What are sedimentary rocks?
Sediment means bits that settle: pebbles, sand, mud, shells, or minerals that come out of water. Sedimentary rocks form when sediment piles up and hardens.
They form at or near Earth's surface, at normal temperatures. That is very different from igneous rocks (cooled from melted rock) and metamorphic rocks (changed by heat and pressure).
Key facts:
- They make up only about 5% of Earth's crust by volume, but cover about 75% of the land surface as a thin skin.
- They are usually in layers called beds or strata.
- They are the only rocks that commonly hold fossils.
- They store most of the world's coal, oil, natural gas and much groundwater, and give building materials such as sandstone, limestone (for cement) and clay.
How sediment becomes rock: from weathering to diagenesis
- Weathering: rock breaks down where it is. Physical: frost wedging, heating and cooling, roots. Chemical: rainwater (a weak acid) dissolves or changes minerals, for example feldspar becomes clay.
- Erosion and transport: rivers, wind, glaciers, waves and gravity move the pieces. On the way, grains get rounder and get sorted by size (fast water carries big grains; slow water only fine ones).
- Deposition: when the water or wind slows, grains settle. Big grains drop first, mud last. Layers form one on top of another.
- Diagenesis (all the changes that turn sediment into rock at low temperature):
- Compaction: the weight of layers above presses grains together. Pore space and water decrease; mud can lose more than half its volume.
- Cementation: water moving through the pores leaves minerals, mainly calcite, silica (quartz) and iron oxides (which colour rocks red). These crystals glue grains together. This step is called lithification (becoming stone).
If burial goes deeper and gets hotter (above about 200 °C), diagenesis ends and metamorphism begins.
Types of sedimentary rocks
1. Clastic (detrital) rocks: made of broken pieces
| Grain size | Sediment | Rock |
|---|---|---|
| > 2 mm | Gravel, pebbles | Conglomerate (rounded), breccia (angular) |
| 1/16 – 2 mm | Sand | Sandstone |
| 1/256 – 1/16 mm | Silt | Siltstone |
| < 1/256 mm | Clay, mud | Shale (splits into thin sheets), mudstone |
2. Chemical rocks: minerals that come out of water
When water evaporates or its chemistry changes, dissolved minerals precipitate: rock salt (halite) and gypsum form as seas or salt lakes dry up (evaporites); some limestone and travertine form from calcium carbonate; chert and flint are silica.
3. Organic (biogenic) rocks: from living things
Coal forms from buried plant remains in swamps (peat → lignite → bituminous coal). Chalk and shelly limestone form from shells and skeletons of sea creatures; coral reefs build limestone too.
Depositional environments
The kind of rock tells us where it formed:
- Mountain rivers, alluvial fans: fast water, coarse gravel → conglomerate, breccia.
- Rivers and deltas: sand and mud → sandstone and shale.
- Beaches: well-sorted, rounded sand → clean sandstone.
- Deserts: wind-blown, frosted sand in large dunes → red sandstone with big cross-beds.
- Lakes and deep sea: calm water, fine mud → shale; deep-sea oozes → chalk, chert.
- Warm shallow seas, reefs: shells and corals → limestone.
- Hot dry lagoons, salt lakes: evaporation → rock salt, gypsum.
- Swamps: plant matter → coal.
- Glaciers: unsorted mix of boulders and clay → tillite.
Sedimentary structures: reading the past
Sedimentary layers keep clues, like a diary written in stone:
- Bedding (stratification): layers, each a time of deposition. By the law of superposition, in undisturbed layers the oldest is at the bottom and the youngest on top. Layers are laid down flat (original horizontality); tilted or folded layers were moved later.
- Ripple marks: small wavy ridges made by water or wind on sand. Symmetrical ripples come from waves moving back and forth (beach, lake shore); asymmetrical ripples come from a current flowing one way, with the steep side facing downstream, showing the flow direction.
- Cross-bedding: thin sloping layers inside a bed, made by moving dunes or sand bars. The slope shows the direction of wind or water.
- Graded bedding: coarse grains at the bottom fining upward, from a flow that slowed (e.g. an underwater mudflow). It tells which way was 'up'.
- Mud cracks: polygon cracks from wet mud drying in the sun: a sign of a shore or lake that dried up.
- Fossils and raindrop marks: past life and climate.
Try it: make layers in a bottle
Put a handful each of small pebbles, sand and soil in a clear bottle, fill it with water, close it and shake hard. Put it down and watch for 10 minutes, then leave it overnight. The pebbles settle first, then sand, then fine mud on top: graded bedding. Swirl the water gently in one direction and look for ripples on the sand. Then press a stack of wet sand and clay layers in a cup with a weight and watch water squeeze out (compaction).
Key formulas and definitions
- Weathering → erosion/transport → deposition → compaction → cementation = sedimentary rock
- Diagenesis = compaction + cementation (+ other low-temperature changes)
- Clastic grain sizes: gravel > 2 mm, sand 1/16–2 mm, silt 1/256–1/16 mm, clay < 1/256 mm
- Law of superposition: lower layer = older (if not overturned)
- Faster water → bigger grains deposited; slower water → finer grains
Worked examples
1. A rock has rounded pebbles 1–3 cm across held in a sandy cement. Name it and its likely environment.
Rounded grains bigger than 2 mm in a cement = conglomerate. Rounding means long transport by fast water, such as a mountain river or a pebbly beach.
2. A layer of shale lies on top of sandstone, which lies on top of conglomerate (not overturned). Which is oldest, and what happened to the water over time?
Conglomerate at the bottom is oldest (superposition). Grain size goes from pebbles to sand to mud upward, so the water became calmer over time, for example as sea level rose or a river slowed.
3. A 10 m thick layer of mud with 60% water compacts into shale with 10% water. Assuming only water is lost, estimate the new thickness.
Solid part = 40% of 10 m = 4 m. In the shale the solids are 90% of the thickness, so thickness ≈ 4 ÷ 0.9 ≈ 4.4 m. Compaction more than halved it.
4. How can you tell if ancient ripple marks were made by waves or by a river current?
Wave ripples are symmetrical with pointed crests (back-and-forth motion). Current ripples are asymmetrical: a gentle upstream slope and a steep downstream slope, which also shows which way the current flowed.
5. Why do we find layers of rock salt and gypsum hundreds of metres thick under some deserts?
They are evaporites. A shallow sea or salt lake kept refilling and drying in a hot climate; each time, dissolved salts precipitated. Thick beds mean this repeated for a very long time.
Common mistakes
- Thinking sedimentary rocks form from cooled magma. That is igneous rock; sedimentary rocks form from sediment at the surface.
- Thinking compaction alone makes all rock. Sand needs cementation too; compaction mainly hardens mud.
- Thinking the top layer is the oldest. In undisturbed layers the bottom is oldest (superposition).
- Thinking all limestone is chemical. Much limestone is organic, made of shells and coral.