale shows very fine, thin layers.7
Cementation. Water still trickles through the tiny gaps between the grains, carrying dissolved minerals. Over time these minerals crystallise in the gaps, especially where grains touch, and glue them together. The most common natural cements are calcite, quartz, hematite (an iron mineral) and clay minerals.72
In the explainer's burial slider, depths are illustrative. Real sediment is usually buried under hundreds of metres or more of younger layers before it turns to stone.
The US National Park Service sorts sedimentary rocks into three groups.2 Some textbooks call the biological group "organic".
| Type | Made from | Examples |
|---|---|---|
| Clastic | Pieces (clasts) of older rock | Conglomerate (pebbles), sandstone (sand), siltstone (silt), shale (clay and mud)1 |
| Chemical | Minerals that come out of water, often as it evaporates | Rock salt (halite), gypsum, cave formations such as stalactites819 |
| Biological (organic) | Remains of living things | Limestone (shells and coral), coquina (shell fragments), coal (plants)2 |
These are what most people picture: broken rock, cemented. They're named by grain size. Conglomerate is pebbles held in sand or mud, sandstone is sand, and shale is mostly clay.1 Yes, sandstone is a sedimentary rock, and the Grand Canyon and Arches National Park are full of it.2
In dry places, some lakes and inland seas have no river flowing out, so water leaves only by evaporating. The water gets saltier and saltier until minerals start to crystallise. Gypsum starts to form when only about a fifth of the water is left, and halite (rock salt) at about a tenth.8 Thick layers of these evaporites become chemical sedimentary rock.
Is limestone a sedimentary rock? Yes. It's the most common chemical sedimentary rock, and most of it is made by living things. Almost all limestone forms in the ocean, mostly in warm, shallow water near coral reefs. Corals, clams, snails and many other sea creatures build shells and skeletons out of calcium carbonate. When they die, their hard parts pile up and are compacted and cemented into limestone.8163 So if you find limestone on land, that place was probably once under the sea.19
Coal forms from plants. In ancient swamps, dead leaves and trunks fell into still, low-oxygen water where they couldn't rot away completely. Buried under more sediment and squeezed over millions of years, the plant material slowly became coal.715 Read the full story in how coal is formed, or see how today's reefs grow in how coral reefs are formed. Sedimentary rocks also hold essentially all of the world's oil and natural gas.17 See how oil is formed.
A fossil forms when a plant or animal is buried quickly in sediment such as sand or mud, before it rots or gets eaten. The soft parts usually decay, but bones, shells and teeth can be slowly replaced by minerals.6 Sedimentary rock is made from exactly that buried sediment, at normal surface temperatures, so the remains survive. That's why fossils are typically found in sedimentary rocks, and only occasionally in some gently altered metamorphic rocks.6 Rocks that were once molten, like granite, don't hold fossils. See more in how fossils are formed.
Because new sediment settles on top of old sediment, the layers at the bottom of a stack are older than the layers above them, unless something later flipped the stack over. This is the law of superposition.9 The Danish scientist Nicolas Steno (also spelled Nicolaus or Niels Stensen) described the idea in his 1669 book, the Prodromus. He also argued that fossils are the remains of living things, and that the layers of Earth's crust record its history.1811
The Grand Canyon is the famous example. Its flat-lying upper walls are sedimentary layers 3,000 to 5,000 feet (900–1,500 m) thick. They were laid down from about 530 to 270 million years ago, roughly 260 million years of history, and record everything from shallow oceans to huge fields of sand dunes.12 The Kaibab Limestone on the rim formed on the sea floor.13 Find out how the river cut through it all in how the Grand Canyon was formed.
The layer stack you can dig through in the explainer is a made-up example to show the idea, not a real place.
Sedimentary rock doesn't stay the same forever. All rocks are linked in a never-ending loop called the rock cycle.3
"Sedimentary rock is just hardened mud." Mud makes shale, but sedimentary rocks can also be made of pebbles, sand, shells, coral, plants or salt.2
"Pressure alone turns sand into rock." Squeezing packs the grains together, but mineral cement growing between them is what glues most sandstone into solid rock.7
"Each layer is one year." Layers record changes in what was being deposited: floods, seasons, shifting rivers or climate. The gaps between beds can stand for anything from decades to thousands of years.9
"Sedimentary rocks only form in the sea." They also form in lakes, rivers, swamps and deserts. Some Grand Canyon layers are ancient sand dunes.312
Rocks on land are broken into small pieces (sediment) by weathering. Rivers, wind and ice carry the pieces to lakes and seas, where they settle in layers. Over thousands to millions of years the layers are buried, squeezed (compaction) and glued together by minerals (cementation) until they become solid rock. Some sedimentary rocks form differently: limestone and coal come from shells and plants, and rock salt is left behind when salty water dries up.
Think of making a sandwich over millions of years. Tiny bits of rock, sand, mud and shells settle at the bottom of a lake or sea, one layer on top of another. The weight of the layers on top squeezes the bottom ones, and minerals in the water act like glue, turning the loose bits into stone.
For clastic rocks like sandstone: weathering (rock breaks into pieces), erosion and transport (water, wind or ice carry the pieces), deposition (the pieces settle in layers), then burial, compaction (the weight of new layers squeezes the older ones) and cementation (minerals glue the grains together). Compaction and cementation often happen together.
Clastic rocks are made of broken pieces of older rock (conglomerate, sandstone, shale). Chemical rocks form when minerals come out of water, for example rock salt when a salty lake dries up. Biological (organic) rocks are made from living things, such as limestone from shells and coral, and coal from plants.
Fossils form when plants or animals are buried quickly in sediment such as sand or mud, before they rot away. Sedimentary rocks are made from exactly that sediment, and they form at normal surface temperatures, so the remains aren't melted or destroyed. Igneous rocks form from molten rock, which almost always destroys any remains.
Igneous rocks form when molten rock (magma or lava) cools and hardens. Metamorphic rocks form when any rock is buried deep and changed by heat and pressure without melting: shale can become slate, and limestone can become marble.
Usually thousands to millions of years. Layers have to pile up, get buried and be cemented by minerals. Some things are faster, like the salt crust on a drying lake, but thick layers of rock take a very long time.