500 million years ago the Latrobe Valley area was covered by a sea with a slimy, mud floor.
100 million years ago the area was a broad, flat valley that was very swampy, and vegetation began to form peat. As the valley sank further due to faults in the Earth's crust, the vegetation sank below the water level in the swamp, and because it wasn't exposed to air, it did not rot away.
35 - 60 million years ago, the vegetation had been squashed so much by overlying material that it had turned into brown coal. The coal lies in 'seams' (layers), varying from 130 metres to 170 metres thick, separated by 'interseams' which contain sedimentary rock deposited when the sea occasionally covered the area .
Black coal is much older than brown coal. The black coal that is found in Queensland and New South Wales is about 300 million years old, compared to the brown coal in the Latrobe Valley which is only about 40 million years old.
As it has been buried for a longer time, black coal is deeper and is covered by more material. This means it has been squeezed more and has lost a lot of its moisture. In comparison, the brown coal in the Latrobe Valley still contains about 65% water.
What does all this mean when it comes to mining and burning coal for electricity? Black coal provides two to three times more energy than brown coal because its water content is lower. However, advantages of brown coal are that it:
