Coal Formation

Coal is a fossil fuel. It is made from the dead remains of prehistoric plants.

Coal originated as forests, similar to this one in Hawkes Bay. Image from Julian Thomson.

Coal originated as forests, similar to this one in Hawkes Bay. Image from Julian Thomson.

New Zealand's coals were formed at different times. Greymouth, Ohai, and Kaitangata are about 70 million years old. Waikato coal is about 40 million years old. Our youngest economically important seams are in Taranaki, Otago, and Southland and are only about 20 million years old. Some lignite near Dargaville is only 1 million years old.

Coal originates as dense swampy forests. After the plants died, the high water table protected many of the plant remains from decomposition. This woody material accumulated to became peat. Peat is soft and easily compressed but when it is buried deeply, the water is squeezed out and the carbon rich substance is changed into coal.

Waikato Coalfield extraction. Image from Steve Edbrooke, GNS Science.

Waikato Coalfield extraction. Image from Steve Edbrooke, GNS Science.

It took about 15 metres of peat formation to produce a one metre layer of coal in the Waikato coalfields.

A coal’s rank (quality) is a result of the temperature and pressure from the depth of burial. The deeper a coal is buried the less water it contains. As the depth, and thus the coal's rank increases, so does its hardness, brittleness, brightness, and energy potential.

Coal Type Depth of Burial Max. temp during burial Moisture NZ examples
Peat <0.2 km 0 - 25°C 50 - 80% Chathams
Lignite 0.2 – 1.5 km 25 – 40°C 30 - 50% Southland, Otago
Sub-Bituminous 1.5 – 2.5 km 45 - 75°C 10 – 30% Waikato
Bituminous 2.5 – 6 km 75 - 180°C 5 – 10% Buller, Greymouth
Anthracite >6 km >180°C <5% None