Blackhead Basalt Columns (Dunedin, Otago) EarthCache

RAMBLES ROUND DUNEDIN: GREEN ISLAND PENINSULA
… The base of the hill is composed of a magnificent range of basaltic columns, standing erect, of much larger size than the beds above, quite as complete as those of Fingal's Cave at Staffa, or the Giant's Causeway in Ireland. Indeed, the rocks here are as worthy of a visit as either of these Old Country lions and there is no doubt, were they as well known, they would be equally famed. The photographer, if his chemicals would not be affected by the damp sea air, would find many an effective grouping, which would make most capital pictures during a sou-wester the contrast between the black rocks and the broken water must be very fine, and give plenty of light and shade.

BASALT COLUMNS, BLACK HEAD, NEAR DUNEDIN. (See Illustrations.) [P.S. This is completely irrelevant, but the same issue of the Otago Witness has photos and photos and a report of the first test Match at Carisbrook, 6 June 1908]

These columns are best seen at the sea level. They can be reached in calm weather at low tide from the extreme north end of the Brighton Beach, though some scrambling is necessary to reach them. Basalt columns have been discovered in nearly all parts of the world, but those of the Giant's Causeway and Cape Raoul [photo] are better known to us than those of other localities. They are to be seen in many places near Dunedin, especially on a peak behind Mount Cargill, at the north head of Otago Harbour, and at Sandymount. In none of these localities, however, have they got the symmetry and regularity of those to be seen at Black Head.

The causes that have resulted in the formation of these columns are well understood. Basalt rock is of volcanic origin, and issued from volcanoes in a molten state. Being relatively fluid, it flowed down the sides of the volcano and accumulated in large pools in depressions near its base. These pools of melted rock gradually cooled down and became solid. As the cooling proceeded the material shrunk or contracted. The tension that this contraction caused was relieved by the formation of crevices, and it is a fact that the tension is most satisfactorily relieved by the formation of a series of vertical crevices intersecting at angles of 120deg. Such a series of crevices separates the rock mass into hexagonal columns. The action is exactly similar to the formation of sun cracks in drying mud, only the shrinkage of mud is caused by the evaporation of some of the water in it, while that of basalt is caused by cooling. Columnar structure is commoner in basalt than in other volcanic rocks, because it is more fluid than other kinds, and collects in pools before it has time to solidify, while less fluid kinds could not flow so far before they became solid. If a lava solidifies while still in motion, its shrinkage crevices are irregular in size and direction, while if it cools very quickly the crevices are arranged in horizontal series, as in the massive rocks of the Ross Creek quarries. Basaltic columns vary widely in size. Those of Black Head are 6in to a foot in diameter, while those at Sandymount measure 2ft. The size probably depends upon the grain of the rock.