Historical Geology/Peat and coal

Vegetable matter in a swamp, southwest France.

In this article we shall look at the deposition of peat in swamps and how it turns into coal.

Some definitions

In this article I shall use the term swamp as a catch-all term for an area of waterlogged ground in which the water is shallow enough for land plants to grow: an ecologist might distinguish more carefully between swamps, marshes, bogs, fens and so forth.

From our point of view, swamps become of interest when the swamp plants deposit plant matter faster than it can completely decay: in that case the partially decomposed plant matter known as peat will build up in the swamp. This matter will become coal on lithification.

The reader should note that there are two types of coal: humic coal, produced by the deposition of the remains of land plants in swamps; and the rarer and less economically important sapropelic coal, formed by the deposition of algae in lakes. The processes of formation are similar, but what we have to say in this article will refer specifically to humic coal.

Deposition of peat

In peat swamps organic matter accumulates faster than it can decay: this is what makes them peat swamps. But why? On the one hand, peat swamps deposit a lot of matter, but perhaps no more than is deposited in an ordinary forest in the form of fallen leaves, branches, and so forth. The crucial difference is that in swamps the deposited vegetation is waterlogged.

The oxygen content of air is about 20%, and this allows aerobic bacteria and fungi and suchlike organisms of decay to function. On the other hand, the oxygen level in oxygenated water can be more conveniently measured in parts per million, and this is the limiting factor in the decay of organic matter in water. The deposited organic matter provides the aerobic bacteria with a potential feast, but in the process of metabolizing the available nutrients they must use up oxygen; and when there are so many of them, dining so heartily, that they are using up all the available oxygen, then that is as fast as they can decompose the matter.

Tollund Man, a bog body from the fourth century B.C.

There will be less oxygen available in the water surrounding more deeply buried plant matter, so that once the accumulation of such matter has started, it is likely to continue; in the same way, at a greater depth in the accumulated pile of sediment, the water will be more acidic, which also retards decay. The power of peat swamps to prevent decay is well-demonstrated by the discovery of well-preserved corpses thousands of years old in the peat bogs of Europe; for example the Danish "Tollund Man", dated to the fourth century B.C, shown in the photograph to the right.

Not all swamps will be peat swamps: this depends on factors such as the rate of deposition of vegetable matter and the rate at which oxygen comes into the system, which will vary according to the rate of flow (if any) of the water. We know of no general formula for determining whether a swamp will be a peat swamp: for our purposes, it is sufficient to note that in some swamps this build-up of plant matter does indeed take place.

Peatification and coalification

Peatification is the process of partial decay that we have described above. The action of bacteria destroys the weaker polymers making up the cell walls, such as cellulose, leaving behind mainly lignin, which is tougher. Because cellulose and lignin share a structural role in the cell walls of plants, the removal of the cellulose leaves the cell structures intact: a look at peat through an electron microscope reveals that even fine details of cell structure are preserved. The resulting matter is known as peat. The reader should note that this term does not refer only to gardeners' peat, which is peatified sphagnum moss, but to any plant matter that has undergone the peatification process.

A coal seam.

Coalification is a chemical process in which hydrogen and oxygen are lost from the original peat fool, increasing the ratio of carbon to other elements. This involves alteration to the remaining molecules of the material, in particular the conversion of lignin to vitrinite. Coalification is not an all-or-nothing process: rather it produces coal of various ranks having a progressively greater proportion of carbon, from lignite through sub-bituminous coal through bituminous coal to the highest rank, anthracite.

In early coalification the process is carried out by bacterial action; when the material is so compacted that water cannot percolate (and so bacteria cannot penetrate) the later stages of coalification are produced by the action of heat and pressure (both of which are produced by sufficiently deep burial). In practice the processes distinguished as early and late coalification can overlap somewhat.

During the coalification process, pressure removes water from the material: peat has 95% water, anthracite less than 1%. At the same time, of course, the material is compressed, so that it may end up with as little as one-twentieth of its original volume.

Because the coalification process involves modification of the chemistry of the coal, all coal might, in a sense, be considered a metamorphic rock; however only anthracite is usually classified as such, since only anthracite approaches the temperatures and pressures that we usually associate with metamorphism.

As this text is intended to be read by readers with little knowledge of chemistry, we have treated the details of peatification and coalification as briefly as possible. The subject has been studied in considerable detail: the reader who is interested will find more information here and here.

Coal from swamps: how do we know?

In the lowest grades of coal, cell structures are still visible under the microscope, revealing their plant origins clearly. As for the higher grades of coal, we should note that from a chemical point of view the various ranks of coal form a continuum: our divisions of coals into lignite, sub-bituminous, and so forth are, as usual in geology, artificial divisions of what is in fact continuous. This is illustrated well by localities where the upper coal beds are lignite and the lower coal beds progress through sub-bituminous to bituminous coal: which fits well with the theory that it is the same substance modified by the increasing heat and pressure associated with burial.

Furthermore, it is possible to simulate coalification in a laboratory. No-one has ever taken a piece of wood and turned it into anthracite coal, since one vital ingredient, that of time, must necessarily be lacking. But it is possible to show that the application of heat and pressure to peat will make it chemically more like lignite, and that similar treatment of lignite will make it more like sub-bituminous coal.

There is, then, no real doubt that coal has its origins as plant matter. We turn now to the question of why geologists ascribe the source of this plant matter to swamps.

In the first place, coalfields are just what we would expect to see if peat deposits were buried to a sufficient depth, since, as we have observed, heat and pressure, which are both produced by deep burial, cause the chemical changes involved in coalification. Coal fields therefore look like peat deposits should look after sufficient heat, time, and pressure.

We can then ask ourselves: how else can such deposits form? To produce the extent and thickness of coal beds that we observe, we require a lot of plant matter to be deposited over a wide area in anoxic conditions, so that it doesn't rot. No other environment fits the bill. On a forest floor, for example, although plant material will be deposited over a long period of time and a wide area, it will be decomposed fairly rapidly, and never attains any great thickness, as can be easily verified with a trowel. When we see a coal seam ten meters thick, which is not unusual, and when we consider how much it has been compacted down (maybe ten or twenty times) from its original volume, we can see that the original plant matter must have been deposited in conditions where only partial decay took place: i.e., in a swamp.

We can imagine peatification taking place in other environments besides swamps: for example, we can imagine a landslide transporting trees down a hillside into a so-called "dead" lake. The trees then might conceivably peatify and, if buried deeply enough under other sediments, coalify. But once again, we find that this would not account for the great depth and lateral extent of coal beds.

Peat swamps therefore stand out as the one plausible explanation for coal. This is confirmed by examination of the beds of rock underlying and overlying coal beds.

Immediately underlying coal beds, we find paleosols, deposits which, as discussed in a previous article, geologists identify as fossilized soils: most obviously, because they have fossilized roots in them. Indeed, the paleosols will sometimes have trees or tree-roots rooted in them, projecting up through the coal beds: furthermore, the trees are consistent with the fossil vegetation found in the coal. This fits well with the swamp theory of the origins of coal. The paleosols underlying coal beds (which are known as seat-earth, or underclay) also show the sort of soil characteristics we should expect to find in waterlogged soils.

Coal seams and related strata.

The picture to the right, taken from Dawson's Acadian Geology, shows the relation of coal beds to paleosols.

The key from the original text identifies the strata from top to bottom as follows:

1. Shale.
2. Shaley coal, 1 foot.
3. Underclay with rootlets, 1 foot 2 inches.
4. Gray sandstone passing downwards into shale, 3 feet. Erect tree with Stigmaria roots (e) on the coal.
5. Coal, 1 inch.
6. Underclay with roots, 10 inches.
7. Gray sandstone, 1 foot 5 inches. Stigmaria rootlet continued from the bed above; erect Calamites.
8. Gray shale, with pyrites. Flattened plants.

The beds overlying coal beds are also consistent with the swamp theory: they are aqueous deposits, either freshwater or marine, depending on the location and nature of the swamp.

For these reasons we can be confident that humic coal does indeed have its origins in the deposition of plant matter in swamps.