Staffa, an entirely volcanic island, is probably best known for its unique geological features such as the many caves and the shape of the basalt columns which are also found in the Giant’s Causeway and Rathlin island in Northern Ireland and closer by on the island of Ulva. It consists of a basement of tuff, underneath colonnades of a black fine-grained Tertiary basalt, overlying which is a third layer of basaltic lava lacking a crystalline structure. By contrast, slow cooling of the second layer of basalt resulted in an extraordinary pattern of predominantly hexagonal columns which form the faces and walls of the principal caves. The lava contracted towards each of a series of equally spaced centres as it cooled and solidified into prismatic columns. The columns typically have three to eight sides, six being most common.
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