How Coal is Formed: A Process That Takes Millions of Years

What is coal?

Coal is a sedimentary rock with a black or brownish-black color. It is found in underground deposits called coal beds or coal seams. Coal is primarily composed of carbon and hydrocarbons along with varying amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.

Due to it’s chemical makeup, coal is highly combustible. As a result, coal is used around the globe as an energy source to fuel electricity generation. The burning of coal produces significant pollutants and greenhouse gases that contribute to global warming.

How is coal formed?

Coal is formed from a process called coalification where ancient buried plant matter slowly transforms into coal over millions of years due to intense pressure and heat.

The process begins with the remains of plants that lived millions of years ago. First, plants absorbed energy from the sun through a process called photosynthesis. Over time, these plants died, and their remains were buried under layers of soil and other debris.

Over millions of years, the plants are transformed into coal through a series of biological, chemical, and physical changes resulting from intense heat and pressure beneath the earth.

This formation process, known as coalification, occurs in several stages:

Coal Formation Process

Swampy plant material —> Peat —> Lignite —> Subbituminous —> Bituminous —> Anthracite

Heat and pressure increases over millions of years as material is buried under more layers of earth —>

Stage 1: Accumulation of plant material

Coal formation begins in swampy areas where waterlogged conditions prevent the decay of dead plant material. Over time, layers of plant debris accumulate. These areas are often referred to as peat bogs or peat swamps.

359 to 299 million years ago during the Carboniferous period on Earth, climactic and geologic conditions were just right for the formation of peat bogs and marshes.

The climate was warm and humid, which are great conditions for the growth of lush vegetation.
Movement of tectonic plates led to the formation of low-lying basins with poor drainage.
There were high levels of precipitation which is favorable for the formation of extensive swampy regions in low-lying areas.

Stage 2: Formation of peat

In those low-lying waterlogged regions, peat begins to form as plants die and sink beneath the surface. Decomposition is slower underwater compared to dry land due to the low oxygen content. Because plants were decomposing slower, this allowed for the development of thick layers of plants.

As new plant material adds to the layers, the weight and pressure increase on the lower layers. This pressure, along with microbial activity (in the initial stages), begins to compact the material, driving out some of the water and gases. The material at this stage is called peat, a brownish-black material that contains about 60% carbon.

Stage 3: Burial under sediments

As the plants layered on top of each other, geological changes, rising water levels, and precipitation continued to bury the peat under further layers of sediment. As the peat layers get buried deeper, they were subjected to increasing temperature and pressure. This stage significantly slows down the microbial activity and starts the chemical transformation of peat into coal.

Stage 4: Coalification

Buried peat undergoes further chemical changes and physical compaction due to the heat and pressure. This process increases the carbon content and decreases the oxygen, hydrogen, and nitrogen content. Moisture content also decreases during coalification. Over millions of years, this process transforms peat into coal.

Peat first turns into lignite coal, the first true type of coal. As the materials are buried deeper with ever increasing pressure and temperatures, lignite further transforms into subbituminous and bituminous coal. These types of coal are harder, more compact, and have a richer carbon content. Finally, if the conditions are right, bituminous coal can metamorphize into anthracite coal, the highest quality type of coal.

Types of coal

Coal is categorized into four main types, ranks, or classifications of coal. The rank of coal is determined by how long it has undergone the formation process.

The four main types of coal in order of quality from highest to lowest are anthracite, bituminous, subbituminous, and lignite.

Factors influencing the formation of coal include:

Quality and quantity of the original organic material
Time
Temperature
Pressure
Climactic conditions
Geologic conditions

Coal that has been under the earth for longer is generally a higher quality with a higher carbon content and heating value.

As tectonic movements and other geologic processes push coal seams deeper underground, they are subjected to even greater pressures and higher temperatures. Higher pressure and hotter temperatures can enhance the quality of coal, which is why older, deeper coal is a higher quality when mined.

Depth beneath the surface, pressure, and temperature are not the only factors that contribute to the quality and characteristics of the resulting coal. Other factors include what the original plant or animal material was and the conditions of its original burial.

Coal we unearth today in underground or surface mines may have began forming over 300 million years ago during the Carboniferous period, a time when the earth was covered in swampy forests.

Anthracite

Highest rank of coal containing 86% to 97% carbon
Highest heating value among all coal types
Less than 1% of coal mining in the United States
Mainly used by the metals industry and is mined in northeastern Pennsylvania

Bituminous

Contains 45% to 86% carbon
Between 100 million and 300 million years old
Bituminous coal accounted for about 45% of total U.S. coal production in 2021
Used to generate electricity and is important for making coking coal used in the iron and steel industry
Top five states for bituminous coal production are West Virginia, Pennsylvania, Illinois, Kentucky, and Indiana

Subbituminous

Subbituminous coal contains 35% to 45% carbon
Lower heating value than bituminous coal
At least 100 million years old.
46% of total U.S. coal production in 2021 was subbituminous
Wyoming and Montana are major producers, some sub-bituminous coal is also mined in Alaska, Colorado, and New Mexico

Lignite

Lowest rank of coal, containing 25% to 35% carbon
Lowest energy content among coal types
Lignite deposits are relatively young and were not subjected to extreme heat or pressure
Crumbly and has a high moisture content, which reduces its heating value
Lignite accounted for 8% of total U.S. coal production in 2021
North Dakota and Texas are the primary producers, with smaller amounts coming from Louisiana, Mississippi, and Montana
Mostly used to generate electricity

Is coal renewable?

Coal is not considered a renewable resource because it takes hundreds of millions of years to form. That means we can consume more coal than the Earth can produce and hypothetically can eventually run out of coal to burn.

Written by Graham Lumley

Graham Lumley is the Growth Product Manager at BKV Energy, a retail electricity provider in Texas' deregulated market, where he has worked since 2023. He analyzes customer usage and billing data, helps develop BKV's fixed-rate plans, and creates tools to simplify Texas electricity shopping. He has written over 200 articles on Texas electricity rates, delivery charges, plan types, energy generation, and more to help Texans understand their bills and pay less. With over 10 years of experience in consumer products, marketing, and energy, he explains the Texas electricity market in plain language.