Origin of Alaska's Coal

Unlike petroleum and natural gas which form from plant life in the marine environment, coal derives from plants that grow on land.

Much of the world's coal is thought to have developed from plants growing in swampy areas near ancient shorelines. Some scientists have suggested that there is evidence of occasional intrusion of the seas into areas where coal is starting to form and that part of the coals may actually have derived from plant debris, marine or terrestrial, laid down by these shallow encroaching seas. But it seems clear that some coal beds are strictly terrestrial, that is, formed entirely on land. The coal fields of central Alaska in the Nenana area are examples of strictly terrestrial coals.

The first requirement for forming a coal bed is to accumulate peat like plant remains in a strata. Then the peat-like deposit must be covered up by a layer of inorganic material such as clay, silt, sand or gravel which might be laid down by a lake or stream. If the original organic bed is not so covered it merely turns to peat.

However, once the coal-to-be plant remains are covered over, they undergo biochemical changes that cause them to give up oxygen and hydrogen to the overlying sediment beds. Microorganisms play a role in this process by decomposing the cellulose, proteins and lignin materials of which the plants are made. The remains of the biochemical decomposition have high carbon content and these start to transform into identifiable coal particles called macerals. Now, not everyone is likely to become overly excited about macerals, but coal miners really dig them.

Several types of macerals form. One type is a charcoal-like fibrous maceral made of plant cell walls and which exhibits the structure of the cell walls. Another maceral is vitreous and shiny; it forms from lignin and cellulose remains.

As the biochemical phase of coal development progresses, another geochemical phase begins. It requires the high temperature that comes from deep burial of the coal bed by progressively built up overburden layers. If a bed is buried to a depth of 10,000 feet, the temperature will rise to the boiling point of water. This is hot enough to create bituminous coal if the temperature persists long enough, perhaps a million years or so. It may even be hot enough to create the higher grade anthracite coal, but the time taken would be much longer.

Most of Alaska's coals started as plants growing during the Tertiary and Cretaceous ages, 30 to 130 million years ago. Most of America's best coals are much older, having been started during the Mississippian or Pennsylvanian periods, 280 to 350 million years ago. Coals are like people, the older they are the better they usually get. Consequently, much of the coal found in other parts of the country ranks higher than most. Alaskan coal is largely lignite, sub-bituminous or bituminous. There is apparently comparatively little anthracite coal in the state.