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.
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.