Is coal a sedimentary rock?

Answered by Kurt Edwards, Geologist and GIS Analyst, INL ESER Program

Coal1he short answer is yes, coal is a sedimentary rock. Coal is formed mainly from plant material that accumulates in a swampy environment. These plants that become coal, lived before and during the time of the dinosaurs. When the plants died they would sink to the bottom of the swamp, accumulate, and become buried by soil. Over a long period of time the plant material would become buried deeper and deeper and be changed by pressure and heat. The pressure and heat would cause the plant material to become solid and form what we know today as coal.

To understand this answer we need to understand what a rock is and the three types of rock. A rock is the solid mineral material forming the surface of the earth and many other planets. Rocks are classified into three types; metamorphic, igneous, and sedimentary.

A metamorphic rock forms when high temperatures and pressure are applied to a pre-existing rock which changes its physical or chemical structure.

An igneous rock forms from a liquid rock, in the form of magma or lava. When the liquid cools it solidifies and becomes an igneous rock.

The last type of rock is sedimentary. Sedimentary rock forms from particles of other rock and organic material. These particles will collect together and under heat, pressure, and time will cement together. Coal is not just a sedimentary rock it is a special kind, it’s organic. Sedimentary rocks are classified further into three main types; Clastic, Chemical, and Organic.

Clastic sedimentary rocks form when older rock is weathered and cemented together with smaller particles. Examples of clastic sedimentary rocks are breccia, sandstone, and shale.

Chemical sedimentary rocks: form when minerals become suspended in a solution and then precipitate, or settle to the bottom, forming rock. Examples of chemical sedimentary rocks are gypsum, halite, and flint.

Organic sedimentary rocks form from accumulations of organic materials such as animal or plant debris. Examples of organic sedimentary rocks are coal, and fossiliferous limestone.

So yes, coal is a sedimentary rock made from organic plant matter; which classifies coal as an organic sedimentary rock.

Coal2Make a Clastic Sedimentary Rock
Sedimentary rocks are laid down in layers, with the oldest layers on the bottom. If you see fossils in one of the layers, the area was covered by a lake or ocean long ago. If you see particles of different sizes, the largest particles are always at the bottom of the layer. Particles higher in the layer become increasingly smaller. A layer like this shows that the water carrying the sediment was slowing down.

Materials: 1 paper cup, 3 mixing cups, 6 tbs Plaster of Paris, 3 tbs water, 4 tbs gravel, 2 tbs sand, 3 drops food coloring

  1. Prepare the plaster of Paris by mixing it with the water.
  2. Mix 2 tablespoons of the gravel with 2 tablespoons of the plaster of Paris and pour the mixture into the paper cup.
  3. Coal3Mix the sand with 2 tablespoons of the plaster of Paris and the food coloring. Add the mixture to the paper cup, on top of the gravel mixture.
  4. Mix the rest of the gravel with the rest of the plaster of Paris. Add the mixture to the paper cup, on top of the sand mixture.
  5. After the mixtures harden for about 5 minutes, tear apart the paper cup and observe the layers.

How is the procedure you used to make your model similar to the way sedimentary rock forms?

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