Cornwall


Cornwall is the westernmost part of the South West Peninsula, between the Bristol Channel to the north and the English Channel to the south. This includes the westernmost point of the peninsula and of England which is named Land’s End. It is a part of South West England, one of the nine regions of England. Cornwall has a rocky coastline with steep cliffs and, to the south, several rias, river valleys which were flooded by the sea after the last ice age. The rocks are granite intrusions surrounded by slate. Both are impermeable for water, so the whole area is characterized by high moorland.

As the geology is not suitable for the formation of caves, we have only a sea cave, which is the result of erosion. Most sites are mining related, the first reference to mining was by the 4th century BC geographer Pytheas wrote: "They prepare the tin, working very carefully the earth in which it is produced". Even Julius Caesar mentioned the tin trade. Actually, tin mining and later copper mining were an economic backbone for millennia, with ups and downs. The mining declined during the 19th century. Today the economy is based on tourism, and the mining heritage is an important part of this tourism.

The Cornwall peninsula is mainly a huge crystalline diapir, an intrusion of magma which cooled very slowly underground. The typical rock of those intrusions is granite, which is chemically similar to basalt, but has a really different shape. The lava at the surface cools very fast and so all minerals in the lava harden to a fine granulated dark rock. Magma underground cools very slowly, as the surrounding rock insulates the heat, so the result contains big patches of different minerals.

The lava cools slowly down, until the melting point of the highest melting mineral is reached. Then this mineral becomes hard, forming crystals floating in the hot lava, growing bigger and bigger. This happens again and again, until only one molten component remains, which fills all the gaps. Then the whole rock is hard, consisting of minerals and filler. This is typically granite, consisting of mica, feldspar and quartz.

The interesting process necessary for the Cornish mines is hydrothermal deposition, which happens when groundwater reaches the still hot intrusion. The rock breaks when it gets contact with the cold water, and the fissures allow the groundwater to flow through the whole intrusion. The water is hot and thus is able to dissolve numerous metals and minerals, which are contained in the granite and the surrounding rock in a very low amount. The water transports those substances through the cracks and deposits them inside the cracks. These are typical ores for Cornwall: tin and copper ores in narrow fissures crossing the granite diapir radially. While the main ores were tin and copper in combination with arsenic, there were also lower amounts of tungsten, wolframite, bismuth, cobalt and uranium. Those side minerals were not processed in older mines, in the 20th century they were only in few mines found in a high enough amount to make processing worthwhile.

Such cracks with minerals and ores are called gangues or lodes. As they are typically vertical they are also called dykes. From the coast they run towards the center of the peninsula. The thickest lodes with the best ore were found deep below the ground and extending under the sea floor. So several Cornish mines were buit up to 60 m deep and extended 1.5 km into the sea underground.