Cornish Geology

“The Cornubian granite batholith provides one of the main high heat flow provinces within the UK.”

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 to produce the geology of Cornwall that we see today.

When you’re out and about in the Duchy, there are five key geological features you can look out for:

  1. Low-grade metamorphic rock, locally known as Killas
  2. Crystalline Granite (this could have very fine or very coarse crystals depending on how quickly it cooled)
  3. Highly-mineralised Lodes which have been heavily mined
  4. Small-scale igneous intrusions, locally known as Elvans
  5. Large fault structures cutting across the rocks
Sedimentary basins of Southwest Britain, Camborne School of Mines (2013).

What Gives Cornwall it's High Geothermal Potential?

Granite.

Almost all of Cornwall and part of Devon is intruded by a large granite body called the Cornubian Batholith. This granite outcrops at the surface in multiple locations but it is a large, three dimensional body with peaks and troughs, so even where you cannot see it at the surface, it may be metres or even kilometres beneath your feet!

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The Cornubian granite batholith stretches from Dartmoor out to the Isles of Scilly
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The batholith contains a variety of minerals, some of which are enriched in critical raw materials such as Lithium, where others contain high concentrations of heat-producing radionuclides such as Potassium (K), Uranium (U) and Thorium (Th). These radionuclides are decaying over time, halving in number over thousands or even millions of years. As they decay, they release heat and it’s this heat that gives Cornwall it’s geothermal potential.

To the left, you can see the difference that the Cornubian Batholith makes to the local surface heat flow in Cornwall compared to the rest of the UK.

On average, the UK has a surface heat flow of 50-60mW/m2 whilst parts of Cornwall reach up to 120mW/m2. This creates a geothermal gradient which is almost 10oC per kilometre hotter than the UK average!

Structural Geology

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.

Overview of some of the major Cornish crosscourses, BGS Regional Memoir 1975.