Structures such as columnar joints are found near gullfoss in the reykjanes peninsula. how were these structures created?

The columnar joints can be defined as the structures formed in basalt. The are composed of columns that are separated by the cracks or fractures that occurs in the rocks when it contracts when it is cooling down.  Basalt are volcanic igneous rocks formed from the erupted lava. The lava cools down after its eruption. When it cools, it starts contracting, which leads to the fractures and cracks. The contraction causes the formation of the columns mostly in the hexagonal shapes. This leads to the formation of the geometrical columnar joints.

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