Before a mine is planned, the first thing engineers ask is what the coal seam actually looks like underground. How flat is it? How thick? How deep? Is it broken up by faults or dykes? These details decide the method of mining, the machinery and, in the end, the cost of every tonne of coal.
This post explains the general form of coal deposits and seams, using Australian coalfields as the example. If you are studying for GATE Mining, DGMS or Mining Sirdar exams, it is a handy base to build on.
What Does a Coal Seam Look Like?
Most coal seams in Australia are tabular deposits. In simple words, they are wide, sheet-like layers of coal spread over large areas. They usually lie at a low dip angle, which means the seam is fairly flat and not steeply inclined. Flat seams are good news for miners because machinery and roadways are far easier to manage on gentle gradients.
Seam Thickness
Thickness changes from place to place, but usually it changes gradually. Seams range from less than 1 m to over 30 m thick.
That does not mean every seam can be mined, though. In Australia the thinnest seam worked has been around 1.3 m, and very few mines work seams thinner than about 1.8 m over long stretches. At the other end of the scale, very thick seams are rarely uniform in quality from top to bottom. So most mines working such seams take only a portion of the full thickness, and few mines, if any, mine more than about 5 m in height.
The one exception to gradual change is a washout. This is a place where part of the seam was eroded away long ago and later filled with stone deposits. Mining into a washout suddenly gives you stone instead of coal.
Breaks in the Seam: Faults and Dykes
Seams generally extend over large areas, but they are often interrupted by geological discontinuities. The two most common ones are:
- Faults: places where the strata has fractured and the rock on one side of the crack has moved relative to the other. The seam on either side no longer lines up, so the mine has to deal with a sudden jump up or down.
- Dykes: igneous intrusions where molten rock once flowed into and through the seam and cooled into a wall of rock. Heat from the intrusion often burns the coal close to it, a process known as cindering, which ruins that coal.
A normal fault displacing a coal seam
A reverse fault displacing a coal seam
An igneous dyke cutting through a coal seam
Splitting and Coalescing of Seams
In some coalfields, a single seam may split into two or more separate seams, divided by bands of rock. The opposite can also happen: separate seams may coalesce, meaning they merge into one thicker seam. Both situations change the thickness you can mine and the quality of the coal, so they matter a lot while planning.
Depth of the Seam
Depth below the surface varies from zero, where the seam outcrops at the surface, down to around 450 to 500 m. Deeper seams do exist, but they have not yet been worked in Australia.
Quick Revision Points
- Coal seams are usually tabular deposits with a low dip angle.
- Thickness ranges from under 1 m to over 30 m, but few mines work more than about 5 m in height.
- Minimum thickness worked is around 1.3 m, and few mines work under about 1.8 m for long stretches.
- A washout is an eroded part of the seam refilled with stone.
- Faults displace the seam, while dykes cut through it and can cinder the nearby coal.
- Seams can split into several seams or coalesce into one.
- Worked depth runs from the outcrop to around 450 to 500 m.
If this post helped you, share it with your classmates and leave a question in the comments. We will keep adding more topics like this on the blog.
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