…Where does coal come from?
…How was coal formed in New Zealand?
…What is the history of exploration for coal in New Zealand?
…What types of coal are there in New Zealand?
…Where is the coal in New Zealand?
…How is coal mined?
…What is coal used for?
…How are coal mines rehabilitated?
…Links to other sites

…ACTIVITIES
…WORKSHEET

Where does coal come from?……

Coal is not a mineral. It is a fossil fuel. It is formed from vegetable material, but it is mined in much the same way as gold and limestone and aggregates, and is treated in much the same way as other minerals. Some geologists classify coal as a fuel mineral.

Coal is formed by plant material being buried by sediment in valleys, basins and river beds. Huge swampy forests of giant ferns and other plants grew and died and fell into the swamp. More grew and died and fell. Over time a thick layer of dead plant material developed. This produced thick layers of peat. Dirt and sand washed into the water covered the peat and stopped the material from completely decomposing. Other layers of plant material may have grown and died and been covered too.

The pressure of the thick layers of sediment and the associated higher temperatures which occurred as the once living material decomposed removed carbon dioxide, hydrogen, oxygen, and sometimes other gases such as methane and butane. There are also a wide range of trace elements from the plant material.This process - called metamorphism - left behind rich carbon deposits - coal.

Increasing metamorphism results in increasing coal rank. Lignite, sub-bituminous, bituminous, and anthracite coals are produced by increased pressure and heat resulting from the depth of burial. As the coal's rank increases so does its hardness, brittleness, brightness, and energy potential.

Geologists estimate that on average about six metres of plant material would be required to produce two metres of peat which would eventually produce a seam of coal about 30 cm thick. About 15 metres of peat formation produced the one metre thickness of coal in the Waikato coalfields.

The oldest known coalfields in the world date from 360 million years ago. Most of the very large deposits in Europe and the United States are from the Paleozoic Age (250 million years ago). In New Zealand most coal deposits are between 35 and 75 million years old (Eocene-Cretaceous). This is very young geologically. These deposits may have been buried very deeply, and perhaps even covered by the sea. Later faulting and folding has brought the coal closer to the surface. There are still very large coal reserves offshore. A huge coal reserve off the Taranaki coast has contributed to the Maui and Kapuni gasfields. (Whereas coal is formed from deposits of decaying plant material, natural gas and oil are thought to have been formed from microscopic marine plants and animals in ocean sediments).

How was coal formed in New Zealand?……

New Zealand's economically important coal seams began to form about 75 million years ago when the sea started to rise gradually against a low-lying land mass. This meant that the rivers had less energy and could not carry their sediment load to the sea. Extensive swampy flats were formed. Peat developed when there was a favourable balance between water level, sediment supply, and plant growth.

In addition New Zealand's hilly terrain provided valleys not much above the rising sea levels. They were quickly flooded with gravels, sand and mud which covered the vegetation layers which had been deposited. Shallow lakes and peat swamps developed. In time these were covered by the sea and deeply buried under marine sediments.

At the same time, the downwarping of some areas such as the present Greymouth Coalfield allowed thick coal measures (non-marine coal-bearing sequences of rocks) to develop. The pattern of the coal seams was complicated by other geological factors, such as the closeness of hilly areas supplying sediment to the coal forming basin, or by faulting disturbing peat accumulation.

So most of New Zealand's coal has its origins in either extensive coastal swamps or in shallow inland basins.

As the land subsided the thickness of sediment cover over many peat deposits increased rapidly. It is the depth at which the coal has been buried (and so the pressure to which the coal has been subjected) which is important, not its age. Because of this some of New Zealand's coal contains as much energy potential as many much older overseas coal deposits.

Much later the sunken land was thrust up above the sea and gradually eroded. This left the coal sufficiently near the surface to be economically viable to mine. Many coal seams remain inaccessible. Oil exploration drilling wells frequently pass through thick layers of coal over 4000m below the surface.

Not all of New Zealand's coals were formed at the same time. Greymouth, Ohai, and Kaitangata are about 70 million years old. Waikato coal is about 40 million years old. Our youngest economically important seams in Taranaki, Otago, and Southland are only about 20 million years old. Some lignite near Dargaville is only 1 million years old.

In a modern day continuation of the coal forming process, large blocks of peat are still developing in places such as the Chatham Islands and Northland.
(parts of this section have been adapted from Alpha 32: Coal geology in New Zealand)

What is the history of exploration for coal in New Zealand?……

In the early days coal was only found when it outcropped or was visible in streams and rivers. In the Waikato, early European settlers and missionaries were shown outcrops of coal by local Maori. Samples were sent back to Auckland for analysis by blacksmiths in the 1840s. The coal was found to be of good quality.

In the South Island whalers broke off lumps of coal at Shag Point in Otago to fuel their blubber pots in the 1830s.

Land wars in the Waikato delayed mining development in the area until the 1870s, but in the South Island many small mines were beginning to recover easily accessible coal. Immigrants from Britain brought with them the skills necessary to mine coal seams.

From these times through to the 1970s there was little economic reason to fully explore the extent of New Zealand's coalfields. Miners had more than enough coal available. The oil shortages of the 1970s provided the reason to develop energy sources which did not depend on overseas supply. The government began a national survey to find out how much coal there was in New Zealand and where it was.

The Coal Resource Survey increased New Zealand's known reserves from 2 billion tonnes in 1973 to 8 billion tonnes in 1978. In the same year State Coal Mines began production at Huntly East and West underground mines, and the next year coal exports to Japan were approved.

The current estimate of New Zealand's in-ground coal resource is 16 billion tonnes, spread across both the North and South Islands, with commercial mining operations concentrated in the Waikato, West Coast, and Southland regions.

For a timeline of coal exploration in New Zealand click here.

What types of coal are there in New Zealand?……

Coal resources in New Zealand are divided into three main types - or ranks - bituminous, sub-bituminous, and lignite. There are no true anthracite coals in New Zealand.The type of coal depends on the plant material it came from and the process it has been subjected to. High ranked coals have less water, less volatiles, and more carbon than lower ranked coals.

Bituminous coal is a high rank coal. It is black in colour, has a high heat output when it burns, and a low water content. Lignite coal is a low rank coal which produces much less heat. It has a high water content, sometimes up to 35%, and the remains of leaves and branches from which it was formed can often be seen. Sub-bituminous coal is between lignite and bituminous coal.

Where is the coal in New Zealand?……

South Island Coals
The South Island's West Coast is one of the country's most geologically active regions. The Buller and Greymouth coalfields have been buried deeply and lifted fairly rapidly. As a result, they contain bituminous coal, much of it well above sea level. The Ohai field in Southland has seen less activity so its coals have not quite reached bituminous rank or been raised as high. The lignites of Eastern Southland have been subject to only shallow burial followed by slight tilting and uplift.

North Island Coals
The coal bearing areas of the North island have all had fairly similar geological histories. The mineable coal is sub-bituminous. The bituminous coals of the North island are buried thousands of metres below onshore Taranaki and offshore from Northland to North West Nelson. In these areas the peat has been buried to great depths and converted to coal but never raised above sea level.

When coal was first discovered the high rank coal of the West Coast was the main type produced. This high heat coal was used in ships, trains, and in town gas works. Today it is the basis of a growing export market. In New Zealand after World War Two shipping turned to oil, railways to electricity and diesel. Later natural gas from Taranaki replaced coal gas.

Production of lower ranked coal has increased and now accounts for about two thirds of production. It is used mainly for steel making at Glenbrook near Auckland, and for electricity generation.

Coal is still used directly for processing by major industries such as cement manufacturing, dairy factories, and meat and wool processing. Many large government and commercial institutions are heated by coal, especially in the South island.

Today the main coal mining areas in New Zealand are around Huntly in the North Island and near Greymouth and Westport in the South Island. Most West Coast coal is exported.

How is coal mined?……

The same geological activity which raised coal seams from under the sea and made them accessible has also made many coal reserves in New Zealand very difficult to mine. Some seams are contorted, making mining impractical. Others are gently folded which means that mining operations must follow the seams up and down. Other seams have been tipped and tilted, broken and faulted. All of these factors contribute to the decision to mine and the techniques used.

Underground Coal Mining
Underground mining methods are used where surface methods are unsuitable because of depth, or the shape of the deposit, or where the land over the resource is very steep or particularly valuable. Underground mines produce less waste material than open pit mines and disturb a smaller amount of land, but mining costs are generally much higher.

Coal is mined using underground methods at Huntly in the North Island and near Greymouth on the West Coast of the South Island. Two underground mining methods are used:

• mechanised bord and pillar mining
• hydro mining

Bord and pillar mining creates a series of tunnels called bords through the coal seam being mined. The overlying rocks (the roof) are supported by pillars of coal that remain between the tunnels. When the tunnels have been mined the pillars are then extracted. The coal miners work back towards the mine access tunnel or shaft, allowing the roof to collapse.

Longwall mining is not used in New Zealand because it requires a coal seam of uniform thickness which extends over a large area. Such formations are rare here because of the country's active geological history and the associated faulting, folding and shearing of the seams. Longwall Mining is very efficient because it can be mechanised and it produces coal at a high rate for a low cost. A rotating cutter moves side to side across a face of coal that may be several hundred metres long. With each pass a fresh slice is cut from the seam. The coal falls onto a conveyor that carries it away from the mine face. The roof is held up by a row of hydraulic supports that move forward as mining advances. The roof can be allowed to collapse behind the supports.

Surface Mining
Surface mining methods are preferred because they are safer and a greater proportion of the deposit can be extracted. Surface mining is generally cheaper than underground mining methods. This is an important consideration as the effort continues to make coal mines more productive.

Open pit or open cast coal mining is carried out in several stages:

• Rocks and soil (called overburden) overlying the coal are removed using earthmoving equipment. This exposes the coal. The soil is stockpiled for later use.
• The coal is excavated and loaded into trucks which take the coal to a stockpile. From here is is transported by road, aerial ropeway, or conveyor belt to a plant where it is prepared for sale.
• When mining is completed the overburden is replaced. The soil which has been stockpiled is returned. The area is tilled, fertiliser is added, and the area is sown with pasture grasses and trees are planted.

When the coal has been mined it is processed. The coal is crushed and then screened into various sizes. At the same time it is washed to remove fine material. It will then be stockpiled before being transported by truck, train, or ship. Sometimes different types of coal are blended together before they are sold.

What is coal used for?……

Some people think that coal is used just for generating heat - whether in an open fire at home or in a steel mill or to generate electricity. While coal is very important in the steelmaking industry and in electricity generation, Coal is also used in the manufacture of a wide range of products, many of which we take for granted in our everyday lives.

Soap, washing powder, perfume, printing ink, sheep dip, antiseptic, weedkiller, adhesives, timber preservative ... the list is extensive.

How are coal mines rehabilitated?……

Open pit coal mines require careful rehabilitation. Factors such as the contour of the land, erosion control, replacement of subsoil and topsoil, and revegetation are all considered in any rehabilitation plan.

The aim of rehabilitation is to return the area to a stable landform which is self sustaining and compatible with surrounding land use. In some cases it is possible to return the land to a better condition than that which existed prior to coal mining commencing. Drainage, fertiliser application, earthworm 'seeding', weed control, the revegetation programme, types of ground cover, and possible end uses such as stock grazing, forestry, or recreation are all considered as part of an effective post-mining rehabilitation plan.

Advances in environmental management and a greater awareness of the need for effective rehabilitation programmes mean that rehabilitation studies are now carried out as part of the feasibility study of any new open pit mine to ensure that the design of the project is suitable.

Links to other sites……

An excellent range of New Zealand-based resources is available on the Solid Energy website http://www.coalnz.com/re-school.htm

For a brief description of coal and links to lots of other sites:
http://www.enviroliteracy.org/coal.html

Lakeview School students in Gillette Wyoming have produced information about their local coal mine as part of the Powder River Coal Co site:
http://web.ccsd.k12.wy.us/mines/PR/Pr.html

There is some additional useful information about coal for students at:
http://testweb.alberta.com/"coalftp/class.htm

For a range of teacher resources, lesson plans, and activity ideas and a wide range of other useful material see:
http://www.coaleducation.org

New Zealand has 8 billion tonnes of proven coal resources. Gasification technology can provide the means to burn coal more cleanly and meet more of our energy needs. Find out more. To visit the Gasification Technology Website click here.

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