Latrobe Valley Coalfields
Our coalfields are much younger than the first coal fields of the Northern Hemisphere. The coal mined for power generation in Victoria is brown coal, in which the process of coalification began 45 million years ago.
500 million years ago the Latrobe Valley was covered by a deep sea. The seabed was covered by slimy mud, the mud was compacted to become slate and mudstone, this process took 100 million years.
350 million years ago the area was lifted and molten rock rose from deep in the Earth's crust. When it cooled the molten rock became an Intrusive Igneous rock called Granodiorite.
Glaciers swept across the Latrobe Valley around 230 million years ago, flattening and eroding the landscape.
The much harder rock Granodiorite was exposed at the surface. Weather easily eroded the softer rocks, shale and mudstone, while the Granodiorite remained intact and is now known as the Baw Baw Plateau.
60 million years ago cracks called faults formed in the Earth's crust. The Latrobe Valley was formed by rock settling between the faults.
The Latrobe Valley naturally became a swampy place, because the streams from the surrounding hills flowed into the area. This was an excellent area for vegetation, which began the first step of the coalification process.
But not all of the coal in the Latrobe Valley is the same age. The faults allowed blocks of land to drop a long way and the swamps became lakes with sand and mud accumulating on the bottom. Different areas of the swamp grew different vegetation this eventually produced various types of coal, all within the same coal seam. This process continued over a period of 45 million years
The black coal of the Northern Hemisphere is approximately 300 million years old, which is much older than the coal of the Latrobe Valley. The main difference between brown and black coal is, black coal has been buried for a longer time by a thicker material. This means that the coal has been squeezed more and has had a lot more of its moisture removed from it. Squeezing is not the only process that causes moisture loss; heat from inside the Earth also helps to dry out the coal, as it is buried deeper into the Earth's crust.
Brown coal having been squeezed and heated less has much higher moisture content than black coal. The brown coal of Latrobe Valley has between 60% - 70% moisture, where as black coal has between 10%- 20% moisture, while the driest black coal, Anthracite has no water in it.
Moisture content is important when considering its energy content. When black coal is burnt in a power station boiler, it releases 23 - 30 megajoules of energy per kilogram of coal. When brown coal is burnt, it releases only 6 - 9 megajoules of energy per kilogram of coal because it is 60% - 70% water.
