Cornwall has a long history of mining and the iconic engine houses still pepper the Cornish landscape as a reminder of the bygone era. The engine houses were constructed to house steam engines to pump out the continual influx of hot water that flooded the mines, even at shallow depths where shafts traverse the crosscourses. More recently, the Hot Dry Rocks (HDR) project – which commenced in Cornwall in the mid-1980’s – enabled research and development of the geothermal potential in the granite batholith.
The GEL Geology team are excited by the geothermal potential of Cornwall, as naturally fractured rock, or fault zones, provide highly permeable pathways for the flow of hydrothermal fluid. These faults, known by the Cornish miners as ‘crosscourses’, enable the circulation of geothermal fluids within the granite, which would otherwise be impermeable.
Cornwall’s surface heat flow is higher than the rest of the UK. As seen below, on average the UK has a surface heat flow of 50-60mW/m2, whilst parts of Cornwall reach up to 120mW/m2. This unique geological setting gives rise to a geothermal gradient almost 10oC per kilometre hotter than the UK average!
More than 350 million years ago, Cornwall was forming through the deposition of sediments within six large basins. Over time, these sediments have been moved, compressed, faulted, and intruded by granite to produce the geology of Cornwall that we see today.
When you are out and about in the Duchy, there are five key geological features you can look out for:
- Low-grade metamorphic rock, locally known as Killas.
- Crystalline Granite, this could have very fine or course crystals depending on how quickly it cooled during its formation.
- Highly mineralised Lodes, which have been heavily mined for tin and copper.
- Small scale igneous intrusions, locally known as Elvans.
- Large fault structures cut across the rocks.
What gives Cornwall its high geothermal potential?
Almost all of Cornwall, the Isles of Scilly, and parts of Devon are intruded by a large granite body called the Cornubian Batholith. This granite outcrops at the surface in multiple locations in Cornwall, however, it is a large, 3-D body with peaks and troughs, so where you cannot see the granite at the surface, it may be meters or even kilometers beneath your feet.
The batholith contains a variety of minerals, some of which are enriched in critical raw materials (CRM) such as Lithium. Cornish granite also has high concentrations of heat-producing radionuclides such as Potassium (K), Uranium (U), and Thorium (Th). These radionuclides are decaying slowly over a long period of time, halving in number over billions of years. As they decay, they release energy in the form of heat, it is this heat that gives Cornwall its geothermal potential.
Structural Geology
Whilst Cornish granite is naturally heat-producing, to harness the geothermal potential we also need pathways for geothermal fluid to flow through at depth, enabling it to transport heat and critical raw materials (CRM). These pathways in the rock are found in the form of fractures or fault zones.
A fault zone is made up of a series of cracks, fractures, and larger planes that cut across rock types forming due to the movement of the crust. Depending on the main orientation of crustal movement at the time, a whole system of faults can form 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. Both lodes and cross course have been widely mapped from the surface down to a maximum of 1000m depth in historic tin and copper mine workings in Cornwall.
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. The crosscourses have been associated with the flooding of mine workings and warm springs, proving there is high permeability.
