What is coal?

Coal 1 is compressed and altered plant material that forms layers in rocks, a process taking many millions of years. 

Dense forests at periods in the Earth's geological history became flooded, and the organic material of which plants are made were buried under layers of sediment. Over time2 the pressure of overlying rock, and accompanying higher temperatures, converted the plant material into an increasingly carbon-rich mineral with impurities, in particular, water, hydrocarbons ranging from methane to bitumen or tar, and sulphur. 

The first stage of coal formation is typically a peat3 bog in which carbon is preserved in situ in oxygen-poor conditions. More plant material accumulates than can be broken down via bacterial decay.

As the peat bog is buried, the pressure of overlying sediment squeezes out water, and drives off the more volatile hydrocarbons and other organic compounds. Over geological time the coal increases in carbon content (or rank) from peat to lignite (link to lignite article), to sub-bituminous and bituminous coal4, and, in parts of the world, anthracite and graphite.

Most of the world’s coal seams were formed during the Carboniferous Period5 (360 – 290 million years ago) when the world was warmer and more humid than today, and when plants had first appeared in quantity on land.

Other important coal-forming periods6 occurred during the more recent Permian Period (290 – 250m years ago), and during the Mesozoic Era (250 – 65m years ago). Coals laid down during the Tertiary Era (less than 65m years ago) are generally less mature, much of it lignite, because there has not been time for this coal to be buried deeply to raise the rank of the coal.

New Zealand coals7 tend to be "young" coals, 30 - 70m years old, and are distinct being made of different plant species to those of earlier times. As a result, New Zealand coals often contain less undesirable impurities, e.g., sulphur, phosphorus, mercury and ash. Our higher-ranked coals are highly sought after internationally as coking coal for steel-making.

New Zealand coals range in rank from lignite to bituminous because of plate tectonics – the geological processes8 that formed our mountains also hardened the coal seams.

View basic coal facts9 suitable for school age children.