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In my con-world I went lengths to introduce a plausible lighter-than-air element and did rather large amounts of thinking about landmasses in order to promote the development of airship technology over that of land-based technology.

This question is about the geological characteristics of most of the mountain ranges and rock in general in this world. Their mix/composition of rock, sand, dirt, etc.

How would mountains and hills have to be made-up in order to impede tunneling them WHILE still allowing them to be mined out?

By tunneling I mean digging tunnels to be used by trains and land-based-traffic to pass under a mountain rather than having to go up and down again.

By mining out I mean that they contain an abundance of useful minerals and ores that will be mined and make up one of the pillars of industry and society.

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A core of granite would allow mining in the outer layers but tunnelling through would be prohibitively expensive and time consuming.

It's almost universally quicker and cheaper to go round granite hills.

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    $\begingroup$ Yeah, a bunch of minerals atop a lump of granite will make mining trivial but tunneling quite hard... $\endgroup$
    – Shalvenay
    Commented May 19, 2016 at 22:10
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Well, think about the direction in which each of these operations needs to go.

Tunnels go horizontally across the mountain. Mines go more diagonally downwards, or even straight down in some cases.

So, if you make the middle of your mountain out of much denser, more solid rock that is really difficult for tunnelling, but add a layer of softer rock underneath that, then you can mine downwards to get the precious metals out, but you can't tunnel through.

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The difference between mining and tunnelling is how long the tunnelers plan to stay

In mining, the primary goal is to extract material. If there's no material of interest further down and stability of the overburden isn't important, then there's nothing to stop miners from using the cheapest/fastest support methods. They only need the supports to last a few hours/days to get out the mineral of interest.

Tunnelling has the opposite goal, in that the material removed isn't the goal, it's a means to an end. Thus, the shape and stability of the hole are of utmost importance to a tunneller. They will invest more resources in maintaining strong supports.

To achieve our ends, we need to find a way to make the permanent nature of tunnels too expensive over relatively transient mining.

Hill/Mountain Composition

It's said that "No one conquers a mountain, they merely sneak up on it." This refers to the dynamic nature of the slopes of a mountain. Even in the summer, boulders can work themselves loose and fall. Rock fall in the winter is even more prolific. In this same vein, we can construct a mountain range that is seismically active enough and violent enough that maintaining tunnels is just too expensive. Given the more transient nature of a mines, when a mine collapses (and the funerals are over), miners can start working on a new mine. In this situation, mines sneak up on the mountain while tunnels attempt to conquer the mountain (which never lasts).

In addition to the seismic activity, if the mountains are a mix of softer and harder stone then we can expect that the frequent shakes will cause nonlinear pressures on the tunnel and mine walls.

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