[MUSIC] So we've talked about how coal forms, how do we get it out of the ground? How is coal mined? How is it obtained? Well, remember, coal is a sedimentary rock. It occurs as sedimentary layers interbedded with other strata. So if those coal layers are close enough to the surface, we can get at them simply by scraping off the layers above and then digging out the coal. Now, the earliest coal mines occurred in places where coal was exposed along a cliff and people just dug into the walls of the cliff. But pretty soon, that kind of mining was not sufficient to provide the supply of coal that people needed. So, other techniques came into play. Well, as I mentioned, if the coal is close to the surface, we simply strip off the overlying layers. Exposing the coal bed and then did it out of the ground. This process is called strip mining. Modern strip mining in industrialized nations utilizes incredibly huge equipment. Particularly, there's a machine called a drag haul, which is effectively a gigantic shovel. As big as two, three, four cars, which scrapes along the ground surface and digs up a huge amount of sediment and rock all in one scoop. Turns around and dumps it elsewhere. Exposing typically a trench, a very broad trench, in which the coalbed lies exposed to the air. Once that coal bed is exposed smaller shovels, usually mechanical shovels go in, dig up the coal, dump it into trucks or dump it onto conveyor belts that take it to a processing plant. Now, the trucks that are used in this transport process are not the typical kind of trucks you see along the road. These are gigantic mining trucks. Some of them can carry 200 tons in a load. This particular photograph is a sort of medium sized mining truck and you can see that a person only comes up to the middle of the wheel. So the coal operation, basically strips away the coal out of the trench. In places where there are reasonable environmental regulations, the miners have to then reclaim the land. So, what happens usually is the drag haul stockpiles the different components of material that it scrapes off in different piles. It'll stockpile the topsoil. Stockpile the subsoil. Stockpile the rocky debris. And then once the coal's been stripped, it goes back and it dumps those back into the ground in sequence. Eventually, smoothing the layer of top soil back over the surface, so that it can be replanted and the land can basically be reclaimed. Now the coal, once it's taken into the processing plant, it's then sorted it into different chunk sizes by various conveyors. Stockpiled in various piles until it can be transported to where it's used, usually by train. Now here's an example of a coal pile, looking a little bit more closely. You see that the coal tends to occur in big chunky blocks. Why is that? Well, part of those blocks are the bedding surfaces in the original coal. The coal is deposited as a layer of strata, so it will have bedding planes. And usually, those bedding planes maybe coated by a layer of clay are a little bit weaker. So, the coal will split on those bedding surfaces. In addition coal, when it forms, usually contracts and as a consequence it cracks. And typically, it forms sets of fractures or sets of cracks that are at right angles to one another. Those fractures or those cracks are called coal cleat. And often, ground water passing through the coal cleat will precipitate clay or other materials in the cleat. In any case, the cleat are planes of weakness. So when the coalbed is broken up, the chunks form bounded on top and bottom by bedding surfaces and on the sides by the coal cleat. Now if the coal layer is deeper down, if it's so deep that it would just take way too much effort to scrape off the overlying layers, then the only way to get at it is to use underground mining. There are two approaches to underground mining. In some cases, the layer of coal is stripped out and the ground above is allowed to collapse behind it. In some cases, the coal is taken out of passage ways leading behind columns in between the support the over lying rock. Coal mining has always been a dark, dirty and dangerous occupation. These days a lot of it is mechanized at least in industrialized countries. Utilizing special grinding equipment that's very low so that it is able to extract only the coal from the layer and not take away the rock above or the rock layer below. What do we use coal for? Originally, coal was used to fuel steam engines. It was also used just to burn in stoves to make heat. Nowadays, coal is used primarily to produce electricity in coal-fired power plants. Here is an example of a power plant, a small one. You can notice in the foreground, there are big piles of coal brought in from coal mines by train, dumped and then conveyed by these inclining conveyor belts into the furnaces. Because of the demand for energy in rapidly industrializing countries, coal use for power has been rapidly increasing. And therefore, coal production has been rapidly increasing too. As this graph shows, most of this increase is currently taking place in China [MUSIC]
