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examBoard: Cambridge
examType: IGCSE
lessonTitle: Formation of Coal
Coal is one of our most important fossil fuels, providing about 27% of the world's energy. But have you ever wondered how this black rock formed? Coal has a fascinating origin story that spans millions of years and begins with ancient swamps and forests.
Key Definitions:
The coal we burn today is made from plants that grew 300-400 million years ago, when dinosaurs hadn't even evolved yet! These ancient plants captured sunlight through photosynthesis and that stored solar energy is what we release when we burn coal today.
Coal formation is a slow process that takes place over millions of years. It begins with the accumulation of plant material and progresses through several stages as pressure and heat transform the material.
The process begins in swampy areas where plants grow and die. In waterlogged, oxygen-poor conditions, dead plant material doesn't fully decompose. Instead, it accumulates as a spongy material called peat. Peat is the first stage in coal formation and contains about 60% carbon.
Over time, layers of sand, clay and other sediments cover the peat. As more layers pile on top, the weight creates pressure. The peat gets squeezed and water is forced out. This compression is a critical part of the coal-forming process.
As the peat gets buried deeper, it experiences higher temperatures. The combination of heat and pressure causes chemical changes in the peat. Carbon content increases while oxygen and hydrogen decrease. This process is called coalification.
Earth's crust movements can bring coal seams closer to the surface, making them accessible for mining. Without these geological processes, much of our coal would remain too deep to reach.
As coal forms, it passes through different stages. Each stage represents increasing carbon content and energy value as more pressure and heat are applied over time.
The first stage - soft, spongy material with high moisture content (60% carbon). Not yet coal, but the precursor.
Soft, brown coal with visible plant remains (70% carbon). Has more energy than peat but less than higher-grade coals.
Black, dense coal with high energy content (80-90% carbon). The most common form used for electricity generation.
The highest grade of coal (over 90% carbon). Hard, shiny and produces the most energy with the least smoke when burned.
As coal matures: Peat (60%) → Lignite (70%) → Bituminous (80-90%) → Anthracite (>90%). Higher carbon means more energy when burned.
Coal doesn't form just anywhere. Specific environmental conditions are necessary:
Most of the world's coal deposits began forming during the Carboniferous Period (359-299 million years ago). During this time, Earth was covered with vast swamp forests filled with giant ferns, club mosses and horsetails that could grow up to 30 metres tall! The climate was warm and humid, creating perfect conditions for lush plant growth. When these plants died, they fell into the swampy waters and began the long process of becoming coal. The name "Carboniferous" literally means "coal-bearing" - that's how important this period was for coal formation!
Coal typically forms in layers called seams, which can vary greatly in thickness and quality:
Coal seams can range from a few centimetres to many metres thick. The thickest coal seam in the world is the Thick Coal seam in the Donbas region of Ukraine, measuring up to 60 metres thick! Most commercial coal mining targets seams at least 1 metre thick.
Geologists can determine coal quality by examining its appearance, carbon content and the presence of impurities. Higher-quality coal is typically darker, harder and more lustrous. The presence of plant fossils can indicate younger, less mature coal.
Coal isn't found everywhere on Earth. Its distribution reflects the ancient geography of our planet and where conditions were right for coal formation.
The largest coal reserves are found in:
In the UK, coal deposits are found in several coal fields including Yorkshire, South Wales and the Midlands. These coal deposits formed when the UK was located near the equator during the Carboniferous Period, with tropical swamp conditions perfect for coal formation.
Coal formation is a significant part of the long-term carbon cycle. When plants photosynthesize, they remove carbon dioxide from the atmosphere and convert it to plant tissue. When plant material becomes coal, this carbon is locked away underground for millions of years. When we mine and burn coal, we release this ancient carbon back into the atmosphere as carbon dioxide, contributing to climate change. This is why coal is considered a non-renewable resource - it takes millions of years to form but only moments to burn.
Understanding coal formation is crucial for environmental management for several reasons:
As we face the challenges of climate change and the need to transition to cleaner energy sources, our understanding of coal - how it formed, where it's found and what happens when we use it - becomes increasingly important for making informed environmental management decisions.
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