The granites started life around 300 million years ago. The first sign that things were really cooking deep down was the intrusion of lamprophyres, strange rocks derived from the earth's mantle. The easiest place to see an example of these rocks is on the beach at the south end of Prisk Cove near Mawnan Smith. The rocks that started life in the high pressure and temperature conditions of the mantle are now very weathered and lying on a beach roamed by cows!
Soon afterwards, the main bodies of the granites started to rise up one after the other, like giant hot air balloons, into the overlying rocks. As they rose, they pushed like fingers into the surrounding slates (killas) of which they swallowed great chunks. You can still see these foreign slate bodies or xenoliths within them. The best places to see giant tapering fingers of granite in the slates are at Megilligar Rocks near Porthleven, or Porthmeor and Wicca Pool on the north coast of Penwith. It was here that Henry De La Beche, who was to become the first director of the British Geological Survey, sketched the jagged granite boundary more than 150 years ago.
What was happening on the surface above the granites? Most of the overlying strata has been removed by erosion but there is evidence at Kingsand and Withnoe, that the granites literally blew their top in the form of violent volcanoes like Vesuvius or Mount St Helens in North America in more recent times. Here rhyolites, the fine-grained version of granite, can be seen as volcanic neck and lava flows.
When the rising granite balloons reached as far upwards as they could go they started to solidify and form a crystalline mass. This initial crystallisation caused the remaining molten rock to change composition and become increasingly volatile. Every so often the pressure got so much that the volatile mineral rich components escaped through fissures within and beyond the granites. This spectacular geology can be seen at places like Cligga, Roche Rock and the mineral lodes and elvans which criss-cross many parts of the county.
A long period of more than 200 million years separates the formation of the granites and associated mineral lodes from today. During much of this huge span of time, which saw the dinosaurs come and go and flowering plants evolve, the climate was tropical and dominated by monsoons. The resulting downpours caused weathering deep into the ground, rotting the granites. This effect was particularly marked in the granites around St Austell where the feldspar minerals were transformed into kaolin, more popularly known as China Clay. The Eden Project is located in one of these deeply weathered pockets from which all the clay has been removed leaving only the unaltered granite.
During the Ice Ages the rotten rock was further broken up by the growth of ice crystals repeatedly forming in the freezing conditions. During the thaws in the arctic summers the rotted material became so waterlogged that it slurped down slope leaving the stronger unweathered parts of granite to stand proud as tors. Excellent examples occur throughout the region, like the Stowe's Hill on Bodmin Moor and Pulpit Rock on the Isles of Scilly.
