Humans have been burning coal for thousands of years; since the industrial revolution, coal has become an important source of both electricity and global warming. But where does coal come from? By studying how coal is formed, scientists can learn both about the deep past and about what to expect when different coals burn.
Coal is formed when swamp plants are buried, compressed and heated to become sedimentary rock in a process called coalification. “Very fundamentally, coal is fossilized plants,” James Hower (opens in a new tab), a petrologist at the University of Kentucky, told LiveScience. The creation of these plants fossils involving, “many accidents of geology,” he said.
Coal formation starts with living plants. “When the tree is still alive, it can be damaged by burning or it can be invaded by insects,” Hower said. “All these things will show up in the coal protocol.” Traces of pollen, leaves, roots and even insect fungi in charcoal, Hower said, can be used to reconstructing old ecosystems. Burn injuries (opens in a new tab)for example, provide clues to ancient climates.
Plants then die. “If the coal is preserved at all, it says something about the overall environment,” Hower said. Plants on mountain slopes or in deserts are unlikely to turn into coal because these environments do not contribute to the formation of peat.
“Of all the coal we see out there, a very, very high percentage came from swamps,” Hower said.
Related: Why is there so much oil in the Arctic?
That’s because when plants die in wetlands, they are covered by water and shielded from oxygen. As a result, they don’t decay as quickly as they would on dry surfaces. Instead, plants build up into layers of peat on the soft bottom of the swamp. That peat, which is sometimes a precursor to coal, has its own long history: it is home to insects, fungi, bacteria and even burrowing tree roots, which all help to break down (opens in a new tab) plants in a process called peat formation. “Any layer we see in a litter could be the product of tens or hundreds or thousands of years,” Hower said.
Minerals that seep into the peat from the water or that are formed by chemical reactions are also captured in coal. Fire charcoal (opens in a new tab) in eastern Kentucky, Hower said, contains rare earth elements from a volcanic eruption millions of years ago; the US Department of Energy is now financing (opens in a new tab) technologies to extract these elements from coal waste for use in solar panels, wind turbines and batteries.
But the minerals in coal also cause problems. Peat exposed to seawater, e.g. often contains (opens in a new tab) more sulfur. Burning coal with sulfur comes with an additional human cost; while mining coal and breathing coal smoke are both parts generally dangerous (opens in a new tab)high sulfur coal may be more likely to do so spontaneous combustion (opens in a new tab) in mines and they can also be linked to (opens in a new tab) heart disease.
Not all peat is transformed into coal; some erode or dry out. To start the process of coalification, the peat must be covered by something inorganic, such as silt from a wide river delta. “The river that just goes back and forth over millions of years, that ends up being your depositional system,” Hower said, referring to layers of built-up sediment.
Over geological time, peat is buried even further. Mountains erode and fill up river valleys; forest grows on top. Over millions of years, new mountains rise. During these millennia, the peat breaks down and gradually turns into coal thanks to two elements: pressure and heat (opens in a new tab). Most coal is between 60 million and 300 million years old.
Pressure makes turf more compact. Heat reorganize (opens in a new tab) the recognizable molecules in plants – such as carbohydrates or cellulose – and releases (opens in a new tab) oxygen and hydrogen, leaving behind carbon and other elements.
Coal that is buried very deep experiences higher temperatures because it is closer to the Earth’s core. But geothermal heat (opens in a new tab) can also reach the earth’s surface through volcanoes, hot springs and geysers. The amount of pressure and heat generally determines the coal’s rank: a measure of how far the coal has come on its journey from soft peat to solid rock.
Lignite is the lowest rank of coal; lignite and sub-bituminous coal still contain recognizable plant parts. Bituminous and sub-bituminous coals have been compressed and heated until they are hard. Anthracite coal, the rarest and highest rank, is smooth and shiny; it has been heated to liquid in a process called metamorphosis. To reach the anthracite craving, Hower said, it is enough to reach a high temperature briefly – even an hour will do the trick.
Anthracite burns without producing soot; they were used historically by coal-powered ships trying to avoid detection (opens in a new tab) in wartime. Lignite and bituminous coal are mostly used for power generation. Lignite and sub-bituminous coal are released little more (opens in a new tab) carbon dioxide than bituminous coals when they burn.
However, these differences are small when coal is compared to other electricity sources that have a lower impact on global warming. In general, coal produces twice as much carbon dioxide per kilowatt hour as natural gas and 90 times as much as wind power, according to US Department of Energy (opens in a new tab).
“Emissions from coal and from industrial processes involved with coal have obviously not been good for the climate,” Hower said. “That’s the reality we live in.”