Coal, one of the world’s most impactful fossil fuels, was formed millions of years ago, in very specific conditions.
Most of the coal on Earth formed approximately 300 million years ago from the remains of trees and other vegetation. Large trees, ferns, and other plants grew in warm, humid places; eventually, they fell down into the swampy water, where they were covered by mud and other sediments.
These remains became trapped on the bottom of swamps, accumulating layer after layer and creating a dense material called peat. Year after year (or rather, millions of years after millions of years), this peat was buried more and more underground, where it was subjected to higher pressure and temperatures. This change in physical conditions triggered chemical reactions that ultimately transform the material into coal.
Coal is still one of the largest sources of energy worldwide, although it’s being phased out in many parts of the world due to its climate impact (we’ll get to that a bit later). But if we want to understand the origins of coal, we have to look back much further — to a period called the Carboniferous.
How coal is formed
The Carboniferous (named after the Latin word for ‘coal’) is a geological period that lasted from approximately 360 to 300 million years ago. It was before the age of the dinosaurs when amphibians were the dominant land vertebrates and vast swaths of huge trees covered the singular mega-continent Pangaea.
The planet’s atmosphere was also different from what it is today; the atmospheric content of oxygen was at its highest level in history: 35%, compared with 21% today. Although it may not be intuitive at first, all these conditions were just right for the formation of coal.
Coal never formed before the Carboniferous, and very rarely formed after it. Two conditions are regarded as crucial for this event. The first of these is the emergence of wooden trees with bark.
Trees started evolving bark and underwent biochemical changes that would make their mass more suitable for an eventual transformation to coal. But the key was timing: a large quantity of wood was buried peacefully in this period because mushrooms and microorganisms hadn’t yet figured out how to decompose trees. In other words, vast areas of fallen trees just lay there and had time to be covered by sediments and undergo the processes that transform plant mass into coal. After these microorganisms did evolve, coal formations became much rarer because the trees would be decomposed before being buried.
The second key element was the lower sea level. 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 they provided the perfect environment where the un-decomposed trees could be buried.
The transformation of trees into coal then became a story of heat, pressure, and time. First, the mass transformed to peat, an accumulation of partially decayed vegetation or organic matter. Peat typically forms in waterlogged conditions. But peat is not coal.
In order for a peat area to continue transforming into coal (and not remain a peatland), more sediment and time is needed. As more sediment piled up on top of the peat, the pressure and heat increase, causing the peat to undergo further chemical changes. These changes resulted in the formation of different types of coal, such as lignite, bituminous coal, and anthracite.
Types of coal
The geological process of changing something under the effect of temperature and pressure is called metamorphism. Coal is considered a sedimentary rock, not a metamorphic rock because it undergoes only low-temperature thermochemical changes — in order for a rock to be considered metamorphic, it would need to undergo far more changes.
Still, coal has been metamorphized somewhat, and the different types of coal are generally classified based on the grade of metamorphism. In general, the higher the grade of metamorphism, the higher-quality the coal (this means a coal that has more embedded energy)
These are the main types of coal:
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 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 is 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.
Coal and climate change
Coal was the key material that sparked the Industrial Revolution, essentially paving the way for what we now consider to be modern society. But coal has become a problem.
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.
But the biggest problem with coal is its greenhouse gas emissions. Coal releases greenhouse gases into the atmosphere, which causes our planet’s atmosphere to retain more heat, triggering what we now call global heating. Even when compared to other fossil fuels like natural gas, coal is much more problematic and yields far more emissions per unit of energy.
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. If we fail to do that and keep relying on fossil fuels like coal, we will face a catastrophic climate future.