Haytor rocks

This lovely image illustrates a geologic feature in southwest England known as the Haytor rocks. 

The rocks are granites, produced almost 300 million years ago as the ocean that preceded the Atlantic was closing and North America and Europe collided.

The processes of continental collision and mountain building produced magma within the Earth’s upper crust. That magma rose up and interacted with the sedimentary rocks above, taking some of the chemical elements that are common in sediments like potassium. These magmas eventually formed potassium-rich granites in this area.

The shape of the rocks you see is a result of fracturing. The vertical fractures in the granite likely formed as the magma was first cooling down; rocks shrink when they cool and that shrinking is enough to cause rocks to crack. The geologists term for cracks like that is “joints”.

The horizontal cracks likely formed much later. They are formed parallel to the Earth’s surface, implying that they are related to removal of weight above this granite. As rocks above this level were eroded, the weight and the pressure on the rocks decreased. The rocks were used to being under pressure, so when the pressure was removed, they cracked and expanded upwards.

The area shows several different types of granite and has been used for both quarrying and recreation, including rock climbing. Some links refer to a fairly spectacular granite-lined path used to transport granite away from the quarry sites.


Image credit: Dave(creative commons)

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