Being composed of carbon, coal forms a carbonaceous deposit. Having been transported and accumulated in a single deposit it is sedimentary. Having undergone metamorphosis and petrification it is a rock. Consequently it is reasonable to classify coal as a carbonaceous sedimentary rock.

Chemical sedimentary rocks are formed by direct chemical precipitation from water. While some limestones and cherts may form in this manner, evaporite deposits consisting of halite, gypsum, and other salts are the most common.

2) Chemical Sedimentary Rocks (see figure above) • When the concentration of dissolved minerals in a body of water reaches saturation, crystal grains precipitate out of solution and settle to the bottom. The resulting layers of chemical sedimentary rocks are called evaporites.

limestone

Common sedimentary rocks include sandstone, limestone, and shale. These rocks often start as sediments carried in rivers and deposited in lakes and oceans. When buried, the sediments lose water and become cemented to form rock. Tuffaceous sandstones contain volcanic ash.

igneous rock

Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is formed when dead plant matter decays into peat and is converted into coal by the heat and pressure of deep burial over millions of years.

The formation of coal begins in areas of swampy wetlands where groundwater is near or slightly above the topsoil. Because of this, the flora present produces organic matter quickly – faster in fact than it can be decomposed. In these areas, layers of organic matter are accumulated and then buried.

It is generally accepted that coal originated from plant debris including ferns, trees, bark, leaves, roots and seeds some of which accumulated and settled in swamps. This unconsolidated accumulation of plant remains is called peat. Peat is being formed today in marshes and bogs.