Whilst the granite provides the heat, to harness the geothermal potential we also need a pathway for the heat to flow through; fault zones.
A fault zone is made up of a series of cracks, fractures and larger planes which cut across rock types, forming due to movement of the crust. Dependent on the main orientation of crustal movement at the time, a whole system of faults are likely to be generated across a region trending in the same direction.
Cornwall has two main fault systems, one trending approximately East-West, forming lodes, and the other trending approximately Northwest-Southeast forming crosscourses. These systems have been widely mapped within the surface 500m in historic mine workings.
Crosscourses have been identified as providing the highest permeability as they trend parallel to the direction of maximum regional stress causing them to be more open than other structures. They are also often associated with mine flooding and warm springs, showing high permeability.