Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How does coal form?

ScienceLesson on requestFree to read

Quick answer

Coal forms from ancient swamp plants. When they die, peat accumulates underwater, where low oxygen slows decay. Burial under sediment compresses the peat and raises its temperature. Over millions of years, heat and pressure expel water and gases, leaving a carbon-rich rock that progresses from lignite to bituminous coal to anthracite.

Coal forms when dead plant material accumulates in swamps, is buried under sediment, and is slowly transformed by heat and pressure over millions of years. This process, called coalification, concentrates carbon and drives off water and gases, turning peat into lignite, then bituminous coal, and finally anthracite.

Step by step

  1. Plants die and fall into waterlogged swamps, where low oxygen prevents full decay and peat accumulates.
  2. Sediment like mud and sand washes over the peat, burying it deeper and deeper over time.
  3. Increasing burial depth raises temperature and pressure, squeezing water and volatile gases out of the peat.
  4. The remaining material becomes progressively richer in carbon, transforming from peat to lignite to bituminous coal.
  5. With deeper burial and higher temperatures, bituminous coal turns into hard, shiny anthracite, the highest rank of coal.

Key things to understand

  1. Coal begins as peat, a soggy layer of partly decayed plants that builds up in waterlogged swamps where oxygen is scarce.
  2. When mud and sand bury the peat, rising temperature and pressure start driving off water, methane, and carbon dioxide.
  3. Over millions of years, the buried peat compacts and loses volatiles, gradually increasing its carbon concentration.
  4. The rank of coal rises through lignite, sub-bituminous, bituminous, and anthracite as burial depth and heat increase.
  5. Most of the world's coal formed during the Carboniferous Period, roughly 300 to 360 million years ago, when vast swamp forests covered continents.

Labelled diagram

  1. Peat

    Partially decayed plant matter accumulates in waterlogged, low-oxygen swamps.

  2. Burial

    Sediment deposits cover the peat, burying it deeper over thousands of years.

  3. Compaction

    Pressure squeezes out water, and temperature rises with depth, driving off gases.

  4. Lignite

    Soft brown coal forms as carbon content increases and moisture decreases.

  5. Bituminous

    Further heat and pressure create denser black coal with higher carbon content.

  6. Anthracite

    Extreme burial metamorphoses coal into hard, shiny, almost pure carbon.

From Peat to Coal: The Coalification Process

Real-world examples

  • The Appalachian coal fields in the eastern United States were formed from swamp forests during the Carboniferous Period and contain large deposits of bituminous coal.
  • The Ruhr region in Germany has deep coal seams that formed during the Carboniferous and supported heavy industry for centuries.
  • The Powder River Basin in Wyoming and Montana holds thick seams of sub-bituminous coal that formed from peat swamps about 60 million years ago.
  • The Jharia coalfield in India contains bituminous coal that formed during the Permian Period, around 250 to 300 million years ago.

Common mistake

✗ Myth: Coal is made from dead dinosaurs.

✓ Fact: Coal actually forms from ancient plants, mainly ferns, trees, and mosses that lived in swamp forests. While dinosaurs lived during the Mesozoic Era, most coal deposits formed earlier, during the Carboniferous Period, from plant material that accumulated before dinosaurs existed.

People also ask

What is coal made of?
Coal is mostly carbon, along with varying amounts of hydrogen, sulfur, oxygen, and nitrogen. It also contains mineral matter that becomes ash when coal is burned. The carbon comes from the original plant material, which was rich in cellulose and lignin. As coalification proceeds, the proportion of carbon increases while other elements are driven off.
How is coal mined?
Coal is extracted either by surface mining or underground mining. Surface mining, used when coal lies close to the surface, involves removing overlying soil and rock with large machines. Underground mining, used for deeper deposits, involves digging tunnels or shafts to reach coal seams. The choice depends on the depth and geology of the deposit.
Why is coal bad for the environment?
Burning coal releases carbon dioxide, a greenhouse gas that drives climate change. It also emits sulfur dioxide and nitrogen oxides, which cause acid rain and respiratory problems, along with mercury and particulate matter. Mining can destroy habitats and pollute water. These impacts make coal one of the most polluting energy sources.
How long does it take for coal to form?
Coal formation takes millions of years. Peat builds up over thousands of years, and turning it into lignite and then bituminous coal usually takes tens of millions of years of burial and heating. That is why coal is non-renewable: we burn it far faster than it forms.
What is the difference between peat and coal?
Peat is the first stage of coalification, a soft, brown, spongy material with low carbon content and high moisture. Coal is peat that has been compressed and heated over geological time, driving off water and volatiles and concentrating carbon. Peat can be burned, but it produces less heat and more smoke than coal.
Why are most coal deposits found in layers?
Coal seams are layers because plant material accumulated in cycles. Swamps would form peat, then seas or rivers would deposit mud and sand over it, then another swamp would develop on top. Each cycle left a distinct layer, and later geological forces folded or tilted these layers, sometimes bringing coal close to the surface.
Does coal form today?
Yes, peat is forming today in places like bogs and coastal swamps, such as the Everglades and parts of Indonesia. However, the process is extremely slow, and most modern peat will never become coal unless it is buried deeply for millions of years. The coal we mine today started forming hundreds of millions of years ago.

Part of the guide: Nature & Earth: Living Things and Rocks →