KS3 · Chemistry

Formation of sedimentary rock

Sand, gravel and clay can become solid rock, stacked in layers like pages in an old book. How do loose grains turn solid, and how do you read the pages?

Read the rock

Step down through a stack of strata

Sedimentary rock builds up in layers. One layer is a stratum; lots of layers together are strata. This stack is a sketch, not a real place — tap each layer and work your way down.

↑ Top of the stack

↓ Bottom of the stack

Zone 1 of 5

Top stratum

This is the most recent deposit — the newest sediment, sitting on top of everything else. It has the fewest layers above it, so it has the least weight pressing on it. Less pressure means the least compaction (compaction is the squeezing of grains closer together by the weight above).

  • Most recent deposit
  • Fewest layers above
  • Least compaction

Going down the stack is going back in time. Each layer was deposited earlier than the one above it, and each one has more layers pressing on it — so the lower the layer, the older it is and the more it has been compacted.

Watch out: The stack tells you which layer came first, not how many years old each one is. That's called a relative timescale.

How did those layers get there? Predict, then check

Wind and water carry broken bits of rock around. The question is when they let go.

Strong wind is blowing sand across open ground. Then it hits a line of trees. What happens to the sand?

Follow one grain

From broken rock to solid rock — put the stages in order

The order the stages happen in, as a piece of rock becomes part of a sedimentary rock

1 · First stage7 · Last stage

  1. Compaction

    The weight of the layers above pushes the grains closer together and squeezes out some of the water

  2. Weathering

    Rock at the surface is weakened or broken by biological, chemical or physical processes

  3. Cementation

    Mineral crystals stick the grains together, and the sediment becomes rock

  4. Deposition

    Wind or water slows down, or ice melts, and the fragments settle as a layer of sediment

  5. Erosion

    The broken pieces are carried to a new place by wind, glaciers, rivers, streams or gravity

  6. Minerals form crystals

    Groundwater flowing between the grains leaves minerals behind as crystals in the gaps

  7. Burial

    Over time, more layers of sediment settle on top

Compaction vs cementation: two different jobs

CompactionvsCementation

Lots of people think squeezing on its own makes rock. Start with the first row.

Focus

Does it turn sediment into rock?

Compaction

Not on its own — it packs the grains closer together

Cementation

Yes — sticking the grains together is what turns sediment into rock

The insight

This is the big one. Squashing and sticking are separate steps, and you need both.

What causes it?

Compaction

Pressure from the weight of the layers above

Cementation

Minerals deposited by groundwater moving between the grains

What happens in the gaps between grains?

Compaction

The grains move closer and some groundwater is squeezed out

Cementation

Mineral crystals, such as quartz or calcite, grow in the gaps (pores)

When does it happen?

Compaction

Once the sediment is buried under more layers

Cementation

After compaction, once crystals have formed in the pores

Sediment in, rock out

Which rock does each sediment become?

Pick a sediment, then choose where it belongs. Try both ways of sorting.

Is this sediment made of rock fragments, or the remains of living organisms?

Still to sort

Rock fragments (0)

Sediments made of pieces of rock

Remains of living organisms (0)

Sediments made of what's left of living things

Where the line is: These rocks still form by compaction and cementation — only the sediment is different.

5 of 5 still to sort.

Every sedimentary rock forms by the same processes. What changes is the sediment you start with.

Watch out: The sediment name is not the rock name. Sand becomes sandstone — but gravel becomes conglomerate, and clay becomes shale.

Your turn

Read a stack you've never seen before

Layer 1 (top): no clues inside. Layer 2: cross-beds. Layer 3: ripples. Layer 4 (bottom): the remains of shellfish.

A scientist sketches four strata in a rock face, numbered from the top down. Suggest four things these strata can tell the scientist. [4 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Weathering breaks or weakens rock; erosion carries the pieces to a new place.
  • Sediment is dropped (deposited) when wind or water slows down, or when ice melts.
  • Compaction (pressure from layers above) and cementation (mineral crystals gluing grains) turn sediment into sedimentary rock.
  • In strata, the lower a layer is, the earlier it was deposited.

The big picture

Sedimentary rock forms as part of the rock cycle. Rock at the surface is weathered and eroded, and the pieces are dropped as sediment when wind or water slows down. Buried under more layers, the sediment is compacted by the weight above and then cemented by mineral crystals from groundwater, becoming sedimentary rock. In a stack of strata, lower layers were laid down earlier, and clues such as ripples, cross-beds and remains of organisms tell scientists about the past.

Key points

1Faster-flowing wind or water carries more and larger rock fragments further; when the flow slows, fragments are deposited as sediment.
2Compaction pushes grains closer together and squeezes out some water; the top layer, with the fewest layers above it, has the least compaction.
3Cementation happens when minerals from groundwater, such as quartz or calcite, form crystals in the pores and stick the grains together.
4Different sediments make different rocks: sand → sandstone, gravel → conglomerate, clay → shale, plankton remains → chalk, shellfish remains → limestone.
5Strata give a relative timescale, and ripples, cross-beds and remains of organisms are clues to past environments and life.

Worked example

Problem

Explain how a layer of gravel on a riverbed can, over a very long time, become the rock conglomerate.

⚠ Watch out

Thinking that squeezing alone makes rock. Compaction only pushes the grains closer together; it takes cementation — mineral crystals from groundwater growing in the pores — to stick them together into rock.

🧠

Memory hook

Drop it, bury it, squash it, glue it. And read strata like a pile of post on a doormat: whatever landed first is at the bottom.

✓

Check yourself

Cover the page. Can you explain why the lowest stratum in a stack is the oldest, and also the one that has been compacted the most?

Flashcards

(14)
What is weathering?
Rock at the Earth's surface being weakened or broken by biological, chemical or physical processes.
What is erosion?
Transporting weathered pieces of rock to a new location — by wind, glaciers, rivers or streams, or gravity.
What is a sediment?
Material deposited by water, wind or ice — usually small grains such as sand, pebbles, mud, organic remains or salts.
Fast flow or slow flow: which carries more, and larger, rock fragments further?
Faster-flowing wind or water. Slower flow carries smaller, lighter fragments more easily.
Give three situations where sediment gets deposited.
Wind hits an obstruction (trees, mountains, buildings); water flows across flatter land; a glacier melts.
What does compaction do to sediment grains?
The weight of the layers above presses down, pushing the grains closer together and squeezing out some groundwater.
Where do the minerals for cementation come from, and where do they go?
From groundwater moving between the grains; they form crystals (such as quartz or calcite) in the pores.
Stratum or strata?
A stratum is one layer of sedimentary rock; strata are many layers.
In a stack of strata, which layer has the least compaction?
The top layer — it has the fewest layers above it, so the least pressure on it.
Name the rocks formed from sand, gravel and clay.
Sandstone, conglomerate and shale.
What are chalk and limestone made from?
Chalk: remains of microscopic plankton. Limestone: remains of larger shellfish.
What do ripples in a stratum suggest?
A current was present — possibly an ancient river, stream or seabed.
How do cross-beds form?
From shifts in the way sediment was deposited — for example, a change in wind direction.
Why do scientists study the objects found in strata?
They help build up an understanding of the organisms living when each layer was deposited.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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