Is coal still being formed today? (original) (raw)
Is the process that is at the origin of coal still existent, why or why not, and where does proof of it exist?
Most of us rarely pick up a piece of coal these days, but if you do find a lump of the black rock, you'll be holding a piece of geological history.
"In general, a high quality black coal seam would take millions of years, if not hundreds of millions of years to form" says Dr Judy Bailey, coal geologist at the Discipline of Earth Science University of Newcastle
The process of coal formation is still taking place today, says Bailey.
"The precursor to coal is called peat, and that is just uncompressed plant matter."
Peat accumulates in wet swampy environments known as mires, and that process is taking place today in areas such as Indonesia and even the Antiplano in the Andes.
"Mires are swamps with trees growing in them, swamps with reeds, stagnant water into which pollen and plant matter fall, and coastal lagoons. Peat can even form in the highlands in rain-fed or glacier-fed lakes in mountain ranges."
However, peat accumulates very slowly at about one millimetre a year on average, says Bailey, although it can happen faster, up to 2 to 3 millimetres per year in the tropics. At that rate, it would take about 12,000-60,000 years to accumulate enough peat to form a three-metre coal seam.
The transformation from peat to coal takes even longer. It generally starts with burial of the peat by other sediments as a result of a volcanic eruption, migration of a river or a change in sea level.
"The pressure of overlying sediment squeezes the water out and causes the peat to compress," says Bailey. The thickness of the peat will be decreased by about ten to one during this process.
The transformation from a plant substance to a metamorphic rock really starts once the peat is buried beneath 3 — 4 kilometres of sediment. At this depth, with an average rate of temperature increase of 30°C per kilometre, the temperature rises to over 100°C and sets off chemical reactions that transform the material into coal.
"The chemical reactions release volatiles," says Bailey, "They help to compress the peat even more and it changes from being a plant substance, like lignin or cellulose, to a geopolymer that contains concentrated carbon. It's very different from peat or plant matter."
The amount of transformation from peat to coal is described by a coal's rank.
"Brown coal and lignite are the lowest rank, then bituminous or black coal. As the temperature and pressure rises even more it changes to anthracite. And eventually some of the earliest coals that would have formed have been metamorphosed into graphite."
The Carboniferous and fungi
The formation of coal seams really kicked off with the diversification of land-based plants around 350 million years ago.
"That was pretty much from the end of the Devonian into the Carboniferous period. Algae was around long before then in shallow seas, so there are coals made completely of algae that date back earlier than the Carboniferous."
The Carboniferous period (300-360 MA) saw the evolution of tall lycopod trees that accelerated the rate at which peat could be formed in tropical equatorial mires. High sea levels and a warmer climate also encouraged coal formation, by extending the area of coastal mires and other wetlands.
Last year, researchers suggested that the evolution of white rot fungi — the fungus that breaks down plant lignin — at the end of the Carboniferous period may have slowed down coal formation. But Bailey disagrees with this theory.
"Coal formation underwent a drastic change at the end of the Carboniferous period, but for other reasons," she says.
"There was a global ice age at the end of the Carboniferous, and the continents were drifting to new locations, so coal accumulation occurred in cold temperate places, closer to the poles.
"The cold favoured a new type of plant called Glossopteris, dominated by gnarly little trees which lost their leaves in winter.
"Plants grow more slowly in the cold, so this could have slowed peat accumulation, but frozen plant matter is less easily decayed and better preserved. It would be hard to distinguish any change in peat accumulation rate due to white rot fungi from the effects that climate change were having on peat."
Peat continued to accumulate strongly throughout the Permian (245-300 MA), when coalfields in the Hunter, Newcastle and Illawarra were forming. However it paused for a period at the Permian — Triassic boundary.
"Coal formation did stop for about 15 million years at the end of the Permian," says Bailey, "but this was due to a global extinction which wiped out most land plants. About 90 per cent of all species on Earth were wiped out at this time."
Once the plants recovered, coal formation began again. This started with the recovery of spore-generated ferns, and a global "fern-spike". Land plants were unusually dominated by ferns until other plants regenerated.
While the coal-forming process is still happening today, we interrupt that process when we mine coal, particularly of lower rank. If we left lower rank brown coal for a few more million years it would turn into black coal.
"Coal takes longer to form than any other rock type," says Bailey.
Ironically, warming of the Earth's climate may increase the number of swampy coastal environments that are perfect for coal formation. But these coal seams won't be ready for a few million years.
"Industrial use of fossil fuels producing carbon dioxide faster than the ocean can dissolve it or plants store it, will regenerate tropical coal-forming conditions.
"This is a typical feedback cycle that regenerates the planet over geological time scales, but does not happen on a fast enough timescale to make dependence on fossil fuels sustainable for humankind right now."
Dr Judy Bailey is a lecturer at Newcastle University's Discipline of Earth Sciences. She's a coal geologist, and coal and sedimentary petrologist. Dr Bailey spoke to Kylie Andrews.