The geological map of the Southwest
Granite is an intrusive, or plutonic rock; it is formed when molten rock buoys to near the surface, and then cools down and becomes solid. That takes a while; that's why granite tends to have had time to form big crystals. Quite unlike basalts, for instance, which are volcanic rocks, and reach the surface in molten form and solidify there. That goes much faster, and hence the completely different, and much more homogenous, look.
Granite. Source: Creative Commons
As granite is formed below the surface, it tends to not be very chemically stable at the surface. And on the time scales of buildings that is not much of an issue, but on the time scale of, say, Dartmoor, it is. These granites have been lying there for roughly 300 million years, and time has not passed by unnoticed. Large parts of the granite have weathered into china clay. Many clay pits still litter the countryside.
One of the big China Clay pits in the Southwest. Notice the infrastructure-poor mass of Dartmoor on top, and the arable lands below, and the clay pit in between. It lies on the contact of the granite and the surrounding rock, where weathering would be most severe.
There are mines, too, in the granite. And where you dig a tunnel into the granite you accelerate the weathering process. So a mine in granite may become crumbly, and prone to collapse.
There is something to say for that these mines are to be avoided. Dying in a collapsing mine does nothing for the wellbeing of one’s loved ones. Doesn’t help the project one works in, either. But then again; such a mine will not exist for much longer. If one has the chance to see one that opportunity must surely be seized! The chances of anything going wrong while someone is inside is slim.
Pics by John
What is wise? Well. The answer is easy. But what does one do? I went in. It was a beautiful place. We might want to go back to explore the bit we couldn’t do this time, but one day within our lifetime we'll have to leave it to its inevitable demise.