Sedimentary Rocks 🪨
Sedimentary rocks are formed from layers of sediment. Many form as sediments cement together over millions of years.
Characteristics
Have multiple layers
Appear as pieces of rock cemented together
Particles do not interlock
Often contain fossils
Layers of different sediments form different sedimentary rocks:
Remains of small shellfish form limestone
Micro plankton form chalk
Gravel forms conglomerate
Sand forms sandstone
Clay forms shale
Plant fragments form coal
Sedimentary rocks are classified according to the type of sediment they form from.
Types of Sedimentary Rocks
There are three types of sedimentary rocks:
Clastic
Biological
Chemical
Clastic Sedimentary Rocks 🧱
Clastic sedimentary rocks are composed of rock fragments weathered from pre-existing rocks due to natural forces such as water, ice, and wind.
Examples
Rock | Composition | Characteristics | Uses |
|---|---|---|---|
Shale | Clay-sized particles | Breaks apart in flat sections, various colors (black, grey, brown, red), good source of fossils | Bricks, roof tiles |
Sandstone | Sand-sized grains | White, grey, pink, red, brown, or black; gritty feel; layers of different colors are easy to see | Construction material |
Conglomerate | Grains larger than 2mm in diameter | Wide range of grain sizes | Ornament |
Breccia | Large angular rock fragments | Fill, road base |
Sedimentary rocks tell us about past environments on Earth's surface and are primary storytellers of past climate, life, and major events. Each environment has particular processes that cause specific sediment types to be deposited.
Biological Sedimentary Rocks 🧫
Biological sedimentary rocks are formed from organic material such as the dead remains of plants and animals, compacted and cemented over time.
Examples
Rock | Composition | Characteristics | Uses |
|---|---|---|---|
Coal | Plant material | Usually hard and black but less dense than other rocks; a fossil fuel | Production of electricity |
Limestone | Organic material (dead plants and animals) | Contains many fossils; varies in color from light grey to dark grey or brown | Building material, filler in paints, rubber, and plastics |
Chemical Sedimentary Rocks 🧪
Chemical sedimentary rocks are formed when materials dissolved in water come out of solution, leaving solid salt crystals as a mineral layer. Salts are normally dissolved in saltwater. However, if the water is trapped in a lake or pond, it can evaporate when the temperature gets high enough.
When water evaporates, it turns from a liquid into a gas and drifts into the atmosphere. Solid salt crystals are left behind as a mineral layer. Repeated rain, then evaporation, can cause many layers of salts to build on top of each other. This process is occurring on the shores of the Dead Sea in Israel, where gypsum domes form.
Examples
Rock | Composition | Characteristics | Uses |
|---|---|---|---|
Dolomite | Mineral dolomite (starts as limestone altered by magnesium) | Similar to limestone | Construction material |
Rock Salt | Residues of salt from evaporated salt lakes or dried-up seabeds | Used on roads and driveways in cold areas to combat ice |
Summary
Sedimentary rocks form from sediment, which includes solid materials that sink to the bottom of oceans, rivers, or lakes, such as dead animals or plants.
The sediments must be cemented together to make rocks.
Over time, these materials become compressed together by the weight of materials on top and cemented together by chemicals like calcium carbonate.
Layers build up, each turning to rock, preserving remains of plants and animals as fossils.