The widespread use of coal was one of the major factors in fueling the Industrial Revolution of the late 1700s and early 1800s. In recent decades, however, coal has received a bad reputation as a major source of acid rain and carbon dioxide in the atmosphere. Then there has always been the proverbial threat of receiving a lump of coal for Christmas if you have been bad.
The formation of coal is fascinating. It has all of the elements that make a good story: forests and swamps, death and decay, and dinosaurs.
Coal can be defined as a rock. The American Geological Institute Dictionary of Geological Terms describes coal as “… a readily combustible rock containing mostly carbonaceous material.” Carbonaceous means it contains carbon. Both the good and evil of coal are a result of it being an excellent source of carbon. The highest grade coals can be up to almost 90 per cent carbon. Yukon coal deposits range from 35 to 80 per cent carbon.
There are many known coal deposits in the Yukon, ranging from south of Whitehorse to the Bonnet Plume, west of Dawson to east of Watson Lake, and lots of places in-between. These various coal deposits all have different environments and times in which each formed. One of the most interesting and best-studied coal deposits in the territory is located just outside of Ross River.
The deposit is near Whiskers Lake on the access road into Ross River from the Robert Campbell Highway. The reason this one has been well studied is because it was actually open-pit mined in the 1980s and early ’90s to provide a source of heat to dry the lead-zinc concentrate at the Faro mine. The coal was mainly in two seams, both one- to two-metres thick.
The story of the Ross River coal begins with the formation of the Tintina Trench (see “Your Backyard Geology” Part 3). The Ross River coal deposit lies right within the fault zone that forms the trench. As ground was pulled apart and moved around by the faulting, some areas were uplifted; other areas dropped and formed basins. It is within one of these basins that the coal was formed.
In the basins, water accumulated and sediments were washed in from the surrounding higher ground. That is why we see conglomerates, sandstones, siltstones, and mudstones adjacent to the coal seams.
Approximately 100-million years ago, the climate was warm and wet around the world. There were jungles and swamps with lush vegetation. Vegetation died and was washed into the basin and accumulated to form peat bogs. At Ross River, you can find many fossil leaf imprints in the mudstones adjacent to the coal deposit.
At times there might be less vegetation and more sand or clay being deposited. If you look at any coal bed, you can quite often see these “partings” where the environment has varied. The main controlling factor would be the fluctuation of the amount of water going into the basin. Not all peat bogs turn into coal. A process called coalification is what turns this vegetation into coal.
Normally, vegetation, leaves and other debris from plants like roots and trunks, when they die, are attacked by fungi first. This turns them into a soggy mat. Then bacteria take over, especially if the decayed vegetation is covered by water, as in a swampy environment.
The oxygen in the vegetation is used up in the biochemical process of breaking down the vegetation. The vegetation, when it was growing, averaged about 50 per cent carbon and 40 per cent oxygen. The carbon increases and the oxygen decreases as the coalification process proceeds. Sulphur from the plant material and minerals, washed into the bogs, is also involved in the process. That is why coal contains sulphur, which can form acid rain when it is burned.
Eventually, due to changes in the environment, the deposit of organic material ceases and the decaying vegetation becomes covered by silt or sand. The biochemical processes slow down but do not stop.
As the organic material becomes covered, pressure becomes the main driver of the coalification process. It is thought that the compaction ratio of peat to (eventually) coal is 10 to one. That is, it would take 10 metres of peat to produce a one-metre-wide coal seam. Today’s peat bogs generally only accumulate at a rate of less than one millimetre per year, or one metre every 1,000 years. In order to form a two-metre-wide coal seam at Ross River would have taken a peat bog around 20,000 years to develop.
Over the millions of years since being deposited, the peat has been covered by hundreds of metres of other rocks. In addition, tectonic pressure from rocks being moved around, by faulting and folding, can create great pressures. Heat is also a driver, either from the pressure itself or nearby hot magmatic rocks.
The peat changes first to lignite, often called brown coal. Then into what is named bituminous coal, and finally to anthracite.
In 1922, a scientific paper, written by Scottish-born botanist Dr. Marie Charlotte Carmichael Stopes, described the knowledge of the chemical composition of coal as being “in the Dark Ages.” She devised the first attempt to describe the composition of coal and categorizing the components with these rather poetic terms: fusain, durain, clarain and vitrain. Although these terms are still used today, the three basic constituents of coal have been simplified to describe it: (1) the woody residue of the original plants, (2) the resinous saps and pollens, and (3) non-combustible material or ash.
Doctor Stopes was also a leading advocate for women’s rights during her life and it is well worth researching her amazing life further.
The exciting thing about the formation of the Ross River coal deposits, and another reason it has been well studied, was the discovery in 1999 of dinosaur tracks in the rocks adjacent to the deposit. They were discovered by a team from the University of Alaska at Fairbanks, lead by Dr. Roland Gangloff, Alaska’s father of dinosaur research. In subsequent years, further work by the Government of Yukon’s Heritage Branch paleontologists and the Yukon Geological Survey were able to date the tracks at approximately 100-million years old.
This was older than geologists originally thought the coal was. The dating was done using pollen found in the sediments, a science called palynology. Several different dinosaurs tracks have been identified including Carnosaur, Hadrosaurs, Ornithomimus and Euoplocephalus. Stop in at the Ross River School; they have some of the actual tracks on display.
All the tracks are found in fine-grained sandstones. Scientists believe these were originally floodplain deposits adjacent to rivers flowing down the Tintina Trench. These elevated linear features made convenient routes for dinosaurs, travelling through the area, to stay out of wet areas and avoid the thick vegetation.
The formation of coal is a mix of biology, geology, chemistry and time. From origins in swampy basins and lush forests roamed by dinosaurs 100-million years ago, to its use for good and bad today. The two main uses today are electrical generation and its combination with iron to form steel. Coal is truly a rock that has many stories.