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How Minerals Form

A9 The Hurlers And New Phoenix Mine - Barry GambleUndersea lava, earthquakes, extreme temperatures, severe weather and relentless wave action have all played their part in creating the rocks, minerals and metals that make up the geology of the Cornish Mining World Heritage Site.

The story of the geology of Cornwall begins nearly 400 million years ago when sand and mud settled on the floor of a Devonian sea and molten rock formed undersea lavas which pushed into the sediments. 

Around 320 million years ago

During Carboniferous times, continents collided and caused a major earth movement in the region. This caused the earlier rock formations to fold and crush together on a roughly north-east to south-west alignment. 

Mudstones became slates which, together with underlying bands of sandstones, are known by the Cornish term ‘killas’.

This accounts for both the orientation of Cornwall’s great granite backbone, or batholith, and the locations of the main tin and copper lodes. Mudstones became slates which, together with underlying bands of sandstones, have long been known by the Cornish term ‘killas’.

Between 300 million and 270 million years ago 

During the late Carboniferous and Permian periods, more continental collision generated great heat and pressure which melted the crust to form granite. 

The intense heat caused water to circulate within cracks in the granite, dissolving metal from surrounding rocks.

Separate granite masses forced up into the rocks above them between 290 million and 270 million years ago. They merged to form a long granite batholith. The intense heat also caused water to circulate within cracks (or fissures) in the granite, dissolving minerals from the surrounding rocks and causing the main tin, copper and tungsten deposits around 270 million years ago.

Around 250 million years ago 

During the late Permian, a mountain chain was created during a period of considerable upward movement of the Earth’s surface. The rocks which once covered the granite were then gradually removed by deep weathering and erosion, exposing the tops of the granite domes. Around 236 million years ago (during the Triassic), the lead, silver, iron and zinc mineralisation formed in a north-south orientation. This alignment – perpendicular to the main tin and copper mineralisation – was due to different patterns of earth movement to when the granite batholith was formed.

Within the past 4 million years

Sea erosion created a relatively flat surface, as well as wave-cut platforms and raised beaches. It is likely that tin placer deposits (small deposits of tin found in riverbeds and valleys or on the sea floor) were formed around the same time – and went on being formed until relatively recently. 

The sea level fell during the Ice Ages of the past one million years – ending around 10,000 years ago – and rose in recent times by about 15 metres. River valleys (known as ‘rias’) were cut and subsequently flooded by these events, including the River Tamar and the Fal estuary.