Abstract

Black ash or cinders from explosive volcanic eruptions form conspicuous surficial deposits in the Tantalus, Makiki, and other parts of Honolulu. The fine-grained black sand deposits southeast of Diamond Head and along the Black Point coast have long been considered to be true black ash of explosive origin. Recently the Diamond Head material has been described as the product of wave action on coastal rocks, drifted inland by the wind to form dunes, hence a sand rather than a true ash formed by explosion. In order to determine which of these views is correct, the writer has repeated and extended earlier field and microscopic examination of the material in question. The result is a complete substantiation of the original view. About 98 per cent of the fragments are glass, in vesicular, pulled, droplet forms, with delicate, unbroken points, and with no sign of abrasion. Structure, bedding, and distribution, as well as the composition, all strongly indicate formation by volcanic explosion, with but a minor part indicating some contemporaneous movement by the wind. Final proof of the volcanic interpretation is afforded by a one-inch layer of volcanic pisolites, or accretionary lapilli, which have been traced in the formation some 500 feet along the Black Point coast. These pisolites are tiny mud balls about a half millimeter in diameter which were formed around raindrops which fell through the dust-laden air during a brief shower which accompanied the eruption. Such pisolites are well known at Kilauea, and their formation has been observed by many geologists. The black ash beds lie on a reef conglomerate containing rounded boulders derived from the earlier Black Point lava flow and hence are the product of a second Black Point eruption. The explosive phase was probably induced when the molten lava entered the sea, the explosive eruption being of the littoral type, like that which formed Puu Hou near Kalae, Hawaii, in 1868. It evidently took place when the sea stood no higher than at present and was the most recent volcanic event in the Diamond Head area.

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