Most coal formed approximately 300 million years ago from the remains of trees and other vegetation. These remains were trapped on the bottom of swamps, accumulating layer after layer and creating a dense material called peat. As this peat was buried more and more underground, the high temperatures and pressure transformed it into coal.
Coal is still the largest source of energy for the generation of electricity worldwide, though it’s being phased out in many parts of the world due to its impact on the climate. But if we want to understand the origins of coal, we have to look back much further — to a period called the Carboniferous.
The Carboniferous (after the Latin name of coal) took place approximately 360 to 300 million years ago. Amphibians were the dominant land vertebrates and vast swaths of huge trees covered the singular mega-continent Pangaea. The atmospheric content of oxygen was at its highest level in history: 35%, compared with 21% today; all the conditions were ripe for the formation of massive coal beds.
Coal never formed before the Carboniferous, and very rarely formed after it. Two conditions are regarded as crucial for this event:
- the emergence of wooden trees with bark; a large quantity of wood was buried in this period because mushrooms and microorganisms hadn’t yet figured out how to decompose trees. After they did, coal formations became much rarer.
- the lower sea levels; the decrease of the sea level created many swampy environments in what is today North America and Europe. These swamps were vital for coal formation.
As mentioned before, these trees were not decomposed by anything and were preserved. In time, they were buried. As they went deeper and deeper, temperature and pressure started building up and started to transform the coal.
Types of coal
The geological process of changing something under the effect of temperature and pressure is called metamorphism. Coal is generally classified into types based on the grade of metamorphism — the higher the grade of metamorphism, the more energy they contain:
- peat is generally considered a precursor of coal, but it has been used as a fuel in some areas — most notably in Ireland and Finland. In its dehydrated form, it can help soak up oil spills.
- lignite is the lowest quality and the first to be formed.
- sub-bituminous coal is most often used as fuel for steam-electric power generation.
- bituminous coal is a dense sedimentary rock, generally of a high-quality.
- “steam coal” is a transition type between bituminous and anthracite.
- anthracite is the highest rank of coal. It’s a hard, glossy rock and highly valued for its properties.
- graphite is not generally considered a type of coal because it cannot be used for heating. It is most often used in pencils or as a lubricant (when powdered).
Coal can be used in its natural form, or it can be either gasified, liquefied or refined. However, no matter the type of coal or how you use it, coal is a non-renewable resource. In realistic terms, no coal is being formed to restock the resources we are using.
The adverse effects of coal
Coal is one of the main contributors to global warming, and coal mining and its fueling of power stations cause major environmental damage.
Historically, coal mining has been very dangerous. The list of coal mine accidents is long, and even today, accidents are still surprisingly common. Many miners also suffer from coalworker’s pneumoconiosis, colloquially known as “black lung”. But the main problem with coal is its emissions.
In 2008 the World Health Organization (WHO) calculated that coal pollution alone is responsible for one million deaths annually across the world; other organizations have come up with similar figures. According to a US report published in 2004, coal-fired power plants shorten nearly 24,000 lives each year in the US (2,800 from lung cancer). In China, the situation is even more dire as smog is a common occurrence in many major Chinese cities.
Burning coal releases great quantities of carbon dioxide into the air and also releases methane — a much more potent greenhouse gas. Methane accounts for 10.5% of greenhouse gas emissions created through human activity. Coal may have allowed the industrial revolution to take place, but if we want to build a sustainable future, we simply have to phase out coal and implement other sources of energy in its stead.