Coal is clearly made up of lots of compressed bits of dead plants. But when we walk around in the world we usually see old bits of wood and leaves decaying and rotting away (that is, getting eaten by bacteria, insects, and all sorts of organisms involved in decay and recycling of organic matter). For thick layers of plant stuff to get fossilized as coal, something must be going on.

The Coal Formation Process.

Coal Formation starts with accumulation of organic matter (bits of dead plants) in a low oxygen setting such as a peat bog. The organic matter accumulates and forms a bed of peat. The peat bed gets buried by other sediments and under heat and pressure begins to transform to a low grade coal - a Lignite. More heat and pressure further metamorphose the lignite into Bituminous coal. Even more heat and pressure metamorphose the bituminous coal into a nice hard shiny Anthracite.

Coal is usually classified into three grades: Lignite, brown coal; Bituminous coal, soft coal; and Anthracite, hard coal. Anthracite is dense, nice and hard, and shiny.

Peat bog in low spot in moorland, Y Mynnydd Dde, Cymru, UK.

The first step in the formation of coal is the accumulation of plant debris in low oxygen conditions, such as in this damp low spot on a moor.

Peat on moorland, exposed in roadcut on the slopes of Y Mynnydd Dde, Cymru, UK.

Peat interbedded with loess in the tundra south of Barrow, Alaska.

Thin lignite in early Paleocene sediments in Montana.

Peat exposed to heat and pressure from burial beneath other sediments becomes compressed and chemicaly changes into low grade coals such as this lignite, and under further heat and pressure is converted to higher grade coals. The pressure from overlying sediments that bury a peat bed will compact the coal. Peats transform to low grade lignites when they are compressed to about 20% of their original thickness. Lignite typicaly transforms to bituminous coal as it is compressed further and heated to between 100 and 200°C. This drives much of the water and other volitiles from the coal. Longer exposure to elevated temperature will further drive volatiles from the coal, and drive chemical reactions that produce anthracite. Anthracite coals are typicaly compressed to 5-10% of the orginal thickness of the peat bed, and contain less than 10% water and other volatiles (Nichols, 1999).

Strip mining a seam of bituminous coal in southern Ilinois.

Coal is often found in beds a meter or more in thickness that are widespread in extent. These are often mined to extract the coal for use as a fossil fuel.

Coal varies considerably, not only in the extent to which it has been metamorphosed, but in the other materials it contains. For example, some coal deposits, such as those of Cretaceous and Tertiary of the western interior of the US, trapped sediment from volcanic ashfalls. Coal can contain not just concentrated plant debris, but also sediments brought in by water, airborne ash falls, elements adsorbed to the carbon of the coal from groundwater, and minerals that form in fractures and spaces in the coal as groundwater flows through it during coalification. Coal seams are typicaly highly variable in composition due to variation in these inputs over short distances (Finkelman, 1993).