Coal Mining. States like Kentucky, Illinois, West Virginia and Pennsylvania lie on top of
coal beds which 300 million years ago were river delta swamps like New Orleans. The typical
coal bed is 1-2 meters thick. In many areas, old-fashioned underground mining has been
replaced by “area mining” (where all the rock is removed from a giant pit, exposing the coal
bed) and “mountain top removal” (where the top of a mountain is sliced off to expose the
coal bed). In both cases, the government now requires that most of the removed material be
put back in its original location.
To what depth of burial of the coal bed is this form of mining energetically efficient (i.e.
the energy from combustion of the coal recovered exceeds the energy expended in mining it)?
Consider both gravitational energy of the rock over the coal, and the energy required to break
the rock covering the coal into transportable pieces.
